Effect of stress state on deformation and fracture of nanocrystalline copper:Molecular dynamics simulation

Deformation in a microcomponent is often constrained by surrounding joined material making the component under mixed loading and multiple stress states. In this study, molecular dynamics （MD） simulation are conducted to probe the effect of stress states on the deformation and fracture of nanocrystalline Cu. Tensile strain is applied on a Cu single crystal, bicrystal and polycrystal respectively, under two different tension boundary conditions. Simulations are first conducted on the bicrystal and polycrystal models without lattice imperfection. The results reveal that, compared with the performance of simulation models under free boundary condition, the transverse stress caused by the constrained boundary condition leads to a much higher tensile stress and can severely limit the plastic deformation, which in return promotes cleavage fracture in the model. Simulations are then performed on Cu single crystal and polycrystal with an initial crack. Under constrained boundary condition, the crack tip propagates rapidly in the single crystal in a cleavage manner while the crack becomes blunting and extends along the grain boundaries in the polycrystal. Under free boundary condition, massive dislocation activities dominate the deformation mechanisms and the crack plays a little role in both single crystals and polycrystals.

A damping boundary condition for atomistic-continuum coupling

The minimization of spurious wave reflection is a challenge in multiscale coupling due to the difference of spatial resolution between atomistic and continuum regions. In this study, a new damping condition is presented for eliminating spurious wave reflection at the interface between atomistic and continuum regions. This damping method starts by a coarse–fine decomposition of the atomic velocity based on the bridging scale method. The fine scale velocity of the atoms in the damping region is reduced by applying nonlinear damping coefficients. The effectiveness of this damping method is verified by one-and two-dimensional simulations.

Optimization of pass schedule to improve the strip shape in hot strip rolling

In this study,the authors proposed a new algorism which can simultaneously meets the strip shape control criterion and the strip crown control criterion.Based on the new algorism,the pass schedule is optimized to maximize the allowable PC angle range.To calculate the objective value for optimization of pass schedule,a slab method has been integrated with the roll stack deformation to simulate the strip profile.

Tribological behavior comparisons of high chromium stainless and mild steels against high-speed steel and ceramics at high temperatures

High-temperature tribology,which is often involved during hot metal forming,is controlled via oxidation on a rubbing surface.However,for high chromium stainless steel(ST),where oxidation is strongly inhibited,the effect of counterface materials on tribological behavior is yet to be elucidated.In this study,the effects of counterfaces on the tribological behavior of 253MA ST and mild steel(MS)are investigated via a ball-on-disc test at 900°C using a 20 N load.The results reveal that high-speed steel(HSS)experiences severe abrasive wear with MS and causes severe sticking problems with ST.Si3N4 and SiC present substantially stronger abrasive wear resistance than HSS with MS,and the friction coefficients are dependent on the type of ceramic.Both ceramics can facilitate the establishment of a thick tribo-oxide layer(>3μm)on ST to prevent sticking;however,this is accompanied by severe pull-out and fracture wear.The effects of the counterface on the mechanical properties of the tribo-oxide layer,near-surface transformation,and the responses of the tribo-oxide layer to friction and wear are discussed.This study contributes to the understanding of interfacial tribological behaviors when different types of tools are used on MS and ST.

Water-in-Water Emulsions,Ultralow Interfacial Tension,and Biolubrication

A water-in-water(W/W)emulsion consists of droplets formed by the spontaneous liquid-liquid separation of two immiscible aqueous phases.The inherent properties of the W/W interfaces,low or ultralowinterfacial tension(γ_(W/W)=1-1000μN/m)and a large thickness of several nanometers,beget the poor inherent stability of emulsions.Herein,we report a nanofibril emulsifier having Schiff base reactivity to generate a W/W emulsion.The W/W emulsion has superior stability(stable>6 months)because collagen nanofibrils,acting as a stabilizer of W/W emulsions,can simultaneously satisfy the requirements of sizeandoverall coverage ratio of the phase interfaces.W/Wemulsions having γ_(W/W)∼10μN/m were used as synthetic synovial fluids,showing superior lubrication performance with a coefficient of friction in the range of 0.003-0.011,which has been demonstrated to be suitable for joint lubrication.An intraarticular injection assessment further confirmed this protective effect on articular cartilage in vivo.Our study reveals the mechanism of emulsion stabilization and opens up the possibility of osteoarthritis(OA)treatment using the biolubrication effects of W/W emulsions for lubricated artificial implant surfaces.

Correlation Between Crystal Rotation and Redundant Shear Strain in Rolled Single Crystals: A Crystal Plasticity FE Analysis

The correlation between crystal rotation and redundant shear strain in rolled single crystals was investigated by using the crystal plasticity finite element(CPFE) model in this paper. The deformation in aluminium single crystals of four representative orientations(rotated-Cube, Goss, Copper, and Brass) after rolling and plain strain compression was simulated, and the predictions have been validated by the experimental observations. In the rotated-Cube and Goss, the redundant shear strain and crystal rotation were in the same pattern, alternating along the thickness, while the relation between them was not obvious for the Copper and Brass due to their asymmetrical distributions of activated slip systems. The relations between slip system activation, crystal rotation, and shear strain were investigated based on the CPFE model, and the correlation between shear strain and crystal rotation has been built.

A crystal plasticity FE study of macro-and micro-subdivision in aluminium single crystals{001}<110>multi-pass rolled to a high reduction

In this study,the substructure formation in a multi-pass rolled aluminium single crystal{001}<110>was investigated by the crystal plasticity finite element model,and the predations were validated by experimental observations at both macro-and micro-scale.A finite element model for multi-pass rolling was developed to follow the real experimental rolling scheme,by which the through-thickness macroscopic subdivision was successfully predicted up to a 90%reduction.The macro-subdivision was featured by forming matrix bands through the thickness,and the deformation behaviours,in terms of slip activity,shear strain and crystal rotation,alternated between matrix bands.The development of matrix bands,stability of crystal orientations,and correlation between slip activity,shear strain and crystal rotation have been investigated.Another modelling method,Submodel,was used to exceedingly increase the mesh resolution in smaller regions of interest,and the experimentally observed microstructure,i.e.,micro-subdivision was explicitly and spatially revealed.Similar predictions were obtained in Submodels with different element sizes,which proves the feasibility of this method in predicting microstructure formation.It was found that the substructure formation by varying slip activity and crystal rotation between domains is energy favourable.The procedure of substructure formation was explained based on the predictions,three types of substructure have been identified,and the substructure formation was discussed.