Microstructural manipulation and improved mechanical properties of a near α titanium alloy

In this study, a newly developed near a titanium alloy with a typical Widmannstatten as-cast microstructure was subjected to isothermal multidirectional forging (IMDF) at 15 degrees C below beta transus temperature. Microstructural observations reveal that the prior beta grain boundaries disappeared and alpha laths transformed into spheroids gradually with increasing the IMDF pass. Ultrafine alpha grains with an average grain size of 1.57 mu m was achieved after four-pass IMDF. Microstructural homogeneity was enhanced and the fraction of recrystallized grains increased remarkably with increasing IMDF pass. The spheroidization process of alpha laths is a result of the recrystallization during IMDF, including the dissociation of alpha laths and the migration of subgrains. Compared to the as-cast titanium alloy, both the tensile strength and elongation to failure at room temperature were improved, which is primarily attributed to the microstructure refinement and spheroidization of alpha laths. In addition to grain refinement, dislocation hardening and substructure strengthening are also the critical mechanisms for the increased strength observed in the samples after IMDP.