Electron Microscopy Observation and X-ray Microanalysis of Aged Martensites in a Cu-12.9 mass%Al-2.5 mass%Mn Shape Memory Alloy

Abstract

Aged martensites in a Cu-12.9 mass%Al-2.5 mass%Mn shape memory alloy have been studied by means of electron microscopy and diffraction and X-ray microanalysis. On ageing an 18R type martensite at 573 K, very small coherent and spherical precipitates are homogeneously formed throughout the martensite. These very small precipitates are evolved by banishing Cu atoms into the martensite matrix. In accordance with such composition variance in the martensite matrix, M_s and A_s temperatures increase on ageing. In connection to the formation of very small precipitates, the 18R martensite gradually varies the crystal structure to a 2H one. Plate-like precipitates are also observed in the martensite matrix at early stages of ageing, which are of a DO₃ structure and rich in Al content. Orientation relationship and habit plane of the plate-like precipitates to the martensite matrix are similar to those of thermoelastic martensites to the β₁ matrix in Cu–Al alloys. At late stages of ageing, equilibrium γ phase appears in a block shape, and the plate-like precipitates with a DO₃ structure are observed in contact with the γ phase.