A study of epitaxial growth behaviors of equiaxed alpha phase at different cooling rates in near alpha titanium alloy

The epitaxial growth behaviors of equiaxed primary α phase (α_p) at different cooling rates (150–0.15 °C/s) in a near α titanium alloy Ti60 were studied by optical micrograph, back scattered electron (BSE) images, high-resolution electron backscatter diffraction technique (EBSD) and electron probe microanalysis (EPMA). Microstructural observations indicated that the size of α_p significantly increased with decreasing cooling rate. The rim-α phase observed by BSE image, which formed at the periphery of α_p during cooling and has an identical crystallographic orientation to the interior region of α_p analyzed by Kikuchi diffraction patterns, is considered to be evidence of epitaxial growth of α_p. EBSD analysis also showed that α_p preferentially grew extending for a distance along the β/β boundary resulting in extension-α phase from α_p. The EPMA confirmed that contrast difference in BSE image within α_p is caused by the difference in composition. The further microanalysis of local composition indicated that epitaxial growth during continuous cooling is mainly controlled by the diffusional redistribution of aluminum and molybdenum atoms between α_p and β matrix. On this basis, the sizes of α_p were theoretically calculated after continuous cooling based on a diffusion-controlled model, and model predictions showed good agreement with experimental measurements.