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To figure out the effect of kink-like defects on twin boundaries, BCC Ta with three different kinds of microstructures: single crystal (SC), perfect twin boundaries (PTB) and defective twin boundaries (DTB), are studied under nanoindentation. Both indenter load and hardness in PTB are enhanced by twin boundaries but reduced in DTB due to intrinsic kink-like defects. According to atomic surface morphology, the relative sink-in depth in DTB is lower than that in PTB but higher than that in SC. By comparing atomic images of prismatic loops in three different stages, it can be found that kink-like defects in DTB perform two opposite functions: acceleration and delay. In initial plastic stage, defective twin boundaries can accelerate pinching-off event and nucleation. In transmission stage, dislocation backward emission is advanced by defective twin boundaries. In obstruction stage, the symmetry of coherent twin boundaries will be broken and asymmetrical shear loops are formed, which will delay prismatic loops moving across twin boundaries. Dislocations of 1 2 〈 111 〉 are the main factor of nanoindentation in [ 1 ¯ 1 ¯ 2 ] direction. Kink-like defects will lead to less drops in DTB on dislocation densities than that in PTB. These findings are helpful to reveal the influence of intrinsic kink-like defects in BCC nanotwinned materials. [Display omitted] • Nanoindentation on nanotwinned Ta with intrinsic kink-like defects is studied. • Kink-like defects prevent twinning partial slip on the top twin boundary. • Prismatic loops will be either accelerated or delayed by kink-like defects. [ABSTRACT FROM AUTHOR]

A new method is proposed to perform the multiscale simulation of nanoindentation with wedge indenter. We designed a potential field to model the wedge indenter based on the repulsive force-field approach which was used to model round indenters. A smooth curve representing the indenter tip is used to avoid the singularity of an ideal wedge indenter. Simulation with our approach correctly shows essential features of wedge nanoindentation. The load responses during retraction is confirmed by elastic model based on the Oliver-Pharr approach. Significant indentation size effect that the contact hardness decreases with the increase of contact depth is compared with existing models. The atomic configurations of the film shows that perfect dislocations nucleate under the indenter with Burges vectors perpendicular to the indentation direction. The strain distribution under the indenter shows that the length of plastic zone in the indentation direction is much larger than the contact width. [ABSTRACT FROM AUTHOR]

Abstract: Anisotropic plastic deformation beneath surface step during nanoindentation process is investigated applying the quasicontinuum (QC) method. Microscopic mechanisms of effects of surface step on FCC Al , (111), and (001) planes under the action of a cylindrical indenter are systematically studied. For steps on plane, dislocations nucleation and emission beneath surface steps is the main plastic deformation characteristic; for steps on (111) plane, the crystal lattice beneath the step undergoes severe distortion and rotation; for steps on (001) plane, deformation twinning and dislocation lock can be observed beneath the surface step. Load–displacement responses of various orientations and roughness are discussed in detail. The concept of geometrically necessary displacement is proposed to describe step flattening process. Strain energy for nanoindentation on stepped and flat surfaces under various crystal orientations is analyzed and compared to further investigate the effect of crystal anisotropic plastic deformation. The description about the atomistic behavior beneath surface step and the discussion about the anisotropic plastic deformation of surface step are meaningful for understanding the influence of surface step on the nanoindentation tests. [Copyright &y& Elsevier]

Abstract: Nanoindentation simulations on flat and stepped surfaces are respectively investigated using the quasicontinuum method based on the embedded-atom method potential. Effect of surface step considering indenter size and step height is studied. Results show that the critical load for the first dislocation emission will be decreased with the increase of step height. However, the effect of surface step will be weakened if the indenter size continues to increase. Initial atomistic structures after dislocation nucleation and emission are discussed systematically. The initial dislocations are not quite identically nucleated under the stepped surface. Stress distribution analysis reveals that the shear stress in the slip planes close to the step is much larger than the shear stress in the slip planes far from the step for nanoindentation on the stepped surface. The multiscale simulation results are consistent with experimental results and analytic solutions. The conclusions about step effect considering indenter size and step height are helpful for understanding the microscopic mechanism of nanoindentation tests on thin films with surface step. [Copyright &y& Elsevier]

The quasicontinuum method has been applied to probe the thin film with surface multi-defects, which is commonly seen in nanoimprint technique and bulk micromachining. Three unilaterally distributed multi-defect models and six bilaterally distributed multi-defect models of Pt thin film have been carried out in nanoindentation. The results show that the nanohardness gradually decreases as the number of unilaterally distributed multi-defects increases, along with the increasingly low decline rate of the nanohardness. The synergy effect of the unilaterally distributed multi-defects has been highly evidenced by the critical load revision for dislocation emission of Pt thin film, and it is predicted into a universal form with the synergy coefficient among the existing multi-defects for FCC metals. Moreover, the nanohardness obviously increases when the bilaterally distributed multi-defects form into symmetrical couple, and it could be even greater than the one with defect-free surface, due to the symmetry-induced enhancement effect on nanohardness. The symmetry-induced enhancement coefficient has been brought out and has well explained the symmetry-induced enhancement effect of bilaterally distributed multi-defects on the nanohardness by a prediction formula. Furthermore, the characteristic length of symmetric relations has been brought out to calculate the symmetry-induced enhancement coefficient and it has been effectively predicted to equal to the sum of the adjacent distance between the surface defect and the indenter, the defect depth near the indenter, and the defect width for FCC metal. [ABSTRACT FROM AUTHOR]

In order to study the effects of kink-like defects in twin boundaries on deformation mechanisms and interaction between dislocations and defects in twin boundaries under localized load, nanotwinned Cu with two defective twin (TDT) boundaries is compared with the nanotwinned Cu with two perfect twin (TPT) boundaries, and nanotwinned Cu with single defective twin (SDT) boundary and single perfect twin boundary by simulating spherical nanoindentations using molecular mechanics. The indenter force-depth and hardness-contact strain responses were analyzed. Results show that the existence of intrinsic defects in twin boundary could reduce the critical load and critical hardness of nanotwinned material. A quantitative parameter was first proposed to evaluate the degree of surface atom accumulation around the indenter during nanoindentation, and it can be inferred that the surface morphology in TDT changes more frequently than the surface morphologies in TPT and SDT. The atomistic configurations of incipient plastic structures of three different models were also analyzed. We found that the intrinsic defects in twin boundary will affect the incipient plastic structures. The formation of twinning partial slip on the defective twin boundary happens before the contact of the dislocation and twin boundary. The kink-like defects could introduce Frank partial dislocation to the twin boundary during interaction between dislocation and twin boundary, which was not detected on the perfect twin boundary. In addition, the area of twinning partial slips on the upper twin boundary in the incipient plastic structures in SDT and TDT are larger than the twinning partial slip area in TPT, which results in the reduction of the critical hardness in SDT and TDT. The kink-like defects could also block the expansion of twinning partial slip on the twin boundary. Furthermore, we investigated the dislocation transmission processes in three different models. It is found that the dislocation transmission event could be delayed in model containing single defective twin boundary, while the transmission process could be advanced in model containing two consecutive defective twin boundaries. The quantitative analysis of dislocation length was also implemented. Result shows that the main emitted dislocation during nanoindentation is Shockley partial, and the dislocation nucleation in SDT and TDT is earlier than the dislocation nucleation in TPT due to the existence of defects. It is inferred that the intrinsic defects on twin boundaries could enhance the interaction between dislocations and twin boundaries, and could strongly change the structure evolution and promote the dislocation nucleation and emission. These findings about kink-like defects in twin boundaries show that the inherent kink-like defects play a crucial role in the deformation mechanisms and it should be taken into consideration in future investigations. Single defective twin boundary structure is recommended to delay the transmission and block the expansion of twin boundary migration. Some of the results are in good agreement with experiments. [ABSTRACT FROM AUTHOR]

Abstract: Multiscale simulations using the quasicontinuum (QC) method with the embedded-atom method (EAM) potential are performed to examine the mechanical response of Cu–Ag bilayer film during nanoindentation tests. An attempt is made, from the viewpoint of collective interaction among misfit dislocations on the interface, to account for the strengthening and weakening mechanisms of interface on Cu–Ag bilayer film. The details of misfit dislocation nucleation, motion and collective interaction on Cu/Ag and Ag/Cu interfaces are discussed systematically, respectively. The investigation shows that the property and performance of Cu–Ag bilayer film mainly depend on the mechanical property of upper film. Both the strengthening and weakening effects are closely related to the collective interaction among misfit dislocations on the interface. Due to the pinning effect of interface on misfit dislocation, both the local interface migration and the voids can be observed at the core region of misfit dislocations. For nanoindentation on Cu/Ag bilayer film, the plastic deformation is localized chiefly in the lower Ag substrate and the void will disappear with the redistribution of misfit dislocations, which indicate that there are distinct protective and strengthening effects of the upper Cu film on the lower Ag substrate. While, for nanoindentation on Ag/Cu bilayer film, both the upper Ag film and the lower Cu substrate experience plastic deformation and the voids will not disappear, which imply that there are an obvious weakening effect of the upper Ag film on the lower Cu substrate. In addition, the multiscale simulation results are consistent with the experimental results. [Copyright &y& Elsevier]

Abstract: The quasi-continuum multiscale method (QC) is applied to investigate position effect of cylindrical indenter on nanoindentation into Cu thin film. Load–displacement responses reflect that indenter position influences the critical load and critical displacement distinctly. The microscopic deformation mechanism shows that it is the retarding of dislocation nucleation beneath the indenter that actually leads to a relative lower critical load. Once the plastic deformation is retarded, the load–displacement curve will undulate several times until dislocations nucleate at the retarded region. It is found that the critical loads periodically change with indenter positions. The period equals to the distance between two adjacent atomic planes in [111] direction. An improved elastic model is proposed to predict the critical load in consideration of the effect of indenter position. The agreement between QC simulation and the present model has shown the effectiveness of the improved model. [Copyright &y& Elsevier]