Nanoindentation and micro-cantilever testing for understanding cold-dwell sensitivity of heat-resistant titanium alloys

The mechanism of cold-dwell fatigue of the Ti alloys Ti-6242S and IMI 834 was investigated by measuring their local strain-rate sensitivities using nanoindentation analysis, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. In addition, microcantilever bending tests involving a load-holding-induced change in plastic strain near grain boundaries were conducted to investigate the plastic behavior of the alloys. The difference in the strain-rate sensitivity exponent between soft and hard grains was large in Ti-6242S and small in IMI 834, although slips were generated in the hard grains of both IMI 834 and Ti-6242S by activated slips in the soft grains during the microcantilever bending tests. The hardness of IMI 834 varied with its Al concentration, whereas no Al-concentration-dependent changes in hardness were observed in Ti-6242S. These results suggest that the stress redistribution was caused by differences in the concentrations of solid elements and the crystal orientation of the hard and soft grains.