NUMERICAL SIMULATIONS OF PLASTIC-DEFORMATION AND FRACTURE EFFECTS IN 2 PHASE GAMMA-TIAL-2-TI3AL LAMELLAR MICROSTRUCTURES(ALPHA)

Deformation characteristics of fully lamellar (FL) and nearly lamellar (NL) morphologies in two phase gamma-TiAl(L 1(0)) + alpha2-Ti3Al(D0(1 .9) polycrystalline aggregates are simulated by finite element methods . Polycrystalline stress-strain response is accurately predicted using , as input parameters, the range of soft (tau(crss)soft) and hard (tau (crss)hard) mode critical resolved shear stresses obtained from single poly-synthetically twinned lamellar crystals, for shear parallel and perpendicular to the lamella. The deformation is severely inhomogeneou s, due in part to the large difference in tau(crss)soft and tau(crss)h ard, with the largest strain accumulations being encountered at grain boundaries, particularly at triple points. Such deformation incompatib ilities between adjacent crystals create large hydrostatic stress conc entrations at grain boundaries, which are likely nucleation sites for fracture, as experimentally verified for both tension and compression loading. Incorporating small volume fractions of gamma-TiAl (with comp liant deformation characteristics, at least at small strains) at grain boundaries, as in the case for NL microstructures, greatly reduces th e magnitude of the peak hydrostatic stresses, and consequently mitigat es fracture initiation. This provides a suitable explanation for the i ncrease in ductility as associated with an increasing volume fraction of gamma-TiAl in lamellar microstructures. It is shown that numericall y computed plots of hydrostatic stress against strain provide a more l ogical and direct correlation between microstructure and ductility res ponse, over the current, more traditional stress-strain plots.