Effects of Temperature and Environment on Fatigue Crack Growth Behavior in TiAl Alloys

Understanding the fatigue crack growth (FCG) mechanism from room to service temperature is essential for airplane engine materials. Recently, we successfully developed the “design principle” for wrought TiAl alloys based on thermodynamics and kinetics. Our model wrought alloys with controlled α₂-Ti₃Al/γ-TiAl lamellar microstructure with ordered bcc structure β-Ti phase at the grain boundaries show excellent room temperature FCG (initiation/propagation) resistance, superior to that of the cast GE 4822 with the fully lamellar microstructure. However, it has not yet been done at high temperatures. In addition, very limited information is available on the FCG behavior of the alloys at elevated temperatures at around 1073 K. In addition, at such temperatures, environmental effects might not be negligible. Therefore, in this study, the effect of temperature and atmosphere on the FCG behavior was investigated using an alloy with microstructure controlled to have β phase at the grain boundaries.<br/>The microstructure of the specimens is similar to the previous ones with β phase along the lamellar boundaries. The β phase was introduced using the cellular reaction (α₂+γ→β+α₂+γ) through multi-step heat treatments. A pre-crack was first introduced to each CT specimen, and FCG tests were done at RT, 873 K, and 1073 K under 20 Hz and <i>R</i> (a tension/tension load ratio) of 0.1. The elevated-temperature tests were conducted in a uniquely designed, environmentally controllable chamber with heating elements. In the case of the test in Ar, the chamber was evacuated and backfilled with Ar flowing. The crack length was measured using the DCPD (direct current potential drop) method.<br/>The stress intensity factor range threshold for fatigue crack growth (Δ<i>K</i>_th) obviously decreased at 873 K in both atmospheres compared to the room temperature. However, it increased to the same value or even better at 1073 K in the air, whereas it remained low in the case of the Ar atmosphere. The Paris slope value (<i>m</i>) was nearly the same regardless of the test conditions. However, the crack growth rate value (d<i>a</i>/d<i>N</i>) value became apparently low by an order of magnitude at the same Δ<i>K</i> value in the Ar atmosphere compared to the value tested in the air. Almost no oxide formation in the cracks was confirmed in the sample tested in Ar, whereas a thick oxide layer was formed in the sample tested in air. From these results, it should be suggested that the FCG behavior obtained in the Ar atmosphere is the nature of the alloys, and the behavior highly affects the oxide formation. The underlying mechanism of FCG will be presented in conjunction with the microstructure analyses.<br/>This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials Integration for revolutionary design system of structural materials” (Funding agency: JST).