Influence of ageing temperature and heating rate on the properties and microstructure of β Ti alloy, Ti–6Cr–5Mo–5V–4Al

A study of the effects of temperature and heating rate upon the ageing response of a Ti–6Cr–5Mo–5V–4Al alloy was undertaken. The degree of strengthening declined with increasing ageing temperature as the precipitate morphologies became coarser, while the extent and thickness of grain boundary α increased with temperature. Ageing at temperatures between 440 °C and 500 °C result in the greatest age hardening response in times of around 4 h due precipitation of the α phase. At 440 °C the hardness increases from the initial solution treated value of 275 HV to more than 450 HV. The rate of heating to reach the isothermal ageing temperature has a substantial impact on the ageing response. Greater scatter was observed in hardness results for the samples aged using a faster heating rate at 500 °C. For ageing at 560 °C a faster rate of heating to the ageing temperature led to rapid declines in the hardness after the peak, equivalent to around 20% of the peak value. Slower heating rates (5 °C min−1) appear to increase the number of nucleation sites for precipitation of the α phase, forming finer, more even distributions of precipitates. Faster heating rates (∼100 °C min−1) produce coarser, less evenly distributed precipitates which coarsen more rapidly.