Effect of ageing on transformation kinetics and internal friction of Ni-rich Ni–Ti alloys

In Ni-rich Ni–Ti alloys the ageing treatments create finely dispersed Ti₃Ni₄ precipitates in the B2-based matrix. Formed precipitates are known to influence the phase transformation temperatures and to facilitate the R-phase transformation. In addition to the change in the phase transformation behaviour, the ageing treatments are also reported to affect internal friction and mechanical properties of Ni–Ti alloys. In the present work, by means of systematic DSC and DMA studies the effects of the ageing treatments on the phase transformation as well as on the internal friction and mechanical properties of Ni-rich Ni–Ti alloy were studied. As an extension to earlier studies, the present study concentrates both on the effects of the ageing time and the ageing temperature systematically. By combining the results of the DSC measurements of phase transformation behaviour to the DMA measurements of internal friction and mechanical behaviour of the Ni-rich Ni–Ti alloy, the present study gives yet unpublished information about the comprehensive effects of the ageing treatments on these alloys. These results enable to control the phase transformation temperatures, internal friction and mechanical properties of the Ni-rich Ni–Ti alloy by selecting the suitable ageing treatment. Results showed that the noticed effects of the ageing treatments on mechanical properties could be explained with the changes in the size, distribution, density and coherency of the formed Ti₃Ni₄ precipitates. The high internal friction values can be explained as the contribution of the internal stress fields and increased amount of mobile interfaces as the result of the formed Ti₃Ni₄ precipitates. The amount of mobile interfaces seems to be more dominant factor for the increased internal friction value than the effect of the internal stress fields. Therefore, the optimal internal friction values can be obtained with a proper aging treatment which will yield a high density of small Ti₃Ni₄ precipitates.