The low-cycle fatigue behavior of an alpha-two + beta titanium aluminide alloy

The role of microstructure in the LCF behavior of Ti-24Al-11Nb has been investigated. Four microstructures were selected in order to vary the size, relative orientation, and distribution of the two phases, as a means of varying the amount of deformation in the alpha-two grains and the ability of the beta phase to accommodate strain incompatibilities between them. Fully-reversed total-strain-controlled LCF and tensile testing were conducted at 25°C and 316°C. Deformation was studied by limited examination of the dislocations present in the material before and after testing and by analysis of the fatigue hysteresis loops to calculate dislocation friction and back stresses. Failure was studied by the use of fractography and the periodic replication of selected fatigue specimens. A comparison between the results of fatigue and tensile tests highlighted differences in the response under monotonic and cyclic loading. A comparison of the data at the two temperatures allowed the effects of moderate increases in alpha-two ductility to be examined, as it increases more rapidly with temperature than that of the beta phase. Microstructural effects were elucidated by differences in the behavior of the microstructures under all test conditions. Results indicate that the LCF behavior for all four microstructures can be adequately described by conventional empirical strain-based fatigue relations. For all microstructures, there was little or no plasticity-controlled fatigue at room temperature. With increasing temperature, plastic strain-life curves became steeper. Although all four microstructures showed fatigue hardening, the amount varied as it was related to the plastic component of the applied strain, which varied with microstructure and temperature. Fatigue and tensile failures occurred by the propagation of cracks, which nucleated at alpha-two slipband/grain boundary intersections. Microstructural effects on fatigue life were significant, when compared with the effect of temperature, and were attributable to differences in plastic strain resulting from a given applied total strain, because of differences in yield behavior and strain accommodation at grain boundaries. The alpha-two slip length and the volume fraction and distribution of the beta phase were found to be critical microstructurel features governing fatigue behavior.