Evaluation of strain-rate sensitivity in superplastic compressive deformation

Plastic-deformation processes in metalworking may be classified into only a few categories on the basis of the type of force applied to the workpiece, these categories being direct compression, indirect compression, tension, bending and shearing processes. For the numerical analysis of superplastic deformation in these metalworking processes, the strain-rate sensitivity, or, determined from a tension test is considered as a reliable unique constant of the mechanical properties. Superplastic flow is believed to be associated with the simultaneous appearance of grain-boundary sliding and some accommodated process. The In value is a macroscopic material constant, and can be determined through simple calculation using tension-test data. It is natural to think that the extent of both grain-boundary sliding and accommodation may alter the rn value. Each superplastic material has its own m value under a specific strain-rate range and temperature. A different strain rate from that of the uniaxial stress, however, can naturally change the extent of grain-boundary sliding and accommodation, and, therefore, change the value of m also. This phenomenon has never been studied. Strain-rate sensitivity determined from a compression test appears to be useful for the analysis of compressive processes such as forging, rolling, extrusion and drawing, In this study, special attention will be paid to the determination of strain-rate sensitivities in uniaxial compressive deformation. The friction at the tool-workpiece interface leads to a barrelled specimen profile generally, and it is difficult to measure compressive stress and strain. A much more suitable test for determining strain-rate sensitivity is the Siebel-type compression test. The advantage of this test is that it eliminates inhomogeneous deformation by using a cone-shaped punch and a corresponding specimen geometry at the same time. Compressive deformation was analyzed with the finite-element method (FEM). Upon comparison of the In values in compression and tension, whilst they are almost the same, a small difference of the values is recognized. (C) 1997 Elsevier Science S.A.