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

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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.