An experimental study of the polycrystalline plasticity of lamellar titanium aluminide

Abstract

Lamellar γ-TiAl has a microstructure that spans multiple length scales: lamellar thickness (10 nm–1 μm), γ domain length (100 nm–10 μm) and colony size (100 μm–1 mm). Only by characterizing the deformation across the different scales can one understand how the microstructure influences plasticity and damage formation in this material. Here, we use digital image correlation and electron backscatter diffraction to map strain and lattice rotation with both sub-colony and sub-lamella spatial resolution in a polycrystalline lamellar γ-TiAl alloy with TiB2 inclusions. Two lamellar thicknesses and temperatures are tested. It is shown that the hard mode-oriented colonies undergo minimal plastic strain. However, as the temperature is increased, macroscopic deformation bands develop across colonies of different orientations, independent of their local lamellar orientations. Increasing magnification, it is apparent that some segments of these bands occur by slip or twinning parallel to the lamellae, and others by transverse lattice rotation. In the thin lamellar condition, such macroscopic deformation bands also occur along colony boundaries; this causes a significant inhomogeneity in the strain distribution. Such strain inhomogeneities are also further characterised here at the microstructural scale of single lamellae, and locally at boride particles.