Variant Selection During Titanium Alloy BCC↔HCP Phase Transformation and Its Effect on Crystal Orientation
ZHENG Guoming1, LI Lei2, MAO Xiaonan1,2, CAI Jianhua2, WU Cong3, LEI Lei4
1 School of Materials Science and Engineering, Northeastern University, Shenyang 110006 2 Northwest Institute for Nonferrous Metal Research, Xi'an 710016 3 School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an 710016 4 School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710016
Abstract: In the actual production of titanium alloys, the material is usually forged or hot processed in the β single-phase region and the α+β dual-phase region, plus a corresponding heat treatment. During the process, titanium alloys usually experienced the α?β phase transformation. In idea state, the variants randomly nucleate and grow, and fine grain and uniform orientation distribution willbe formed,which lead to good mechanical properties of materials. However, due to the existence of various factors in the actual thermal processing, the phenomenon of variant selection during phase transformation occurs, which will affect the distribution of crystal orientation and thus impair the mechanical properties of the material. To sum up, the factors influencing the selection of variants during the phase transition process can be divided into: (1) various internal defects formed in the alloy and the residual phase; (2) the influence of processing technology.
The internal defects formed during the processing of titanium alloys mainly include dislocations, various grain boundaries and phase boundaries, and inclusions. In addition to α and β titanium alloys, residual phases exist at room temperature. From a thermodynamic point of view, these defects cause the energy imbalance of the entire system. In order to balance the system energy, some variants need to be precipitated, resulting in the phenomenon of variant selection. So far, various micro-mechanism studies have found that titanium alloys tend to form adjacent β crystal grains that share the same 〈110〉 direction during the thermal processing, which usually causes α-phase to be precipitated with the c-axis parallel to β〈110〉 direction, and the frequency of the alpha-variant to be preferentially selected is very high, and the rotting cause of this β/β-crystal couple has not been clarified. In addition, the titanium alloy phase transition is very sensitive to the thermal processing parameters, such as cooling rate, processing temperature, holding time and deformation and so on, which will cause variant selection. Through in-depth analysis, the influe-nce of the processing technology on the selection of variants can ultimately be attributed to the fact that it leads to abnormalities in the internal structure of the material, which in turn leads to variant selection.
Reports of improving the mechanical properties and crystal orientation distribution of the material through controlling the selection of variants in the phase transition process have not yet appeared. The research results on the phase transformation of titanium alloys are mainly completed by developed countries, and most of the outstanding results appeared before 2010. After 2010, there are few new results. This is mainly due to the fact that it is difficult to directly obtain the information of the α?β transition at high temperature, which has become a heavy obstacle for the in-depth study of the selection of phase change variants. However, the emergence of in-situ EBSD technology in recent years has provided new opportunities for the study of this issue. It is believed that a number of new results will emerge in the coming years.
In this paper, the mechanism of thevariant selection caused by various internal and external factors in the phase transformation of titanium alloys are summarized. The relationship between the titanium alloy texture evolution and the evolution of α?β in titanium alloys is described. The current paperintroduced the essence, causes ofvariantselection, and its effects on the distribution of crystal orientation, and pointed out the current problems in the direction of the study and the future research focus.
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2019, Vol. 33 
