Influence of bending fatigue on residual mechanical properties of a γ-TiAl intermetallic alloy

The aim of this study is mainly the investigation of the influence of bending fatigue on residual mechanical properties of a near fully lamellar (NFL) γ-TiAl intermetallic material. For this purpose three Ti-47Al-2Cr-2Nb-1B ingots were prepared by Vacuum Induction Melting (VIM) in a ceramic crucible, permanent mould casting and heat treatment without hot-isostatic-pressing (HIP) process. In order to obtain three different NFL microstructures metallic mould was preheated at three different temperatures. Four-point bending strength, bending strain to fracture, fracture toughness, hardness and low-cycle bending fatigue strength (σmax)L of near fully lamellar Ti-47Al-2Cr-2Nb-1B with different colony sizes and interlamellar spacings were measured on un-HIP'ed test bars. The S-N curves are fairly flat with a low value of stress life exponent; some previous experiments revealed that the shape of the S-N curve of FL γ-TiAl in the low-cycle fatigue range can be quite different from that in the high-cycle fatigue regime. The effects of low-cycle fatigue, 2×105 alternate bending cycles (R=0.1), at high stress intensity levels, i.e. the fatigue limit, on mechanical behaviour of the three different microstructures were evaluated. The data reported and discussed in this paper show that bending fatigue "training" enhances the mechanical behaviour of this material, mainly through the appearance of a plastic deformation during the bending test. According to experimental evidence, the higher the maximum applied stress, normalized by the ultimate flexural strength during fatigue cycling, the larger the improvement of mechanical properties of NFL microstructures The effects of the microstructures on bending deformation, fracture behaviour and fatigue resistance were discussed as well.