More Like this

1. Influence of Microstructure and Strain Hardening Rate on Acoustic Nonlinearity Parameter in Stainless Steel 316L during Tensile Loading

   https://doi.org/10.32548/RS.2023.036  

   Do, Hyelim; Kim, Changgong; Matlack, Kathryn H (September 2023, The American Society for Nondestructive Testing Inc.)

   The accumulation of dislocations, which are atomic defects in materials subjected to plastic deformation, can cause structural failures. Early detection of such dislocation-related damage is essential to prevent these failures. The acoustic nonlinearity parameter β has been shown to be sensitive to the nonlinearity of dislocation motions, and prior research has shown a relationship between β and dislocation parameters in various damage mechanisms. While most work thus far reports that β generally increases with increased plastic deformation, recent research showed that β can decrease during monotonic tensile loading in stainless steel 316L characterized by in situ nonlinear ultrasonic measurements. The objective of this research is to examine the correlation between the decrease of β with plastic strain as reported in this recent study, and the initial microstructure and strain hardening rate. The initial microstructure, characterized with electron backscatter diffraction (EBSD), shows an increase in dislocation density and a reduction of grain area, which can possibly result in a decrease in β. Further, it is shown that the decrease rate of β monotonically decreases with hardening rate, providing a evidence that the decrease in β may relate to the shift from planar slip to wavy slip. These results help interpret the underlying mechanisms for the decrease in β during tensile loading. 

   more » « less
2. Study of the Thermomechanical Behavior of Single-Crystal and Polycrystal Copper

   https://doi.org/10.3390/met14091086  

   Kunda, Sudip; Schmelzer, Noah J; Pedgaonkar, Akhilesh; Rees, Jack E; Dunham, Samuel D; Lieou, Charles_K C; Langbaum, Justin_C M; Bronkhorst, Curt A (September 2024, Metals)

   This research paper presents an experimental, theoretical, and numerical study of the thermomechanical behavior of single-crystal and polycrystal copper under uniaxial stress compression loading at varying rates of deformation. The thermomechanical theory is based on a thermodynamically consistent framework for single-crystal face-centered cubic metals, and assumes that all plastic power is partitioned between stored energy due to dislocation structure evolution (configurational) and thermal (kinetic vibrational) energy. An expression for the Taylor–Quinney factor is proposed, which is a simple function of effective temperature and is allowed by second-law restrictions. This single-crystal model is used for the study of single- and polycrystal copper. New polycrystal thermomechanical experimental results are presented at varying strain rates. The temperature evolution on the surface of the polycrystal samples is measured using mounted thermocouples. Thermomechanical numerical single- and polycrystal simulations were performed for all experimental conditions ranging between 10−3 and 5 × 103 s−1. A Taylor homogenization model is used to represent polycrystal behavior. The numerical simulations of all conditions compare reasonable well with experimental results for both stress and temperature evolution. Given our lack of understanding of the mechanisms responsible for the coupling of dislocation glide and atomic vibration, this implies that the proposed theory is a reasonably accurate approximation of the single-crystal thermomechanics. 

   more » « less
3. In Situ Study of Twin Boundary Stability in Nanotwinned Copper Pillars under Different Strain Rates

   https://doi.org/10.3390/nano13010190  

   Chang, Shou-Yi; Huang, Yi-Chung; Lin, Shao-Yi; Lu, Chia-Ling; Chen, Chih; Dao, Ming (January 2023, Nanomaterials)

   The nanoscopic deformation of ⟨111⟩ nanotwinned copper nanopillars under strain rates between 10^−5/s and 5×10^−4/s was studied by using in situ transmission electron microscopy. The correlation among dislocation activity, twin boundary instability due to incoherent twin boundary migration and corresponding mechanical responses was investigated. Dislocations piled up in the nanotwinned copper, giving rise to significant hardening at relatively high strain rates of 3–5×10^−4/s. Lower strain rates resulted in detwinning and reduced hardening, while corresponding deformation mechanisms are proposed based on experimental results. At low/ultralow strain rates below 6×10^−5/s, dislocation activity almost ceased operating, but the migration of twin boundaries via the 1/4 ⟨10-1⟩ kink-like motion of atoms is suggested as the detwinning mechanism. At medium strain rates of 1–2×10^−4/s, detwinning was decelerated likely due to the interfered kink-like motion of atoms by activated partial dislocations, while dislocation climb may alternatively dominate detwinning. These results indicate that, even for the same nanoscale twin boundary spacing, different nanomechanical deformation mechanisms can operate at different strain rates. 

   more » « less
4. Laser Shock Peening Induced Back Stress Mitigation in Rolled Stainless Steel

   https://doi.org/10.1115/1.4052909  

   Over, Veronica; Lawrence Yao, Y. (June 2022, Journal of Manufacturing Science and Engineering)

   Abstract Laser shock peening (LSP) is investigated as a potential tool for reducing tensile back stress, shown here applied to rolled and annealed 304L austenitic steel. The back stress of treated and untreated dog-bone samples is extracted from hysteresis tensile testing. Electron back-scatter diffraction (EBSD) and orientation imaging microscopy (OIM) analysis quantify the geometrically necessary dislocation (GND) density distribution of unstrained and strained as well as unpeened and peened conditions. Finite element analysis (FEA) simulation models back stress and residual stress development through tensile testing and LSP treatment using known LSP pressure models and Ziegler's nonlinear kinematic hardening law. Nonlinear regression fitting of tensile testing stress–strain in as-received specimens extracts the kinematic hardening parameters that are used in numerical study. This research shows LSP may be used to overcome manufacturing design challenges presented by yield asymmetry due to back stress in rolled steel. 

   more » « less
5. Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions

   https://doi.org/10.1038/s41467-018-05027-5  

   Turnage, S. A.; Rajagopalan, M.; Darling, K. A.; Garg, P.; Kale, C.; Bazehhour, B. G.; Adlakha, I.; Hornbuckle, B. C.; Williams, C. L.; Peralta, P.; et al (July 2018, Nature Communications)

   Abstract Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically, ~10³ s⁻¹, resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (~1356 K). 

   more » « less