Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

Abstract

We present the microscale fracture mechanics aspects of LCF cracks non-propagating till failure in the cylindrical specimens of Ni based superalloy Haynes 282 and attempt to unravel the underlying crystallographic factors. Two key parameters, Crack Tip Opening Angle (CTOA) and maximum tangential stress (θMTS) have been opted for characterization. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime and the fraction of the stabilized regime increases in lower strain amplitude where the fatigue life is more. Mixity of local KI and KII fields is directly proportional to θMTS and that has been assessed by measuring local deflections. The modal mixity has been found to be microstructure sensitive and an entirely statistical parameter. Different growth modes of these non-propagating cracks have been identified from the microstructural observations. The role of elasto-plastic incompatibility of neighboring grains has been addressed by conducting crystallographic analyses. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favoring subsurface crack propagation below which grains (mostly with high elastic compliance and low SF) are not participating in crack propagation. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. From the results of a detailed slip transfer analysis based on the Luster-Morris parameter (LMP), it is observed that there exists microstructurally controlled lower bounds that inhibits slip transfer (LMP value <0.1). Dislocation debris analysis using TEM has also been carried out to understand micromechanisms clearly. Both non coplanar and planar mode of dislocation activity has been observed and twin boundaries has been see to act as barrier to the planar dislocation motion.