【作者】 刘佳毅；

【导师】 金学军；

【作者基本信息】 上海交通大学 ， 材料物理化学， 2014， 博士

【摘要】 马氏体预相变是广泛存在于形状记忆合金、超导、巨磁电阻材料中的一种相变临界现象。从上世纪60年代马氏体预相变首次被发现以来，围绕其机制的讨论一直没有间断。之前的学者们提出了很多解释马氏体预相变的理论，但是到目前为止还未有一个理论能被实验所完全证实。对马氏体预相变的研究不仅仅可以丰富物理理论，还对类似体系（如铁电体系）的研究具有参考意义。本文研究分三部分，分别为：(1) Au-Cu-Al合金马氏体预相变花呢状组织与应变玻璃行为；(2)母相至中间相转变的“pseudospinodal decomposition”机制；(3)中间相结构的演化及中间相与母相界面的表征。马氏体预相变的花呢状组织是马氏体预相变的重要特征之一。近年来，一些理论假说认为这种花呢状组织是由点缺陷或成分起伏钉扎马氏体相变造成，并且这种花呢状组织可以类比“自旋玻璃”，属于一种全新的物理现象—应变玻璃态。但是，也有观点认为马氏体预相变的花呢状组织与应变玻璃态并不兼容，两者分别属于两种不同的机制。本文通过系统的实验研究和理论分析，在Au-Cu-Al合金体系中观察到马氏体预相变花呢状组织，并且发现花呢状组织表现出应变玻璃行为。因此本文认为马氏体预相变花呢状组织与应变玻璃行为在本质上是一致的。另外，一些学者预测马氏体预相变的本质并不是马氏体相变的前驱效应，而是马氏体相变之前的母相发生失稳，即从母相中均匀析出纳米中间相。这种中间相的晶体结构对称性低于母相的晶体结构对称性，并且母相→中间相转变是一种全新的相变机制—“Pseudospinodal decomposition”。然而，这个假说尚未被实验证实。本文设计了一系列不同成分的Au-Cu-Al合金，研究了马氏体预相变花呢状组织在不同热处理条件下的演化过程。实验发现，花呢状组织的本质就是一种纳米中间相在母相中的均匀形核，而非点缺陷钉扎马氏体相变造成。随着母相初始成分的不同，花呢状组织最终可以演化出不同的组织结构（如棋盘花纹纳米线或孪晶）。通过实验并综合文献结果，本文证实了之前学者关于“Pseudospinodaldecomposition”的假说。本文表征了中间相的晶体结构，发现其随着成分变化很大，最终稳定的孪晶结构有三斜与四方两种。因此推测Au-Cu-Al合金中有类似于铁电体系中准同型相界(MPB)。三斜晶系与四方晶系的交界可能就是造成花呢状组织的准同型相界(MPB)。本文还观察了母相/中间相的界面。对比实验结果，认为母相/中间相界面处的6M亚稳超点阵结构可能来自于淬火过程中扩散不完全。更多还原

【Abstract】 Premartensitic transition is a critical phenomenon which has been documented ina wide range of materials, including shape memory alloy, superconductors, andcolossal magnetoresistance materials, and its mechanism has been frequentlydiscussed since it was observed in1960s。Many theories have been proposed toaccount for such a transition, yet none of them has been well experimentally confimed.Investigation of Premartensitic transition not only enriches the phase transformationtheories, but also sheds light on similar systems such as ferroelectric systems.The content of this paper includes:(1) the relationship between the precursortweed morphology and strain glassy behavior;(2) the “pseudospinodaldecomposition” mechanism of parent→intermediate phase transition;(3) structuralcharacterization of intermediate phase and the interface of intermediate/parent phase.The premartensitic tweed, as an important feature of premartensitic transition,was usually interpreted as indication of local static lattice strain of martensitestabilized by point defects or compositional disorder. Based on this viewpoint, a newconcept “strain glass” was proposed to explain this anomaly. Yet another theorysuggested that premartensitic tweed and strain glass are incompatible, in other words,the premartensitic tweed and strain glass stem from different mechanisms. However, apremartensitic strain glassy behavior was observed in the Au7Cu5Al4alloy via a seriesof experiments in the present paper. Therefore, we suggest that the premartensitictweed and strain glass origin from the same nature.Some scholars also suggested that the premartensitic tweed is not a precursorstate of martensite, and on the contrary, should be associated with the embryos ofsingle-domain nano-particles whose symmetry is lower than that of parent, at theearly stage of “pseudospinodal decomposition” developing at the temperature aboveMartensitic transformation where, according to thermodynamics, the compositionalhomogeneous parent phase is unstable against decomposition. However, thishypothesis has not been verified by experiments. In this paper, the premartensitic tweed in Au-Cu-Al alloys, contrary to previous thought that relies on defects, isconfirmed to be a result of intermediate phase embedding in the parent phase via“pseudospinodal decomposition”, being different from the martensite formed at lowertemperature. Accompanying the parent phase→intermediate phase transition, thetweed can further transform to chessboard nanowires or twins, in consistence withprevious theoretical prediction on “pseudospinodal decomposition”.The crystal structure of intermediate phase in Au-Cu-Al alloys was characterized,which varies obviously as a function of composition. The final stable intermediatephase has a trigonal or tetragonal twinned structure. Thus we infer that the phaseboundary between trigonal and tetragonal intermediate phase can be paralleled withMPB in ferroelectric system, where tweed structure is often formed. In addition, theparent/intermediate interface was also observed, where the6M superstructure wasfound and is probably due to incomplete diffusion during quenching process.更多还原