In situ neutron diffraction study of texture evolution and variant selection during the α → β → α phase transformation in Ti-6Al-4V

In the present study, in situ phase transformation experiments have been carried out using neutron diffraction to monitor the texture evolution during the α → β → α phase transformation in Ti-6Al-4V with and without 0.4% yttrium additions. The aim of adding yttrium was to control β grain growth above the β transus by Zener pinning. First, both alloys were thermomechanically processed to generate a similar starting α texture and grain morphology. Subsequently, both materials were heat treated above the β transus up to 1250°C followed by furnace cooling to 210°C to promote diffusional phase transformation starting from β grain boundaries. In situ texture measurements were taken during α → β → α phase transformation starting at room temperature, 800°C, 950°C, above β transus (1050 and 1250°C), and back to near room temperature. The degree of variant selection was determined by comparing the predicted transformation texture during heating and cooling based on the Burgers relationship and the assumption of no variant selection with the measured textures. It was found that during heating β grows from the pre-existing β and that the β texture evolved even before the β transus was exceeded. The β texture strengthened noticeably above the β transus in the case of conventional Ti-6Al-4V but not Ti-6Al-4V-0.4Y, which was related to β grain coarsening. The level of variant selection was clearly affected by grain coarsening and the formation of β texture components that contribute to the 〈1 1 1〉//normal direction (ND) γ fibre texture rotated about 10°away from ND.