Orthorhombic martensite formation upon aging in a Ti-30Nb-4Sn alloy

The characteristics of orthorhombic martensite (alpha '') formed by step-quenching in a Ti-30Nb-4Sn (wt%) alloy have been investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). According to literature, alpha '' lattice parameters depend mainly on the composition of the parent 3 phase. In this study, samples subjected to step quenching heat treatment presented alpha '' phase formation in the proximity of alpha phase laths, driven by two combined factors: solute rejection and lattice strain. Our results indicate that as the aging is prolonged, alpha '' becomes richer in solute content, which makes it more similar to the parent alpha phase. An average 2.55% lattice strain along [110]beta directions was found to be necessary in order to obtain alpha '' from the beta phase after 24 h of aging at 400 degrees C, followed by water-quenching. The initial lattice strain along the same direction was estimated at approximately 3.60% with zero aging time. The precipitation of the alpha phase does not inhibit a solute rich alpha '' phase formation. (C) 2016 Elsevier B.V. All rights reserved.