Non-isothermal kinetic analysis of high temperature oxidation of Ti-6Al-4V alloy

In this study, non-isothermal kinetic analysis of high temperature oxidation of Ti-6Al-4V alloy by thermogravimetric analysis (TGA) at heating rates of 10, 15, and 20 degrees C/min up to 1550 degrees C in air atmosphere was studied. The results revealed that the formed oxide layer consists of several different parts; an inner part of TiO2, an outer part of TiVO4, and an intermediate part of Al2O3. Also, the thickness of oxide layer increased by an increase in the maximum heating temperature, while its density decreased by formation of the pores and cracks in higher maximum temperature due to the mismatch between the thermal expansion coefficient of the oxide layer and the substrate. This mismatch led to formation of large stresses in the oxide layer at high temperatures (>= 1000 degrees C) and thereby isolation of the oxide layer during cooling. The microhardness profiles shift to higher microhardness values by an increase in the maximum temperature due to the increasing of oxygen solubility in the substrate. On the other hand, the kinetic results performed by isoconversional Starink, KAS, FWO, Tang, and Friedman methods in combination with the invariant kinetic parameters (RP) method and fitting model estimated the activation energy (E) and pre-exponential factor (lnA) of the oxidation process equal to 205-235 kJ/mol and 12-13 min(-1), respectively. Furthermore, it was approved that this reaction is controlled by a diffusion control model (Dl model). (C) 2019 Published by Elsevier B.V.