SHEAR LOCALIZATION,MICROSTRUCTURE EVOLUTION AND FRACTURE UNDER HIGH-STRAIN RATE

Abstract

Investigations on microstructural aspects of adiabatic shear localization under strain rate 10~3s~(-1) to 10~4s~(-1)using EBSD-SEM,TEM and HREM are critically reviewed.The principal features are:the critical condition for the occurrence of shear-bands should involve both shear strain and strain rate;the deformed-band forms first and the white-etching band follows with further shear deformation;the presence of white-etching bands is generally indicated by different etching response of the narrow bands;shear band seems to initiate with a crystallographic slip in a favored grain and then propagates into the adjacent grains by co-operative slip or cross-slip,leading to the shear localization over the whole cross-section;both grain refinement and transition from crystalline to amorphous can occur in shear bands;theγ→e→α′transformation may also occur and the new transformed products have a certain orientation relationship with their parent matrix;the sharp drop in the load-carrying capability of the specimen seems to be closely associated with the appearance of a critical coalescence of microcracks or voids in the band;the equiaxed-and distorted-free grains are proposed to attribute to the rotational recrystallization.On the other hand,the micro-mechanism of shear localization and fracture subjected to quasi-static and fatigue loadings is different from the dynamic case,namely caused by the dislocation planner slip and not related to thermal effects.Additionally,some further research are suggested.