NUCLEATION OF MARTENSITE IN CUZNAL ALLOYS AS INVESTIGATED BY YOUNGS MODULUS AND ACOUSTIC-EMISSION MEASUREMENTS

Young's modulus (E) has been measured during coding/heating cycles car ried out between a fixed upper temperature T-2 (T-2 = 340 K) and a gra dually decreasing lower temperature T-1 (T-1 = 320, 310, 300, 290, 285 , 281 and 279.5 K). The E(T) curves show closed thermal hysteretic loo ps, which are needle-like for T-1 > Ms while they assume a downward be nt shape for T-1 < Ms. The occurrence of thermal hysteresis in the coo ling/heating E(T) curves is interpreted as evidence of the formation, far above Ms, of static martensite embryos, which are stabilized by pr eexisting lattice defects. The formation, on cooling, and the delayed disappearance, on heating of these static embryos is believed to be re sponsible for the needle shaped E(T) loops. Acoustic emission is found to occur only during the heating stages of partial thermal cycles car ried out above Ms, suggesting that only the disappearance of a stabili zed embryo is associated with an abrupt release of the stored energy.