Sub-transus transformation-driven softening in Ti-6Al-2Sn-4Zr-2Mo alloy additively manufactured using laser powder bed fusion

In the present work, the high-temperature phase stability and the correlated flow stress softening of Ti-6 %Al-2 %Sn-4 %Zr-2 %Mo alloy (Ti-6242) fabricated through the laser powder bed fusion process were studied. Toward this end, a set of hot compression tests were conducted at 700, 800, 900 and 1000 °C under the strain rate of 0.001s-1. A distinct observation was the occurrence of a significant strength drop and notable flow softening after reaching the maximum stress at all thermomechanical conditions. The peak stress was significantly dropped (∼490 MPa) with increasing temperature from 700 °C to 800 °C, proposing activation of a new emerging softening mechanism. The α′ martensitic starting microstructure extensively started to decompose at around 800 °C, leading to the formation of α and β grains, where the β phase fraction was considerably increased by approaching the β transus temperature. The reverse transformation of the martensite served as an effective softening mechanism, owing to the fact that the α and β phases represent higher capability for strain accommodation. At 900 °C and 1000 °C, the penetration of β phases through the α lamellae and the occurrence of α-globularization was clearly evident, contributing to the flow softening at elevated temperatures. A preferred texture towards the <2-1-10> and <0001> orientations was also recognized in the microstructure deformed at 800 °C and 900 °C, which proposed the contribution of texture weakening as an additional factor in flow softening.