WO2021025499A1 - High-strength and high-formability beta titanium alloy - Google Patents

Abstract

The present invention relates to a beta titanium alloy having high strength and high formability while having a low manufacturing cost, and to a manufacturing method for the titanium alloy. According to one example of the present invention, a Ti-Mo-Fe-Cr-Al based beta titanium alloy is characterized by comprising, in wt%, Ti-(1%≤Mo<4%)-(1%≤Fe<3%)-(1%≤Cr<4%)-(0%<Al≤4%), wherein an equivalent weight of Mo is 12.0 or more.

Description

High strength, high formability beta titanium alloy

The present invention relates to a titanium alloy having high strength and excellent formability at the same time and having a low manufacturing cost.

Titanium (Ti, titanium) alloys have low density, high strength, excellent specific strength, and biocompatibility, and are therefore used in many industrial fields.

Currently widely used titanium alloys contain a lot of commercially pure (CP) titanium in a pure (or commercially pure) state, an alpha + beta titanium alloy in which a high temperature beta phase and a low temperature alpha phase coexist, and a beta stabilizing element. It is divided into beta titanium alloys, etc.

Among the titanium alloys, beta titanium alloys are widely used in the field of structural materials requiring high strength, such as landing gears for aircraft.

In recent years, the demand for a beta titanium alloy that can realize not only high strength but also high formability characteristics at the same time has increased, and thus research on this has been actively conducted.

Among them, a transformation mechanism that simultaneously generates TWIP (twining induced plasticity), which increases strength and workability by generating twins during deformation, and TRIP (transformation induced plasticity), in which the high temperature phase transforms into martensite during deformation Attempts to utilize (mechanism) in beta titanium alloys are being actively conducted.

This is because, if the above TWIP and TRIP effects are applicable to the beta titanium alloy, the high strength and high formability of the beta titanium alloy can be simultaneously implemented.

Molybdenum (Mo, molybdenum) has been known as a key element capable of simultaneously achieving the TWIP and TRIP effects in a beta titanium alloy. Therefore, in the conventional beta titanium alloy, most of Mo was added in an amount of 8 to 15% by weight (hereinafter referred to as'wt.%' or'%') to realize the TWIP and TRIP effects at the same time.

However, there is a problem that the melting point of Mo is much higher than that of the known titanium, so that it is difficult to manufacture an ingot having a uniform microstructure. Furthermore, the Mo generates segregation during casting or melting, thereby creating a non-uniform microstructure in both macroscopic and microscopic ways.

The segregation and non-uniform microstructure problem of the Mo-added beta titanium alloy must be solved through a subsequent process (for example, thermal mechanical process (TMP) or homogenization heat treatment), which in turn causes productivity and economic problems. Furthermore, since Mo is an element that is more expensive than other alloying elements added to the titanium alloy, there is a problem of deteriorating the economic efficiency of the titanium alloy.

Accordingly, there is an increasing need for development of a beta titanium alloy capable of simultaneously securing the TWIP and TRIP effects while reducing the amount of Mo added.

It is an object of the present invention accordingly to provide a new beta titanium alloy having a low cost while replacing expensive Mo.

Specifically, it is an object of the present invention to provide a new beta titanium alloy capable of simultaneously implementing the TWIP and TRIP effects while containing significantly less Mo than the existing beta titanium alloy.

More specifically, the present invention is to provide a new beta titanium alloy having no segregation compared to the existing beta titanium and having a uniform microstructure and excellent economical efficiency in which high strength and high formability characteristics can be realized.

The objects of the present invention are not limited to the above-mentioned objects, and other objects and advantages of the present invention that are not mentioned can be understood by the following description, and will be more clearly understood by examples of the present invention. In addition, it will be easily understood that the objects and advantages of the present invention can be realized by the means shown in the claims and combinations thereof.

Ti-Mo-Fe-Cr-Al-based titanium alloy according to an embodiment of the present invention for achieving the above object, in weight %, Ti-(1%≤Mo<4%)-(1%≤Fe Ti-Mo-Fe-Cr-Al type containing <3%)-(1%≤Cr<4%)-(0%<Al≤4%) and having an Mo equivalent of 12.0 or more, excellent in strength and formability It is a beta titanium alloy.

Preferably, the alloy is Ti-Mo-Fe-Cr-Al-based beta titanium having excellent strength and formability, including O≤0.2% by weight %.

Preferably, the alloy is a Ti-Mo-Fe-Cr-Al-based beta titanium alloy having excellent strength and formability, in which the microstructure before deformation includes an alpha phase in the beta phase matrix.

In particular, the average grain size of the beta matrix is within 200 ㎛, Ti-Mo-Fe-Cr-Al-based beta titanium alloy excellent in strength and formability.

Preferably, the alloy is a Ti-Mo-Fe-Cr-Al-based beta titanium alloy having excellent strength and formability, wherein the microstructure after deformation includes tween and martensite together with the beta phase matrix and the alpha phase.

Preferably, the alloy has a tensile strength of 850 MPa or more, and is a Ti-Mo-Fe-Cr-Al-based beta titanium alloy having excellent strength and formability.

At this time, the alloy is a Ti-Mo-Fe-Cr-Al-based beta titanium alloy having excellent strength and formability, in which the product of the elongation of 15% or more and the tensile strength (MPa) and the elongation (%) is 15,000 MPa% or more.

According to the present invention, a new Ti-Mo-Fe-Cr-Al-based beta titanium alloy capable of securing mechanical properties equivalent to or higher than that of a beta titanium alloy with a high amount of Mo added (more than 8% Mo) while replacing expensive Mo. There is an advantage it can provide.

Particularly, the Ti-Mo-Fe-Cr-Al-based beta titanium alloy of the present invention increases the stability of the beta phase by including a part of the alpha phase, thereby generating TWIP and TRIP even with a small amount of Mo added, resulting in high tensile strength properties and excellent elongation at the same time. There is an advantage that can be secured.

In addition to the above-described effects, specific effects of the present invention will be described together while describing specific details for carrying out the present invention.

1 is a view showing a method of manufacturing a Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention.

2 shows the product of the tensile strength (UTS, ultimate tensile strength, MPa) and the elongation (%) according to the Mo composition range of the beta titanium alloys of Examples and Comparative Examples of the present invention.

3 shows the product of tensile strength (UTS, ultimate tensile strength, MPa) and elongation (%) according to the value of Mo equivalent in the beta phase after solution treatment and quenching in the beta titanium alloys of Examples and Comparative Examples of the present invention. will be.

4 shows a microstructure of a 2-1-2 beta titanium alloy, which is a comparative example of the present invention, quenched after solution treatment.

5 is a photograph of a microstructure of a 2-2-1 beta titanium alloy, which is a comparative example of the present invention, quenched after solution treatment.

6 shows the tensile strength (UTS, ultimate tensile strength, MPa) value according to the alpha phase fraction of the beta titanium alloys of Examples and Comparative Examples of the present invention.

7 is a photograph of a microstructure of Example 2-5-1 beta titanium alloy of the present invention before deformation (quickly cooled after solution treatment) and 2% deformation.

8 is a microstructure photograph of Example 2-2-2 of the present invention before deformation (quick-cooled after solution treatment) and 15% of the beta titanium alloy.

9 is a view showing tensile strength and elongation of Ti-Mo-Fe-Cr-Al beta titanium alloys of all Examples and Comparative Examples of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings so that those of ordinary skill in the art may easily implement the present invention. The present invention may be implemented in various different forms, and is not limited to the embodiments described herein.

In order to clearly describe the present invention, parts irrelevant to the description have been omitted, and the same reference numerals are assigned to the same or similar components throughout the specification. Further, some embodiments of the present invention will be described in detail with reference to exemplary drawings. In adding reference numerals to elements of each drawing, the same elements may have the same numerals as possible even if they are indicated on different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known configuration or function may obscure the subject matter of the present invention, a detailed description thereof may be omitted.

In describing the constituent elements of the present invention, terms such as first, second, A, B, (a), (b) may be used. These terms are only for distinguishing the component from other components, and the nature, order, order, or number of the component is not limited by the term. When a component is described as being "connected", "coupled" or "connected" to another component, the component may be directly connected or connected to that other component, but other components between each component It is to be understood that is "interposed", or that each component may be "connected", "coupled" or "connected" through other components.

Titanium is a polymorphous element with two crystal structures: a high temperature beta phase with a body centered cublic (BCC) structure and a low temperature alpha phase with a hexagonal closed packed (HCP) structure. to be.

When a transition metal is added to the titanium, a region in which the high-temperature beta phase of the titanium is stabilized is widened. In other words, the beta transus temperature, which is the temperature at which the high temperature beta phase is transformed into the low temperature alpha phase, decreases.

At this time, the degree of stabilization of the beta phase varies depending on the alloying element added, and the degree of stabilization of the beta phase for each alloying element based on Mo (molybdenum) is referred to as the following Mo equivalency.

[Mo] _eq = [Mo] + 0.67 [V] + 0.44 [W] + 0.28 [Nb] + 0.22 [Ta] + 2.9 [Fe] + 1.6 [Cr] + 1.25 [Ni] + 1.7 [Mn] + 1.7 [Co] - 1.0 [Al]

The Mo equivalent value is determined by the composition and composition range of the titanium alloy, but is a value obtained by normalizing various alloying elements and composition ranges of each component with one parameter. Therefore, the Mo equivalent value may be a parameter value capable of determining the phase stability, deformation mechanism, and mechanical properties of the titanium alloy.

Meanwhile, Al is a representative alloying element that stabilizes the low-temperature alpha phase in a titanium alloy.

Therefore, when calculating the Mo equivalent, Al is calculated to have a negative value unlike other beta-stabilizing element values.

On the other hand, most transition metals are alloying elements that stabilize the high temperature beta phase in titanium alloys.

Therefore, when calculating the Mo equivalent, transition metals are calculated to have a positive value, unlike Al.

Conventional beta titanium alloys for realizing the TWIP and TRIP effects at the same time contain at least 8 to 15% of Mo and almost no alpha stabilizing elements such as Al. Therefore, as calculated from the Mo equivalents, the conventional beta titanium alloy has a very high amount of Mo equivalents of at least 12, preferably at least 15.

The beta titanium alloy of the present invention is one technical feature in terms of composition that the Mo content is significantly reduced compared to the existing high Mo (hereinafter, high Mo) beta titanium alloy.

Further, the beta titanium alloy of the present invention is another technical feature in terms of composition in that Mo is substituted with other alloying elements in order to simultaneously realize the TWIP and TRIP effects.

Furthermore, the beta titanium alloy of the present invention is another technical feature in terms of composition in that the beta titanium alloy contains Al, which is an alpha stabilizing element rather than in the beta titanium alloy for high strength.

Hereinafter, the technical characteristics of the component and composition range of the beta titanium alloy of the present invention will be described.

The beta titanium alloy of the present invention contains Al as an essential component.

The reason for adding Al in the present invention is as follows.

In general, titanium alloys are strengthened by interstitial oxygen strengthening, solid solution strengthening, precipitation strengthening, and dislocation density or grain refinement strengthening mechanisms.

Among the above strengthening mechanisms, strengthening by interstitial oxygen is not preferable because it may cause brittleness.

On the other hand, aluminum is the most representative solid solution strengthening element in titanium alloys.

When aluminum is added to titanium, aluminum is mainly dissolved in the alpha phase and strengthens the alpha phase. In addition, aluminum can increase oxidation resistance and creep resistance in titanium alloys.

If the amount of Al added is more than 4%, the formability of the beta titanium alloy is deteriorated, and further, excessive addition of Al has a problem in that Ti ₃ Al phase formation may be promoted.

In addition, the beta titanium alloy of the present invention contains Mo as an essential component.

Mo is an element capable of simultaneously implementing the TWIP and TRIP effects of the beta titanium alloy of the present invention.

To this end, the beta titanium alloy of the present invention is characterized in that Mo is added by 1% or more and less than 4%.

If the amount of Mo added is less than 1%, the TWIP and TRIP effects cannot be simultaneously realized, making it difficult to secure excellent mechanical properties of the beta titanium alloy.

On the other hand, when the addition amount of Mo is 4% or more, castability decreases, making it difficult to manufacture an ingot, and a problem of segregation of Mo occurs. Furthermore, the beta titanium alloy of the present invention contains other beta stabilizing elements in addition to Mo. Therefore, if more than 4% of Mo is added together with other beta stabilizing elements other than Mo, the beta stability of the beta titanium alloy of the present invention is too high, so that TWIP and TRIP do not occur when the beta titanium alloy is deformed, and only slip occurs. Accordingly, there is a problem that the strength and formability of the beta titanium alloy of the present invention are deteriorated.

In addition, the beta titanium alloy of the present invention contains Cr and Fe as essential components in addition to Mo.

Both Cr and Fe replace Mo and are additive elements that allow TWIP and TRIP to occur simultaneously when the beta titanium alloy of the present invention is deformed.

If the amount of each of Cr and Fe is less than 1%, the TWIP and TRIP effects cannot be realized at the same time, making it difficult to secure excellent mechanical properties of the beta titanium alloy.

On the other hand, when the addition amount of Cr exceeds 4% or the addition amount of Fe exceeds 3%, it causes a composition segregation problem during melting, resulting in lower castability, which makes it difficult to manufacture an ingot.

Meanwhile, the beta titanium alloy of the present invention contains O (oxygen).

Oxygen is an element that can be regarded as a kind of impurity that is unavoidably included in generally pure titanium (commercially pure) and all titanium alloys.

When oxygen is included in the titanium or titanium alloy, the strength increases due to the interstitial strengthening effect by oxygen in the titanium or titanium alloy.

However, if oxygen is excessively added, the formability of titanium or titanium alloy is deteriorated. Therefore, the beta titanium alloy of the present invention contains at most 0.2% or less of oxygen.

The beta titanium alloy of the present invention is based on the above compositional characteristics, in weight %, Ti-(1%≤Mo<4%)-(1%≤Fe<3%)-(1%≤Cr<4% )-(0%<Al≤4%).

Meanwhile, the Ti-Mo-Fe-Cr-Al-based beta titanium alloy of the present invention can be manufactured by various manufacturing methods. However, similar to a conventional titanium alloy, the mechanical properties of the Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention are also affected by the microstructure.

1 is a view showing a method of manufacturing a Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention.

As shown in FIG. 1, the manufacturing method according to an embodiment of the present invention first performs a hot forging process for breaking an as-cast structure.

In the forging process, beta forging is performed in the beta single-phase region based on the beta transus temperature, which is the lowest temperature at which the high-temperature beta-phase single-phase region appears, and forging is performed in a region above the alpha + beta below the beta transus temperature. It includes the alpha + beta forging process performed. However, as the term “hot forging” means, in alpha + beta forging, the forging temperature must be higher than the recrystallization temperature.

Beta forging in the present invention is preferably carried out at a temperature 50 ~ 100 ℃ higher than the beta transus temperature.

If it is performed near the beta transus temperature, the beta single-phase region cannot be guaranteed. On the other hand, if it is performed above the beta transus temperature, there is a problem that grain growth is excessively promoted due to an excessively high temperature, thereby deteriorating all mechanical properties of the final alloy.

Alpha + beta forging in the present invention is preferably carried out at a temperature 30 ~ 50 ℃ lower than the beta transus temperature.

If it is performed near the beta transus temperature, it is not possible to guarantee a region above the alpha + beta. On the other hand, if it is performed at a temperature too low than the beta transus temperature, there is a problem that recrystallization does not occur during forging.

Meanwhile, in the hot forging process in the manufacturing method according to an embodiment of the present invention, all forging methods such as upset forging and/or side forging can be applied.

After the hot forging process in the present invention, a solution treatment process is performed.

As a term, the solution treatment process is a process of heating a hot-forged beta matrix or alpha + beta matrix alloy at a high temperature.

It is preferable that the solution treatment according to an embodiment of the present invention is performed in a temperature range above alpha + beta. This is because the solution treatment in the above alpha + beta region is effective in suppressing grain growth compared to the solution treatment in the beta single phase region. Furthermore, the solution treatment of the above alpha+beta region in the present invention enhances the alpha phase stabilizing alloy element and the beta stabilizing alloy element in the alpha phase and beta phase, respectively, so that TWIP and TRIP are generated when deformed even with a small Mo content. Because it can promote.

However, the solution treatment in the present invention is not necessarily limited to a region above alpha + beta. If crystal grain growth can be suppressed, and furthermore, solution treatment in the beta region can be applied under conditions in which a large amount of beta stabilizing elements are added in the composition range of the Ti-Mo-Fe-Cr-Al titanium alloy of the present invention. .

The solution treatment process in an embodiment of the present invention is preferably 20 to 90 ℃ lower than the beta transus temperature.

If the solution treatment temperature is higher than the upper limit value, there is a problem that almost all of the alpha phase generated in hot forging in the alpha + beta region is removed due to an excessively high solution treatment temperature.

Furthermore, there is a problem in that crystal grain growth may occur due to an excessively high solution treatment temperature.

Furthermore, too high a solution treatment temperature does not sufficiently ensure the concentration of the beta-stabilizing element into the beta phase, so that the beta phase is transformed into martensite during cooling, thereby preventing TWIP and TRIP from occurring.

On the other hand, if the solution treatment temperature is lower than the lower limit, the fraction of the beta phase decreases and further, the beta stabilizing element is excessively concentrated in the beta phase, so that TWIP and TRIP do not occur during deformation, and only slip occurs, resulting in a decrease in mechanical properties. There is a problem.

After the solution treatment process in the present invention, a rapid cooling process is performed.

The quenching process after the solution treatment is a process for maintaining the microstructure during the solution treatment process, more specifically, the high-temperature beta phase as it is at room temperature. When the beta phase maintained at room temperature through the rapid cooling generates TWIP and TRIP effects upon subsequent deformation, high strength and high formability of the beta titanium alloy of the present invention can be secured.

Hereinafter, a Ti-Mo-Fe-Cr-Al-based beta titanium alloy according to an embodiment of the present invention will be described in more detail through the following experimental examples.

<Experimental Example>

Table 1 is a table summarizing components and composition ranges corresponding to Examples and Comparative Examples of the Ti-Mo-Fe-Cr-Al-based beta titanium alloy of the present invention.

<Table 1>

Compositions 1-1 to 1-3 in Table 1 correspond to the composition of the comparative example of the Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention. On the other hand, the compositions 2-1-1 to 2-5-1 in Table 1 correspond to the compositions of the examples of the Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention.

Table 2 summarizes the mechanical properties and heat treatment conditions of the examples and comparative examples of the Ti-Mo-Fe-Cr-Al-based titanium alloy of the present invention in Table 1, and the proportion of the alpha phase and the Mo equivalent in the beta phase. It is a table.

<Table 2>

The titanium alloy that does not contain Mo of Comparative Example 1-1 of Table 1 exhibits remarkably low ductility because TWIP and TRIP do not occur. In addition, Comparative Example 1-2 of Table 1 that does not contain Al and Comparative Example 1-3 of Table 1 that contains 4% Mo also exhibits low strength and/or elongation properties.

FIG. 2 shows the product of tensile strength (UTS, ultimate tensile strength, MPa) and elongation (%) according to the Mo composition range of the examples and comparative examples of Tables 1 and 2 of the present invention.

As shown in Fig. 2, it can be seen that a beta titanium alloy having a composition range of 1% or more and less than 4% of Mo has a very high tensile strength*elongation value. However, since the beta titanium alloy corresponding to Comparative Example 1-2 does not contain Al, it has a lower tensile strength * elongation value than the beta titanium alloy corresponding to the embodiment of the present invention.

3 is a tensile strength (UTS, ultimate tensile strength, MPa) and elongation according to the value of Mo equivalent in the beta phase after solution treatment and quenching in the beta titanium alloys of Examples and Comparative Examples of Tables 1 and 2 of the present invention ( %).

In general, the Mo equivalent value is known as a value that determines the phase stability, deformation mechanism, and mechanical properties of a titanium alloy.

Particularly, the beta-titanium alloy to which the present invention belongs is intended to improve mechanical properties and workability by generating TWIP and TRIP effects. At this time, the TWIP and TRIP effects occur during processing in the beta phase. Therefore, in the beta titanium alloy of the present invention, the composition of the beta phase in the final microstructure is an important factor determining the occurrence of the TWIP and TRIP effects. The composition of the beta phase in the final microstructure is determined according to the conditions of the hot forging and solution treatment as well as the composition (or nominal composition) of the whole alloy. For example, as quantitatively illustrated in 2-2-2 to 2-2-4, 2-3-3 to 2-3-4, 2-4-2 to 2-4-5, etc. in Table 2, , Even in the same alloy composition, the lower the alpha + beta solution treatment temperature, the more the beta stabilizing element is concentrated in the beta phase, and the Mo equivalent value in the beta phase increases.

As shown in FIG. 3, the Ti-Mo-Fe-Cr-Al beta titanium alloy of the present invention was measured to have remarkably excellent mechanical properties in the range of the beta phase Mo equivalent value of 12.0 or more.

On the other hand, the Ti-Mo-Fe-Cr-Al beta titanium alloy corresponding to the comparative examples of 2-1-1 and 2-1-2 in the range of Mo equivalent values lower than 12.0 has relatively low beta phase stability, and thus Accordingly, during rapid cooling after solution treatment, the beta phase is transformed into martensite, and TWIP and TRIP do not occur. As a result, the Ti-Mo-Fe-Cr-Al beta titanium alloy corresponding to the comparative example is deteriorated in mechanical properties.

As a specific example, Comparative Examples 2-1-1 and 2-1-2 beta titanium alloys as described in Table 1, the composition of the alloy Ti-Mo-Fe-Cr-Al beta titanium corresponding to the embodiment of the present invention It satisfies all of the composition and composition range of the alloy. On the other hand, Comparative Examples 2-1-1 and 2-1-2 beta titanium alloys were measured to have Mo equivalents in the beta phase after solution treatment as described in Table 2 to have 7.6 and 9.2, respectively, and as a result, yield strength and It was determined that the elongation was low.

4 shows a microstructure of a 2-1-2 beta titanium alloy, which is a comparative example of the present invention, quenched after solution treatment.

As shown in FIG. 4, the 2-1-2 beta titanium alloy, which is a comparative example of the present invention, has low beta phase stability due to a low Mo equivalent in the beta phase, so that the beta phase cannot be maintained during rapid cooling, and the transformation from the beta phase to martensite Occurred.

Meanwhile, in FIG. 3, 2-2-1 corresponding to the comparative example of the present invention was measured to be superior in product of the ultimate tensile strength (UTS) and elongation (%) compared to some examples of the beta titanium of the present invention. However, Comparative Example 2-2-1, as shown in the mechanical property evaluation results in Table 2, has a very low mechanical strength that does not satisfy the minimum mechanical strength required of the beta titanium alloy.

The reason that Comparative Example 2-2-1 has lower mechanical strength than Examples 2-2-2 to 2-2-4 having the same alloy composition is due to the microstructure of Comparative Example 2-2-1.

5 is a microstructure photograph of 2-2-1, which is a comparative example of the present invention.

As shown in Fig. 5, Comparative Example 2-2-1 has very coarse grains. The coarse crystal grains of Comparative Example 2-2-1 originate from the solution treatment temperature in Table 2. The comparative example 2-2-1 was quenched after solution treatment in a beta single-phase region of 840° C. higher than 820° C., which is a beta transus temperature determined from the alloy composition and composition range. Accordingly, it is determined that Comparative Example 2-2-1 has a microstructure having coarse grains by solution treatment in the beta single phase region, resulting in low mechanical strength.

6 shows the ultimate tensile strength (UTS) value according to the alpha phase fraction of the beta titanium alloys of Examples and Comparative Examples of Tables 1 and 2 of the present invention.

As shown in FIG. 6, all of the beta titanium alloys corresponding to the embodiment of the present invention contain an alpha phase. In other words, as shown in Table 2, the beta-titanium alloys of the embodiment of the present invention are rapidly cooled after solution treatment in the alpha + beta or higher region to include all of the alpha phase. This is not only effectively suppressed the crystal grain growth of the beta titanium alloys of the embodiment of the present invention during the solution treatment, but also the beta stabilizing element is concentrated to the beta phase to stabilize the beta phase, and the beta phase may cause TWIP and TRIP phenomena during subsequent deformation. Means. As a result, all of the beta titanium alloys of the examples of the present invention were measured to have a tensile strength of at least 850 MPa or more.

7 is a photo of a microstructure of 2-5-1 beta titanium alloy according to an embodiment of the present invention before deformation (quickly cooled after solution treatment) and 2% deformation.

8 is a microstructure photograph of a 2-2-2 beta titanium alloy, which is an example of the present invention, before deformation (quickly cooled after solution treatment) and 15% deformed.

First, as shown in the microstructure photographs before deformation of FIGS. 7 and 8, the beta titanium alloy of the embodiment of the present invention has an alpha phase uniformly present in the beta phase matrix before deformation, and the crystal grain size of the beta phase does not exceed a maximum of 200 μm. Can be seen.

When the beta titanium alloy of the embodiment of the present invention is plastically deformed before the deformation, twins and martensite are generated in the beta phase due to deformation as shown in FIGS. 7 and 8. The presence of tween and martensite, respectively, is evidence that directly proves that the TWIP and TRIP phenomena occur in the beta titanium alloy of the embodiment of the present invention.

On the other hand, comparing FIGS. 7 and 8, it can be seen that as the amount of deformation increases, more TWIP and TRIP phenomena occur in the beta titanium alloy of the embodiment of the present invention, and as a result, more tween and martensite exist.

9 is a view showing tensile strength and elongation of Ti-Mo-Fe-Cr-Al beta titanium alloys of all Examples and Comparative Examples of the present invention described in Tables 1 and 2 above.

As shown in Figure 9, the beta titanium alloys corresponding to the embodiment of the present invention all have a tensile strength of 850 MPa or more and an elongation of 15% or more, whereas the beta titanium alloys corresponding to the comparative example are at least one of tensile strength or elongation One property was measured to be poor.

As described above with reference to the drawings illustrated for the present invention, the present invention is not limited by the embodiments and drawings disclosed in the present specification, and various by a person skilled in the art within the scope of the technical idea of the present invention. It is obvious that transformation can be made. In addition, even if not explicitly described and described the effect of the configuration of the present invention while describing the embodiments of the present invention, it is natural that the predictable effect by the configuration should also be recognized.

Claims (7)

1. In weight %, Ti-(1%≤Mo<4%)-(1%≤Fe<3%)-(1%≤Cr<4%)-(0%<Al≤4%),

   Mo equivalent is 12.0 or more,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
2. The method of claim 1,

   The alloy contains O≤0.2% by weight%,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
3. The method of claim 1,

   The alloy includes an alpha phase in the beta phase matrix in the microstructure before deformation,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
4. The method of claim 3,

   The average crystal grain size of the beta matrix is within 200㎛ ,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
5. The method of claim 1,

   The alloy includes tween and martensite with a beta-phase matrix and an alpha-phase microstructure after deformation,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
6. The method of claim 1,

   The tensile strength of the alloy is 850 MPa or more,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.
7. The method of claim 6,

   The alloy has an elongation of 15% or more,

   The product of tensile strength (MPa) and elongation (%) is 15,000 or more,

   Ti-Mo-Fe-Cr-Al-based beta titanium alloy with excellent strength and formability.

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