Inapparent Strengthening Effect of Twin Interface in Cu/Pd Multilayered Films with a Large Lattice Mismatch

It has been found that there are two kinds of interfaces in a Cu/Pd multilayered film, namely, cube-on-cube and twin. However, the effects of the interfacial structure and modulation period on the mechanical properties of a Cu/Pd multilayered film remain unclear. In this work, molecular dynamics simulations of Cu/Pd multilayered film with different interfaces and modulation periods under in-plane tension are performed to investigate the effects of the interfacial structure and modulation period. The interface misfit dislocation net exhibits a periodic triangular distribution, while the residual internal stress can be released through the bending of dislocation lines. With the increase of the modulation period, the maximum stress shows an upward trend, while the flow stress declines. It was found that the maximum stress and flow stress of the sample with a cube-on-cube interface is higher than that of the sample with a twin interface, which is different from the traditional cognition. This unusual phenomenon is mainly attributed to the discontinuity and unevenness of the twin boundaries caused by the extremely severe lattice mismatch

Investigation of Interaction between Dislocation Loop and Coherent Twin Boundary in BCC Ta Film during Nanoindentation

In this work, the interaction between dislocation loop (DL) and coherent twin boundary (CTB) in a body-centered cubic (BCC) tantalum (Ta) film during nanoindentation was investigated with molecular dynamics (MD) simulation. The formation and propagation of <111> full DLs in the nanotwinned (nt) Ta film during the indentation was observed, and it was found that CTB can strongly affect the stress distribution in the Ta film, and thus change the motion and type of dislocations. There are three kinds of mechanisms for the interaction between DL and CTB in a twinned BCC Ta film: (i) dislocation absorption, (ii) dislocation desorption, and (iii) direct slip transmission. The nucleation of twin boundary dislocations and the formation of the steps in CTB were also observed during the indentation. The mechanisms presented in this work can provide atomic images for understanding the plastic deformation of BCC metals with mirror-symmetry grain boundary structures, and provide available information for the evaluation and design of high-performance nt BCC metallic thin film coatings

Anisotropic and asymmetric deformation mechanisms of nanolaminated graphene/Cu composites

We conducted molecular dynamics (MD) simulations of tension and compression along the direction and MD simulations of compression along the and directions on nanolaminated graphene/Cu (NGCu) composites to investigate the effects of the incorporated graphene and the deformation mechanisms related to the loading direction. The deformation behavior and the defect structures were found to be strongly dependent on the loading conditions. An asymmetric tension-compression deformation behavior was thus found in graphene/Cu nanolaminates under the loading, which was dominated by stacking faults and deformation twins formed by dislocation slide under tension and compression, respectively. High density and ordered nanotwins were formed at the graphene/Cu interfaces. Two different formation mechanisms of the twins were found under the compression, and the nucleated twins were easy to be thickened with the assistance of the graphene wrinkles. Multiple twins were formed under the compression by the dislocation cross-slip. This study provides a way to introduce graphene reinforcement and twin boundary to Cu matrix composites and design nanotwinned graphene/Cu composites with excellent mechanical performance

Design for the reduction of volume shrinkage-induced distortion in digital light processing 3D printing

Due to its high printing resolution and fast printing speed, digital light processing (DLP) has become one of the most widely used additive manufacturing technologies. In a typical DLP printing, resin is photocured from liquid into solid accompanied by a large volume shrinkage, which often leads to shape distortion of printed structures. In this study, we investigated the volume shrinkage-induced distortion of DLP-printed parts by conducting experiments, theoretical modeling, and finite element analysis (FEA) simulations. Material property evolution coupled with volume shrinkage during photocuring was first modeled constitutively. The constitutive theory was then implemented into FEA simulations of the layer-by-layer DLP printing process to study the development of shape distortion due to volume shrinkage during printing. Experiments validated the efficiency of the proposed FEA simulations. FEA was further applied to help predict the shape distortion in printed microfluidic channels and overhanging structures where printing parameters for compensating for distortion can be designed based on FEA.H.J.Q., X.K, S.M and L.Y. acknowledge the support of AFOSR, USA grants (FA9550-19-1-0151 and FA9550-20-1-0306; Dr. B.-L. ‘‘Les’’ Lee, Program Manager

Mode-II interlaminar fracture toughness of GFRP/Al laminates improved by surface modified VGCF interleaves

Mode-II interlaminar fracture toughness of GFRP/Al (aluminum) laminates is improved by employing acid treatment to an Al plate and inserting surface modified vapor grown carbon fiber (S-VGCF) between the Al plate and a GFRP layer. End notched flexure (ENF) tests are carried out to investigate the effect of S-VGCF addition on the Mode-II interlaminar fracture toughness. The experimental results demonstrate that the critical load and Mode-II fracture toughness of the specimen with acid treatment of Al plate and 10 g/m2 addition of surface modified VGCF are enhanced by 135.51% and 425.16%, respectively, compared to those of corresponding pristine specimen. Due to the presence of the oxygen containing functional groups, the specimens with addition of S-VGCF possess much higher Mode-II fracture toughness compared to the specimens with addition of pristine VGCF. Laser scanning and scanning electron microscopies are employed to observe the fracture surface of specimens to reveal the improvement mechanisms. Moreover, numerical simulations are performed based on the cohesive zone model to verify the experimental results. The interfacial shear strength between the Al plate and the GFRP layer are also predicted by the numerical simulations

Strengthening effects of twin interface in Cu/Ni multilayer thin films – A\ud molecular dynamics study

Molecular dynamics (MD) simulations of Cu/Ni multilayers with coherent, semi-coherent and coherent twin interfaces\ud under tension at temperatures of 10 K, 100 K and 300 K are carried out to study the effects of the interfaces\ud on the overallmechanical behavior. The microstructure and evolution of defects are investigated in detail to\ud understand the strengthening mechanisms. It is found that all kinds of interfaces can act as potent impediments\ud to the motion of dislocations. However, only the coherent twin interface shows significant strengthening effects\ud via plastic deformation. The reason is attributed to the fact that the twin interface can absorb inclined dislocations\ud and decrease its density, and therefore strengthen the layered structures