High temperature fatigue crack growth behaviour of TIMETAL 21S in an oxidizing environment.

Abstract

The high temperature fatigue crack growth behaviour of the newly developed, metastable titanium-based alloy, TIMETAL 21S, was investigated in an inert and an oxidizing environment. The investigation adopted a two pronged approached, namely, to initially establish the pure microstructural behaviour under oxidizing and inert environments at various elevated temperatures, and consequently, to establish the environmental effects on the fatigue crack growth behaviour in the various environments at high temperature. The effect of the oxidizing environment on the metastable alloy and on the mechanical and chemical events occurring at the fatigue crack were studied by using optical and scanning electron microscopy, including ED X analysis, x-ray diffraction, and Auger Electron Spectroscopy (AES) . For the microstructural investigation, the TIMETAL 21S samples were exposed for 5 hours to a pure argon and argon + 20% O2 environment at 300°C to 750°C in increments of 50°C. The results showed that in the oxidizing environment a more homogeneous nucleation of the alpha phase had occurred at higher temperatures and that the oxide Ti02, in addition to the alpha case, had predominantly formed on the exposed surfaces. AES analysis showed that dissolution of the oxygen into the alloy occurred even at low temperatures. An LEFM approach was used to investigate fatigue crack growth rate (FCGR) of C(T) specimens at 375°C, 450°C, 550°C and 620°C in the argon and argon + 20% oxygen environment. The crack growth rates were monitored using load-line compliance and the beachmarking method - a method by which beach marks were impressed on the fracture surface to track the progressing crack. The results showed that the crack growth rates were lower in the oxidizing environment and was influenced by a synergistic effect of the temperature, stress intensity at the crack tip and the environment. In addition to the phenomena of crack tip shielding (a process whereby the effective crack tip driving force experienced at the crack tip was locally reduced), other mechanisms such as slip character modification and secondary cracking ahead of the crack tip, leading to crack tip blunting and branching, had to be incorporated to fully explain the crack growth behaviour. The tests conducted in the inert environment effectively excluded the effect of oxygen on the crack growth behaviour and substantiated that various mechanisms ultimately determined the FCGR in TIMETAL 21S at elevated temperatures.