JPH03170650A - Production of high strength ti alloy - Google Patents

Abstract

PURPOSE:To obtain a Ti alloy having high strength and high ductility by applying a repetition of strain induced martensitic transformation and inverse martensitic transformation to a beta Ti alloy and introducing dislocation by means of the transformations. CONSTITUTION:For example, a beta Ti alloy which has a composition consisting of, by weight, 10% V, 2% Fe, 3% Al, and the balance Ti and also has a structure consisting of two phases of alpha and beta phases in an equilibrium state at room temp. and causes retained beta phase nonequilibrium at room temp. by means of cooling from a temp. in the vicinity of the beta transformation point and also causes strain induced martensitic transformation by means of plastic working is subjected, at least two or more times, to a repetition of a process in which strain induced martensitic transformation is caused by means of cold working, such as drawing, and then inverse martensitic transformation returning to beta phase is caused by means of heat treatment, by which the Ti alloy having high-level yield strength and tensile. strength can be produced without causing deterioration in elongation percentage.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a method for producing a β-type Ti alloy, and in particular, by combining appropriate processing and aging treatment, a high-strength Ti alloy can be produced without impairing ductility. It concerns the method by which it can be obtained.

[Prior Art] Unlike α-type Ti alloys, β-type Tt alloys can be cold-worked, and high strength can be obtained by heat treatment after processing, so they are used in various equipment in the aircraft field, etc. Demand is expected to increase as a component material for ships, automobiles, etc. Particularly in the automobile field, it is expected that applications such as valve springs will be expanded by taking advantage of the characteristics of a small Young's modulus and high strength. A high strength exceeding ' has been obtained. Such high strength is achieved through work hardening due to cold working and precipitation hardening due to the precipitation of α phase during aging treatment, but this results in an extremely low ductility, making it difficult to use as actual parts. It becomes unbearable.

[Problems to be Solved by the Invention] The present invention was developed by paying attention to the above-mentioned circumstances, and its purpose is to develop Ti that exhibits high strength and excellent ductility.
The aim is to establish a manufacturing method for alloys. [Means for Solving the Problems] The structure of the method of the present invention that can solve the above problems uses a β-type τi alloy that exhibits deformation-induced martensitic transformation, and cold working causes deformation-induced martensitic transformation. The gist of this method is to repeat at least two times the step of heat-treating the martensitic material and then returning it to the β phase to cause reverse martensitic transformation.

[Operation] In the case of a β-type Ti alloy in which deformation occurs due to slip, lattice defects (dislocations) introduced by plastic deformation are extremely non-uniform. The reason for this is something that should be theoretically clarified in the future and has not yet been clarified, but as a result, α phase precipitates using the dislocation area as a precipitation site during aging treatment (precipitation hardening heat treatment). Therefore, the α phase is locally unevenly distributed, and the aged structure becomes extremely non-uniform.

As a result, precipitation hardening due to α phase precipitation becomes nonuniform, which not only hinders high strength, but also causes strain to concentrate in areas where α phase precipitation is coarse (i.e., areas where precipitation hardening is insufficient), resulting in fracture. As a result, ductility is also very poor.

The present invention improves these difficulties, and it is possible to obtain a substructure in which dislocations, which serve as α-phase precipitation sites during aging, are uniformly and densely introduced, thereby making it possible to distribute the α-phase uniformly and densely throughout the entire body. The aim is to establish a technology capable of precipitating the precipitation, and specifically, using a β-type Ti alloy that exhibits deformation-induced martensitic transformation as a Ti alloy, as described above, and cold working to induce deformation-induced martensitic transformation. The process of generating reverse martensitic transformation, which is then heat-treated to return it to the β-type, is repeated at least twice.

Here, the β-type Ti alloy exhibiting deformation-induced martensitic transformation consists of two phases, α and β, in an equilibrium state at room temperature.
Even if the temperature is above the β transformation point or below the β transformation point, cooling from the vicinity produces a non-equilibrium residual β phase at room temperature, and when this is plastically worked, deformation-induced martensitic transformation occurs.
Refers to type Ti alloy. Such β-type Ti alloy is usually 10
% of cold working results in almost the entire martensitic structure, and further processing causes slip deformation and dislocations are introduced unevenly. However, in the present invention, plastic working is carried out until deformation-induced martensitic transformation occurs. After stopping and introducing dislocations through this transformation alone, subsequent heat treatment causes reverse martensitic transformation to return to the β-type, and then plastic working is added to this to cause deformation-induced martensitic transformation again, thereby creating new dislocations. will be introduced. The dislocations introduced by slip deformation are non-uniform as mentioned above, but the dislocations introduced by transformation are uniformly given throughout, and these dislocations are also converted to α phase precipitation sites by reverse martensitic transformation by subsequent heat treatment. It will be inherited as is. Then, by repeating the process of introducing dislocations through deformation-induced martensitic transformation and the process of introducing dislocations through reverse martensitic transformation two or more times, new dislocations are sequentially and uniformly introduced, α-phase precipitation sites are accumulated, and uniform Moreover, a substructure with dense α-phase precipitation sites can be obtained. At this time, in the parts where transformation did not progress much in the first plastic working and dislocations were introduced roughly, martensitic transformation progresses preferentially in the next processing and dislocations are introduced. In other words, areas where dislocations are coarse have a small degree of work hardening and have lower strength than areas where dislocations are densely introduced, making it easier for work-induced martensitic transformation to occur during subsequent plastic working. As a result, by repeating the above operations, the introduction positions of dislocations become more uniform, and it becomes possible to introduce α phase precipitation sites extremely uniformly and with high density.

The uniformity and densification of α-phase precipitation sites can be effectively achieved by repeating the deformation-induced martensitic transformation and reverse martensitic transformation at least twice, but the more the number of repetitions increases, the more The effect is enhanced and reflected in the physical properties of the Ti alloy finally obtained.

That is, in a Ti alloy in which dislocations are uniformly and densely introduced in this manner, when this is subjected to an aging treatment, α phase precipitates uniformly and densely over the entire structure using the dislocations as precipitation sites. As a result, the Ti alloy has undergone precipitation hardening due to alpha phase precipitation evenly over the entire area, and has extremely high strength without any strength defects compared to conventional materials that have a mixture of localized precipitation hardened areas and insufficient precipitation hardened areas. Becomes the property of Moreover, even when this alloy is subsequently subjected to a load, the internal strain will be distributed evenly within the metal structure and will be evenly dispersed in each of the precipitated α phases. As a result of no longer causing local fracture due to stress concentration, ductility is also significantly improved. The type of β-type Ti alloy used in the present invention is not particularly limited as long as it exhibits deformation-induced martensitic transformation, and the β-stabilizing elements in Ti are MO, V, and N, which are all solid solutions.
b, Ta, eutectoid Fe, Cr, Mn, Co, Ni, or neutral elements Sn, Zn, etc. can be used in various β-type Ti alloys, and α-stabilizing elements can be used. A certain AI
It is preferable to contain a small amount (approximately 3%) of α-phase because it promotes the precipitation of α phase during aging treatment and the effects of the present invention are more effectively exhibited. [Example] Ti-1 0V-2Fe-3AI alloy was used as a β-type Ti alloy that exhibits deformation-induced martensitic transformation, and Ti-1 5V, which only causes sliding deformation and does not exhibit deformation-induced martensitic transformation, was used as a comparative material. -3Cr-3Sn-3
Using AI alloys, we investigated the effects of the number of repetitions (the number of repetitions of deformation-induced martensitic transformation and reverse martensitic transformation) and the amount of cold working on the tensile properties after aging treatment. In addition, the drawing method is used for cold working, and Ti-10
The V-2Fe-3AI alloy was air-cooled at 510°C for 8 hours, and the Ti-15V-3Cr-3Sn-3AI alloy was air-cooled at 480tx for 8 hours. Table 1 shows the processing conditions and physical properties of the aged material obtained. From Table 1, it can be considered as follows.

No. Examples 1 to 3 are examples that satisfy the specified requirements of the present invention, and by repeating deformation-induced martensitic transformation and reverse martensitic transformation two or more times, a high level of proof stress and tensile strength can be achieved without significantly reducing the elongation rate. is obtained.

On the other hand, even when β-type Ti alloys exhibiting the same deformation-induced martensitic transformation are used, those in which dislocations are introduced by sliding deformation (Nos. 4 to 6) show a significant decrease in elongation due to cold working. The elongation rate decreased to 0.3% with a total processing amount of 18.5%. Moreover, the total machining amount was almost the same in Monoku Experiment No. Comparing the yield strength and tensile strength with Experiment No. 1 to 3), Experiment No. It can be seen that numbers 4 to 6 are considerably lower. It is thought that these differences reflect the uniform and dense introduction of dislocations and the resulting uniformity and high density of α phase precipitation according to the present invention. Also, experiment no. 7 to 9 are comparative examples using a β-type Ti alloy that does not show deformation-induced martensitic transformation and only causes sliding deformation, and although the elongation rate is good at low deformation rates, the elongation rate rapidly decreases as the deformation rate increases. decreases to . Moreover, the α-phase precipitation sites are concentrated in the sliding deformation part, and other parts are insufficiently strengthened by precipitation, so the yield strength and tensile strength of the present invention material (N
o. It is quite low compared to 1 to 3).

[Effects of the Invention] The present invention is structured as described above, and by repeating process-induced martensitic transformation and reverse martensitic transformation, and introducing dislocations through the transformation, α-phase precipitation sites are uniformly and densely distributed over the entire area. As a result, precipitation hardening due to alpha phase precipitation can be made uniform, promoting high strength, and the stress strain generated during secondary processing can also be well dispersed, increasing ductility and increasing the strength. Strong and ductile Ti
We were able to provide the alloy.

Claims (1)

[Claims] A β-type Ti alloy exhibiting deformation-induced martensitic transformation is used, and at least two steps are performed in which the deformation-induced martensitic transformation is caused by cold working, and then the reverse martensitic transformation is caused by heat treatment to return it to the β phase. A method for producing a high-strength Ti alloy, the method comprising repeating the process more than once.