Twinning in Crack Tip Plasticity of Two-Phase Titanium Aluminides
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Twinning in Crack Tip Plasticity of Two-Phase Titanium Aluminides Fritz Appel Institute for Materials Research, GKSS Research Centre, Max-Planck-Straße, D-21502 Geesthacht, GERMANY ABSTRACT Intermetallic titanium aluminides based on γ(TiAl) are prone to cleavage fracture on low index lattice planes. Unfavourably oriented grains may therefore provide easy crack paths so that the cracks can rapidly grow to a length which is critical for failure. The effect of crack tip plasticity on crack propagation in γ(TiAl) was investigated by conventional and high-resolution electron microscopy. Crack tip shielding due to mechanical twinning was recognized as toughening mechanism, which occur at the atomic scale and apparently is capable to stabilize fastly growing cracks. The potential of the mechanism will be discussed in the context of novel design concepts for improving the strength properties of γ-base titanium aluminide alloys. 1. INTRODUCTION Titanium aluminides based on the intermetallic phases γ(TiAl) and α2(Ti3Al) meet many demands for high temperature technology. However, as with many other intermetallics an inherent drawback for practical use is their tendency to undergo brittle transgranular fracture at low and ambient temperatures [1-3]. It is now recognized that there are at least three causes of brittleness in such alloys: - the occurence of cleavage on low index planes - insufficient deformation modes which can simultaneously operate at given stress - low dislocation mobilities. Extensive fractographic studies have demonstrated that the microstructure exerts a significant effect on crack propagation [4-6]. For toughening the material a lamellar morphology of the γ(TiAl) and α2(Ti3Al) phases is beneficial, because shear ligament bridging, crack deflection and microcrack shielding occur. These processes take place on a scale ranging from ten to some hundred microns; thus, continuum fracture mechanics is capable of capturing these prominent features of crack propagation [4]. However, plastic dissipation at the crack tip may also occur at the atomic level and constitute a major fraction of the total work of fracture. Rice and Thomson [7] have proposed that the intrinsic brittleness or ductility of a material is closely related to the competition between cleavage decohesion and crack tip shielding or blunting due to dislocation emission. Stable crack growth requires the plastic zone to keep up with the cleavage crack, which is difficult if the mobility and multiplication rate of the dislocations are low. In this respect mechanical twinning might be an important mechanism, because the growth rate of twins is often an appreciable fraction of the elastic wave velocity [8]. There have been relatively few studies on the contribution of twinning to crack tip plasticity, although twinning is a prominent deformation mode in γ(TiAl). This results probably from the fact that brittle fracture is a rapid process and most analytical techniques are not capable of imaging the details on the appropriate N1.8.1
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