Phase transformation textures in Ti-6Al-4V alloy

Titanium alloys are widely used in various industrial, domestic, and medical applications such as turbine blades, bicycle frames, knee implants, etc. The two-phase titanium alloy Ti-6Al-4V (wt. percent) is considered to be a workhorse alloy for many applications in these diverse fields. Despite the large body of work on this alloy, the question of the transformation mechanism from the hep a to the bcc β phase, occurring on heating to temperatures above the α/β transus at ∼980°C, is still unresolved. Due to experimental difficulties, it has not yet been clearly determined whether the increase in β volume fraction occurs by fresh nucleation of β crystals within α phase grains or the growth of preexisting β grains. Since the Burgers orientation relationship holds only if the β grains are nucleated within the α grains, the outcome of this question greatly affects texture-modeling efforts for this system. The Burgers orientation relationship predicts that the {0001} crystal direction in a grain of the a phase becomes a {110} crystal direction in a grain of the β phase after the transformation. In this work we present experimental results from in-situ texture measurements performed on the HIPPO neutron diffractometer at LANSCE. Using the combination of time-of-flight neutrons and full-pattern Rietveld analysis allowed us to determine the orientation distribution functions of both phases at room temperature, 800°C, 1020°C and again at room temperature. We found strong indications that the β phase indeed grows from grains preexisting at room temperature. Upon re-transformation from β to α we found that the Burgers relationship is followed.