Dynamic phase evolution in titanium alloy Ti-6A1-4V

Industrial processing of the Ti-6A1-4V alloy consists of different steps of heat treatments and hot deformation. The heat treatments are dependent on the kinetic of the allotropic phase transformation, which can be characterized by static and dynamic methods. The methods of post morten samples such as metallography and EPMA combined with Image Analysis were used to quantify the α phase by freezing the microstructure after different heat treatments. The dynamic methods allowed detecting the temperature of phase transformations during heating or cooling. The aim of this work is to determine the phase transformation evolution of Ti-6A1-4V of defined starting microstructures. Samples heat treated in α+β and β fields with controlled cooling rates and water quenching were characterized by α combination of dilatometry, differential scanning calorimetry (DSC) and metallography. Pre annealing heat treatments in the 930-1030°C range using different cooling rates result in a wide combination of primary α, β, martensite and secondary α phases with different thickness. Continuous heating was carried out by DSC and the peaks observed were correlated with the microstructure obtained from interrupted tests. Transformation of α into β was identified as endothermic peaks, while the transformation of martensite into β and α produced an exothermic peak. Both phenomena provoke a detectable retardation in the rate of the length expansion, denoted by dilatometry experiments. Secondary α laths transformed into β at lower temperatures than primary α, and became thinner and shorter by increasing the temperature.