In situ assessment of isochronal phase transformations in a lamellar Ti–5Al–5Mo-5V–3Cr–1Zr alloy using synchrotron X-ray diffraction

Phase transformations in a lamellar Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy upon heating at continuous rates of 5, 20 and 100 °C/min has been studied by in situ high energy X-ray diffraction (HEXRD), aided by scanning and transmission electron microscopy, atom probe tomography and hardness measurements. During slow heating, the β phase has shown a behavior suggestive of additional transformations other than the continuous α → β decomposition observed at higher heating rates. Such behavior has been attributed to the precipitation of an embryonic isothermal ω phase. APT analyses have shown that no elemental partitioning occurs between the β matrix and the lamellar α phase, but it does between β and grain boundary α, suggesting that ω phase precipitates with solute rejection in regions outside lamellar colonies embedded in an enriched matrix. Above 500 °C, diffusion becomes significantly enhanced and α decomposition takes place.