Gaseous Carbo-Oxidizing of Ti-6al-4v: On the Role of Initial Microstructure

Abstract

The role of microstructure resulting from different heat treatment procedures on the thermochemical surface hardening response was investigated for Ti-6Al-4V alpha-beta alloy. Four different types of microstructures were addressed, i.e. equiaxial, lamellar, bimodal and martensitic and these were realized by adopting different heat treatment procedures. These different conditions were subsequently carbo-oxidized in an atmosphere consisting of 50%CO-50%CO2 at ambient pressure in the temperature range 900-1050 K at 50 K intervals. The microstructural evolution of the bulk and the hardened case for the different samples was investigated. Hardness profiles were recorded on cross-sections of the samples and the microstructure was investigated with light optical microscopy (LOM) and scanning electron microscopy (SEM). X-ray diffraction (XRD) depth profiling for in-depth phase determination was carried out by successive sublayer removal. A diffusion zone with interstitials in solid solution in α-Ti located underneath three oxide layers developed for all conditions treated. Layers of rutile TiO2 were detected in the outermost surface as well as an inner layer. The outer/inner rutile layers were separated by a thin layer of α-Al2O3. The lamellar microstructure had a different carbo-oxidizing response than the other 3 conditions, by forming a significantly thinner case, which is interpreted in terms of a lower effective oxygen diffusion in a coarse lamellar α/β microstructure. For all initial conditions, thermochemical treatment was observed to affect the bulk microstructure and hardness. The present investigation suggests that surface hardening is not just an ‘add-on’ process, but has to be designed / tailored in relation to the microstructure determined by previous heat treatment steps.

Keywords: Ti-6Al-4V, carbo-oxidizing, surface hardening, Heat treatment, Microstructure