CN111041395B

CN111041395B - Ultra-high density twin crystal titanium and preparation method thereof

Info

Publication number: CN111041395B
Application number: CN201811187137.7A
Authority: CN
Inventors: 曹阳; 黄照文; 李玉胜; 雍鹏凌; 聂金凤; 周浩
Original assignee: Nanjing University of Science and Technology
Current assignee: Nanjing University of Science and Technology
Priority date: 2018-10-12
Filing date: 2018-10-12
Publication date: 2021-07-06
Anticipated expiration: 2038-10-12
Also published as: CN111041395A

Abstract

The invention discloses an ultrahigh-density twin crystal titanium and a preparation method thereof, wherein the ultrahigh-density twin crystal titanium comprises three procedures of rotary rolling, annealing treatment and cryogenic low-temperature cold rolling, and the texture is regulated and controlled through the rotary rolling to obtain an ultrahigh-strength texture titanium material with the basal plane texture strength of 21.73; annealing treatment recovers and eliminates microscopic defects and substructures, but retains high-strength texture with basal plane texture strength of 14.33; the cryogenic low-temperature cold rolling deformation introduces an ultra-high density twin crystal structure with twin crystal area fraction as high as 67.3 percent on the basis of the previous two steps. The yield strength of the ultra-high density twin crystal titanium prepared by the invention is above 850MPa, and the tensile strength is close to 1 GP.

Description

Ultra-high density twin crystal titanium and preparation method thereof

Technical Field

The invention relates to a preparation method of ultrahigh-density twin crystal titanium, in particular to a preparation method of a material for introducing ultrahigh-density twin crystal by regulating and controlling texture through rotary rolling and combining deep cooling rolling, and belongs to the field of material preparation.

Background

Titanium is increasingly used in aerospace, automobiles, ships and biomedical fields because of its light weight, high specific strength, good corrosion resistance and excellent biocompatibility. However, compared with general structural engineering materials, such as stainless steel, die steel, etc., titanium has low strength, which limits further application in the industrial field. In order to improve the strength of titanium, it is an effective means to refine titanium crystal grains by plastic deformation. The large plastic deformation technology has obvious capability of refining grains, can refine the grain structure of the material to submicron or even nanometer level, and is recognized as the most promising method for preparing block nano-crystalline and ultra-fine crystalline materials by the international material science community. However, the large plastic deformation technique usually requires a high strain amount to be applied to the material, and has many disadvantages of high cost of a mold and equipment, poor process continuity, small sample size, and the like. Therefore, it is challenging to obtain a material with high strength and high toughness by refining grains with high strain amount of large plastic deformation technology in practical production.

L, Lu et al Science, 2009,323: in the publication "reforming the maximum strength in nano-twinned copper" published in 607-610, the introduction of nano-twins into the material is introduced, which can greatly improve the strength of the material and maintain the plasticity of the material. The strengthening mechanism is mainly as follows: (1) the twin crystal boundary can block dislocation sliding in material deformation, so that the rheological stress required by the material deformation is improved, and the improvement of the material strength is realized; (2) the twin crystal cutting realizes the grain refinement, and according to the Hall-Petch relation, the smaller grain size can effectively improve the strength of the material. And the titanium with a close-packed hexagonal crystal structure has fewer independent sliding systems and poorer processing deformability, and microcracks are easily formed in the deformation process, so that the material fails in advance. By introducing twin crystals into the material, the plasticity and the processing deformability of the material can be improved while the strength of the material is improved. However, in the case of titanium, the critical shear stress required for the nucleation of the twin crystal is high, and the factors affecting the nucleation of the twin crystal (such as stress state, grain size, material texture, etc.) are large, so that it is difficult to obtain a high-density twin crystal material by general plastic deformation.

The study on microstructure and mechanical properties of high-strength textured industrial pure titanium processed at room temperature and ultra-low temperature shows that the twin crystal density in the rolled industrial pure titanium can be effectively increased by reducing the rolling temperature. The reason for this is that: (1) the material flow deformation stress is improved in the low-temperature deformation process, and the critical shear stress of twin crystal nucleation is reached; (2) the low-temperature deformation is beneficial to exciting the low-temperature twin crystal nucleation. Compared with room temperature rolling, the real stress and the real strain of the titanium sample rolled at the ultralow temperature in the tensile test are obviously improved. However, the twin crystal density of titanium is greatly increased as compared with room temperature deformation merely by lowering the deformation temperature, but the ultimate density is still not satisfactory, and it is difficult to obtain high-density twin crystals uniformly distributed throughout the inside of the crystal grains. And the due strengthening and toughening effect of the nanometer twin crystal is not reflected on the improvement degree of the mechanical property.

Disclosure of Invention

The invention aims to provide a method for preparing ultra-high density twin crystal titanium by combining rotary rolling and cryogenic rolling. According to the invention, the initial texture of the sample is regulated and controlled through rotary rolling, redundant substructures introduced in the rolling process are eliminated through subsequent annealing treatment, the material with uniform grain size distribution and high-strength texture is obtained, and finally, rolling deformation is carried out at the liquid nitrogen temperature, so that the high-strength and high-toughness titanium plate with an ultrahigh-density twin crystal structure is obtained.

The invention is realized by the following technical scheme, and the preparation method of the ultra-high density twin crystal titanium comprises the following steps:

step one, rotary rolling: rolling the rolled plate by 2-5%, rotating the rolled plate by 10-170 degrees along the reverse or clockwise direction, and rolling the rolled plate by 2-5% again, rotating the rolled plate by 10-170 degrees along the rolling direction continuously in the reverse or clockwise direction, wherein the rotating direction of each time is consistent with that of the previous rotation; repeating the process until the reduction reaches 60-90% so as to ensure that the material with a specific high-strength texture is obtained;

step two, annealing treatment: according to the thickness of the plate, annealing the rolled material at 525-575 ℃ for 30-60 minutes;

step three, cold rolling at a deep low temperature: and (3) performing multi-pass rolling treatment on the annealed material at the liquid nitrogen temperature until the rolling reduction is 25-50%, thus obtaining the titanium material with the ultrahigh-density twin crystal structure.

Further, in the third step, the annealed material is soaked in liquid nitrogen for 10mins to ensure that the material is at a low temperature during each rolling treatment, and the reduction of each rolling treatment is 5%.

Compared with the prior art, the invention has the following technical effects:

(1) the invention adopts a rolling process which is mature and applied in industry and carries out multi-pass and small-reduction rotary rolling treatment. On one hand, on the premise of ensuring that the material does not lose efficacy, the texture of the material is designed controllably to obtain the material with stronger texture favorable for twin crystal nucleation, and on the other hand, the material can be uniformly introduced with substructure inside through rotary rolling treatment, thereby providing necessary conditions for recrystallization of the material.

(2) The method adopts the annealing treatment of 525-575 ℃ for 30-60 minutes on the material subjected to the rotary rolling treatment, the heat treatment process is obtained on the premise of comprehensively considering the recrystallization and the grain growth rate of titanium, when the temperature is higher than the temperature range, the grain growth phenomenon of the titanium sample subjected to the rotary rolling treatment is obvious, and the sample with overlarge grain size is difficult to obtain a high-density twin crystal structure through plastic deformation; when the temperature is lower than this temperature range, recrystallization of the spin-rolling treated titanium sample is difficult to occur. In addition, the heat treatment time is based on the criterion, the size of the recrystallized grains is reduced as much as possible on the premise of eliminating redundant substructures in the material and obtaining the recrystallized grains, and the annealing time selected according to the thickness of the material is 30-60 minutes.

(3) The method skillfully combines the influence of material texture, grain size and deformation temperature on twin crystal nucleation, successfully prepares the titanium material with ultra-high density twin crystal, and solves the problem of low titanium strength. Compared with the high-strength material obtained by a general large plastic deformation mode, the ultrahigh-density twin crystal material has less micro defects, so that the material can be kept in shape to the maximum extent while high strength is obtained. In addition, the twin boundary energy is lower, so that the material can be ensured to be used at higher temperature without failure. And the sample size can be easily scaled up to more closely approach industrial applications.

(4) Simple processing technology, less investment, high safety factor and high production efficiency.

(5) The ultra-high density twin crystal material can be prepared by using the rolling treatment method widely applied in the industry at present, an additional production line is not required to be established, and various materials can be conveniently prepared; the processing temperature is easy to control, and the repeatability of the process is high.

Drawings

FIG. 1 is a schematic view of an exemplary process flow.

FIG. 2 (a) is a texture distribution diagram of a titanium sample subjected to a rolling treatment by rotation, and (b) is a texture distribution diagram of a titanium sample subjected to a rolling treatment by general.

FIG. 3 (a) is an ultrahigh density twin crystal titanium sample, (b) is a back scattering electron diffraction topography of a general twin crystal structure titanium sample, and (c) is a corresponding twin crystal area fraction distribution diagram.

FIG. 4 is a tensile engineering stress-strain curve diagram of an ultra-high density twin crystal titanium sample and a general twin crystal structure titanium sample.

Detailed Description

The following examples are given for the detailed implementation and specific operation of the present invention, but the scope of the present invention is not limited to the following examples.

As shown in fig. 1, the following example involves a two-step process comprising: the texture is regulated and controlled through rotary rolling and annealing treatment, and the ultrahigh-density twin-crystal titanium is prepared through deep low-temperature rolling, wherein:

a sample of 15 mm. times.60 mm. times.100 mm was cut out of the resulting plate, and the surface and corners were polished and then treated. FIG. 1 shows a flow chart of a process for preparing ultra-high density twin crystal titanium. The method comprises the following specific implementation steps: 1) along the sheet material RD₁Rolling treatment with the rolling reduction of 5% is carried out in the direction; 2) after rolling, the plate is rotated by 135 degrees in the anticlockwise direction, and the rolling direction is from RD₁Becomes RD₂(ii) a 3) Along the sheet material RD₂Rolling in the direction of 5%, rotating the rolled plate counterclockwise by 135 ° along the rolling direction, wherein the rolling direction is RD₂Becomes RD₃(ii) a Repeating the steps in the same way until the total reduction of the plate reaches 80%; 4) after the rotary rolling, annealing the sample at 550 ℃ for 30mins in a nitrogen protective atmosphere, eliminating a substructure introduced in the rotary rolling process of the sample, and obtaining a high-strength texture material with an isometric crystal structure; 5) then, carrying out cryogenic rolling treatment on the sample: and (3) soaking the sample in liquid nitrogen for 10mins before each rolling to ensure that the sample is at a low temperature during rolling treatment, wherein the reduction of each rolling is 5%, and the final reduction is 25-50%, so that the titanium with the ultrahigh-density twin crystal structure is obtained.

Fig. 2a is a graph of titanium texture distribution for spin rolling and annealing processes, where the basal plane of the sample (0002) is almost perpendicular to the rolling Normal (ND) and the texture intensity is 14.33, representing a typical unimodal basal plane texture. Fig. 2b is a typical rolling process titanium texture profile where the basal plane normal of the sample (0002) is turned from the rolling Transverse Direction (TD) to ND at an angle of about 40 ° and the texture intensity is only 7.86, representing a typical bimodal rolling texture. Therefore, the texture of the sample can be effectively regulated and controlled through the rotary rolling and annealing treatment, and the material with stronger specific texture is obtained.

Fig. 3 is a comparison of two samples of cryogenic rolling deformation, wherein fig. 3a is a sample (hereinafter referred to as ultra-high density twin crystal titanium) which is subjected to deep cold rolling after texture is regulated by rotary rolling according to the present patent, and fig. 3b is a sample (hereinafter referred to as general twin crystal structure titanium) which is directly subjected to cryogenic rolling without rotary rolling. FIG. 3c is comparative statistics of twin area fractions. Therefore, the twin crystal density in the titanium sample after the deep low temperature treatment can be obviously improved by regulating and controlling the initial texture of the material, and the titanium material with the ultrahigh density twin crystal structure is obtained.

The tensile engineering stress-strain curves of the ultra-high density twin-crystal titanium and the general twin-crystal structure titanium sample are shown in FIG. 4. The result shows that the yield strength and the tensile strength of the ultra-high density twin crystal titanium sample are both improved by more than 100MPa, and the tensile strength is close to 1000 MPa; and the elongation at break is also improved from 11% to 14%. Therefore, the strength and the tensile plasticity of the titanium can be improved simultaneously by introducing the high-density twin crystal structure, so that the high-strength and high-toughness titanium material is prepared.

Claims

1. The preparation method of the ultrahigh-density twin crystal titanium is characterized by comprising the following steps of:

s1, rolling by rotation

Rolling the rolled plate by 2-5%, rotating the rolled plate by 10-170 degrees along the reverse or clockwise direction, and rolling the rolled plate by 2-5% again, rotating the rolled plate by 10-170 degrees along the rolling direction continuously in the reverse or clockwise direction, wherein the rotating direction of each time is consistent with that of the previous rotation; repeating the process until the reduction amount reaches 60-90%;

s2, annealing treatment

According to the thickness of the plate, annealing treatment is carried out on the material subjected to rotary rolling for 30-60 minutes at 525-575 ℃;

s3, cold rolling at deep low temperature

And (3) performing multi-pass rolling treatment on the annealed material at the liquid nitrogen temperature until the rolling reduction is 25-50%, thus obtaining the titanium material with the ultrahigh-density twin crystal structure.

2. The method of claim 1, wherein the annealed material is subjected to a multi-pass rolling process after being immersed in liquid nitrogen for 10mins in step S3.

3. The method of claim 1, wherein in step S3, the reduction of each rolling pass is 5% until the reduction is 25-50%.

4. The ultra-high density twin crystal titanium produced by the production method as claimed in any one of claims 1 to 3.