形狀記憶合金Au30Cu25Zn45中熱彈性馬氏體相變的相場模擬

形狀記憶合金Au30Cu25Zn45中熱彈性馬氏體相變的相場模擬PHASE-FIELD MODELING OF THE MARTENSITIC TRANSFORMATION IN SHAPE MEMORY ALLOY Au30Cu25Zn45

使用相場模擬方法研究了形狀記憶合金 Au30Cu25Zn45 馬氏體相變過程中的組織演變, 并與實驗結(jié)果進行比較. 模擬發(fā)現(xiàn), Au30Cu25Zn45 合金馬氏體相變后形成的特殊的彎曲狀組織, 是由相變形成的四變體結(jié)結(jié)構(gòu)(quad-junction)中的變體對逐層疊加長大而成, 先后形成的變體層沿同一孿晶面生長, 并且先形成的變體層尺寸較大, 從而形成凸起的馬氏體組織. 進一步研究得到, 馬氏體變體中存在 6 組能夠形成這種quad-junction 的變體組合, 每一組合中有 4 個變體, 且兩兩之間形成 4 對不同的 1 類/2 類孿生變體對與 2 對復(fù)合變體對, quad-junction 由其中 4 種兩兩具有相同孿晶面法向的變體對組成, 且這 2 組孿晶面法向相互垂直.

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 excellent functional stability. In the theory, several conditions that corresponding to extremely low hysteresis are specified. 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 the shape 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 conditions. The simulation results show that during the martensitic transformation, quad-junctions composed of four different 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 Au30Cu25Zn45. Further analysis based on the crystallographic theory of martensitic transformation shows that in Au30Cu25Zn45 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 perpendicular to each other.

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