查看更多>>摘要：Spatially gradient microstructures have shown a promising application in enhancing strengthductility synergy of engineering metals such as austenitic stainless steels. However, existing approaches are limiting in producing a thick gradient nanostructured (GNS) layer with a high strengthening capability, and the underlying deformation mechanisms are still not clear in GNS austenitic stainless steels. In this work, we developed a new approach, i.e., plate surface mechanical rolling treatment, to produce a bulk gradient nanostructure in a 304 stainless steel plate of -1.90 mm in thickness. Uniaxial tensile tests revealed that an ultra-high yield strength of -1073 MPa with a considerable uniform elongation of -21% was achieved in the GNS sample. Subsequently, the evolutions of microstructure, phase, microhardness, and local strain distribution were systematically studied in the GNS plate during tensile tests. The results demonstrated that the mechanical incompatibilities, relating with the gradient microstructure and martensiteenclosing-austenite domains, contribute to an extra strain-hardening capability, leading to the outstanding strength-ductility synergy in the GNS 304 stainless steel. Furthermore, analyses based on experimental observations and theoretical calculations revealed that dislocation activities, instead of deformation-induced martensite transformation, microstructure refinement, and twinning, play a dominant role in the strain-hardening mechanisms of the GNS plate during tension.
查看更多>>摘要：The evolution of local deformation and damage in commercially pure titanium was studied for flat un-notched, U-notch and V-notch tensile specimens with two distinct initial textures namely off-basal (BA) and prismatic-pyramidal (PP) orientations to establish the micro-mechanisms of anisotropic deformation and damage in titanium for different notch severity. BA orientation showed higher apparent yield stress, greater plastic deformation, better notch strength ratio (NSR) value than PP orientation for all the cases. Electron backscatter diffraction investigation showed an increase in the number of twins per grain and the thickness of twins with notch severity irrespective of orientation with BA samples showing higher twin activity than corresponding PP samples. In addition, extensive slip activity was observed within twins and parent grains in BA orientation unlike PP orientation that showed slip activity near grain boundaries. These microstructural observations correlated well with strain measurement from digital image correlation which also showed that the local maximum longitudinal strain for BA orientation increased with notch severity unlike PP orientation. Viscoplastic self-consistent crystal plasticity simulations at the mesoscale indicated that the activity of basal, pyramidal slip < c+a > systems and twinning increased at the expense of prismatic slip with an increase in notch severity for both the orientations. The experimental observations show extensive diffuse necking in BA orientation, while higher fraction of fluted voids with c-axis aligned along the major axis of the void are observed in the PP orientation. Prismatic slip promotes nucleation and growth of voids in the prism plane which is perpendicular to the tensile axis for PP orientation contributing to void growth along c-axis forming fluted voids, while higher twinning in BA orientation leads to fewer fluted voids, thus highlighting the significant role of crystallographic texture in controlling anisotropic deformation and damage mechanisms in titanium.
查看更多>>摘要：Large-scale molecular dynamics simulations are conducted on single crystal copper along eight representative orientations to investigate dislocation-dominated void nucleation during shock loading and spall failure, including mechanisms and anisotropy. Spall strength estimated from free surface velocity decreases in the order of group I ([001]), group IV ([012] and [011]), group III ([122] and [123]) and group II ([114], [112] and [111]), respectively, in good agreement with anisotropy of spall strength in previous experiments and simulations. A lower spall strength is statistically associated with a higher void nucleation rate and a higher density of stable immobile dislocations (1/6<110> and 1/3<100>) formed before void nucleation. Stable immobile dislocation plays a key role in void nucleation. The formation of stable immobile dislocations requires three conditions: high resolved shear stress for activating mobile dislocations, two or more main slip planes for dislocation reaction, and high angle between activated slip directions for high stability of immobile dislocations. Based on crystal elastic-plastic theory, resolved shear stress on all slip systems is calculated to analyze orientation effects on the three conditions. The three conditions can be fulfilled by group II orientations, but cannot by group I, III, and IV orientations, leading to anisotropy in void nucleation and spall strength.
查看更多>>摘要：Because typical solder joints undergo thermal aging during use, models simulating the behaviors of solder under mechanical stress must accurately reflect the changes in properties caused by thermal aging effects. The uniaxial deformation of SAC305 (96.5Sn-3.0Ag-0.5Cu wt.%) solder under different aging conditions indicates that the mechanical properties of solder are weakened with aging. In this study, the nanoindentation method was used to evaluate the effect of aging on the mechanical properties of the beta-Sn dendritic regions and eutectic regions of SAC305. Based on this evaluation, a dual-phase (DP) representative volume element (RVE) crystal plasticity finiteelement method (CPFEM) model was developed to simulate the thermal aging effects of SAC305 solder. In situ tensile tests performed with scanning electron microscopy (SEM) showed that the tensile deformation was concentrated in the beta-Sn dendritic regions of the solder. The DP CPFEM model was established based on the microstructures shown by electron backscatter diffraction (EBSD) analysis performed during the in situ tensile tests. This model was used to verify the accuracy of the DP CPFEM in simulating the deformation localization by comparison with the strain field distribution observed via SEM-digital image correlation (DIC) technology. The developed DP CPFEM model allows the prediction of the potential failure locations of a microscale solder joint.
查看更多>>摘要：The strain rate sensitivity (SRS) of binary Mg-Gd and Mg-Y alloys and the effect of solute on the rate-controlling mechanisms have been studied by analyzing rate changes during tension and compression tests at 78K and 298K. The steady-state SRS evaluated from the slope of Haasen plots at 78K increases with the solute concentration. The reverse behavior is observed at 298K, and the concentrated alloys exhibit the negative SRS. The activation work and the activation distance evaluated from strain rate jump tests at 78K discriminate regimes of plastic flow determined by solute-dislocation and dislocation-dislocation interactions. At the onset of plastic flow, dislocation-solute interactions control the plastic flow properties. This regime of low flow stress is represented by the activation work in the magnitude of 10(-2) eV and the activation distance in the range 10(-2)b -10(-1)b. The deformation by mechanical twinning occurring early in the compression conforms to these parameters and suggest no fundamental difference in the thermally activated glide of ordinary and twinning dislocations through the field of solute obstacles. The results suggest that by-passing of Gd and Y atoms by basal dislocations occurs by sequential activation of partial dislocations. At higher flow stress, the dislocation-dislocation interactions determine the work-hardening. The thermally activated motion of jogs produced during interactions of basal and prismatic dislocations is thought to be the rate-controlling process. The activation work and the activation distance signatures of this process are W* similar to 0.4 eV - 0.8 eV and d similar to 0.5b - 1b, depending on the solute content.
查看更多>>摘要：In newly developed hydrogel-based devices, hydrogels are commonly used in load-bearing components subjected to prolonged cyclic deformations. The anti-fatigue capability of hydrogels is crucial for extending the service life of the devices. While recent developments in the synthesis and characterisation of tough hydrogels have facilitated continuous improvements of the fatigue resistance, the underlying mechanisms that dominate the fatigue fracture of hydrogels are still inconclusive. This work aims to model the complex constitutive response and predict the fatigue crack behaviour of hydrogels. The contribution of this work is twofold. (i) A physically-based poro-visco-hyperelastic model is developed within the framework of the Theory of Porous Media (TPM) at finite strains to describe the solid-fluid-coupled material behaviour of hydrogels. The fluid transport in hydrogels is governed by Darcy's law. The non equilibrium mechanical response induced by polymer chain rearrangement is considered by introducing internal variables based on a multiplicative decomposition of the solid deformation gradient tensor into elastic and inelastic parts. The time-dependent breaking/reforming kinetics of physical chains is described by a Bell model-based chain evolution law. (ii) An energy-based fatigue crack growth model is proposed to predict the fatigue crack growth of hydrogels. A volume averaging method is used to calculate the elastic energy density surrounding the crack tip as the driving force. In particular, the cyclic evolution of the averaged energy density due to the breaking/reforming kinetics of physical chains is incorporated in the crack growth model for hydrogels with physical networks. The computational results demonstrate that the predicted fatigue crack growth of different hydrogels reasonably agrees with experimental data. Moreover, the effects of fluid transport, time-dependent deformations and chain kinetics on the fatigue fracture behaviour of hydrogels are systematically analysed. The prediction results indicate that these time-dependent mechanisms cannot be ignored in modelling the fatigue behaviour of anti-fatigue hydrogels.
查看更多>>摘要：This paper presents a multi-scale modelling approach to investigate the underpinning mechanisms of microstructure-sensitive damage of single crystal Sn-3Ag-0.5Cu (wt%, SAC305) solder joints of a Ball Grid Array (BGA) board assembly subject to thermal cycling. The multi-scale scheme couples board-scale modelling at the continuum macro-scale and individual solder modelling at the crystal micro-scale. Systematic studies of tin crystal orientation and its role in fatigue damage have been compared to experimental observations. Crystallographic orientation is examined with respect to damage development, providing evidence-based optimal solder microstructural design for in-service thermomechanical fatigue.
查看更多>>摘要：The plastic deformation behavior of single crystals of Mo5Si3 with the tetragonal D8m structure has been investigated in compression as a function of crystal orientation and temperature (1200-1500??) in the bulk form and as a function of crystal orientation and specimen size at room temperature in the micropillar form. The slip system of {112}< 111 > is identified to be the only one that operates at high temperatures above 1200? in bulk crystals, while any plastic flow is not detected at room temperature in micropillar crystals. The critical resolved shear stress (CRSS) for {112}< 111 > slip at room temperature estimated from the extrapolation of the temperature dependence of CRSS obtained for bulk crystals is considerably higher than fracture stresses obtained for micropillar crystals, indicating that the room-temperature brittleness is due in principle to the difficulty in dislocation motion arising from the very high CRSS value. The value of fracture toughness evaluated with a chevron-notched micro-beam specimen with a notch plane parallel to (001) is 1.54 MPa.m(1/2) , which is comparable to those reported for other transition-metal (TM) silicides of the TM5Si3-type. The selection of {112} slip plane and the dissociation of the 1/2 < 111 > dislocation on the slip plane are discussed based on generalized stacking fault energy (GSFE) curves theoretically calculated by first-principles calculations.
查看更多>>摘要：Molecular dynamics (MD) simulations are employed to study the interaction mechanisms between dislocations and nano-precipitates in CuFe alloys. On one hand, the critical shear stress to activate nucleation of dislocations around alpha-Fe nano-precipitates is substantially reduced, which promotes twinning deformation and dislocation gliding away from the nano-precipitates. On the other hand, nano-precipitates could also prevent dislocations from moving towards themselves. A hybrid model is developed to demonstrate the strengthening behaviors with an enhancing efficiency factor (beta) introduced. Large alpha-Fe nano-precipitates lead to high strengthening efficiency, but an optimized size is obtained owing to the softening effect of large nano-precipitates. Moreover, the strengthening effects of nano-precipitates will disappear if the loading speed exceeds a critical value.
查看更多>>摘要：The mechanical behavior and microstructure at fracture of a rolled AZ31B magnesium alloy were experimentally investigated using tubular specimens subjected to combined axial-torsion loading with different ratios of the axial and shear stress components. The stress state influences the stress-strain responses. The equivalent stress-equivalent plastic strain curves under tension torsion loading show a sigmoidal shape and the peak value of the equivalent strain hardening rate increases as the ratio of the axial stress to the shear stress increases. Under compression torsion, the equivalent stress-equivalent plastic strain curves show a concave-down shape, and the equivalent strain hardening rate decreases at faster rates as the ratio of the axial stress to the shear stress increases. Among all the loading conditions investigated, pure tension and pure compression result in the highest strength, and torsion combined with a slight tension has the lowest strength. The trend of ductility is inversely proportional to the strength with respect to the influence of the combined axial-torsion loading. Detailed twinning structures reveal that extensive tension twins are induced under tension-torsion loading paths. Conversely, a combination of tension and compression twins is observed under compression-torsion loading due to the high Schmid factors of the compression twins resulted from the grains with favorable orientations. The significant effect of the stress state on the post-fracture texture is explained in terms of the twin variant favorability which is dominated by the crystal orientation relative to the orientation of the applied principal stresses. The experimental results are critical for the development and validation of constitutive models for magnesium alloys under multiaxial loading.
NSTL
Elsevier