LOW-CYCLE FATIGUE BEHAVIOR OF POLYCRYSTALLINE NI3AL ALLOYS AT AMBIENTAND ELEVATED-TEMPERATURES

The low cycle fatigue (LCF) resistance of polycrystalline Ni3Al has be en evaluated at ambient, intermediate (300 degrees C), and elevated (6 00 degrees C) temperatures using strain rates of 10(-2)/s and 10(-4)/s . Testing was conducted on a binary and a Cr-containing alloy of simil ar stoichiometry and B content (hypostoichiometric, 200 wppm B). Test results were combined with electron microscope investigations in order to evaluate microstructural changes during LCF. At ambient and interm ediate temperatures, the cyclic constitutive response of both alloys w as similar, and the LCF behavior was virtually rate independent. Under these conditions, the alloys rapidly hardened and then gradually soft ened for the remainder of the life. Initial hardening resulted from th e accumulation of dislocation debris within the deformed microstructur e, whereas softening was related to localized disordering. For these e xperimental conditions, crack initiation resulted within persistent sl ip bands (PSBs). At the elevated temperature, diffusion-assisted de fo rmation resulted in a rate-dependent constitutive response and crack-i nitiation characteristics; At the high strain rate (10(-2)/s), continu ous cyclic hardening resulted from the accumulation of dislocation deb ris. At the low strain rate (10(-4)/s), the diffusion of dislocation d ebris to grain; boundaries resulted in cyclic softening. The elevated temperature LCF resistance was determined by the effect of the constit utive response on the driving force for environmental embrittlement. C hromium additions were observed to enhance LCF performance only under conditions where crack initiation was environmentally driven.