Effect of heat treatment on the two internal friction peaks in a Cu–Al–Mn shape memory alloy

Two internal friction peaks are observed in the Cu–11.9Al–2.5Mn (wt%) shape memory alloy at around 150 and 325 °C during heating. The high-temperature peak originates from the phase transition in which martensite transforms to austenite while the low-temperature peak results from the pseudo-first-order phase transition associated with the thinning process of the twins. In the present study the effect of heat treatment on the two internal friction peaks was studied and correlated with the martensite microstructures. It was found that relative high quenching temperature and rapid cooling speed gave rise to both increased ambient damping and low-temperature peak due to the formation of favorable twin structure characterized by coarse twin boundaries and high twin content, indicating that the motion of twin boundaries was the predominant damping mechanism in the martensite state. The high-temperature peak, however, followed a non-monotonous dependence on the quenching temperature and cooling modes. It was proposed that the high-temperature peak depends on the content of martensite and the concentration of the quenched-in vacancies, rather than the twin structures.