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

TL;DR

This study investigates the micro-mechanisms of low cycle fatigue crack growth in a Ni-based superalloy using advanced characterization techniques and parametric approaches to understand crack propagation behavior.

Contribution

It introduces a combined use of CTOA and { heta}MTS approaches, along with EBSD and slip transfer analysis, to elucidate micro-mechanisms of fatigue crack growth in Haynes 282.

Findings

Crack tip opening angle (CTOA) shows non-linear decay during propagation.

Favorable grain microstructural conditions influence subsurface crack growth.

Twin-matrix incompatibility and slip transfer are key to understanding crack propagation.

Abstract

The present work discusses the micro-mechanism of low cycle fatigue (LCF) crack growth in smooth bar specimens of Haynes 282. Two parametric approaches, i.e. crack tip opening angle (CTOA) and maximum tangential stress ({\theta}MTS) have been opted to characterize the cracks. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime. Mixicity of local KI and KII fields is directly proportional to {\theta}MTS and that can be assessed by measuring local deflections. Around the crack, the role of grain incompatibility has been addressed through EBSD and slip transfer analysis. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favouring subsurface crack propagation, and these values exist beyond a limiting threshold. The SF-EM maps mark the regions of cracked and uncracked grains in the material. 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. A detailed slip transfer analysis based on the Luster-Morris parameter (LMP) has been carried out for investigating the interrelation between slip activity, elasto-plastic incompatibility, and grain boundary geometry.