形状记忆合金Au_(30)Cu_(25)Zn_(45)中热弹性马氏体相变的相场模拟

PHASE-FIELD MODELLING OF THE MARTENSITIC TRANSFORMATION IN SHAPE MEMORY ALLOY Au_(30)Cu_(25)Zn_(45)

使用相场模拟方法研究了形状记忆合金Au30Cu25Zn45马氏体相变过程中的组织演变,并与实验结果进行比较.模拟发现,Au30Cu25Zn45合金马氏体相变后形成的特殊的弯曲状组织,是由相变形成的四变体结结构(quad-junction)中的变体对逐层叠加长大而成,先后形成的变体层沿同一孪晶面生长,并且先形成的变体层尺寸较大,从而形成凸起的马氏体组织.进一步研究得到,马氏体变体中存在6组能够形成这种quad-junction的变体组合,每一组合中有4个变体,且两两之间形成4对不同的1类/2类孪生变体对与2对复合变体对,quad-junction由其中4种两两具有相同孪晶面法向的变体对组成,且这2组孪晶面法向相互垂直.

Applications of shape memory alloys require them have the ability to undergo back and forth through the solid-to-solid martensitic phase transformations for many times without degradation of properties （termed ＂reversibility＂）. Low hysteresis and small migration of transformation temperature under cycling are the macroscopic manifestation of high reversibility. By the crystallographic theory of martensite, materials with certain crystalline symmetry and geometric compatibility tend to form no-stressed transformation interface and have exce- llent functional stability. In the theory, several conditions that corresponding to extremely low hysteresis are speci- fied. Stronger compatibility conditions which lead to even better reversibility have been theoretically proposed, those conditions are called ＂cofactor conditions＂. Recently, for the first time, experimental results find out theshape memory alloy Au30Cu25Zn45 that closely satisfy the cofactor conditions. Enhanced reversibility with thermal hysteresis of 2.045 ℃, and the unusual riverine microstructure are found in Au30Cu25Zn45. However, their studies are limited to crystallographic analysis, and haven＇t provided enough details of microstructural evolution in martensitic transformation. Furthermore, it is the evolution of microstructures that leads to an extremely low thermal hysteresis in this alloy. Thus, making clear of evolution of microstructures in martensitic transformation in this alloy is of great importance. So, in the present work, the phase field method was applied, in which the microstructure is described by Landau theory of martensitic transformation, Khachaturyan-Shatalov＇s phase field microelasticity theory, and thermodynamics gradient to study the microstructural evolution of martensitic transformation in Au30Cu25Zn45, trying to figure out pathway of formation of the unusual microstructure with satisfying cofactor condi- tions. The simulation results show that during the martensitic transformation, quad-junctions composed of four dif- ferent variants are formed. These junctions grow layer by layer, and the previously formed layer has larger size, thus leading to the formation of the experimentally reported ＂riverine＂ microstructure of martensite in Au30Cu25Zr45. Further analysis based on the crystallographic theory of martensitic transformation shows that in Au30Cu25Zr45 6 groups of variants can form such kind of quad-junction, and each group of variants can form 4 kinds of type 1/type 2 twin pairs and two kinds of compound twin pairs. All of the quad-junctions in this transformation are composed of four of those 6 twin pairs in each variant group, and the twin walls of these four twin pairs are per- pendicular to each other.