A study on the nickel-rich ternary Ti–Ni–Al shape memory alloys

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The transformation sequence and hardening effects of 400 °C aged Ti47.5Ni50.65Al1.85 and Ti49.5Ni50.13Al0.37 shape memory alloys have been investigated by electrical resistivity tests, internal friction measurements, hardness tests and TEM observations. Both solution hardening and precipitation hardening are found to occur in these alloys. The hardening effects of Ti47.5Ni50.65Al1.85 alloy are obvious and much higher than those of Ti49.5Ni50.13Al0.37 alloy due to the former having the larger Ni/Ti ratio and a higher Al solute content in its matrix. The transformation sequence of 400 °C aged Ti47.5Ni50.65Al1.85 alloy shows B2↔R-phase only for an ageing time of more than 10 h and that of 400°C aged Ti49.5Ni50.13Al0.37 alloy shows the sequence B2↔R-phase↔B19′ or B2↔R-phase with different ageing times. All of these characteristics are associated with Ti11Ni14 precipitates during the ageing process. These aged Ti–Ni–Al alloys exhibit very good shape memory effects, in which the maximal shape recovery occurs at the peak of hardness.

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