In our previous study, we observed a lack of $\left\{ {10\bar{1}2} \right\}$ twinning in a deformed Mg–Y alloy, which contributed to the observed yield “symmetry.” However, the effects of texture and grain size on polycrystalline deformation made it difficult to fully understand why twinning was not active. Therefore, we report herein in-depth study by in situ transmission electron microscopy, i.e., in situ TEM. The in situ deformation of nano-sized Mg–Y pillars revealed that prismatic slip was favored over twinning, namely, the critical stress required to activate prismatic slip was lower than that for twinning. This finding diametrically differs from that reported in other nano/micro-pillar deformation studies, where twinning is always the dominant deformation mechanism. By measuring the critical stresses for basal, prismatic, and pyramidal slip systems, this in situ TEM study also sheds light on the effects of the alloying element Y on reducing the intrinsic plastic anisotropy in the Mg matrix.
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A crystal plasticity investigation of grain size-texture interaction in magnesium alloys
This work investigates the microstructure-property linkages in magnesium (Mg) with an emphasis on understanding interaction effects between the grain size, texture, and loading orientation. A single crys- tal plasticity framework endowed with experimentally informed micro Hall-Petch type relations for the activation thresholds for slip and twinning is adopted to resolve polycrystalline microstructures over a broad texture-grain size space. The macroscopic trends from the simulations corroborate with experi- ments. The synergistic effects of microstructural engineering on the micromechanical characteristics are mapped, which reveal their role in the emergent macroscopic behaviors. The simulations predict reduced extension twinning with grain size refinement even though the micro Hall-Petch coefficient for twinning is smaller than that for the non-basal slip modes. While grain refinement and textural weakening gen- erally reduce the net plastic anisotropy and tension-compression asymmetry, the degree to which these macroscopic behaviors are tempered depends on the loading orientation. The results offer preliminary insight into the roles that texture and grain size may play in the damage behavior of engineered Mg microstructures.
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- Award ID(s):
- 1932976
- NSF-PAR ID:
- 10284675
- Date Published:
- Journal Name:
- Acta materialia
- Volume:
- 208
- ISSN:
- 1873-2453
- Page Range / eLocation ID:
- 116743
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED−9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED−9.4%specimens are similar, and they experience twinning exhaustion →
tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED−9.4%specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens. -
null (Ed.)Stress-strain responses and twinning characteristics are studied for a rolled AZ31B magnesium alloy under three different stress states: tension along the normal direction (NDT), compression along the rolled direction (RDC), and torsion about the normal direction (NDTOR) using companion specimens interrupted at incremental strain levels. Tension twinning is extensively induced in twinning-favorable NDT and RDC. All the six variants of tension twin are activated under NDT, whereas a maximum of four variants is activated under RDC. Under NDTOR, both tension twins and compression twins are activated at relatively large strains and twinning occurs in a small fraction of favored grains rather than in the majority of grains. Secondary and tertiary twins are observed in the favorably-orientated grains at high strain levels. Deformation under each stress state shows three stages of strain hardening rate: fast decrease (Stage I), sequential increase (Stage II), and progressive decrease (Stage III). The increase in the hardening rate, which is more significant under NDT and RDC as compared to NDTOR, is attributed to the hardening effect of twin boundaries and twinning texture-induced slip activities. The hardening effect of twin boundaries include the dynamic Hall-Petch hardening induced by the multiplication of twin boundaries (TBs) and twin-twin boundaries (TTBs) as well as the hardening effect associated with the energetically unfavorable TTB formation. When the applied plastic strain is larger than 0.05 under NDT and RDC, the tension twin volume fraction is higher than 50%. The twinning-induced texture leads to the activation of non-basal slips mainly in the twinned volume, i.e. prismatic slips under NDT and pyramidal slips under RDC. The low work hardening under NDTOR is due to the prevailing basal slips with reduced twinning activities under NDTOR.more » « less
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This study investigates the effect of microstructure, specifically the grain size and TiAlx impurity, on the compressive strength and hysteretic behavior of Ti2AlC at room temperature. Given the plate-like nature of the MAX phase grains, the length and thicknesses of over 100 grains for each microstructure were measured. A Hall-Petch like relationship between compressive strength and the grain length was observed, but not such a relationship was observed with the grain thickness. Results from cyclic compression testing in combination with resonant ultrasound spectroscopy show that room temperature mechanical response of Ti2AlC can be divided into four stress regions regardless of the variation in grain size and/or amount of impurities. The grain size effect on the transition stresses for stress regions was also investigated. It was found that all transition stresses, between the different stress regions, also follow different Hall-Petch-type relationships.more » « less
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ABSTRACT Mechanical stretching and topographical cues are both effective mechanical stimulations for regulating cell morphology, orientation, and behaviors. The competition of these two mechanical stimulations remains largely underexplored. Previous studies have suggested that a small cyclic mechanical strain is not able to reorient cells that have been pre‐aligned by relatively large linear microstructures, but can reorient those pre‐aligned by small linear micro/nanostructures if the characteristic dimension of these structures is below a certain threshold. Likewise, for micro/nanostructures with a given characteristic dimension, the strain must exceed a certain magnitude to overrule the topographic cues. There are however no in‐depth investigations of such “thresholds” due to the lack of close examination of dynamic cell orientation during and shortly after the mechanical loading. In this study, the time‐dependent combinatory effects of active and passive mechanical stimulations on cell orientation are investigated by developing a micromechanical stimulator. The results show that the cells pre‐aligned by linear micro/nanostructures can be altered by cyclic in‐plane strain, regardless of the structure size. During the loading, the micro/nanostructures can resist the reorientation effects by cyclic in‐plane strain while the resistive capability (measured by the mean orientation angle change and the reorientation speed) increases with the increasing characteristic dimension. The micro/nanostructures also can recover the cell orientation after the cessation of cyclic in‐plane strain, while the recovering capability increases with the characteristic dimension. The previously observed thresholds are largely dependent on the observation time points. In order to accurately evaluate the combinatory effects of the two mechanical stimulations, observations during the active loading with a short time interval or endpoint observations shortly after the loading are preferred. This study provides a microengineering solution to investigate the time‐dependent combinatory effects of the active and passive mechanical stimulations and is expected to enhance our understanding of cell responses to complex mechanical environments. Biotechnol. Bioeng. 2016;113: 2191–2201. © 2016 Wiley Periodicals, Inc.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1016/j.actamat.2021.116743
