Microstructure and properties of Ti55531 alloy subjected to deep cryogenic treatment

The cryogenic treatment of near β Ti–5Al–5Mo–5V–3Cr–1Zr (Ti55531) alloy was conducted using the liquid nitrogen refrigeration method. The microstructure and properties were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results show that the alloy achieves optimal properties at a cryogenic time of 8h. Compared to samples that were not cryogenically treated, microhardness, ultimate tensile strength (UTS), and elongation increased by 12.9%, 6.8%, and 8.0% respectively. Detailed microstructural characterization indicated that the cryogenic processing can lead to grain refinement, enhanced dislocation density, and accelerate the transformation of the metastable β phase into the stable α phase, resulting in a higher volume fraction of the α phase. After cryogenic treatment, the lath-shaped α phase reorients from a vertical to a parallel orientation, and its average width is reduced. Simultaneously, cryogenic treatment diminished texture strength but enhanced the uniformity of the alloy. Improvements induced by cryogenic treatment are attributed to alloy volume and lattice contraction caused by cold compression forces at cryogenic temperatures, which generate internal stress and grain deflection.