An official website of the United States government
Abstract Fatigue short‐cracks in Mg alloys display complex growth behavior due to high plastic anisotropy and crack path dependence on local microstructural features. In this study, the three‐dimensional crystallography of short‐crack paths in Mg alloy WE43 was characterized by mapping near‐field high‐energy X‐ray diffraction microscopy (HEDM) reconstructed grain maps to high‐resolution X‐ray CT reconstructions of the fracture surfaces in the crack initiation and short‐crack growth regions of six ultrasonic fatigue specimens. Crack–grain–boundary intersections were analyzed at 81 locations across the six crack paths. The basal intragranular, non‐basal intragranular, or intergranular character of short‐crack growth following each boundary intersection was correlated to crystallographic and geometric parameters of the trailing and leading grains, three‐dimensional grain boundary plane, and advancing crack front. The results indicate that crack paths are dependent on the combined crystallographic and geometric character of the local microstructure, and crack path prediction can be improved by use of dimensionality reduction on subsets of high‐linear‐correlation microstructural parameters.
more »
« less
Grain size effect on strain localization, slip-grain boundary interaction and damage in the Alloy 718 Ni-based superalloy at 650 °C
Grain size effects on the early plastic strain localization and slip transfer at grain boundaries were investigated for the Alloy 718 Ni-based superalloy at 650C. Three microstructures with different grain sizes underwent monotonic tensile tests at 650C, both in air and under vacuum, until rupture. All the microstructure variants exhibit fully intragranular fracture under vacuum and partially intergranular fracture in air. In this latter case, predominant intergranular fracture mode was found in the fine-grain microstructures. Interrupted tensile tests were also conducted under vacuum with ex-situ SEM high-resolution digital image correlation (HR-DIC) measurements to assess in-plane kinematics fields at the microstructure scale. Out-of-plane displacement jumps were obtained using laser scanning confocal microscopy. Both crystallographic slip within grains and near twin boundaries (TBs) and morphological sliding happening at grain boundaries (GBs) were documented. Statistical analysis of all plastic events aimed at quantifying strain localization distribution as a function of the microstructure. The fine-grain microstructure was found to have extensive strain localization at grain boundaries, while the coarse-grain microstructure is more prone to intragranular slip development and slip localization near TBs. Different scenarios of slip band/grain boundary interactions were evidenced.
more »
« less
- Award ID(s):
- 2338346
- PAR ID:
- 10568661
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Materials Science and Engineering: A
- Volume:
- 912
- Issue:
- C
- ISSN:
- 0921-5093
- Page Range / eLocation ID:
- 146927
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The deformation behavior of Ti-6Al-4V titanium alloy is significantly influenced by slip localized within crystallographic slip bands. Experimental observations reveal that intense slip bands in Ti-6Al-4V form at strains well below the macroscopic yield strain and may serially propagate across grain boundaries, resulting in long-range localization that percolates through the microstructure. These connected, localized slip bands serve as potential sites for crack initiation. Although slip localization in Ti-6Al-4V is known to be influenced by various factors, an investigation of optimal microstructures that limit localization remains lacking. In this work, we develop a novel strategy that integrates an explicit slip band crystal plasticity technique, graph networks, and neural network models to identify Ti-6Al-4V microstructures that reduce the propensity for strain localization. Simulations are conducted on a dataset of 3D polycrystals, each represented as a graph to account for grain neighborhood and connectivity. The results are then used to train neural network surrogate models that accurately predict localization-based properties of a polycrystal, given its microstructure. These properties include the ratio of slip accumulated in the band to that in the matrix, fraction of total applied strain accommodated by slip bands, and spatial connectivity of slip bands throughout the microstructure. The initial dataset is enriched by synthetic data generated by the surrogate models, and a grid search optimization is subsequently performed to find optimal microstructures. Describing a 3D polycrystal with only a few features and a combination of graph and neural network models offer robustness compared to the alternative approaches without compromising accuracy. We show that while each material property is optimized through a unique microstructure solution, elongated grain shape emerges as a recurring feature among all optimal microstructures. This finding suggests that designing microstructures with elongated grains could potentially mitigate strain localization without compromising strength.more » « less
-
In this work, molecular dynamics simulations to explore the crack propagation and fracture behavior of Cu/Nb metallic nanolayered composites (MNCs) were performed. The results of this study are consistent with the previous experimental results, which illustrated that cracks in Cu and Nb layers may exhibit different propagation paths and distances under the isostrain loading condition. The analysis reveals that the interface can increase the fracture resistance of the Nb layer in Cu/Nb MNCs by providing the dislocation sources to generate the plastic strain at the front of the crack. Increasing the layer thickness can enhance the fracture resistance of both Cu and Nb layers, as the critical stress for activating the dislocation motion decreases with the increment of the layer thickness. In addition, grain boundaries (GBs) in polycrystalline Cu/Nb samples would decrease the fracture resistance of Nb layer by promoting the crack propagate along the GBs, i.e., intergranular fracture, while the effect of interface and layer thickness on the fracture resistance of MNCs will not be altered by introducing the GBs in MNCs.more » « less
-
The softening effect of ultrasonic vibration on pure copper is studied from a new perspective with micro-tensile tests, where the gauge length of the specimen is one order of magnitude smaller than the ultrasonic wavelength. With this configuration, the amount of flow stress reduction increases linearly with vibration amplitude whereas the flow stress reduction is insensitive to the studied strain rate ranging from 0.06/s to 1/s. Temperature rise associated with ultrasonic vibration is minimal from infrared thermal imaging. In situ digital image correlation (DIC) analysis shows strain localization near ultrasonic source whereas uniform strain distribution was observed during conventional tensile test. Optical microstructure characterization shows that area fraction of annealing twins in the deformed copper reduced from 3.3% to 1.8% with ultrasonic vibration. This is possibly attributed to enhanced interaction of dislocation between twin boundaries which act as non-regenerative dislocation source. Electron backscatter diffraction (EBSD) results show that ultrasonic vibration promotes preferential grain re-orientation and reduces the misorientation within grains.more » « less
-
Abstract The influence of grain size distribution on ductile intergranular crack growth resistance is investigated using full-field microstructure-based finite element calculations and a simpler model based on discrete unit events and graph search. The finite element calculations are carried out for a plane strain slice with planar grains subjected to mode I small-scale yielding conditions. The finite element formulation accounts for finite deformations, and the constitutive relation models the loss of stress carrying capacity due to progressive void nucleation, growth, and coalescence. The discrete unit events are characterized by a set of finite element calculations for crack growth at a single-grain boundary junction. A directed graph of the connectivity of grain boundary junctions and the distances between them is used to create a directed graph in J-resistance space. For a specified grain boundary distribution, this enables crack growth resistance curves to be calculated for all possible crack paths. Crack growth resistance curves are calculated based on various path choice criteria and compared with the results of full-field finite element calculations of the initial boundary value problem. The effect of unimodal and bimodal grain size distributions on intergranular crack growth is considered. It is found that a significant increase in crack growth resistance is obtained if the difference in grain sizes in the bimodal grain size distribution is sufficiently large.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.msea.2024.146927
