A systematic study of hcp crystal orientation and morphology effects in polycrystal deformation and fatigue

Abstract

Elastically anisotropic, physically based, length-scale- and rate-dependent crystal plasticity finite element investigations of a model hcp polycrystal are presented and a systematic study was carried out on the effects of combinations of crystallographic orientations on local, grain-level stresses and accumulated slip in cycles containing cold dwell. It is shown that the most damaging combination is the one comprising a primary hard grain with c -axis near-parallel to the loading direction and an adjacent soft grain having c -axis near-normal to the load and a prismatic slip plane at approximately 70° to the normal to the load. We term such a combination a rogue grain combination. In passing, we compare results with the Stroh model and show that even under conditions of plasticity in the hcp polycrystal, the Stroh model qualitatively predicts some of the observed behaviours. It is shown that under very particular circumstances, a morphological – crystallographic interaction occurs which leads to particularly localized accumulated slip in the soft grain and the penetration of the slip into the adjacent hard grain. The interaction effect occurs only when the (morphological) orientation of the grain boundary in the rogue grain combination coincides (within approximately ±5°) with the (crystallographic) orientation of an active slip system in the soft grain. It is argued that the rogue grain combination and the morphological–crystallographic interaction are responsible for fatigue facet formation in Ti alloys with cold dwell, and a possible mechanism for facet formation is presented. The experimental observations of fatigue facet formation have been reviewed and they provide considerable support for the conclusions from the crystal plasticity modelling. In particular, faceting was found to occur at precisely those locations predicted by the model, i.e. at a rogue grain combination. Some experimental evidence for the need for a crystallographic–morphological interaction in faceting is also presented.

Crystal plasticity modeling of cyclic deformation for a polycrystalline nickel-based superalloy at high temperature

“…This arrangement of neighboring hard and soft grains introduces large mismatches of deformation and generates pronounced stress and strain concentrations between the neighboring grains [31][32][33]. Overall, the stress and strain heterogeneities remain similar for all three cyclic loading histories.…”

Crystal plasticity analysis of micro-deformation, lattice rotation and geometrically necessary dislocation density

“…Major drivers for this are in, for example, development of understanding of deformation, fatigue and texture evolution (Rugg et al 2007). Here, we confine ourselves to length scales of order grain size and hence relevant to polycrystalline materials for which there is significant interest in slip transfer, slip localization, grain boundary sliding, twinning, fatigue crack nucleation and micro-texture (Dunne et al 2007a;Dunne & Rugg 2008;Gong & Wilkinson 2009;Bache & Dunne 2010;McDowell & Dunne 2010;Wang et al 2010). Also of great interest is the ability to calculate accurately the evolving densities of dislocations, be they geometrically necessary (GND) or statistically stored (SSD), because of the potential roles played in latent hardening, initiation of recrystallization and nucleation of fatigue cracks, for example.…”

A crystal plasticity study of cyclic constitutive behaviour, crack-tip deformation and crack-growth path for a polycrystalline nickel-based superalloy

“…A review of literature shows that the physically-based crystal plasticity theory has been generally used to describe the mechanical behaviour of materials at grain level. With the assistance of the finite element (FE) method, the theory is able to predict the global and local stress-strain response [10][11][12][13][14][15], the evolution of crystallographic grain texture [10,16] and micro-structural crack nucleation [17][18][19] in polycrystalline materials under monotonic, creep and fatigue loading conditions. Recently, application of the theory has also been extended to polycrystalline nickel superalloy, where material microstructure was considered as one of the major factors influencing the fatigue and creep properties of the material.…”