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 alpha phase (alpha(p)) at different cooling rates (150-0.15 degrees C/s) in a near alpha titanium alloy Ti60 were studied by optical micrograph, back scattered electron (BSE) images, high-resolution electron backscatter diffraction technique (EBSD) and electron probe micro-analysis (EPMA). Microstructural observations indicated that the size of alpha(p) significantly increased with decreasing cooling rate. The rim-alpha phase observed by BSE image, which formed at the periphery of ap during cooling and has an identical crystallographic orientation to the interior region of alpha(p) analyzed by Kikuchi diffraction patterns, is considered to be evidence of epitaxial growth of alpha(p). EBSD analysis also showed that ap preferentially grew extending for a distance along the WO boundary resulting in extension-alpha phase from alpha(p). The EPMA confirmed that contrast difference in BSE image within alpha(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 alpha(p) and 13 matrix. On this basis, the sizes of alpha(p) were theoretically calculated after continuous cooling based on a diffusion-controlled model, and model predictions showed good agreement with experimental measurements. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.