Development of geometrically necessary dislocation density and micromechanical fields in solid state thermal cycle

Abstract

This study presents a comprehensive analysis of time evolution and spatial distribution of crystallographic geometrically necessary dislocation (GND) density induced by solid-state thermal cycling in an IN718 polycrystal. We examine the associated residual states of stress, statistically stored dislocation (SSD) density and lattice rotation along the build direction. A thermomechanical crystal plasticity finite element model is employed to capture the thermal-stress response under a simulated single-step thermal cycle. Two distinct methodologies for calculating GND density are explored and compared: one based on the plastic deformation gradient ( ρ G N D p ) and the other on the orientation gradient ( ρ G N D e ). This computational approach expands existing postmortem and 2D experimental analyses by providing a fully time-resolved, three-dimensional perspective on the evolution of lattice rotations and dislocation densities under thermal cycling without external mechanical loading. Results reveal that while both methods yield residual GND densities at the same order of magnitude, ρ G N D e shows higher sensitivity to transient thermoelastic expansion. The spatial GND density distribution along the build direction increases with the thermal gradient, with peak temperatures during SSTC near the top of the microstructure leading to significantly elevated ρ G N D e and ρ G N D p . Additionally, the study highlights the inherently three-dimensional nature of lattice rotation induced by thermal cycling, with notable out-of-plane lattice rotation gradients developing even in the presence of a unidirectional temperature gradient. We observe an average residual stress of approximately 50% of the initial flow stress and the corresponding spatial distributions of residual stresses and SSD density exhibit notable hotspots near regions of microstructural constraints and vary with thermal gradients along the build direction. The degree of plasticity, as reflected in the stored SSD and GND densities and local plastic strain levels, reaches magnitudes comparable to those observed under moderate monotonic deformation. These findings underscore the critical role of thermal cycling in shaping microstructural evolution and residual states in additively manufactured components, offering new insights into aspects that currently cannot be directly captured via experimental measurements. • Two methods for GND density calculation reveal distinct sensitivities to thermal expansion during thermal cycle. • GND density correlates strongly with the through-height thermal gradient. • Thermal cycling induces notable out-of-plane lattice rotations under unidirectional temperature gradient. • Residual stresses reach approximately 50% of initial flow stress, with hotspots near microstructural constraints. • SSD density and residual stress distributions vary significantly with thermal gradients along the build direction.

Topics & Concepts