Nucleation of Martensite in CuZnAl Alloys as Investigated by Young's Modulus and Acoustic Emission Measurements

Abstract
Young's modulus (E) has been measured during cooling/heating cycles carried out between a fixed upper temperature T₂ (T₂=340 K) and a gradually decreasing lower temperature T₁ (T₁ =320, 310, 300, 290, 285, 281 and 279.5 K). The E(T) curves show closed thermal hysteretic loops, which are needle-like for T₁> while they assume a downward bent shape for T₁<M_s. The occurrence of thermal hysteresis in the cooling/heating E(T) curves is interpreted as evidence of the formation, far above Ms, of static martensite embryos, which are stabilued by preexisting lattice defects. The formation, on cooling, and the delayed disappearance, on heating, of these static embryos is believed to be responsible for the needle shaped E(T) loops. Acoustic emission is found to occur only during the heating stages of partial thermal cycles carried out above Ms, suggesting that only the disappearance of a stabilized embryo is associated with an abrupt release of the stored energy.