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Nanoindentation was performed on individual grains of a polycrystalline Mg sample with c-axis declination angles ranging from parallel (0°) to perpendicular (90°) to the c-axis. Hardness was highest at ∼0°, decreased up to ∼55°, and then increased at ∼90° to an intermediate level. At ∼0°, high-density 〈c + a〉 dislocations extended deep into the crystal, contributing to high hardness. At ∼55°, 〈c + a〉 dislocations were confined near the indent, and occasional extension twinning reoriented the crystal to ∼45°, promoting 〈a〉 slip in both matrix and twin, leading to low hardness. At ∼90°, extension twinning reoriented the crystal to ∼0°, inducing texture hardening and intermediate hardness. Despite the complex stress state in nanoindentation, which fundamentally differs from the uniaxial stress in bulk tensile and compression tests, the combined contributions of dislocation and twinning still give rise to measurable hardness anisotropy, suggesting nanoindentation as a high-throughput technique for probing orientation-dependent mechanical behavior in Mg.
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In situ TEM observations and molecular dynamics simulations of deformation defect activities in Mg via nanoindentation
In this work, we performed in situ nanoindentation in TEM to capture the real-time 〈c + a〉 dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of 〈c + a〉 dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both 〈c + a〉 dislocation retraction and detwinning. Moreover, we note that the plastic zone comprised of 〈c + a〉 dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that, in addition to 〈a〉 dislocations, the I1 stacking fault bounded with a 〈1/2c+p〉 Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant 〈c + a〉 dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of 〈c + a〉 dislocation slip and twinning in Mg and alloys.
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- Award ID(s):
- 2144973
- PAR ID:
- 10503858
- Publisher / Repository:
- KeAi
- Date Published:
- Journal Name:
- Journal of Magnesium and Alloys
- Volume:
- 11
- Issue:
- 12
- ISSN:
- 2213-9567
- Page Range / eLocation ID:
- 4513 to 4524
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jma.2023.08.013
