In-situ deformation of TiAl PST crystals in TEM

Abstract

The deformation process of TiAl polysynthetically twinned (PST) crystals was in-situ observed in a transmission electron microscope (TEM) with a tensile holder. Microcracks, resulted from different reasons, were observed to nucleate at several different sites. In many cases microcracks were observed to nucleate on lamellar boundaries and on the boundary between the deformation twin and the matrix. The intersecting parts of two deformation twins within the same TiAl lamella have been found to be favorite nucleation sites of microcracks. Microcracks were also observed to nucleate on the domain boundary of two TiAl domains coexisting in a TiAl lamella and at the triple point among TiAl domain boundary and lamellar boundary. It has revealed that cracks tend to propagate along the lamellar boundaries when the angle between tensile axis and lamellar boundaries is about 78°. When crack propagation was hindered, one or more microcracks nucleated ahead of the main crack. Then the microcracks grew up, connected each other and connected with the main crack through shear rupture of the ledgement between them.