Ordering and Martensitic Transformations of the Cu-Al-Mn Base Shape Memory Alloys.

热弹性相变引起的形状记忆效应已经被研究了30年，人们普遍注意到，形状记忆效应的起源已经得到了澄清。最近，我们发现在Cu-Al-Mn合金中，完全无序的bcc相转化为无序的fcc相，表现出近乎完美的SM效应。这一结果与关于SM效应机制的常识不同。本研究的目的是通过晶体学和热力学分析来阐明无序Cu-Al-Mn合金完美SM效应的来源。得到的结果如下：(1)确定了母相和马氏体的晶体结构。Al含量在17at%以上的合金由母体A2 （bcc）向马氏体Al （fcc）组织转变，Al含量在16at%以下的合金由L21组织向18R组织转变。(2)澄清了A2/A1相变的体积变化非常小，小于1%，且相变是热弹性的。(3)对许多合金的SM性能进行了研究。结果表明，大多数Al含量大于14at%的合金具有优异的SM效应。(4)采用Bragg-Williams-Gorsky模型对有序相的稳定性进行了热力学分析，并确定了Cu、Al和Mn之间的对相互作用能。(5)在式(4)的基础上估计了短程有序化引起的焓变。澄清了焓变值与A2/A1马氏体相变的焓变值相当。所有这些结果表明，短程有序足以使母体和马氏体晶格之间的晶格对应具有可逆性，从而导致SM效应。

The shape memory (SM) effect due to the thermoelastic transformation has been investigated for 30 years, and it is generally noticed that the origin of the SM effect has been already clarified. Recently, we found in the Cu-Al-Mn alloys that a fully disordered bcc phase which transforms a disordered fcc phase shows an almost perfect SM effect. This result is peculiar against a common sense on the mechanism of the SM effect.The purpose of this study is to clarify an origin of the perfect SM effect of the disordered Cu-Al-Mn alloys by crystallographic and thermodynamic analysis. The Pbtained results are as follows :(1) The crystal structures of the parent and martensite phases have been determined. Alloys with high Al content over 17at% transform from the parent A2 (bcc) to martensite Al (fcc) structure, and those under 16at% transform from the L21 to 18R structure.(2) It has been clarified that volume change by the A2/A1 transformation is very small less than 1% and the transformation is thermoelastic.(3) The SM properties of many alloys have been examined. It has been clarified that the most alloys with the Al content over 14at% show an excellent SM effect.(4) A thermodynamic analysis of the stability of ordered phase has been performed Bragg-Williams-Gorsky model and pair interaction energies between Cu, Al, and Mn were determined.(5) Enthalpy changes due to short-range ordering have been estimated on the basis of the result of (4). It was clarified that values of the enthalpy changes are comparable to enthalpy changes due to the A2/A1 martensitic transformations.All these results suggest that the short-range ordering is enough to give a reversibility on the lattice correspondence between the parent and martensite lattices, resulting in the SM effect.