Thermal fatigue of components with compositional gradients of multiphase NiAl-X and alloy IN738

Following a short evaluation of the advantages and weaknesses of materials based on NiAl for high temperature applications, it is proposed that compositionally graded design of components might be a viable solution to the problem of parts which are subjected to varying loads at both low and high temperature. Following this principle, specimens have been prepared by powder metallurgical techniques with composition and microstructure gradually changing from NiAl reinforced with ternary Laves phases (with Nb and Ta) for the hot section, to superalloy IN 738 for the low temperature section of the component. These specimens have been subjected to thermal fatigue testing between 200 and 1100 C. Crack initiation and propagation have been observed by SEM and quantitatively evaluated. The advantage of the graded solution with respect to crack arrest is obvious. The different factors governing thermal fatigue behavior of graded components are discussed in detail. (orig.)