Optimisation of the oxygen diffusion process for surface hardening of the Ti-6A1-4V alloy

Includes abstract.Includes bibliographical references (leaves 87-89).A well established method of surface hardening for titanium alloys is through solid solution strengthening by alloying with oxygen. This is achieved through a two step oxygen boost diffusion hardening (OBDH) process. During the oxidation step, the formation of a thick adherent rutile TiO2 layer occurs which is followed by subsequent boost diffusion (step 2) wherein the oxide layer is decomposed to free up oxygen for boost diffusion hardening. Step 1 is characteristically conducted at 850°C for 30 minutes in an oxygen rich environment followed by cooling. Step 2 is conducted in an oxygen free environment at 850°C for 20 hours. The approach of the present study was to refine the OBDH process and gain understanding of the mechanism responsible for boost diffusion hardening in order to optimise boost diffusion hardening. This was done by introducing and comparing the hardening results for a cyclic isothermal OBDH treatment and a continuous thermal OBDH treatment to the resultant hardening of the original two step process which included cooling after step 1