When subject to inappropriate heat treatments or harsh operating conditions involving elevated temperatures, titanium alloys have a tendency to form a hardened surface layer, termed an alpha case. The relationships between the alloy composition and the rate of alpha case formation are poorly understood, as is the effect of alpha case formation on tensile properties. There is also limited data available characterising the development of alpha casing at high temperatures above the â transus temperature, where titanium alloys adopt an alternate crystal structure.
Heat treatments were carried out on Grade 2 and Grade 5 titanium alloys in atmospheric conditions at a range of temperatures both below and above the âtransus temperature. This resulted in varying amounts of alpha casing and oxygen diffusion. Microstructural and microhardness analysis were performed and models of alpha case development were created for temperatures both above and below the β transus temperature using Fick’s Second Law of Diffusion and an Arrhenius law describing the temperature dependence of the diffusivity of oxygen in titanium.
Further experimentation was performed to observe the effects of alpha case formation and oxygen diffusion on the tensile properties of Grade 2 titanium. Properties were investigated for samples treated both above and below the β phase transus temperature of the alloy. The alpha casing was found to have detrimental effects on properties. It was found that the alpha case layer held no structural integrity, even at early stages of tensile deformation. The alpha casing also caused premature crack initiation in affected samples. A model of reduced cross sectional area was applied to investigate the reduction in the apparent ultimate tensile strength (UTS) of samples. Samples treated below the β transus temperature were found to follow this model closely. For samples treated above the β transus temperature, it was found that other mechanisms such as fracture mechanics and solution strengthening due to further inward diffusion of oxygen were more dominant.
A nomogram showing what depth of alpha case can be expected after exposing Grade 2 titanium below the β transus temperature for various times and temperatures was produced using the empirically determined diffusion constants found in this study. This chart is a practical tool that can be used to estimate the depth of alpha case and how much material should be removed in final machining operations. The nomogram compares well with the limited range of recommendations provided by ASM, however is more general, being applicable over a much wider range of temperatures and times.
The methods of the thesis could be used to develop models of oxygen diffusion and alpha case formation in a range of titanium alloys and this could be the subject of future research. Suggestions for novel methods of preventing alpha case development are briefly discussed.