The martensitic transition of a Cu-Al-Ni single crystal has been studied by isothermal
mechanical spectroscopy and in situ transmission electron microscopy (TEM) observations. A
damping instability (with time) has been found at low frequency at the beginning of the transition.
At the same time, TEM study has evidenced the presence of thin martensite plates which appear
just at the beginning of the (massive) martensite formation and growth. The martensite plate growth
rate is quite low and non continuous. While high internal friction in the transition domain is usually
associated with the mobility between martensite/austenite interfaces, the present internal friction
anomaly has been ascribed to the unstable motions of the first martensite plates.