Twin-twin interaction in an Mg-3Al-Zn1 magnesium alloy subjected to ultra-high strain rate deformation (~106/s) was investigated. The material was processed by laser shock peening and the ...microstructure at various depths was characterized by electron backscattered diffraction. The results show that different 101¯2 twin variants were activated in individual parent grains, and the interfaces between these twin variants present very abnormal morphologies. One variant can be totally surrounded by the other, forming isolated or disconnected islands which have the same crystallographic orientation. A mechanism was proposed to account for the abnormal twin-twin interaction.
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Achieving high strength and large ductility simultaneously has been a long-standing goal in Al alloy. In the present study, a superior combination of ultimate strength (354 ± 1 MPa) and elongation ...(34 ± 0.9%) are achieved in a fine-grained (4.7 μm) Al-6.5 Mg alloy prepared by high strain rate hot rolling (HSRR) at 400 °C. The ultimate strength can be further improved to 454 ± 2 MPa after the subsequent cold rolling (CR), accompanied by an acceptable good ductility of 9.2 ± 1.1%. The microstructural evolution is characterized by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It has been shown that the high ductility of HSRRed alloy is attributed primarily to the high content of solute Mg atoms, high fraction of fine grains with high-angle grain boundaries (HAGBs) and the weak texture, while the high strength is mainly due to the combination strengthening effect from strain hardening, grain refinement and high content of solute Mg atoms. This study provides a possible strategy to gain high strength and large ductility, i.e. obtaining fine grains by enhanced recrystallization resulted from high strain rate and high content of solute Mg atoms, and then followed by cold working hardening.
The effects of the rolling parameters (rolling temperature, strain rate and thickness reduction) on the dynamic recrystallization (DRX) behaviors of the direct-chill cast high Mg alloyed Al-Mg alloy ...(Al-9.2Mg-0.8Mn-0.2Zr-0.15Ti) are carefully investigated. The as-rolled microstructures are characterized by optical microscope (OM), transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Strain rate plays a crucial role in enhancing DRX and the alloy experienced high strain rate rolling (HSRR) at 400 °C with a strain rate of 10 s−1 is featured with a fine-grained microstructure, with the DRX volume fraction of 95.4%, the average grain size of 2.63 μm and the fraction of high-angle grain boundaries (HAGBs) up to 91.9%. The relatively high fraction of HAGBs is associated with the full DRX related to the continuous DRX. The discontinuous DRX and the continuous DRX occur simultaneously in the high Mg alloyed Al-Mg alloy during HSRR and the strain-induced deformation bands (DBs) plays an important role in enhancing the DRX process.
The unique combination of very large strains, high temperatures and high strain rates inherent to friction stir welding (FSW) and friction stir processing (FSP) and their dependency on the processing ...parameters provides an opportunity to tailor the microstructure, and hence the performance of welds and surfaces to an extent not possible with fusion processes. While a great deal of attention has previously been focused on the FSW parameters and their effect on weld quality and joint performance, here the focus is on developing a comprehensive understanding of the fundamentals of the microstructural evolution during FSW/P. Through a consideration of the mechanisms underlying the development of grain structures and textures, phases, phase transformations and precipitation, microstructural control across a very wide range of similar and dissimilar material joints is examined. In particular, when considering the joining of dissimilar metals and alloys, special attention is focused on the control and dispersion of deleterious intermetallic compounds. Similarly, we consider how FSP can be used to locally refine the microstructure as well as provide an opportunity to form metal matrix composites (MMCs) for near surface reinforcement. Finally, the current gaps in our knowledge are considered in the context of the future outlook for FSW/P.
Quenching and partitioning (Q&P) steels possess high strength and good ductility because of the transformation of metastable austenite to martensite, which is referred to transformation-induced ...plasticity (TRIP) effect. In literature, TRIP effect generally results in an enhancement of work hardening rate during tensile test. Nevertheless, the present work observes an abnormal TRIP effect in a 1500 MPa Q&P steel. Although a considerable amount of retained austenite transformed to martensite during the tensile test at a strain rate of 1000 s − 1, no obvious enhancement of work hardening rate was observed. To explore the underlying mechanisms for such an abnormal TRIP effect, the evolution of dislocation density and martensitic transformation were characterized by synchrotron X-ray diffraction and electron microscopy. Comparing the quasi-static and high-strain-rate results, it is found that the dislocation density in the martensite matrix is suppressed at 1000 s − 1, resulting in a lower work hardening. Furthermore, the transformed martensite deforms plastically at 1000 s − 1. Without the composite-like deformation behavior (elastically in hard transformed martensite and plastically in soft martensite matrix), the corresponding work hardening is reduced.
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This study details an investigation about the formation and development of adiabatic shearing bands (ASB) in 7003-T4 aluminum alloy under high strain rate impacting. The influence of the strain and ...strain rate on the evolution of the ASB were investigated through dynamic impact tests performed on a split Hopkinson pressure bar (SHPB) system. The results show that, during high strain rate impacting, the local temperature rising of the sample, which results in the thermal softening overcomes the strain rate hardening and strain hardening, is the key factor for the formation of ASB. The threshold of the strain rate and strain for the formation of the ASB is 1353 s−1 and 0.17 respectively. The microstructure of the ASB is mainly comprised of elongated grains and incomplete dislocation cells. As the strain increased further, the temperature of the ASB exceeded the recrystallization temperature. It caused the microstructure of the ASB changed into equiaxed recrystallization grains, when the strain rate and strain was increased to 2535 s−1 and 0.32 respectively. This study also reveals that the strain is an important parameter for the shape development of the ASB. When impacted with strain rate of 2535 s−1, the ASB appeared as the strain reached 0.17 with an appearance of straight line in the 45° direction of the samples. As the strain further increased upon 0.28, it changed into a parabolic shearing band in shape.
Dynamic compression experiments of fine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy were measured by the split-Hopkinson pressure bar test at the strain rates in the range 1000–2000 s−1 and the temperature ...range 293–573 K along the transverse direction. The microstructure of the alloy was characterized by electron back-scattering diffraction and transmission electron microscopy. The results showed that the deformation hardening mechanisms dominated by pyramidal <c + a> slip and assisted by many mechanisms such as tension twinning, while the deformation softening mechanism just dominated by a partial dynamic recrystallization at the grain boundaries. During the entire deformation process at different temperatures, softening was found as the only accompanying mechanism of hardening. Even at 573 K, the fully recrystallized structure was not achieved, and the hardening mechanism was always dominant until they tend to balance. Based on these deformation mechanisms, especially the continuous attenuation mechanism of dynamic recrystallization softening associated with hardening, the Johnson–Cook model was modified, and a unified constitutive equation for deformation under high strain rate at different temperatures was constructed. The resulted obtained by this equation were in good agreement with the experimental results.
•EBSD and TEM were used to analyze the deformation mechanism of Mg–7Gd–5Y–1.2Nd–0.5Zr alloy.•The deformation hardening mechanisms were dominated by pyramidal < c + a > slip and assisted by many mechanisms such as tension twinning, while the deformation softening mechanism was just dominated by dynamic recrystallization.•The Johnson–Cook model was modified, which was in good agreement with the measured results.
•A dislocation-based crystal plasticity model with DRX and thermal softening is developed to predict shear localization at high strain rates.•DRX dominates the softening overtaking the thermal ...softening, promoting the formation of shear localization.•Both the grain refinement and the reduction of dislocation density appear in the process of DRX.•The formation of shear localization at high-strain rates significantly depends on the loading mode and texture.
Shear localization is an important failure mode, or even the dominant mode in metals at high-strain rates. However, it is a great challenge to accurately predict the occurrence and evolution of shear localization in metals at the high-strain rate deformation. Here, a dislocation-based crystal plasticity constitutive model with a crucial mechanism of shear instability, namely dynamic recrystallization, was developed. The evolution equations of dislocation density and grain size in the process of dynamic recrystallization were proposed and incorporated into the new constitutive model. The threshold of the stored energy in crystals was used as the criterion for the occurrence of dynamic recrystallization. Dynamic compression of a nanograin Cu-Al alloy was performed using the crystal plasticity finite element method based on the new constitutive model, and good agreement of the numerical prediction with the existing experimental data validates the new constitutive model. The results show dynamic recrystallization can be a more dominant mechanism for the occurrence of shear instability than thermal softening. In addition, dynamic tension and shear of the Cu-Al alloys with five typical textures were also simulated, showing that both loading mode and texture can significantly affect the formation of shear localization. This work is helpful for us to understand the role of microstructural evolution in the formation of shear localization at high-strain rates and to design the microstructure for artificially controlling or preventing the formation of shear localization.
•Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time.•The strain rates up to 10,000 s−1 were achieved with ...gas powder gun, where the shock deformation mode is predominant.•Shock deformation mode results in stiffness increases in comparison to the quasi-static response.•Validated computational models were used for critical strain rate analysis, determination of critical loading velocities and analysis of deformation modes together with analytical constitutive crushing models of cellular structures.
Auxetic cellular structures build from inverted tetrapods were experimentally tested at high strain rate compression loading for the first time. The strain rates up to 10,000 s−1 were achieved with gas powder gun, where the shock deformation mode is predominant. The deformation localizes in the deformation front between the impacting specimen and the fixed plate due to the inertia effects. This deformation mode results in stiffness increases in comparison to the quasi-static response. The results from experimental testing were used for validation of developed computational models in finite element explicit code LS-DYNA. Furthermore, the validated computational models were used for critical strain rate analysis, determination of critical loading velocities and analysis of deformation modes together with analytical constitutive crushing models of cellular structures.
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