JP2014080669A - β TYPE TITANIUM ALLOY AND THERMAL MECHANICAL TREATMENT METHOD OF THE SAME - Google Patents

Abstract

The present invention can be used in various mechanical engineering fields for producing semi-finished products and parts from β (beta) type titanium alloys by thermomechanical processing accompanied by changes in material properties. An object of the present invention is to improve the strength and fatigue characteristics of a β-type titanium alloy and to use it in engineering.
A β-type titanium alloy according to the present invention has a fine grain structure, an average particle size of β particles is 0.5 μm or less, and an average particle size of spherical α-phase precipitates is 0.5 μm or less. And it is characterized in that the volume fraction in the tissue is 40% or more. Moreover, the thermomechanical processing method of the β-type titanium alloy according to the present invention includes a high strain processing and a heat treatment step, and continues before the processing for 1 minute or more per 1 mm of billet diameter (β transus + 5 to β transus + 15 ) Heated to ℃, after water quenching, by the equal channel angle extrusion method, the change of deformation direction after each processing stage is 90 °, and the cumulative strain is at the temperature of (β transus -200 to β transus -150) ℃ It is characterized in that it is subjected to high strain processing until it becomes 3.5 or more and then water quenching.
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Description

  The present invention relates to a β-type titanium alloy and a thermomechanical processing method thereof, and can be used in the field of mechanical engineering for improving the mechanical properties of a material and manufacturing a semi-finished product or member from the alloyed titanium alloy. is there.

  High alloys (high strength) β alloys belong to the class of titanium alloys in which the β phase is stable at room temperature and transform only by heating. Therefore, the strength increases as a result of quenching and aging (U. Zwicker, Titan und Titanlegierungen / Berlin: Springer Verlag.-1974. 717 с .; Boyer R., Welsch G., Collings E. Materials Properties Handbook : Titanium Alloys. ASM International. 1998. 1048 p.). It is also known that the particle size of the matrix β phase and the form of secondary (secondary) α phase precipitates have a great influence on the strength and ductility of the alloy after aging treatment. In particular, it has been proved that when the β phase particle size is reduced to 10 μm, high strength can be obtained with sufficient ductility (OM Ivasishin, PE Markovsky, SL Semiatin, CH Ward Aging response of coarse- and fine-grained titanium). alloys // Materials Science Engeneering A Vol. 405 (2005), p. 296-305 ISSN 0921-5093).

  As a method to improve physical and mechanical properties for commercially available metals and alloys, a fine plastic (UFG) structure is generated by severe plastic deformation (SPD). It is known. This demonstrates that very high plastic strain can be obtained at relatively low temperatures (usually 0.3-0.4 Tm, K) and high load pressures (RZ Valiev, Y. Estrin, Z. Horita, TG Langdon, MJ Zehetbauer, YT Zhu Producing bulk ultrafine-grained materials by severe plastic deformation. // JOM-2006. 58, No4, p33.). SPD technologies such as High Pressure Torsion (HPT), Equal Channel Angular Pressing (ECAP), multi-directional multi-stage forging, and variations thereof have been actively developed. I came. In particular, the ECAP technique for deforming a bulk sample by simple shearing is a multi-stage extrusion process performed in a special die set, and is a method of extruding in two channels that normally intersect at 90 ° with the same cross-sectional area ( Patent RF No.2181314. Published. BI 2002, No.16). However, β-type alloys with fine grains, that is, grain sizes of less than 1 μm are not yet known in the literature. In such a structure, the β-phase decomposition process during heating occurs differently from the case of a coarse-grained structure, and the decomposition behavior and dynamics are determined depending on the presence of crystal lattice defects. Further, the presence of fine grains contributes to uniform diffusion decomposition of the β phase, but this is almost impossible in a coarse grain structure.

A technique for processing a member made of a titanium alloy, in which a thermomechanical treatment is performed over 11 stages (Russian Patent No. 2384647 MPK (МПК) C22F1 / 18 (2006.01) , Published: 20.03.2010). In the first stage beta transus (T _beta) above, was heated to _{T β + 150~T β + 250 ℃} , the deformation direction changing 90 °, upsetting and stretching (upset and drawing), 20~ the strain at each step 50% and alternately deform 4 times.

In a second stage, heated to _{T β + 140~T β + 230 ℃} , the deformation direction changing 90 °, upsetting and stretching (upset and drawing), and the strain at each step to 25-50%, alternately Deform 4 times.

In the 3rd stage, it heats to T ( _beta) -20-T ( _beta) -40 degreeC, and makes distortion 20-60%.

In the fourth stage, and heated to _{T β + 60~T β + 120 ℃} , the strain 20 to 60%.

In the 5th stage, it heats to T ( _beta) -20-T ( _beta) -40 degreeC, and makes distortion 20-60%.

In the sixth stage, and heated to _{T β + 30~T β + 90 ℃} , the strain 20 to 60%.

In the seventh stage, and heated to _T β -20~T β -40 ℃, the strain 20 to 60%.

In the eighth stage, and heated to _{T β + 30~T β + 80 ℃} , the strain 20 to 70%.

In the 9th stage, it heats to T ( _beta) -20-T ( _beta) -40 degreeC, and makes distortion 20-50%.

In the 10th stage, it heats to T ( _beta) -20-T ( _beta) -200 degreeC, and cools by air.

In the eleventh stage, heated to _T β -270~T β -550 ℃, the immersion time (soaking time) 5-20 hours. In this case, the deformation process of at least three stages performed from the third stage to the ninth stage is performed with a change angle of 90 degrees in the deformation direction.

  Although this method contributes to the improvement of the strength of the processed material, it is very time consuming, consumes a large amount of power, and further, the required strength and fatigue characteristics cannot be achieved.

There is also a thermomechanical treatment method for β-titanium alloys (OM Ivasishin PE Markovsky, Yu.V. Matviychuk, SL Semiatin, CH Ward, S. Fox.A comparative study of the mechanical properties of high-strength titanium alloys / / Journal of Alloys and Compounds, 2007 ISSN: 0925-8388). This _{method, (T β + 60~T β +100} ) and water quenching from ° C., until a temperature corresponding to the recrystallization process occurs temperature after cold rolling and / or withdrawal rapidly heated, followed, by quenching, beta Enabling a tissue having a phase average size of about 10 μm. Thereby, high strength (about 1500 MPa) and sufficient ductility are obtained.

  The disadvantage of this method is the heating process up to the temperature corresponding to the temperature at which the recrystallization process occurs, and very accurate measurement and control of all process parameters including heating rate, immersion / holding time at a given temperature, and cooling rate. Assumption. Considering the combination of these parameters, the allowable range of these parameters is very narrow, and it is expected that the possibility of obtaining the expected mechanical property level is low.

The method closest to the method according to the present proposal is a thermomechanical processing method of a β-type titanium alloy, which includes hot forming by extrusion forming 98% reduction at 880 ° C., and subsequently a material sample (T _β incomplete quenching from _-40~T β -60) ℃, 1000 hours at 500 ° C., and performs an aging treatment (T. Nishimura, M.Nishigaki, Y.Moriguchi, Characteristics of beta Titanium alloy Ti-15Mo- 5Zr-3Al // R & D. Vol.32 No.1).

  Although the β-type titanium alloy treated according to the above process has high strength and ductility, it does not provide the necessary yield strength. Furthermore, this processing method is time consuming and has low productivity.

  Furthermore, Patent Document 1 discloses an invention relating to a method for producing a biocompatible high-strength titanium material. In this method, a biocompatible titanium material made of titanium or a titanium alloy containing no vanadium is obtained by subjecting a biocompatible titanium material made of titanium or a vanadium-free material to strong processing four or more times under a temperature condition of 300 ° C. to 700 ° C. using a rotary ECAP apparatus. A biocompatible titanium material having a metal structure refined to 10 μm or less is characterized.

JP-A-2005-105385

  In the technique described in Patent Document 1, sufficient refinement of β grains has not been achieved, and as a result, there has been a problem that it is difficult to obtain sufficiently high fatigue strength.

  The present invention is to develop a β-type titanium alloy having a fine grain structure and a thermomechanical processing method for a β-type titanium alloy, which has excellent material strength and fatigue resistance and exceeds the characteristics of conventional β-type alloys. With the goal.

  The above problem is solved by the following means.

  Β-type titanium alloy having a fine grain structure, wherein the average grain size of β grains is 0.5 μm or less, the average grain size of spherical α-phase precipitates is 0.5 μm or less, and the volume fraction in the structure Β-type titanium alloy, characterized in that is 40% or more.

  This is a thermomechanical processing method for β-type titanium alloy, including strong strain processing and heat treatment process, and before processing, it is continued for 1 minute or more per billet diameter of 1 mm and heated to (βtransus + 5−βtransus + 15) ° C. Then, after water quenching, the cumulative strain is 3.5 or more at a temperature of (β transus-200 to β transus-150) ° C. with a change in deformation direction after each processing step of 90 ° by an equal channel angular extrusion method. The method is characterized by subjecting to strong strain processing until it becomes, followed by water quenching.

  According to the present invention, the β grains are refined as the spherical α phase having an average particle size of 0.5 μm or less is dispersed and precipitated at the grain boundaries / sub-grain boundaries and the structure defects. As a result, there are provided a β-type titanium alloy having a fine grain structure and a thermomechanical processing method for the β-type titanium alloy, which are excellent in strength and fatigue resistance of a material exceeding the characteristics of a conventional β-type alloy.

  One embodiment of the present invention relates to a β-type titanium alloy. Specifically, it is a β-type titanium alloy having a fine grain structure, the average particle size of β grains is 0.5 μm or less, the average particle size of spherical α-phase precipitates is 0.5 μm or less, and The present invention relates to a β-type titanium alloy characterized by having a volume fraction in a tissue of 40% or more. Here, the average particle diameter of β grains is preferably 0.3 to 0.5 μm, and more preferably 0.3 to 0.4 μm. The average particle size of the spherical α phase precipitate is preferably 0.1 to 0.5 μm, more preferably 0.1 to 0.3 μm. Furthermore, the volume fraction of the spherical α phase precipitate in the structure is preferably 0.5 to 20%, more preferably 1.0 to 10%. In addition, there is no restriction | limiting in particular about the specific composition of the beta type titanium alloy which concerns on this form, Although conventionally well-known knowledge can be referred suitably, As an example, Ti-15mass% Mo-5mass% Zr-3mass% Al etc. Is mentioned.

  Another embodiment of the present invention relates to a thermomechanical processing method for a β-type titanium alloy. Specifically, it includes high-strain processing and heat treatment process, and before processing, it is continued for 1 minute or more per billet diameter and heated to (βtransus + 5−βtransus + 15) ° C. and after water quenching, equal channel angle By the extrusion method, the deformation direction after each machining stage is changed 90 °, and at a temperature of (β transus-200 to β transus-150) ° C., a high strain process is performed until the accumulated strain becomes 3.5 or more, The present invention relates to a method characterized by subsequent water quenching.

  Here, the crystal structure of titanium has a dense hexagonal crystal (hcp) structure at a low temperature range below the transformation point temperature, and allotropic transformation into a body-centered cubic (bcc) structure at a high temperature range above the transformation point temperature. This transformation temperature is β transus (β transformation point). In a titanium alloy, β transus can be changed by adding various alloy elements.

  In this specification, the average particle diameter of β grains and spherical α phase precipitates is calculated by a conventional method using a transmission electron microscope (TEM). That is, it is calculated as being half the sum of the major axis and minor axis of the particle. The remarkable refinement of β grains is that a secondary α phase with a diameter of 0.5 μm or less and a spherical shape (the ratio of the particle length is 2 or less) is dispersed and precipitated at grain boundaries / sub-grain boundaries and in structure defects. It is accompanied. The degree of uniform dispersion of secondary α-phase particles is predicted by calculating the number of particles on a random cutting line on any two cross sections, and according to this, the phase needs to be 50% or more. . It is the grain boundary hardening mechanism due to the decrease in β grain size based on the so-called Hall-Petch law and the precipitation hardening mechanism related to the precipitation of α phase secondary particles that contribute to the increase in the strength of the alloy (Cocks Yu.V Physics of strength and ductility. Translated from English. М .: Metallurgy, 1972. 304 p.). Sufficient ductility of the fine-grained alloy is a secondary contribution that contributes to the formation of relative equilibrium β-phase grain boundaries as a result of the recovery process that occurs when the decomposition temperature rises and to the relaxation of stress concentration between the boundaries during deformation by tension. Obtained by the spherical shape of the target α phase.

  The point that the fatigue resistance limit (fatigue strength) depends on the grain size is often described by an equation similar to the Hall-Petch law for yield stress (proof stress). According to the equation, in most cases, the fatigue properties of metallic materials increase when the particle size decreases to a fine range (less than 1 μm) (A. Vinogradov, S. Hashimoto, Multiscale phenomena in fatigue of ultra-fine grain materials-an overview.// Materials Transactions. 2001. V. 42 (1). pp.74-84). However, the formation of microstructures in metals and alloys may not necessarily lead to an increase in fatigue resistance, which is thought to be due to limitations in ductility. This depends on the individuality of the organization, such as the distribution of secondary phases (Semenova IP, Yakushina EB, Nurgaleeva VV, Valiev RZ Nanostructuring of Ti-alloys by SPD processing to achieve superior fatigue properties // International Joint Materials Research ( formerly Z. Metallk.), Vol. 100 (2009), 12, P.1691-1696.

  By applying the β-type titanium alloy treatment proposed by the present invention, a fine grain structure can be formed, the β phase particle size is less than 0.5 μm, and the volume fraction is about 45%. is there. By forming such a fine structure (UFG), not only high strength but also ductility is improved, and as a result, fatigue resistance is improved (Research (formerly Z. Metallk.), Vol. 100 (2009), 12, P.1691-1696.).

Therefore, by combining heat treatment and high strain processing within the range described above, the formation of fine grains in the β phase matrix, the dispersion precipitation of secondary α phase particles, and the combination of the hardening mechanism of grain boundaries and dispersion , High strength (UTS> 1500 MPa), and fatigue resistance limit (S ₋₁ > 700 MPa) can be obtained.

The technical effects achieved by the present invention are as follows. _{(T β + 5~T β +15)} β tissue of a single phase is obtained by water quenching from ° C., consisting of coaxial crystals beta grain size is equipped with a body-centered cubic crystals (bcc) lattice 40 to 60 [mu] m for. The obtained single-phase β structure of billet alloy shows good workability and deformability (EW Collings, The physical metallurgy of titanium alloys, Moscow: Metallurgy, 1988.-224pp .; Boyer R., Welsch G., Collings E. Materials Properties Handbook: Titanium Alloys. ASM International. 1998. 1048 p.). (Β transus-200 to β transus-150) During severe strain processing at a temperature of ° C., β grains are divided and recrystallization between phases occurs, and new grains and fine grains with a grain size of 0.5 μm or less are formed. The Deformation at a temperature below a specific range greatly reduces the deformability of the material due to thermally activated recovery and suppression of the recrystallization process. Processing deformation beyond a specific range according to the present invention makes grain growth uncontrollable, resulting in non-uniform tissue refinement.

  The technical effect of the present invention is to improve the strength and fatigue characteristics of β-type titanium alloy by forming a fine grain structure, and β grains having a size of 0.5 μm or less and a volume fraction in the structure ( The volumetric α) is composed of spherical α phases having an average particle size of 0.5 μm or less and uniformly dispersed with a volume ratio of 40% or more. In addition, the fatigue endurance limit (fatigue strength) measured by the rotational bending test of the fine grain (UFG) alloy exceeds the fatigue strength obtained by the conventional hardening thermomechanical treatment by 40 to 80 MPa, and the present invention It is clear that it has a high advantage over the prior art.

  The method is performed as follows.

First, a billet comprising a beta titanium alloy (T _β + 5~T _β +15) to a temperature of ° C., per billet diameter 1 mm, and heating continues for more than 1 minute, then perform water quenching. As a result, the texture of the material exhibits a single-phase β matrix. The obtained bcc lattice structure has higher ductility and deformability than the hcp structure of α titanium, and in this state, the material can accumulate higher strain. Thereafter, the billet is processed as a high strain processing at (β transus-200 to β transus-150) ° C. by an equal channel angular extrusion method at 90 ° in all deformation stages. The number of execution cycles is carried out until it becomes 3.5 or more when defined by cumulative strain. After that, the billet is rapidly cooled with water at room temperature, and the state brought about by the high strain processing is set and fixed. After this treatment, a fine grain structure is formed in the billet, leaving the original geometric shape unchanged. This structure is mainly composed of β grains not exceeding 0.5 μm, granular α phase precipitates having an average particle diameter of 0.5 μm or less and a volume fraction in the structure of 40% or more and uniformly dispersed in a fine-grain matrix. Consists of. This has the following characteristics. That is, it has fatigue properties with high strength and good ductility. After the treatment, mechanical property measurement and microstructure observation are performed by a tensile test at room temperature.

Application example A near β-type titanium alloy (Ti-15Mo-5Zr-3Al) having a diameter of 20 mm and a length of 100 mm was used. The β transus temperature was 785 ° C. The bar was water quenched from 800 ° C. (after heating for about 20 minutes). Thereafter, the bar was subjected to strength strain deformation according to the above method. The processing / deformation temperature was 600 ° C. The number of stages was n = 5, and the accumulated strain was e = 3.5.

  Thereafter, uniform control of the fine structure was performed over the cross section of the billet. As a result of the treatment, a fine structure is obtained. The features are β grains having an average particle diameter of 0.3 to 0.5 μm as a main phase and secondary particles uniformly dispersed in a diameter of 0.1 to 0.5 μm. It is a precipitate of α phase, and the volume fractions of α phase and β phase occupy volume ratios of 45% and 55%, respectively. Table 1 shows the values of the mechanical properties obtained by the tensile test at room temperature. For comparison, Table 1 also shows mechanical properties according to the present invention before the thermomechanical treatment according to the present invention (T. Nishimura, M. Nishigaki, Y. Moriguchi, Characteristics of Beta Titanium alloy Ti-15Mo-). 5Zr-3Al // R & D. Vol.32 No.1]).

What can be said from the data in Table 1 is that the treatment according to the embodiment of the present invention can obtain a higher tensile strength (UTS) and fatigue strength while maintaining a sufficient level of ductility as compared with the treatment using the conventional product. It is. Thereby, a low elastic modulus can also be maintained.

  Therefore, the β-type titanium alloy thermomechanical processing method according to the present invention can improve the strength and fatigue property level and uniformity of the processed material while maintaining ductility.

Claims (2)

Β-type titanium alloy having a fine grain structure,
β particles have an average particle size of 0.5 μm or less,
A β-type titanium alloy characterized in that the spherical α-phase precipitate has an average particle size of 0.5 μm or less and a volume fraction in the structure of 40% or more. A thermomechanical processing method for β-type titanium alloy,
Including strong strain processing and heat treatment process, before processing, continue for 1 minute or more per 1 mm billet diameter, heat to (βtransus + 5−βtransus + 15) ° C., and after water quenching, by equal channel angle extrusion method, The deformation direction after the processing stage is changed by 90 °, and at a temperature of (β transus -200 to β transus -150) ° C., the high strain processing is performed until the cumulative strain becomes 3.5 or more, and then water quenching is performed. A method characterized by that.