Anomalous Dilatometric Response of Hot-Worked Ti-5Al-5Mo-5V-3Cr Alloy: In Terms of Evolution of Microstructure, Texture and Residual Stress

The Ti-5553 alloy has high strength close to that of the β Ti alloy for applications in high-strength structural components. In this article, results of detailed investigations of dilatometric responses of this alloy are presented. A Ti-5553 alloy ingot (150 mm diameter and 250 mm length) was double vacuum arc remelted (VAR), homogenized, forged and rolled. A very interesting dilatometric response was observed in this deformed material during heating in a dilatometer. Monitoring the sample length during continuous heating at a constant heating rate showed expansion and contraction in different temperature ranges. The observed contraction could not be attributed to any known phase transformation feasible in this alloy over the corresponding temperature range. Interestingly, the dilatometric contraction during heating was found to be orientation dependent. Precise lattice parameter measurements done on the basis of synchrotron data showed that the α to β phase transformation is associated with a slight expansion. To understand physical phenomena causing the dilatometric expansion/contraction, evolutions of microstructure and texture were investigated by intermittent sample removal at various intermediate temperatures along the heating path. Dilatometric responses were analyzed in terms of α phase dissolution, recovery, recrystallization of both α and β phases and release of anisotropic residual stress present in the starting material. The present investigation showed that the dilatometric examination coupled with microscopic observations can be employed for identifying the temperature ranges over which these processes occur—the information relevant for devising thermomechanical treatments of this alloy. This article also describes a method of assessing micro-residual stresses present in the two-phase Ti alloys from the dilatometric response of a small sample from the thermomechanically processed material.