Microstructure evolution and phase transformation kinetics of low cost Ti-35421 titanium alloy during continuous heating

Abstract

The thermal expansion behavior, microstructure evolution and β → ωiso phase transformation kinetics of the low cost Ti–3Al–5Mo–4Cr–2Zr–1Fe (Ti-35421, wt.%) titanium alloy during continuous heating have been investigated by using dilatometric (DIL) method. The results revealed that corresponding transformation sequence can be verified as β → ωiso, β + ωiso → α + β, β → α and α → β using TEM and SEM analysis during the heating process. The microstructure and local composition distribution was detected by STEM mapping. The phase transition curve exhibited a typical S-type pattern, indicating the phase transformation was controlled by the nucleation and growth mechanism. It was found that the S-curve shifted to a higher temperature and the phase transition interval became shorter as the heating rate increased. Furthermore, the average activation energy required for the β → ωiso phase transition was calculated by Kissinger–Akahira–Sunose (KAS) equation is about 90.21 kJ/mol. The Avrami exponent n in the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was used to study the nucleation growth mechanism of the ω phase during continuous heating. The exponent n can be divided into three stages: the initial phase transition (0.02 < f < 0.06), the middle phase transition (0.06 < f < 0.95) and the final phase transition (0.95 < f < 1), which indicated that the phase transition mechanism is different. Moreover, it can be seen that the experimental results of TEM and SEM for Ti-35421 were consistent with the ones of DIL method. Finally, the continuous heating transformation (CHT) curve was obtained by the phase transition curve. The microstructure evolution and phase transformation obtained in the present work could be used to optimize processing.