Effects of heat treatments on martensitic transformation temperature (Ms) of Cu-Zn-Al and Cu-Al-Ni-Ti shape memory alloys having two different compositions respectively were investigated by electrical resistivity measurement, X-ray diffraction and optical microscopy. Annealing or partial solution treatment were carried out at temperatures below the β transition temperature (Tβ) and aging at temperatures below the eutectoid temperature, respectively.
By annealing at temperatures below Tβ, the Ms was either lowered in Cu-Zn-Al alloys or raised in Cu-Al-Ni-Ti alloys from that after complete solution treatment above Tβ. This behavior was explained by the increase or decrease of solute concentration in the β solid solution due to the precipitation of α or γ2 phase, respectively. The same tendency of the Ms change was also observed by aging at temperatures up to 673K. A Cu-27.7 mass%Zn-4.32 mass%Al alloy showed, in the initial stage of aging, reversible and rapid change in Ms wth aging temperature, which was attributed to a change in the ordered state, and after a prolonged aging, showed an irreversible decrease in Ms with aging phenomena accompanying long range diffusion. In Cu-Al-Ni-Ti alloys, the Ms increase by the change in the ordered state was slower than that of Cu-Zn-Al alloys. Namely, Cu-Al-Ni-Ti alloys showed a better thermal stability than that of Cu-Zn-Al alloys up to higher temperatures for longer periods of aging. Ms changes by precipitation of α or γ2 phase by aging were especially remarkable in alloys of composition remote from eutectoid point. A Cu-14.9 mass%Al-3.26 mass%Ni-0.45 mass%Ti alloy showed an increase of Ms due to precipitation of the γ2 phase by more than 200 K.
