Microstructure evolution of a fully martensitic Ti-16Nb shape memory alloy during thermo-mechanical treatment

The microstructures of Ti-16Nb shape memory alloy cold-rolled to different extents and subsequently annealed at different temperatures have been investigated. Experimental evidences from transmission electron microscopy (TEM) indicate that rolling deformation proceeds mainly through twinning inside the martensite plate rather than movement of the original twin boundaries. The deformation twins that are twinned by {111} type I and <211> type II twinning mode with the matrix nucleate inside the martensite plate in the specimen subjected to 5% thickness reduction. Such deformation twins propagate throughout the whole martensite plate gradually while the rolling deformation aggravates. The boundaries of the original martesite plates become blurry and distorted owing to the formation of deformation twins and they are hardly to be identified in the 20% cold-rolled specimen. The α″ martensite decomposes into α and β phases when the cold-rolled specimens are annealed at 450 and 550 ^oC. The α precipitates exhibit a fiber-like morphology, alternating with the β matrix. In the specimens annealed at 650, 700 and 750 ^oC, the martensite shows the parallel morphology, the boundaries of which are straight and well-defined. The martensite has the wedge-like microstructure in the specimens annealed at temperatures above 750 ^oC, which is similar to that of the solution treated specimen. The density of the residual defects is responsible for the microstructure evolution during the post-deformation annealing.