The uniformity of crystal orientation is another key factor affecting the stability of the mechanical properties of metal materials in addition to the structure morphology and micro-domain composition uniformity. The uniformity of crystal orientation of polycrystalline materials can be described by microtexture, which is expressed as the difference between the crystallographic preferred orientation of a given region and the adjacent regions. Microtextures are ubiquitous in forged structures of near-α titanium alloys and picture titanium alloys. It is easy to achieve relatively uniform microstructures in different parts of large-scale bars and forgings through thermal processing optimization, but their crystal orientation uniformity is difficult to control. Studies have shown that micro-texture can lead to high clutter in ultrasonic flaw detection of titanium alloys, fatigue (especially load fatigue) performance, tensile performance, and stability reduction.
In order to further improve the performance and stability of titanium alloy rods and forgings, the high-temperature titanium alloy team of the Lightweight and High-Strength Materials Research Department of the Institute of Metal Research, Chinese Academy of Sciences conducted a systematic study on the formation and evolution mechanism of the microtexture in the titanium alloy forging structure. Work. Through thermal compression experiments on Ti60 titanium alloy with a lath-like structure, the spheroidization mechanism of α-phase and the evolution law of crystal orientation in the initial stage of deformation in the two-phase region of the two-phase region were studied (Fig. 1), and it was found that when the slip along the orthogonal direction accumulates enough deformation, the subgrain size formed in the α-phase lath is much smaller than its own width, and the subgrain can accumulate enough misorientation through the polymerization process. At this time, the lath picture phase is continuously regenerated spheroidization of the crystallization mechanism yields an equiaxed α-phase with dispersed orientation. When deformed at a higher temperature, the intragranular slip of the lath picture phase is less, and the size of the formed subgrain is equivalent to the width of the lath α phase. At this time, the spheroidization of the lath picture phase is based on the "grain boundary separation" mechanism Mainly, the same slab generally retains similar crystal orientation after α-cluster spheroidization.
In addition, the microtexture in the forged structure of titanium alloy has a strong "stubbornness" in the subsequent thermal processing process, which is mainly manifested in that the crystal orientation of the original β grains is difficult to randomize, and even after more than ten times of repeated heat treatment Upsetting deformation is also difficult to eliminate. Through the study of the microtexture evolution law in the two-state structure of TC17 alloy (Fig. 2), the team found that the pinning effect of the primary α phase is the main reason for the strong "stubbornness" of the crystal orientation of the original β grains. The pinning effect of the primary picture phase confines the growth of β grains, which is beneficial to the homogenization of β grain size, but also hinders the aggregation process of β subgrains and limits the randomization of the orientation of original β grains. By analyzing the evolution of the substructure of the β grain with the deformation parameters, the research team discussed the influence of various deformation parameters on the crystal orientation of the β phase under the pinning action of the primary α phase, in order to understand the mechanism of titanium alloys with strong organizational heredity Provides a new perspective.
Based on the above understanding, the research team combined the evolution characteristics of the crystal orientation of titanium alloys in different systems, systematically sorted out the forging process of titanium alloys, and carried out large-scale rods of 250-550 mm in size for titanium alloys such as TC25G, Ti60, Ti65, Ti2AlNb, and TC11. (Figure 3) and the development and industrial production verification of forgings have solved technical problems such as many times of processing of titanium alloy bars and forgings, high flaw detection clutter, poor structure uniformity, and unstable performance. Innovative technology has successively applied for the "Forging Process of High-quality Titanium Alloy Large-scale Bar" (ZL 202010882185.9); Creep resistance, high fracture toughness TC25G titanium alloy forging preparation process" (ZL 202010195030.8); "A Ti2AlNb-based alloy forging preparation process" (ZL 202010882080.3) and more than 20 core technology patents, related technologies in many countries Applied in engineering.
Related basic research results were recently published online in Metallurgical and Materials Transactions A, titled "Influence of Globularization Process on Local Texture Evolution of a Near-α Titanium Alloy with a Transformed Microstructure" and "Microtexture Evolution of Titanium Alloy During Hot Deformation: For Better Understanding the Role of Primary α Grains". The first author of the paper is researcher Zhao Zibo, the corresponding author is researcher Wang Qingjiang, researcher Liu Jianrong guided the design of the experiment, and researcher Yang Rui guided the entire research work. This work was funded by a special basic research project, the talent project of the Youth Promotion Association, and the Chinese Academy of Sciences stable support for youth teams.
Fig.1 The relationship between the spheroidization mechanism of α-phase and the formation of micro-texture: (a) the difference in macroscopic texture and crystal orientation distribution of α-phase in samples with different deformation temperatures; (b) the difference in spheroidization mechanism of α-phase during deformation at different temperatures
Fig. 2 The relationship between the "stubbornness" of microtexture and the pinning effect of primary α phase: (a) crystal orientation of primary α phase and β phase under the condition of high deformation amount and high deformation rate; (b) grain size distribution of β phase; (c) Measurement and distribution of primary α-phase spacing; (d) Distribution of β-phase misorientation
Fig. 3 Comparison of crystal orientation of bars before and after forging process optimization: (a) low-magnification structure and crystal orientation of Φ300mm bars before optimization; (b) low-magnification structures and crystal orientations of Φ300mm and Φ550mm bars after optimization
