Microtexture and macrozones in two-phase titanium alloys

Two-phase titanium alloys dominate the aerospace engine industry and their use in jet engines is not replaceable in the short to medium term. Cyclic loading during flight makes fatigue performance one of the key considerations in aerospace applications. The effect of titanium alloy microstructure on fatigue behaviour has been extensively investigated but there remain questions about the effects of Regions of crystallographically aligned α grains, which are known as ‘macrozones’. We currently have limited knowledge about the contributions of macrozones to crack initiation and crack propagation. The aim of this thesis is to provide new understanding of the effect of macrozones on fatigue behaviour and to propose a processing window for macrozone-free microstructures. It is expected that the presence of macrozones will influence both fatigue crack initiation and propagation. The hypothesis is that dislocations pile up on the interface between two macrozones and accumulate strain on this interface. The accumulated strain consequently leads to crack initiation. The microcrack growth in macrozones is subsequently faster than in random orientations because the aligned grains in macrozones provide lower resistance to transmission across grain boundaries. In order to characterize the effect of macrozones on fatigue behaviour we conducted four-point bending high-cycle fatigue tests, which have the advantage of uniform tensile stress on the tensile surface. Samples made from as-forged Ti-6Al-4V were used for the investigation of fatigue crack initiation, and samples with micronotches made from both as-forged and hot isostatic pressed Ti-6Al-4V were used for the investigation of crack propagation. The crack initiation and propagation were tracked by replicas taken at intervals during testing, and crack propagation rates were calculated. Electron Backscattered Diffraction was used to identify the texture of macrozones, from which the Schmid factor distributions were calculated to identify the potential activated slip systems combined with other slip analysis like slip trace analysis. Hot compression tests were carried out at different temperatures and strain rates on cylindrical Ti-6Al-4V samples with as-cast microstructures to identify a processing window for a macrozone- free material. It was found that fatigue cracks can be initiated between two macrozones with different primary deformation modes. Slip is effectively continuous in a macrozone due to aligned α grains but arrested on the macrozone boundary with high-angle misorientation, which leads to strain accumulation on the boundary and initiates a crack. A misorientation crack factor parameter is proposed based on Electron Backscattered Diffraction data in this work to quantify the effect of macrozones on crack initiations based on deformation modes, misorientation and boundary trace. The calculated misorientation crack factor results of macrozones on crack initiation areas on tested samples are consistent with the experimental results. Furthermore, we found that microcrack growth is sensitive to the presence of macrozones: microcrack propagation was faster in a macrozone than in random orientations. Transgranular microcrack growth is mostly along either basal or prismatic planes in a macrozone with reasonable effective stress intensity factor and not arrested by grain boundaries. The β phase on α grain boundaries in macrozones in two-phase titanium alloys provides minimal resistance to microcrack propagation. Finally, this thesis describes a processing window, which gives macrozone-free titanium alloys based on hot compression results. Hot compression tests of the two- phase titanium alloy Ti-6Al-4V with as-cast microstructures indicate that macrozones mostly form at low temperature (i.e. 850 °C and below) and are enhanced by subsequent annealing. Hot working close to the β transus gives more chance of a macrozone-free product.