the adiabatic turbocompound engine is a high-temperature ceramics utilized in ap- 16. J. A. Coppola and C. H.

McMurty, in National
good likelihood for commercialization by plications that would have been uni- Symposium on Ceramics in the Service of Man
(Carnegie Institution of Washington, Washing-
the 1990's. Ceramic components in truck magined a decade ago. ton, D.C., 1976).
and small industrial gas turbine engines 17. W. B. Hillig, in (2), pp. 979-1000.
18. R. N. Katz and G. E. Gazza, in (5), pp. 417-431.
can be phased in gradually and are also Refences ad Note 19. A. Tsuge, K. Nishida, M. Komatsu, J. Am. Ce-
ram. Soc. 58, 323 (1975).
likely to be commercialized. The major 1. Should We Have a New Engine?-An Automo- 20. D. R. Clarke and G. Thomas, ibid. 60, 491
question is whether the gas turbine, with tive Power Systems Evaluation (Rep. JPL-SP (1977).
43-17, Jet Propulsion Laboratory, Pasadena, 21. G. E. Gazza, Am. Ceram. Soc. Bull. 54, 778
ceramic components, will be adapted as Calif., 1975), vols. I and 2. (1975).
an alternative engine for the automotive 2. J. J. Burke, E. M. Lenoe, R. N. Katz, Eds., Ce- 22. R. J. Bratton, C. A. Anderson, F. F. Lange, in
ramics for High Performance Applications-II (2), pp. 805-825.
industry. The Department of Energy's (Brook Hill, Chestnut Hill, Mass., 1978). 23. G. E. Gazza, H. Knoch, G. D. Quinn, Am. Ce-
3. Proceedings of the Workshop on Ceramics for ram. Soc. Bull. 57, 1059 (1978).
Advanced Automotive Gas Turbine Pro- Advanced Heat Engines (Rep. CONF 770110, 24. A. Tsuge and K. Nishida, ibid., p. 424.
gram (11) is currently funding the devel- ERDA, Washington, D.C., 1977). 25. D. J. Godfrey, J. Br. Interplanet. Soc. 22, 353
4. Proceedings of the Conference on Basic Re- (1969).
opment and evaluation of two such en- search Directions for Advanced Automotive 26. D. R. Messier and P. Wong, in (15), pp. 181-194;
gines, one by AiResearch Company with Technology (Department of Transportation, J. Mangels, in ibid., pp. 195-206.
Washington, D.C., April 1979). 27. G. R. Terwiliger and F. F. Lange J. Mater. Sci.
Ford Motor Company as a subcon- 5. R. W. Davidge, in Nitrogen Ceramics, F. Riley, 10, 1169 (1975).
tractor, and one by Detroit Diesel Alli- Ed. (Noordhoff, Leyden, 1977), pp. 653-657. 28. H. F. Priest, G. L. Priest, G. E. Gazza, J. Am.
6. J. L. Klann, Advanced Gas Turbine Perform- Ceram. Soc. 60, 81(1977).
son in conjunction with Pontiac. These ance Analysis (8th Summary Rep., Automotive 29. M. Mitomo, M. Tsutsumi, E. Bannai, T. Tan-
Power Systems Contractor's Coordination aka, Am. Ceram. Soc. Bull. 55, 313 (1976).
engines integrated into vehicles are due Meeting, ERDA-64, May 1975), p. 173. 30. A. Giachello and P. Popper, paper presented at
for demonstration around 1982 to 1984. 7. H. E. Helms and F. A. Rockwood, Heavy Duty the 4th International Meeting on Modern Ce-
Gas Turbine Engine Program Progress Report ramic Technologies, St. Vincent, Italy, 28 May
Assuming technical success of these pro- (Rep. DDAEDR 9346, contract NAS3-20064, to I June 1979.
grams, there is still the issue of the in- NASA-Lewis Research Center, February 1978). 31. J. A. Mangels and G. J. Tennenhouse, Am. Ce-
8. F. B. Wallace et al., in (2), pp. 593-624. ram. Soc. Bull. 58, 884 (1979).
vestment costs for this new technology 9. R. Kamo, in (2), pp. 907-922. 32. K. HJ. Jack, in (5), pp. 103-128.
versus the costs of advanced piston en- 10. R. A. Penty and J. W. Bjerklie, in New Horizons 33. L. J. Gauckler, S. Boskovic, G. Petzow, T. Y.
Materials and Processes for the Eighties, Pro- Tien, in (2), pp. 559-572.
gine technology. Conversion of the na- ceedings of the 11th National SAMPE Technical 34. A. F. McLean and J. R. Secord, U.S. Army Ma-
tion's automotive engine production Conference (SAMPE, Azusa, Calif., 1979); R. ter. Mech. Res. Cent. Rep. TR 79-12 (1979).
A. Penty, personal communications. 35. Presentations on these programs were made at
lines is a multibillion-dollar undertaking. 11. G. Thur, paper presented at the 17th Depart-
ment of Energy Highway Vehicle Systems Con-
the Conference on Ceramics for High Perform-
ance Applications III-Reliability, held at Orcas
Whether ceramic gas turbine technology tractors' Coordination Meeting, Dearborn, Island, Wash., July 1979.
is fully utilized will depend on complex Mich., October 1979. 36. W. L. Wallace, J. E. Harper, F. B. Wallace,
12. G. Q. Weaver and B. A. Olson, in Silicon Car- "Ceramic Gas Turbine Engine Demonstration
cost-benefit trade-offs between it and al- bide-1973, R. C. Marshal, J. W. Faust, Jr., C. Program," Interim Rep. 13 on contract N00024-
ternative technologies. What is impor- E. Ryan, Eds. (Univ. of South Carolina Press, 76-C-5352, May 1979.
Columbia, 1974), pp. 367-374. 37. C. F. Bersch, in (2), pp. 397-406.
tant in our increasingly energy- and re- 13. S. Prochazka, in ibid., pp. 391-402. 38. H.-C. Miao, Y.-H. Uu, T.-C. Chiaug, paper
14. R. J. Bratton and D. G. Miller, in (2), p. 719. presented at the 4th International Meeting on
source-scarce world is that such options 15. S. Prochazka, in Ceramics for High Perform- Modern Ceramic Technologies, St. Vincent,
are at least available. In any event, over ance Applications, J. J. Burke, A. E. Gorum, R. Italy, 28 May to I June 1979.
N. Katz, Eds. (Brook Hill, Chestnut Hill, 39. R. J. Lumby, B. North, A. J. Taylor, in (2), pp.
the next decade we will begin to see Mass., 1974), pp. 239-252. 893-906.

veloped which became known later as

the superalloys. At the same time, a new
class of titanium alloys of high specific
strength became available in usable
structural forms. The superalloys proved
to be of great utility in the hottest parts
Aircraft Gas Turbine Materials of the engine, such as burner and turbine
sections, whereas the titanium alloys
and Processes were ideal for the cooler compressor sec-
tion of the engine. The effect of these de-
velopments was to cause a sharp in-
B. H. Kear and E. R. Thompson crease in the use of superalloys and tita-
nium alloys in engines, at the expense of
conventional nickel- and iron-base alloys
(Fig. 1).
For more than three decades the de- ing levels of engineering sophistication Starting in the mid-1960's, the empha-
velopment of the gas turbine engine has and performance. As an indication of the sis gradually shifted from alloy develop-
been paced by the availability of materi- remarkable progress that has been made ment toward process development. In
als and the ability to process them into over the years, it may be noted that, the productive period that followed, sev-
useful shapes. The most challenging ma- since the 1950's, thrust-to-weight ratios eral important advances were made in
terials problems have been encountered have tripled, fuel efficiencies have more the materials processing area. Perhaps
in aircraft gas turbines. This is because than doubled, and the time between the most striking innovation was the in-
of the need to maintain high operating ef- overhauls has increased from 100 to troduction of directional solidification
ficiencies without incurring unacceptable more than 10,000 hours. processing of turbine blades and vanes.
weight penalties. It is to the credit of ma- The most significant developments in
terials technologists that these challeng- alloy design occurred in the early 1950's. B. H. Kear is a senior consulting scientist and E.
es have continued to be met, as engine Entirely new classes of heat-resistant R. Thompson is a group leader at United Tech-
nologies Research Center, East Hartford, Con-
designs have progressed to ever-increas- nickel- and cobalt-base alloys were de- necticut 06108.
SCIENCE, VOL. 208, 23 MAY 1980 0036 8075/80/0523-0847$00.S500 Copyright G 1980 AAAS 847
Another development of equal signifi- The effect is to drive the turbine, which are attached to the compressor casing
cance was the adaptation of powder met- in turn drives the compressor. The high- between adjacent rotor stages. The flow
allurgy processing techniques, such as velocity gases expelled through the ex- path, defined by successive stages of the
hot isostatic pressing and superplastic haust nozzle generate engine thrust. compressor, decreases in the direction of
forging, to the production of turbine Additional thrust is derived from the air flow, corresponding to the reduction
disks. Along with these developments, relatively low-velocity bypass air driv- in volume as compression occurs from
which were focused primarily on the su- en by the fan and ducted outside the stage to stage (Fig. 2). The high-pressure
peralloys, important gains in processing engine. compressor runs hotter and at higher
capabilities were also achieved for tita- The indicated temperature and pres- speeds than the low-pressure compres-
nium alloys. At the same time, advanced sure variations correspond to maximum sor.
composites were developed, and these thrust developed on takeoff and repre- Blades and vanes must be capable of
have found useful applications in the en- sent important design parameters. From resisting aerodynamic loading. In addi-
gine. the materials standpoint, the crucial fac- tion, blades must be able to withstand
The situation today is that superalloys tor is the peak temperature developed at high centrifugal loading and the effect of
account for about 50 percent by weight different locations in the engine, since vibratory stresses caused by high-veloc-
ity air streaming from the spaces be-
tween blades. Disks must possess high
Summary. Materials and processing innovations that have been incorporated into load-bearing capacity, since they have to
the manufacture of critical components for high-performance aircraft gas turbine en- resist the centrifugal loading of the disk
gines are described. The materials of interest are the nickel- and cobalt-base super- and blades. Some consideration must al-
alloys for turbine and burner sections of the engine, and titanium alloys and com- so be given to the effects of high strains
posites for compressor and fan sections of the engine. Advanced processing methods developed at the critical points of attach-
considered include directional solidification, hot isostatic pressing, superplastic forg- ment of the blades to the disk. Steels and
ing, directional recrystallization, and diffusion brazing. Future trends in gas turbine titanium alloys have proved to be satis-
technology are discussed in terms of materials availability, substitution, and further factory materials for blades, vanes, and
advances in air-cooled hardware. disks in the low-pressure compressor,
but the more heat-resistant nickel-base
alloys are preferred for the hotter sec-
of advanced engines, with the balance this sets an upper limit on the required tions of the high-pressure compressor.
roughly equally divided between tita- temperature capability of the material. Burner. The burner section is essen-
nium alloys, composites, and steels (Fig. Other materials design considerations of tially an annular combustion chamber, in
1). The trend appears to be in the direc- equal significance are (i) the magnitude which several burner cans are arranged
tion of somewhat higher weight fractions of stresses developed by centrifugal side by side around the inside of the
of superalloys and composites, at the ex- forces, vibratory forces, and thermal chamber. Each burner can, which is a
pense of titanium alloys and steels. In gradients and (ii) the potential for oxida- separate combustion chamber, is per-
the superalloy area, the trend is toward tion (or hot corrosion) and erosion in the forated with holes and slots to admit air
increasing applications for directionally high-velocity gas stream. for cooling. About one-third of the total
solidified (DS) and powder metallurgy Fan. The fan section of the engine is volume of air entering the burner from
(PM) products. integral with the front part of the low- the compressor discharge is mixed with
This article presents an overview of pressure compressor. This permits the the fuel to sustain combustion. The re-
materials developments as they are re- fan blades to rotate at low tip speed, con- mainder of the air bypasses the fuel noz-
lated to specific components in the gas sistent with optimum fan efficiency. The zles and is used downstream to cool the
turbine engine, such as blades, vanes, fan blades are relatively long and thin burner can surfaces and combustion
disks, and combustors. A brief descrip- components and are braced (shrouded) products before the hot gases enter the
tion will first be given of the design and at the midspan for support and to pre- turbine. The system is designed to main-
operation of component parts in an en- vent vibration. Air passing through the tain an even temperature distribution in
gine to provide the reader with a better fan and exhausted through the ducts the hot gases leaving the combustor and
appreciation of the factors involved in tends to carry away ingested foreign ma- to ensure that the peak temperature does
materials selection. terial, which otherwise might damage the not exceed the allowable limit at the tur-
engine core. bine inlet.
Materials for fan blades must be Burner materials must be formable,
Engine Components strong, elastically stiff, and resistant to weldable, and resistant to corrosion, dis-
damage by foreign objects. Experience tortion, and thermal fatigue at high tem-
Gas turbine engines such as the JT9D has shown that titanium alloys satisfy peratures. The strength requirements are
turbofan engine (Fig. 2) comprise three these requirements. Composites with relatively low, but the strength must be
main sections: compressor (fan), burner, high specific strength, such as carbon- maintained up to approximately 1l00°C.
and turbine (1). The compressor raises epoxy, have also been considered for A special class of heat-resistant sheet al-
the pressure and temperature of the in- this application, but at present lack suf- loys have been developed for this pur-
coming air and delivers it to the burner. ficient resistance to foreign object dam- pose.
In the combustion chamber, a fine spray age. Turbine. The configuration of the main
of fuel is thoroughly mixed with the high- Compressor. The compressor consists components of the turbine-the rotor
pressure air, and the mixture is ignited. of a series of rotating blades and station- and interstage guide vanes-is similar to
The hot gases leaving the combustion ary vanes, which are arranged in stages that of the compressor, except that the
chamber undergo rapid expansion in the concentric with the axis of rotation. The gas path increases in the direction of
turbine section, accompanied by a sharp blades are fastened into slots around the flow to accommodate the expansion of
drop in gas pressure and temperature. periphery of individual disks; the vanes gases between stages in the turbine. The
848 SCIENCE, VOL. 208
 blades are attached to the disks, using a Materials and Applications in structure between the face-centered
characteristic fir-tree root design, which cubic y and ordered face-centered cubic
leaves space for expansion. Vanes are Superalloys. The high-temperature al- y' phases. The cobalt-base alloys derive
slotted into the turbine casing in stages loys (2-5), known as the superalloys, oc- their strength from combined solid solu-
between the rotors. To reduce vibrations cur in two broad classes: nickel-base and tion hardening and carbide dispersion
and to increase turbine efficiency, the cobalt-base (Table 1). Nickel-base alloys strengthening. Fine networks of carbide
blade tips are sometimes shrouded. To are strengthened by precipitation of the phases (Fig. 4b) appear to be particularly
permit a higher turbine inlet temper- Ni3(Al,X) y' phase, where X is a solid effective in strengthening at high temper-
ature, which is critical to improve oper- solution hardening element, such as Ti, atures.
ating efficiency, inlet guide vanes and Nb, or Ta. In most advanced alloys, y' The nickel-base alloys have out-
first-stage blades are air-cooled. This is precipitation hardening is supplemented standing strength characteristics at tem-
accomplished by passing compressor by solid solution hardening of the ry ma- peratures in the range of 7500 to 1000°C.
bleed air through longitudinal holes, trix with refractory elements, such as W, The cobalt-base alloys are stronger and
tubes, or cavities in the airfoil sections Mo, or Re. The y' particles precipitate more corrosion-resistant at temperatures
(Fig. 3). The cooling air exits through on a fine scale (Fig. 4a), and both precipi- above about 1050°C. Accordingly, the
small holes and slots at the leading and tate and matrix phase are coherent. This nickel-base alloys are used for inter-
trailing edges of the airfoil. Air cooling is is a consequence of the close similarity mediate-temperature blade and disk ap-
necessary only in the turbine inlet sec-
tion, since the energy extracted from the
hot gases by the first- and second-stage
rotors reduces the temperature to a toler-
able level. It is illustrative of the ef-
fectiveness of air cooling that even with
a turbine inlet temperature as high as Fig. 1. Weight per-
1300°C, metal temperatures for inlet cent of materials used Wt. pct.
guide vanes and first-stage blades are in most advanced air- materials
easily maintained at 11000 and 950°C, re- craft gas turbine en-
spectively. gines.
The materials requirements for turbine
blades and vanes are similar, except for
the reduced strength requirements in Year
vanes. Stresses in vanes are less than 35
megapascals, whereas blades are sub-
jected to longitudinal stresses up to 200
MPa in the midspan of the airfoil section.
The blade root, which is attached to the
disk, is outside the gas path and experi-
ences a maximum temperature of ap-
proximately 750°C, but tensile stresses
are much higher, in the range 275 to 550 Fig. 2. Internal pressure
(P) and temperature (T)
MPa. The primary requirements are variations in the Pratt &
creep strength at high temperatures, Whitney JT9D Turbofan
thermal fatigue resistance, and resist- engine at a sea-level take-
ance to environmental degradation by off thrust of 19,500 kilo-
grams and with a bypass
oxidation, corrosion, and erosion. Sec- ratio of 5: 1.
ondary requirements include castability,
impact strength, and microstructural sta- P (MPa) 0.1 0.16 0.22 0.15 2.2 2.1 0.15
bility to ensure that properties are main- T (OC) 15 54 99 54 471 1077 454
tained for long periods of time. Some | Ti-Ti+Fe--Ni+Fe
vane designs also require weldability. Ni- Ni +Co +Fel
Turbine disks operate at temperatures
that do not exceed approximately 750°C.
The maximum temperature occurs at
the outer edge or rim of the disk. The Diffusion
stresses developed by centrifugal loading bonding
are high at the rim and even higher in the
bore. Primary materials requirements
are for high burst strength at the oper-
ating temperature of the bore and good Fig. 3. Advanced air-
creep strength at rim operating condi- cooled turbine blades.
tions. The materials should also possess
good fatigue strength, both low-cycle
and high-cycle. A bewildering variety of
superalloys, both nickel- and cobalt-
base, have been designed for turbine ap- Two-part
plications. blade
23 MAY 1980 849
 Table 1. Nominal compostions of some nickebse and coba-base aloys. be made between blade and disk aloys.
Composition (% by weight) Blade alloys were selected for inter-
mdiate-temperature creep strength,
Ni Cr Co C Ti Al Mo W Other disk alloys for high strength at somewMhat
Blade alloys lower temperatres. As operating tem-
Waspaloy Bal.* 19.5 13.5 0.08 3.0 1.4 4.0 0.08 Zr, 0.007 B perates in the engine have incrased,
B-1900 Bal. 8.0 10 0.11 1.0 6.0 6.0 0.07 Zr, 0.015 B, 4.3 Ta this distinction has become blurred be-
PWA 1422 Bal. 9.0 10 0.11 2.0 5.0 12.5 0.015 B, 1.0Cb,2.0Hf cauwse of the need to provide additional
Disk alloys ceep strength in the disk alloys. The so-
lacoloy 901 Bal. 12.5 0.10 2.6 6.0 34 Fe, 0.015 B
Waspaloy Bal. 19.5 13.5 0.08 3.0 1.4 4.0 0.08 Zr, 0.007 B lution to this problem has been to in-
IN-100 Bal. 12.4 18.5 0.07 4.3 5.0 3.2 0.06 Zr, 0.02 B, 0.8 V crease the 'y' volume faciion of disk al-
Vane alloys loys at the expense of hot workability.
X-40 10 25 Bal. 0.50 7.5 1.5 Fe As a consequence, conventional hot
WI-52 21 Bal. 0.45 11 1.75 Fe,2.0Cb
MAR-M509 10 24 Bal. 0.60 0.2 7 0.5 Zr, 3.5 Ta forging of disks, which is limited to
Burner alloys workable low y' alloys, has been gadu-
HastelloyX Bal. 22 1.5 0.10 9 0.6 18.5 Fe ally replaced by more flexible PM pro-
Haynes l88 22 22 Bal. 0.10 14.5 0.08 La cessing methods (Fig. 6). The new pro-
Inconel 617 -Bal. 22 12.5 0.07 1.0 9
cesses, which are known as superplastic
Balance. forging and hot isostatic pressing, are ap-
plicable to all nickel-base alloys. Al-
though appreciable gains in strength
plications, whereas the cobalt-base al- the overall picture, it can be seen that have been achieved with the new alloys
loys are generally preferred for high-tem- such processing innovations, along with and processing methods, this benefit is
perture vane applications. advances in alloy design, have been re- perhaps not as significant as the im-
The essential requirement for blade al- sponsible for an increase of about l50"C provement in creep strength, because of
loys is adequate creep strength at ele- in the permissible operating temperature the higher rim temperatures encountered
vated temperatures. Experience has of blades. Current interest in this area is in today's engines. For this reason, an-
shown that this requirement is best satis- focused on exploiting the higher strength other likely development in the 1980's is
fied by exploiting the nickel-base alloys capabilities of directionally solidified the dual-property disk, in which the bore
with a high y' volume fraction. In the eutectics (6) and directionally recrys- of the disk is optimized for load-bearing
1950's, alloys of this type, containing tallized (7) conventional superalloys. capacity and the rim for creep strength.
about 30 percent y', were fabricated into The eutectic microstructure is composite Cobalt-base alloys have found their
blades by hot forging (Fig. 5). In the in nature, consisting of a y/Iy' matrix and widest application as vane materials.
1960's following the development of im- reinforcing fibers, or lamellae of a refrac- They are attractive for this application
proved alloys containing up to 60 percent tory phase. An example is the y/y'-a eu- because they can be processed by rela-
v', forging was replaced by investment tectic, which is reinforced with thin fila- tively inexpensive investment casting
casting as the preferred method of blade ments of a-Mo (Fig. 4c). Such alloys techniques, without having to resort to
fabrication. Later this was to evolve into promise to increase the temperature ca- vacuum melting. The early alloys, such
directional solidification processing of pability of blade alloys by a further 75°C as X-40 and WI-52 (Fig. 7), are examples
blades, including both columnar-grained in the 1980's (Fig. 5). of alloys in this air-melting category. For
and single-crystal materials. Looking at At one time, a clear distinction could newer alloys, such as MAR-M509, which

-0p25ii _______________ 2.5 1

Fg. 4. Representative microstructures of (a) y'-strengthened nckkel-base aLoy, (b) carbide-strengthened cobalt-base alloy, and (c) fiber-strength-
ened y/y'-a (Mo) eutectic alloy.
8SO SCIENCE, VOL. 20B
 CONVENTIONAL SUPERPLASTIC FORGE
WROUGHT CAST DIRECTIONALLY SoLIDIFIED FORGE HOT ISOSTATIC PRESS
_ _ OR RECRYSTALLIZED
iioo1- Y/Y-ar RSR 143
1400
|_ ~~~~IN.100NII
r- I
00 r---
0 MERL 76 MERL 76
P)'WA 1480 L) 1200 WASPALOY (LCF (LCF)1
Li 1 000 PWA 1422 L 0
LO
cr-
a:
B-1 900 CD AF 115
IL (CREEP)
m 900 o 1000 IN901I I
z
UJ
WASPALOY IL cc
w
DUAL
CD
z z PROPERTY
m1<Z 0z
800 uc,)
- 950's 0 cc
00 C) 0O 00
cZr 800

1950's 1960's 1970's 1980's ~

1950's 1960's 1970's 1980's 1 990's
Fig. 5 (left). Evolution of blade alloys. Fig. 6 (right). Evolution of disk alloys.

contain more reactive elements, vacuum

melting and casting is necessary. At
present, the future of cobalt-base alloys
in engines is uncertain, because of cur-
rent and projected shortages of cobalt. It 0
seems clear, however, that greater use
LLi
will be made of directional solidification cc
of vane alloys, because of the real im- Fig. 7. Evolution of
provements in thermal fatigue resistance vane alloys. IJ
IL
exhibited by DS structures. It also seems
likely that some use will be made of the LU
C.
new generation of oxide dispersion
strengthened (ODS) alloys, which are
now becoming available commercially.
A lot will depend on the cost of fabricat-
ing these materials into vane shapes.
These new alloys combine high-temper-
ature strength with remarkable resist-
ance to oxidation and hot corrosion. as mechanical alloying, promise sub- disk components may be near-a or a-
Combustor components are fabricated stantial reduction in the cost of these ma- lean ,8 alloys. Examples of these respec-
from sheet alloys by a variety of conven- terials (8, 9). tive alloys are Ti-8Al-lMo-lV, which
tional shaping and joining operations. Titanium alloys. Titanium alloys are contains small amounts of the (-stabiliz-
High workability and good weldability categorized by the microstructural ing elements Mo and V, and Ti-6Al-2Sn-
are essential for burner alloys, which phases that predominate near room tem- 4Mo-2Zr, where solid solution strength-
precludes the use of precipitation-hard- perature (10, 11). Titanium exhibits an ening of the phase by Al, Sn, and Zr
a

enable systems. Experience has shown allotropic phase transformation; the (, produces improved creep resistance.
that the necessary strength at high tem- high-temperature phase, is body-cen- The relatively good strength of Ti-8Al-
peratures can be achieved by refractory tered cubic, and a, the low-temperature lMo-lV and its higher elastic modulus
metal (W or Mo) solid solution strength- phase, is close-packed hexagonal. In and lower density, as indicated in Table
ening of either nickel- or cobalt-base al- pure titanium this transformation occurs 2, favor its use for applications where
loys. High Cr levels are necessary to at 882°C. Alloying elements may act to material stiffness is important.
maintain adequate resistance to oxida- stabilize either the or the a phase. The application of titanium alloys to
tion and hot corrosion. Representative Aluminum is the most important of the the blades and disks of the compressor
alloy compositions for service up to a-stabilizing elements. The 8 alloys have section of the engine has been a major
1100'C are given in Table 1. low creep strengths at elevated temper- contributor to its evolution. This usage
Distortion of combustors due to ther- atures and are therefore limited to low- was prompted by the low density of tita-
mal cycling and local hot spots remains a temperature applications. Table 2 lists nium alloys, which leads to superior spe-
problem. Attempts are being made to im- selected properties of three conventional cific strength of these alloys at temper-
prove the situation by exploiting the titanium alloys that are representative of atures up to about 500°C. In rotating gas
more heat-resistant ODS alloys, such as current compressor disk and blade mate- turbine components, tensile, creep, low-
HA 8077 (NiCrAl with Y203 dispersion). rials. and high-cycle fatigue, fracture tough-
The ODS alloys have the potential for Materials used for moderate-temper- ness, and erosion properties are the im-
use at an 100°C higher temperature
-
ature applications are often the heat- portant selection criteria. Alloying and
than conventional sheet alloys. Manu- treatable a + alloys, such as Ti-6Al- thermomechanical processing are used
facture of ODS sheet products has been 4V, with fine-grained microstructures of to achieve an appropriate balance of
demonstrated, but costs are high. Im- a and transformed The alloys selected
(. these properties and to improve the tem-
proved manufacturing techniques, such for high-temperature compressor and perature capability of titanium alloys.
23 MAY 1980 851
 brittlement in the presence of hydrogen Table 3. Properties of reinforcing fibers.
concentrations higher than 150 parts per
million and alloy element segregation Strength Density
and contamination, were solved by care-
Fiber
Fbr (MPa) modulus
mdls
(GPa) (kgtml
i3
ful vacuum arc melting. Titanium alloys Graphite 2100-2400 200-400 1700-1900
are now commonly produced by multiple Boron 3500 400 2500
consumable electrode arc melting under Silicon 3450 400 3000
vacuum. This procedure degasses and carbide
homogenizes the cast structure.
Gas turbine components of titanium
alloys are usually produced from hot-
forged and heat-treated products. The 325°C, is being evaluated for exhaust
properties of these alloys depend on mi- flaps in an advanced military engine. An-
Fig. 8. Composite fan blade.
crostructural morphology as well as other component subject to substantial
composition. The morphology is subject development activity is the fan blade of
to the thermomechanical processing his- boron/aluminum (Fig. 8). In comparison
Despite the improvements that have tory, and microstructural variations that to the present titanium blade, the com-
been made, the upper use temperature occur in some products, in turn, result in posite blade is capable of a higher tip
for titanium alloys is disappointingly low variability of properties. This sensitivity speed and may, because of its higher
relative to the melting point of titanium. is a consequence of effects such as that modulus, be designed without the mid-
Some promise for overcoming this tem- of cooling rate on the 18-to-a transforma- span stiffener, which leads to improved
perature limitation is provided by the tion. Advances have been made in the aerodynamic efficiency. The 1980's
Ti,Al,rbased intermetallics, TiAl and fabrication of titanium alloys by PM should see increased applications of
Ti3Al. These intermetallics combine low techniques, and this approach promises composites as engine components. Cost
densities with a high-temperature capa- to improve their compositional and mi- and reliability will be important consid-
bility. They show loss of ductility above crostructural homogeneity. erations for these applications.
room temperature, and this problem is Composites. The high strengths, high Coatings. The development of coat-
being addressed. Another approach, stiffnesses, and low densities character- ings to extend the life of a gas turbine air-
which may further enhance the specific istic of high-performance fiber-rein- foil began in about the mid-1950's. How-
properties of titanium alloys and their forced materials make them attractive ever, it was not until the late 1950's that
upper use temperature, involves the use candidates for gas turbine structural ap- the first practical application to cobalt-
of fibers, such as silicon carbide, for re- plications (12) (Table 3). The most im- base alloy vanes was realized. Since
inforcement. The eventual application of portant current composite systems are then, successful applications of coatings
such composite systems awaits exten- graphite/epoxy, graphite/polyimide, and have been made to blades, burner cans,
sive materials development, component boron/aluminum. Graphite/epoxy is find- and other critical components in the gas
design, and evaluation efforts. ing application as a material for fan exit turbine engine (13).
The initial developments in titanium guide vanes. This system has a use tem- The earliest coating methods involved
alloys were not without significant prob- perature up to 175°C. The graphite/poly- some type of aluminizing treatment; that
lems. Some of these, such as alloy em- imide system, with a use temperature to is, diffusion of aluminum into the surface
layers of the alloy substrate. The effect
of such a treatment on nickel- and co-
Table 2. Selected properties of representative titanium alloys. balt-base alloys is to produce protective
layers consisting mainly of the inter-
Annealing/solution 20°C Typical metallic compounds NiAl and CoAl (Fig.
Alloy Density
(kg/im3) treatment
temperatures
elastic
modulus strength (MPa) 9a). These compounds impart good oxi-
(OC) (GPa) 20°C 5380C dation and hot corrosion (sulfidation) re-
sistance, because they form a continuous
Ti-6A1-4V 4428 732-760/899-968 11.3 1000 480 impervious scale of alumina on exposure
Ti-8Al-lMo-lV 4373 760-788/non-heat-treatable 12.7 1000 630 to a high-temperature oxidizing environ-
Ti-6AI-2Sn-4Mo-2Zr 4539 704-843/829-913 11.3 1000 680
ment. Two coating methods have been
widely used for aluminizing superalloys:
slurry fusion and pack cementation. In
the slurry process, the part is sprayed
a b with a suspension of aluminum or alumi-
VAPOR
num alloy and subjected to a high-tem-
DIFFUSION DEPOSITED perature treatment to produce melting
ALUMINIDE OVERLAY
COATING
and interdiffusion between deposit and
COATING
(MAINLY ^) (0+Y) substrate. In the pack cementation pro-
cess, the part is reacted with aluminum
or aluminum alloy powder in the pres-
ALLOY
ence of an ammonium halide activator at
ALLOY SUBSTRATE an elevated temperature. The operation
SUBSTRATE (Y+ y') is carried out in an hermetically sealed
(Y y')
+

container to maintain a controlled activi-

Fig. 9. Microstructures of coatings: (a) diffusion aluminide and (b) vapor-deposited MCrAlY ty of the aluminum in the vapor phase.
overlay. Typical coating thicknesses are in the
852 SCIENCE, VOL. 208
range 25 to 100 micrometers. Advanced are applied by plasma-spraying layers itation and cracking under creep condi-
coatings treatments include additions of 125 to 500 ,m thick. To minimize the ef- tions. In 1967 it was shown that further
Cr and Pt to further enhance the oxida- fects of stresses due to thermal expan- improvements in properties could be
tion and hot corrosion resistance of the sion mismatch between alloy substrate achieved by eliminating the grain bound-
coating. Major improvements in hot cor- and zirconia deposit, coatings composi- aries altogether; that is, by directional
rosion resistance can be achieved by tions are normally graded-that is, the solidification to produce single crystals
proper control of the Pt distribution in composition gradually changes from (16). Following these leads, in the late
the aluminide layer. The Pt-rich coatings base metal to pure zirconia over the 1960's, Pratt & Whitney Aircraft devel-
are synthesized by electroplating a thin coating thickness. Thermal barrier coat- oped various commercial processes for
(about 5 ,um) layer of Pt before aluminiz- ings are now being developed for turbine the directional solidification of bars, in-
ing. airfoils, to reduce metal temperatures gots, slabs, and more complex shapes.
A limitation of the diffusion aluminide and thereby reduce cooling-air require- The most notable achievement was the
coatings is that they seriously lack duc- ments. Some of the best results have directional solidification of cast-to-size
tility at temperatures below about 750°C. been obtained with a 250-gm plasma- turbine blades (17) (Fig. 11).
Thus, they are highly susceptible to sur- sprayed coating of yttria-stabilized zir- The DS process evolved from conven-
face cracking under thermal cycling con- conia over a 100-Itm layer of NiCrAl tional investment casting practice. In
ditions. Another problem is the relative- coating (Fig. 10). Metal temperature re- conventional casting, the melt is poured
ly poor adherence of the protective oxide ductions of - 100°C have been achieved into a ceramic shell mold, and solidi-
scale to the coating alloy. Under thermal in an engine test of experimental air- fication is allowed to occur in a relatively
cycling conditions, it is not uncommon cooled vanes. Increased applications of uncontrolled manner by radiation to the
to encounter repetitive detachment or ceramic coatings for thermal insulation walls of the vacuum chamber (Fig. 12a).
spallation of the alumina scale, leading to and as abradable seals are expected in Since the mold preheat is usually about
rapid degradation of the coating due to the 1980's. half the melt temperature, the melt expe-
loss of Al. To resolve these problems, riences a chilling effect in the mold,
the emphasis in coating technology which results in fairly rapid solidification
shifted in the mid-1960's to direct bond- Advanced Processes and the formation of a fine poly-
ing or cladding of optimized coating crystalline structure. In directional solid-
compositions to the superalloy sub- Directional solidification. In 1960 it ification the situation is similar, except
strate. Ductility was improved by mak- was demonstrated that the creep proper- for a higher mold preheat temperature
ing adjustments in coating compositions ties of superalloys can be improved and some provision for controlling tem-
so as to develop microstructures in markedly by directional solidification to perature gradients in the mold. This is
which the brittle ,8-NiAl or 3-CoAl is em- align all the grain boundaries parallel to accomplished by attaching the ceramic
bedded in a ductile y solid solution ma- the direction of the applied tensile stress mold, open at its base, to a water-cooled
trix. Oxide adherence was improved by (15). The effect was attributed to the copper chill plate. Following the in-
making trace additions of rare earth ele- elimination of grain boundaries trans- troduction of the melt into the mold and
ments, such as yttrium. The final result verse to the applied stress, since such the commencement of solidification in
of this work was the introduction of the boundaries are highly susceptible to cav- the chill zone, the temperature gradient
MCrAlY coatings, where M is Ni, Co, or
Ni + Co (Fig. 9b). As in the case of dif-
fusion coating, overlay coatings also
benefit from additions of Pt.
Many methods of processing MCrAlY Hot gas
overlay coatings have been tested, in-
cluding diffusion bonding, powder sinter-
ing, plasma spraying, vacuum evapora- Fig. 10. Thermal bar-
tion, sputtering, and ion plating. Of rier coating.
these, electron beam evaporation has
emerged as the preferred method of coat-
ing superalloy blades and vanes. In a
typical arrangement, using continuous Cooling air-
ingot feed, it is necessary to impart a passage
complex motion to the part in order to
achieve a uniform coating deposit.
An oxide scale on the alloy coating
acts to some extent as a thermal barrier,
because of its relatively poor heat trans-
fer characteristics. Its effectiveness,
however, is limited because its thickness
is typically < 1 am. To improve the situ- Fig. 11. Effect of processing
ation, the concept of applying thermal on grain structure of cast-to-
size turbine blades.
barrier coatings, on top of existing coat-
ings, has emerged (14). So far the meth-
od has been applied successfully to sheet
metal components, such as burner cans
and exhaust liners. The coatings are
based on yttria-stabilized zirconia, and
23 MAY 1980 853
 hour, while columnar-grained material
Investment can be produced at rates up to 40 cm/
mold hour.
Nickel-base superalloys respond well
Radiation to DS processing. This applies to both
conventional y' precipitation-hardened
alloys and eutectic superalloys, such as
Radiation yly'-a (Mo) and y/y'-TaC. Eutectic su-
baffle A il
Water cooled
peralloys, however, require steeper tem-
a chill b perature gradients and much lower
Fig. 12. Comparison of (a) conventional cast- growth rates (typically 2 cm/hour) to
-

ing and (b) directional solidification. obtain optimally aligned structures. Sig-
nificant improvements in creep rupture
properties by DS processing have been
in the mold is adjusted to promote the obtained for both types of superalloys
advance of the solid-liquid interface in (Fig. 5). Conventional superalloys also
a direction normal to the chill sur- exhibit improved thermal fatigue proper-
face. Thus, a characteristic directionally ties in the <100> orientation. These
aligned or columnar-grained structure is property advantages have been exploited
developed in the casting. A useful con- in DS superalloy castings of first- and Fig. 14. Diffusion brazing of matched blade
sequence of the steep temperature gradi- second-stage turbine blades and inlet halves.
ents developed in the chill zone is the guide vanes. At present, superalloy cast-
formation of a strong < 100> or cube tex- ings of the columnar grain type are the
ture, which is incorporated into the col- most widely used in commercial engines. ature capabilities of the alloys have con-
umnar grain structure. Current practice However, the situation is changing, and tinued to improve. The picture may be
favors the arrangement depicted in Fig. it appears that before long single-crystal changing because of continuing short-
12b. The desired temperature gradient is castings will displace them for the most ages in the supply of cobalt. It now
maintained by gradually lowering the critical applications in the engine. This is seems likely that the 1980's will see the
mold from the hot zone, through an array because of the higher temperature capa- gradual adoption of DS nickel-based su-
of heat shields, into the vacuum cham- bilities that have been achieved in a new peralloys, such as PWA 1422, for vane
ber, where radiation losses can occur class of superalloys, designed specifical- applications, along with ODS nickel-
freely. ly to exploit the single-crystal character base alloys, such as HA 8077.
The procedure for processing single- of the material. An example of an alloy in Superplastic forging. In 1963 it was
crystal superalloys emerged quite natu- this category is PWA 1480, which offers found that fine-grained superalloys ex-
rally from the basic DS process. A crys- a 25C advantage over its columnar- hibit high ductility or superplastic behav-
tal selector is incorporated into the mold grained predecessor, PWA 1422 (Fig. 5). ior when deformed at high temperatures
just above the chill zone. After devel- It remains to be seen whether the dem- under appropriately low strain rate con-
oping the desired <100> texture in the onstrated advantages in creep strength of ditions (18). This paved the way for the
chill zone, the crystal selector is used to certain eutectic superalloys, such as subsequent development of a new pro-
exclude all but one grain, which then ex- yly'-a, will offset the higher manufactur- cess for the hot deformation processing
pands to fill the mold. To obtain single ing costs due to the lower growth rates. of superalloys, which has become known
crystals in any desired orientation, con- In contrast to these developments, the as the Gatorizing forging process.
ventional seeding techniques must be situation with respect to cobalt-base su- The desired fine-grained micro-
used in conjunction with the DS process. peralloys for vane applications has re- structure can be obtained most conve-
The oriented seed crystal is attached to mained relatively static. As indicated in niently by hot extrusion. Typically, the
the base of the mold in contact with the Fig. 7, conventional investment casting material is subjected to high deformation
chill plate. The maximum allowable has maintained a dominant position in rates at temperatures just below the y'
growth rate for the DS processing of the processing of cobalt-base superalloys solution temperature. This is a crucial
single crystals is 500 centimeters per since the 1950's, although the temper- aspect of the process, since the high de-
formation rate causes spontaneous re-
crystallization on a very fine scale and
the presence of the y' particles serves to
blank stabilize the fine grain structure. The re-
sulting grain size is in the range I to 5
Superplastically Am, depending on the extrusion ratio
rolled sht n and temperature. When the optimum
RSR
fine-grained structure has been achieved,
powder the material may be worked into shape
by exploiting its superplastic character.
This is accomplished by controlled strain
rate, isothermal forging-that is, super-
Induction ECM plastic forging. After forging, the creep
melt machined strength of the material is recovered
blade by a solution heat treatment to coars-
en the grain structure, followed by
Fig. 13. Sequence of steps involved in fabrication of a wafer blade. quenching and aging to obtain the opti-
854 SCIENCE, VOL. 208
mum distribution of the y' hardening phase. to some extent for the healing of casting utilizing two different powder com-
Superplastic forging has been used to defects. positions.
fabricate superalloy turbine disks and The superalloy powder is packed in- Diffusion brazing. Conventional braz-
even complete rotors starting from pre- side a thin-walled collapsible container, ing is widely used in the fabrication of
alloyed powder. A particular advantage which is a geometrically expanded ver- gas turbine components and parts, but
of the process, compared with conven- sion of the final shape. After vacuum de- not usually for joining critical superalloy
tional hot forging, is its ability to gener- gassing at an elevated temperature, the components that are exposed to high
ate a forged profile that more closely container is sealed, pressure checked, temperatures and corrosive environ-
matches the final shape of the article. and subjected to the simultaneous appli- ments. This is because it is difficult to de-
This reduces machining costs, because cation of pressure and temperature for a sign a filler material that satisfies all
less material has to be removed in the time sufficient to cause full densification property requirements, both physical
finish machining operation. of the powder. Finally, the shaped part is and mechanical, including complete
A characteristic feature of the fine- obtained by stripping off the container. compatibility with the workpiece. In rec-
grained superplastic material, at least At this time, processing of superalloy ognition of these shortcomings in con-
when produced from prealloyed powder, disks, near-final shape, has become a ventional brazing, a new process called
is its remarkable chemical and micro- firmly established technique. The devel- diffusion brazing was developed in 1974
structural uniformity. Another favorable opment phase of the process, however, (24).
aspect is its ability to form a directionallyis far from over. Remaining to be formu- Diffusion brazing, as applied to the
aligned or columnar-grained structure lated are procedures for more effectively joining of superalloy components, is a
when subjected to directional recrys- controlling the size and distribution of fa- vacuum brazing operation. The process
tallization (DR) under the influence of a tigue-limiting flaws, such as ceramic in- makes use of a filler, or interlayer materi-
steep thermal gradient (19). The resulting clusions, which are invariably found al, that closely matches the composition
grain structure bears a superficial resem- mixed in with the alloy powder. The of the workpiece except for the addition
blance to that obtained by directional so- most promising immediate solution to of an appropriate melt depressant, such
lidification. However, there are impor- this problem is to screen out the finer as boron, to form a eutectic with a low
tant distinctions. The DR grain structure size fractions (44 ,um) of alloy powder for melting point. The interlayer is placed
is finer in scale and much more homoge- fabrication purposes. In this way, an up- between the mating surfaces of the work-
neous than the DS structure. Further- per limit can be set for the maximum piece, and a slight normal pressure is ex-
more, the DR structure exhibits not one, flaw (inclusion) size, which controls fa- erted to maintain physical contact. The
but several grain textures, including, tigue crack initiation. This is an impor- temperature is increased to the pre-
<100>, <110>, and <111>, depending tant design consideration in highly determined brazing temperature, where
on processing parameters and alloy com- stressed parts, such as disks. the eutectic melts and alloys with the
position. Hot isostatic pressing has also found workpiece. Under isothermal condi-
This ability to develop textured colum- useful application in the healing of de- tions, the melting point of the eutectic
nar-grained structures entirely by solid- fects in conventionally cast blades and gradually rises as the boron diffuses
state processing has stimulated interest vanes, such as small shrinkage pores. away into the workpiece. The process is
in new methods for the fabrication of air- When such pores are not surface-con- considered to be complete when no eu-
cooled blades and vanes of advanced nected, HIP may be used to close them. tectic remains in the joint. A subsequent
design, starting from superplastically Surface-connected pores can be healed heat treatment is employed to erase all
formed sheet stock (20, 21). One such only if a coating is applied before HIP. traces of the original interface.
processing scheme is depicted in Fig. 13. This treatment results in a significant im- Diffusion brazing produces good joints
The critical step in the process is the dif- provement in rupture life. in a variety of nickel-base superalloys,
fusion bonding of thin photoetched wa- In addition to these current appli- including dissimilar alloy combinations.
fers to produce the desired configuration cations, HIP is also being considered for Furthermore, bond strengths com-
of internal cooling passages. Following the rejuvenation of parts, such as blades, parable with base-metal strength have
directional recrystallization of the bond- vanes, and disks, that have sustained been achieved, even in high-temperature
ed structure, the actual blade profile is creep or fatigue damage in service. Blade creep-rupture tests. Successful appli-
formed by electrochemical machining. and vane materials that have been ex- cations for diffusion brazing have includ-
An experimental air-cooled wafer blade, tended well into steady-state creep can ed joining of vane clusters, attachment of
similar to that shown in Fig. 13, has al- be restored to their original condition by hardened tips to blades, and bonding of
ready withstood higher turbine inlet tem- HIP and reheat treatment. The HIP two-part blades (25) (Fig. 14). With re-
peratures than even the most sophisti- cycle evidently heals the micropores that spect to the two-part blade application,
cated one-piece or two-piece blade cast- develop during creep. Fatigue-induced good joints have been obtained irrespec-
ings (Fig. 3). This is because of the much cracks in disk materials have also been tive of whether the matching blade
more efficient air-cooling schemes that healed by HIP. However, it is not yet halves have DS columnar-grained or
are attainable with the multiple-wafer clear how best to handle the surface-con- single-crystal structures.
fabrication technique. nected cracks, which tend to be oxidized
Hot isostatic pressing. In 1955, a pro- or otherwise degraded. Another area of
cess was invented for the gas-pressure interest is the fabrication of laminated Future Perspective
diffusion bonding of materials, which lat- composite structures, or graded mono-
er became known as hot isostatic press- lithic structures. A good example of the Among the factors that will influence
ing (HIP) (22, 23). After 20 years, the latter is the dual-property disk concept, the application of materials to future air-
process has found its greatest com- which envisages a bore that possesses craft gas turbines will be the cost and
mercial significance in the production of high load-bearing capacity and a rim that availability of certain strategic elements,
fully dense, shaped parts from pre- has good creep strength. Conceivably, such as Cr, Co, Ta, and Pt. This may dic-
alloyed powders. It has also been used this structure could be made by HIP, tate the limitation of specific alloys to the
23 MAY 1980 855
most critical engine components. Such engines will be designed primarily for 8. J. S. Benjamin, Met. Trans. 1, 2943 (1970).
9. __, Sci. Am. 234, 40 (May 1976).
constraints will promote the develop- fuel efficiency, possibly even at the ex- 10. C. Hammond and J. Nutting, Met. Sci. 11, 474
ment of new material systems and pro- pense of some sacrifice in component du- (1977).
11. S. R. Seagle and L. J. Bartlo, Met. Eng. Q. 8, 1
cesses. As design schemes for gas tur- rability. An important consideration will (1968).
bine components increase in sophisti- be the attainment of higher metal tem- 12. G. M. Ault and J. C. Freche, J. Astronaut.
Aeronaut. 17 (No. 10), 48 (October 1979).
cation, it seems clear that manufacturing peratures for turbine blades, to reduce 13. Z. A. Foroulis and F. S. Pettit, Eds., Pro-
innovations will play an increasingly im- cooling-air requirements. The achieve- ceedings of Symposium on Properties of High
Temperature Alloys with Emphasis on Environ-
portant role. ment of higher design strengths for disks mental Effects (Electrochemical Society,
Princeton, N.J., 1976).
Some anticipated changes are outlined and higher resistance to environmental 14. C. H. Liebert and F. S. Stepka, NASA Tech.
below: degradation for vanes will also be impor- Memo. TM X-3352 (1976).
15. F. L. VerSnyder and R. W. Guard, Trans. Am.
1) Low- to moderate-temperature tant goals, because of the benefits of re- Soc. Met. 52, 485 (1960).
static and rotating components will be duced weight, lower engine cost, and in- 16. B. H. Kear and B. J. Piearcey, Trans. AIME
238, 1209 (1967).
manufactured from high-performance creased operating life. 17. F. L. VerSnyder and M. E. Shank, Mater. Sci.
composite materials. Eng. 6, 213 (1970).
18. J. B. Moore and R. L. Athey, U.S. Patent
2) Near-final shape disks will be pro- References and Notes 3,519,503 (1970).
duced that are graded in composition, 1. The Aircraft Gas Turbine Engine and Its Opera- 19. M. M. Allen, V. E. Woodings, J. A. Miller, U.S.
tion (Pratt & Whitney Aircraft Manual No. 200, Patent 3,975,219 (1976).
microstructure, and anisotropy to opti- United Technologies Corp., Hartford, Conn., 20. W. H. Brown and D. B. Brown, U.S. Patent
mize mechanical properties. 1970). 3,872,563 (1975).
2. C. T. Sims and W. C. Hagel, Eds., The Super- 21. A. R. Cox et al., Pratt & Whitney Aircraft re-
3) New burner configurations will per- alloys (Wiley, New York, 1972). port on DARPA contract F33615-76-C-5136
mit the use of new, improved sheet mate- 3. P. R. Sahm and M. 0. Speidel, Eds., High Tem- (1979).
perature Materials in Gas Turbines (Elsevier, 22. H. A. Saller, S. J. Paprocki, R. W. Dayton, E.
rials. Amsterdam, 1976). S. Hodge, U.S. Patent 687,842 (1966).
4. B. H. Kear, D. R. Muzyka, J. K. Tien, S. T. 23. H. D. Hanes, D. A. Seifert, C. R. Watts, Hot
4) Multipiece construction of vanes Wlodek, Eds., Superalloys: Metallurgy and Isostatic Pressing (Metals and Ceramics Infor-
and blades to provide highly efficient Manufacture (Claitor's, Baton Rouge, La., mation Center Rep. MCIC-77-34, Battelle Co-
1976). lumbus Laboratories, Columbus, Ohio, 1977).
cooling will become common practice. 5. E. F. Bradley, Ed., Source Book on Materials 24. D. S. Duvall, W. A. Owczarski,;D. F. Paulonis,
The emphasis in military engines will for Elevated Temperature Applications (Ameri- Weld. J. 54, 203 (1974).
can Society for Metals, Metals Park, Ohio, 25. J. Mayfield,Aviat. Week Space Technol. 111, 41
be on improving system reliability and 1979). (3 December 1979).
performance. Higher turbine inlet tem- 6. J. L. Walter, M. F. Gigliotti, B. F. Oliver, H. 26. We thank C. P. Sullivan, M. J. Donachie, Jr., G.
Bibring, Eds., Proceedings of the Third Confer- W. Goward, and M. L. Gel for their construc-
peratures will continue to be the primary ence on In-Situ Composites (Ginn, Lexington, tive criticism of the manuscript and for valuable
goal, since this is the most effective way Mass., 1979). information incorporated in several figures. In
7. A. R. Cox, J. B. Moore, E. C. VanReuth, addition, we appreciate the helpful comments
to increase power output. Commercial AGARD Conf. Proc. No. 256 (1978). made by C. E. Sohl and R. G. Bourdeau.

ist in liquids, they are not necessarily

"famorphous," but instead may contain
well-defined short-range order.
Perhaps the prime virtue of metallic
glasses is that they can be produced in
Metallic Glasses useful forms economically. As a result,
on a comparative cost basis, they are po-
tentially the strongest, toughest, most
John J. Gilman corrosion-resistant, and most easily
magnetized materials known to man.
Their costs are very low because they
are formed directly from the liquid with-
During almost all of the 8000 years that They showed that very fast cooling out passing through the many steps that
metals have been used by humans, their (- 106 °C per second) can yield metallic are used in conventional metallurgy.
structures have consisted of aggregates materials that are rigid and have liquid- They can be made from the least ex-
of crystals. Therefore, the discovery by like molecular structures. pensive of all metallic raw materials,
iron.
To make a thin strip of steel in the con-
Summary. The novel intemal structures of metallic glasses lead to exceptional ventional way, an ingot is first cast; this
strength, corrosion resistance, and ease of magnetization. Combined with low manu- is hot-rolled to form a billet, the billet is
facturing costs, these properties make glassy ribbons attractive for many applications. flattened by further rolling into a narrow
These materials also have scientific fascination because their compositions, struc- plate, and then a series of cold-rolling
tures, and properties have unexpected features. and annealing steps is used to reduce the
plate to thin strip stock. In all, six or
eight steps are needed.
Klement et al. (1) that selected metal al- By analogy with other supercooled liq- In contrast, thin strips of metallic glass
loys can be quenched fast enough to cir- uids, quenched metallic alloys are called are cast in one step. An entire spool can
cumvent crystallization caused consid- glasses. Since they are derived from liq- be produced in a matter of a few min-
erable exictement among metallurgists. uids rather than gases or plasmas, they utes. It is estimated that about four to
do not necessarily have the same struc- five times less energy is consumed in go-
John J. Gilman is director of the Corporate Devel- tures as other noncrystalline metals. Al- ing from a liquid metal to a thin, metallic
opment Center, Allied Chemical Corporation, Mor-
nstown, New Jersey 07960. so, since associations of atoms often ex- glass strip than would be consumed by
8S6 0036.8075/80/O523-0856S01.50/0 Copyright 1980 AAAS SCIENCE, VOL. 208, 23 MAY 1980