Formation of Alpha-Case Layer During Investment Casting of Pure Ti and Ti-6Al-4V Using Comparative XRD and EXAFS Investigation

In titanium casting, a reaction byproduct known as alpha-case is a hard and brittle phase layer that exists on the surface of cast objects. This reaction layer may be detrimental to the mechanical properties of cast objects and may complicate the finishing processes. To investigate the formation of the alpha-case layer during the casting of commercially pure titanium (CP-Ti) and Ti-6Al-4V alloy, both metal ingots were cast in spinel-based investments in a controlled environment. The cast samples were metallographically prepared and analyzed for the thicknesses of alpha-case layer depth and the metal matrix. The samples were characterized using light microscopy, Vickers microhardness testing, X-ray diffractometry, and extended X-ray absorption fine structure spectrometry. The alpha-case layer on the CP-Ti cast sample was thicker than that of the Ti-6Al-4V cast sample due to the ability of the α-titanium matrix to uptake oxygen faster, which in turn is attributed to the higher diffusion coefficient of the α-titanium phase. A metastable Ti₃O phase was observed as an intermediate reaction product which could be transformed into the alpha-case layer and the α-titanium phase.