Order-disorder transition in Cu-Zn-Al martensite under electron irradiation

The Cu-Zn-Al alloy is one of the most popular shape memory alloys. In this alloy, the parent phase has the B2 ordered structure; it transforms to Ll
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martensite phase by cooling. Since the martensitic transformation does not involve long-range atomic diffusion, the martensite phase inherits the ordered atomic arrangements directly from the parent phase; however, the stability of the atomic arrangements is not necessarily the same in the martensite phase as in the parent phase. The purpose of the present study is to investigate the stability of long-range ordered structure in the Cu-Zn-Al martensite under electron irradiation in 1 MeV HVEM.In the case of high-flux irradiation, the martensite phase disordered at all temperatures tested. Meanwhile, for the low-flux irradiation, the martensite remained ordered when the irradiation temperature was between 203 K and 333 K. Thus two order-disorder transition temperatures exist under electron irradiation.The long-range order parameter was estimated as a function of temperature and electron flux using the model proposed by Banerjee and Urban. The observed higher transition temperature was close to that of the thermal equilibrium one while the lower one was much higher than the prediction. This discrepancy suggests that more mobile defects than vacancies may be also responsible for the irradiation enhanced reordering process.