Effects of heat treatment on evolution of microstructure of boron free and boron containing biomedical Ti-13Zr-13Nb alloys

In the present study, the effects of heat treatment on the microstructure of Ti-13Zr-13Nb (TZN) and Ti-13Zr-13Nb-0.5B (TZNB) alloys have been investigated. Depending on the heat treatment conditions, the microstructure of the heat treated TZN alloy consisted mainly of elongated and/or equiaxed α, β or martensite. Slow cooling (furnace or air cooling) from the solution treatment temperature produced α and β phases in the microstructure. Rapid cooling (water quenching) resulted in martensite and retained β when the solution treatment temperature was above or close to β transus. However, martensite was not formed after water quenching from a solution treatment temperature which was below β transus due to partitioning effect of the alloying elements. Increasing the cooling rate from the furnace cooling to the air cooling led to finer microstructure. Aging of water quenched samples transformed the martensite, if present, into α and β, and the morphology of α phase changed from elongated to equiaxed and enhanced the growth of α. The microstructure of all the TZNB samples consisted of dispersed precipitated particles of TiB in the matrix. The majority of the boride particles showed an acicular (needle like) morphology. The other phases present in the TZNB alloy were similar to those in the similarly heat treated TZN alloy. Moreover, a growth of α phase was observed in the microstructure of TZNB alloy when compared with that of TZN alloy.