MECHANISMS OF SUBSTRUCTURE DEVELOPMENT AND STRENGTHENING IN SHOCK LOADED AND CONVENTIONALLY DEFORMED MOLYBDENUM AND MOLYBDENUM -35 RHENIUM.

摘要

The formation of defect substructures and the resulting strengthening have been studied in shock loaded Mo and shock loaded and conventionally deformed Mo-35Re. The hardening of shocked Mo and Mo-35Re increases nearly linearly with shock pressure although the hardness of Mo is somewhat less than Mo-35Re at 20GPa. Both the defect generation and the resulting hardening increase with shock pulse duration at short durations in both Mo and Mo-35Re, however, saturation occurs at pulse durations of (TURNEQ)0.2(mu)sec and (TURNEQ)0.1(mu)sec, respectively. This time dependence of the plastic flow processes emphasizes further the difficulties associated with the assumptions commonly made in the modeling of shock processes. Shock hardening in Mo results from loose tangles of dislocations, and is proportional to the square root of the dislocation density. In shocked Mo-35Re, both dislocations and twin boundaries make a significant contribution to the hardening. The strength coefficient of the twin boundaries is about 5% of that for high angle grain boundaries. It is proposed that twin nucleation occurs by the three-fold dissociation of the edge components associated with dislocation dipoles. This mechanism predicts the formation of three-layer twin fragments at many layers within a slip band, these then coalesce to form thicker twins. The maximum shear stresses developed in both materials at 20GPa-0.018(mu)sec are (TURNEQ)2.5GPa. It is shown that these stresses are not large enough to cause either homogeneous nucleation of dislocations, or super sonic dislocation velocities.;It has been shown that Mo-35Re can be cold rolled to a strain of at least 1.0 with no sign of failure, and more importantly that the strain hardening continues to increase with strain. The hardening at small strains (< 0.28) results from loose tangles of dislocations, while at larger strains a dislocation cell structure is formed that provides an additional contribution to the hardening. Twinning does not make a significant contribution to the ductility of the alloy.