Vacancy migration and long-range ordering due to ageing in AuCd shape memory alloys

Vacancy concentrations in the martensite phases ζ'₂ and γ'₂ of Au₁ - _cCd_c alloys (0.475 ≤ c ≤ 0.500) are measured for samples furnace-cooled (FC) and water-quenched (WQ) from 763 K. There is a tendency for c_v to increase with composition. It shows a much higher concentration in WQ samples than in FC samples, indicating the existence of large amount of quenched-in vacancies. Furthermore, the vacancy migration and the long-range ordering during ageing in martensite and parent B2 phases for FC and WQ samples of c=0.495 and 0.475 are examined by measuring the lattice constants and the integrated intensities by the X-ray diffraction method. In both samples, the martensite ageing does not bring about an effective vacancy migration and a detectable change in long-range order (LRO). For the B2 phase of the WQ sample, the quenched-in vacancies are drastically annealed-out by ageing, which accompanies a promotion of the LRO. The difference in ease of the vacancy migration between the martensite and the parent phases is due to the difference in the crystal structure.