Numerical simulations of plastic deformation and fracture effects in two phase γ-TiAl + α₂-Ti₃Al lamellar microstructures

Abstract

Deformation characteristics of fully lamellar (FL) and nearly lamellar (NL) morphologies in two phase γ-TiAl(L1₀) + α₂-Ti₃Al(D0₁₉) polycrystalline aggregates are simulated by finite element methods. Polycrystalline stress-strain response is accurately predicted using, as input parameters, the range of soft (τ^hard _crss) and hard (τ^soft _crss) 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 inhomogeneous, due in part to the large difference in (τ^soft _crss) and (τ^hard _crss), with the largest strain accumulations being encountered at grain boundaries, particularly at triple points. Such deformation incompatibilities between adjacent crystals create large hydrostatic stress concentrations at grain boundaries, which are likely nucleation sites for fracture, as experimentally verified for both tension and compression loading. Incorporating small volume fractions of γ-TiAl (with compliant deformation characteristics, at least at small strains) at grain boundaries, as in the case for NL microstructures, greatly reduces the magnitude of the peak hydrostatic stresses, and consequently mitigates fracture initiation. This provides a suitable explanation for the increase in ductility as associated with an increasing volume fraction of γ-TiAl in lamellar microstructures. It is shown that numerically computed plots of hydrostatic stress against strain provide a more logical and direct correlation between microstructure and ductility response, over the current, more traditional stress-strain plots.