Browsing by Author "Woo, Wanchuck"

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Dynamic Strain Evolution around a Crack Tip under Steady- and Overloaded-Fatigue Conditions
(2015-12-01)
Lee, Soo Yeol
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Huang, E-Wen
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Woo, Wanchuck
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Yoon, Cheol
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Chae, Hobyung
In-situ neutron diffraction studies on high-temperature deformation behavior in a CoCrFeMnNi high entropy alloy
(2015-07-01)
Woo, Wanchuck
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Huang, E-Wen
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Yeh, Jien-Wei
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Choo, Hahn
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Lee, Chi
An equiatomic, face-centered-cubic, CoCrFeMnNi high-entropy alloy was tensile tested at 800 K and 1,000 K with a nominal strain rate of 6.7 x 10(-6) s(-1). In addition to the macroscopic (bulk) behavior, mesoscopic (lattice) phenomena during the pseudo-creep deformation were investigated simultaneously using in-situ neutron diffraction. The evolutions of lattice strains, peak widths, and intensities of several hkl reflections suggest that the dominant deformation mode is the dislocation glide at 800 K and diffusion-controlled dislocation creep at 1,000 K. (C) 2015 Elsevier Ltd. All rights reserved.
Irreversible phase transformation in a CoCrFeMnNi high entropy alloy under hydrostatic compression
(2018-03-01)
Huang, E-Wen
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Lin, Chih-Ming
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Jain, Jayant
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Shieh, Sean R.
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Wang, Ching-Pao
An equal-molar CoCrFeMnNi, face-centered-cubic high-entropy alloy system is investigated using in-situ angular-dispersive X-ray diffraction under hydrostatic compression up to 20 GPa via diamond anvil cell. The evolutions of multiple diffraction peaks are collected simultaneously to elucidate the phase stability field. The results indicated that an irreversible phase transformation had occurred in the high entropy alloy upon decompression to ambient pressure. A reference material (n-type silicon-doped gallium arsenide) was investigated following the same protocol to demonstrate the different deformation mechanisms. It is suggested that the atomic bonding characteristics on the phase stability may play an important role in the high entropy alloys.
Neutron diffraction residual stress analysis during fatigue crack growth retardation of stainless steel
(2017-11-01)
Seo, Sukho
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Huang, E-Wen
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Woo, Wanchuck
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Lee, Soo Yeol
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材料科學與工程學系
After tensile overloading during fatigue crack growth, retardation, of the crack growth rate was significant. Neutron diffraction was employed to examine the evolution of crack-tip residual stress fields during constant-amplitude cyclic loading and during fatigue crack growth following the overload. It was found that the tensile overload induces larger compressive residual stress and zone size near the crack tip in the crack-growth and crack-opening direction. For the maximum crack growth retardation, the largest compressive residual stresses were measured in the region between an overloading point and the current propagating crack tip, for all three of the orthogonal directions. Such large compressive residual stresses in the crack-wake region are thought to reduce the crack tip driving force, thereby retarding the crack propagation rate significantly. Residual stress mapping was performed to examine the effect of the fatigue stress state on the residual stresses in the three different regions from the centerline to the surface, along the through-thickness direction in the compact-tension specimen. Much larger compressive residual stresses were measured at the surface than at mid-thickness. It is thought that larger compressive residual stresses at the surface result from the plane stress condition, resulting in a slower fatigue crack growth rate at the surface of the specimen. (C) 2017 Elsevier Ltd. All rights reserved.
On plastic anisotropy and deformation history-driven anelasticity of an extruded magnesium alloy
(2020-02-01)
Wang, Huai
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Lee, Soo Yeol
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Wang, Huamiao
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Woo, Wanchuck
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Huang, E-Wen
The plastic anisotropy and unloading anelasticity of an extruded Mg-9 wt.%Al alloy (with two annealed textures) were investigated using in-situ neutron diffraction measurement and crystal plasticity modelling. The distinctive yielding and hardening behaviors during compressive and tensile loading-unloading cycles were interpreted by the alternative operations of different slip/twinning modes. We reveal that the micromechanical responses of differently-oriented grains strongly depend on their neighboring conditions and operative deformation mechanisms. Based on the modelling results, unloading anelasticity is hypothesized to be controlled by the plastic contribution of the hard deformation mode and its impeding effects on easier deformation modes. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Plastic anisotropy and deformation-induced phase transformation of additive manufactured stainless steel
(2019-08-05)
Chae, Hobyung
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Huang, E-Wen
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Jain, Jayant
;
Wang, Huamiao
;
Woo, Wanchuck
Plastic anisotropy and deformation-induced phase transformation of additively manufactured (AM) stainless steels were investigated via in-situ neutron diffraction, electron backscatter diffraction, metallography, and fractography. Two types of tensile specimens were manufactured: (1) One sample was vertically fabricated with its tensile axis parallel to the z-direction (AM-V), (2) The other sample was horizontally fabricated with its tensile axis perpendicular to the z-direction (AM-H). A commercial 15-5PH stainless steel (CA) was used for comparison. AM steel revealed enhanced yield strength, tensile strength, and uniform elongation over CA, which was mainly due to grain refinement and transformation induced plasticity (TRIP). Different onsets of strain nonlinearity between AM-V and AM-H were closely related to martensitic phase transformation. Stresses estimated from lattice strains measured by neutron diffraction matched well with the applied stress-strain curves. After plastic deformation, voids were formed and congregated near the solidified line where fine grains were populated. Higher dislocation density was observed in the fine grain zone, and lower density was shown in the relatively coarse grain zone. AM steels exhibited significant anisotropic fracture behavior in terms of loading direction. In contrast to isotropic failure for CA and AM-V, AM-H revealed anisotropic failure with elliptical formation of the fracture feature. The fracture surface of AM-H possessed many secondary cracks propagating perpendicular to the building direction. The occurrence of secondary cracks in AM-H resulted in rapid load drop during tensile loading after necking.