ANOMALOUS PLASTIC FLOW OF CERIUM NEAR THE ISOMORPHIC PHASE TRANSFORMATIONS UNDER HIGH HYDROSTATIC PRESSURE

摘要

Compression tests have been carried out on cerium specimens at room temperature (0.27Tm) under high hydrostatic pressures up to 1.2 GPa. A strong increase of the yield strength was observed for both isomorphic γ and y phases at pressures approaching the γ←→ α isomorphic phase transformations. That increase was in good agreement with the theory of dislocations when the dependence of elastic prop- erties and a lattice parameter of cerium on pressure was applied to calculate the effect of pressure on the yield stress controlled by the edge dislocations. An anomalous strong decrease of the yield stress was observed in both　γ and α phases in the vicinity of both γ←→ α phase transformations. That phenomenon was explained as an effect of pressure induced new phase atoms through spreading the cores of edge dislo- cations. A complete disappearance of work hardening in both γ and α phases was also observed in the wide range of pressures. The influence of hydrostatic pressure on the energy of grain boundaries of both phases was considered to be responsible for that property. The ratio of the grain boundary energy to the Peierls energy is suggested to be a criterion of the work hardening ability of f.c.c. polycrystals.