Improving the high temperature oxidation resistance of pure titanium by shot-peening treatments

Abstract

Shot-peening (SP) treatments have shown their capacity to improve the oxidation resistance of titanium and zirconium thanks to the large compressive stresses and the surface hardening induced by this mechanical process. However, shot-peening treatments can produce a surface chemical deposit, which can modify the high temperature oxidation resistance.Here, we study pure titanium samples shot-peened with different type of balls: tungsten carbide, alumina or glass. The oxidation behavior was studied at 700°C in dry air by thermo gravimetric analysis for short isotherm oxidation periods up to 100h. Also, long oxidation tests (3000h) at 700°C were performed with an intermittent monitoring of the samples mass. The oxidized samples were characterized by XRD, SEM/EDS, XPS and nuclear reaction analysis. After 100h of oxidation the alumina ball shot-peening treatment produced the most resistant samples. The smallest α-case area was found for the samples that received the most energetic mechanical treatment. The formation of a continuous nitride layer observed underneath the oxide layer can explain the oxidation resistance improvement. Shot-peening treatments with alumina balls produce an alumina deposit on the samples surface that participates to reduce the oxidation. The chemical and structural surface modifications brought by the mechanical treatment participate to the oxidation resistance improvement. For long oxidation periods (3000h), the shot-peening with WC balls followed by a stripping with glass balls was the only treatment that showed an oxidation resistance improvement.