INELASTIC DEFORMATION OF POLYCRYSTALLINE FACE-CENTERED-CUBIC MATERIALS BY SLIP AND TWINNING

There have been considerable recent advances in the understanding of a nisotropy due to crystallographic texturing, and a reasonably successf ul elasto-viscoplasticity theory for the deformation of face-centered- cubic (f.c.c.) single crystals and polycrystals with high stacking fau lt energies is now at hand. The high stacking fault energy f.c.c. mate rials (e.g. Cu, Al) deform predominantly by crystallographic slip. In contrast, for materials with low stacking energies, e.g. alpha-brass, in addition to crystallographic slip, deformation twinning plays an im portant role in maintaining generalized plastic flow. A direct manifes tation of twinning is the different crystallographic texture that is o bserved in 70-30 brass as compared to pure copper. In this paper we fo rmulate a rate-independent constitutive model which accounts for both slip and twinning. We have also developed a new scheme to determine th e active systems and the shear increments on the active slip and twin systems. We have implemented our constitutive equations and computatio nal procedures in the finite-element program ABAQUS/Explicit (1995). B y using comparisons between model predictions and macroscopically-meas ured stress-strain curves and texture evolution we have deduced inform ation about the values of the single-crystal parameters associated wit h slip and twin system deformation resistances and hardening due to sl ip and twinning. We show that our model is able to reproduce both the experimentally measured pole figures and the stress-strain curves in p lane strain compression for alpha-brass. With the model so calibrated, we show that the predictions for the texture and stress-strain curves from the model are also in reasonably good agreement with experiments in simple compression. We have also evaluated the applicability of a Taylor-type model for combined slip and twinning. Our calculations sho w that for the high-symmetry f.c.c. brass, a Taylor-type model for cry stals deforming by combined slip and twinning is able to reasonably we ll predict the macroscopic stress-strain curves and crystallographic t exture evolution. Our calculations show that in plane strain as well a s simple compression, the crystallographic texture that develops is a result of lattice rotation due to both slip and twinning, and that as suggested by Wassermann (1963), in contrast to copper which does not t win under normal circumstances, it is twinning which is responsible fo r the brass-type texture that is observed in f.c.c. metals with low st acking fault energies. (C) 1998 Elsevier Science Ltd. All rights reser ved.