THE DEVELOPMENT OF THE ORDERED STRUCTURE FROM COLD-WORKED DISORDERED ALPHA IN NI4MO

The Ni-20 at.% Mo alloy has an order-disorder critical temperature of 1141 K (868-degrees-C). The disordered phase alpha can be retained by quenching and in this condition samples were cold worked by swaging 43% reduction in area, then aged at 1123 K (850-degrees-C) and 973 K (700-degrees-C) for up to 172 800 s (48 h). The kinetics of the disorder-order (alpha to beta) transformation has allowed an examination of the interaction of recrystallization and ordering, which was followed by micro-hardness measurements and transmission electron microscopy observations. At 1123 K, the alpha recrystallized with accompanying softening, then ordered domains (beta phase) formed with an accompanying sharp increase in hardness. The hardness began to decrease as the domains increased in size. However, some deformed material was still present after 48 h. At 973 K, the deformed alpha first changed to deformed beta, with a considerable increase in hardness. Then recrystallized beta domains formed in the deformed beta, and eventually the hardness began to decrease as the domains became large. It was also found that recrystallization in the deformed beta was considerably retarded compared to that in deformed alpha. The general difference in the behaviour at the two temperatures is consistent with the stored energy of cold work being greater than the free energy change of the disorder-order transformation at 1123 K, whereas the opposite effect is found at 973 K.