Incipient Plastic Deformation Research Articles

Comparative analysis of nanoindentation-induced incipient deformation of zirconium-based bulk metallic glass in various structural states

The incipient plastic deformation behaviour of as-cast and annealed samples of Zr50Cu40Al10 (atom %) bulk metallic glass was investigated by nanoindentation. A validated load-over-displacement versus displacement plot showed the existence of a precursor event to incipient plasticity in both samples. The activation stress remained consistent in both samples, indicating that the event corresponds to local behaviour in regions with the same energy state, and it is not influenced by the overall microstructure. Conversely, the first serration detected in the as-relaxed sample exhibited a greater magnitude, potentially arising from a distinct energy-dissipation rate related to larger-scale microstructural characteristics.

Atomic study on deformation behavior and anisotropy effect of 3C-SiC under nanoindentation

3C silicon carbide (3C-SiC) has great potential and value for innovated high-precision devices. However, the high hardness make it a daunting task to understand its deformation behavior and anisotropy effect, which impedes its processing and application. In this study, we performed molecular dynamic simulations of nanoindentation to study the deformation behavior and anisotropy effect of 3C-SiC. It is found that the elastic deformation and incipient plastic deformation are orientation-dependent, exhibiting fourfold symmetry, central symmetry and threefold symmetry while indenting on 3C-SiC (010), (110) and (111) surfaces. The dislocations nucleate and propagate due to the high shear stress induced by indentation, which in turn leads to the dissipation of shear stress in the sample. In the plastic stage, the dislocations evolve from embryonic dislocations to ribbon dislocations, and finally to prismatic dislocations. The prismatic dislocation loops are surrounded by stacking faults in {111} planes, most of which are enclosed by four stair-rod dislocations in <110> orientations. Moreover, we find that Thompson tetrahedron is suitable for understanding the anisotropy effect of ceramics with cubic diamond structure, and it is applied to investigate the formation and evolution of dislocations systematically in 3C-SiC. This research is meaningful for understanding the deformation behavior and anisotropy effect of 3C-SiC. The results may provide theoretical support for the processing and application of 3C-SiC.

Strain rate and anisotropic effects on incipient plastic deformation of Zn–Cu–Ti alloy sheets

Zinc alloys shows strong strain rate dependent and anisotropic mechanical behaviours, the mechanistic origin of which is however not fully understood. In this work, a combination of the experiments and crystal plasticity finite element simulations were used to gain an insight into the relation between deformation mechanisms, tensile deformation behaviour and texture evolution, especially for the incipient plastic deformation stage. To this end, specimens were cut from Zn–Cu–Ti alloy sheets at various orientations with respect to the rolling direction, and uniaxial tensile tests were conducted at different strain rates ranging from 10−3 to 5 × 10−2 s−1. The microstructure evolution with the deformation was examined by the electron backscatter diffraction technique. Crystal plasticity model was calibrated considering the effect of strain rate on the flow stress, and finite element simulations were carried out based on a representative volume element of the polycrystal zinc alloy. It is found out that the flow stress increases significantly, as the loading direction approaches the transverse direction, which results in a lower relative activity of basal slip and higher relative activity of non-basal ones. This, in turn, reduces the rotation of the c-axis of the crystallites towards ND during tensile tests. On the other hand, a positive strain rate sensitivity is found in the zinc alloy. Meanwhile, increasing strain rate leads to an obvious decrease in the activity of the pyramidal π-II slip systems, accompanied by more activated twinning systems. The lower activity of the pyramidal π-II slip at a higher strain rate probably hinders the development of CDRX and grain fragmentation at large strain levels.

Hydrogen-enhanced homogeneous dislocation nucleation of nanoindentation in nickel

Hydrogen-dislocation interactions are the essential problems to comprehend the multi-scale/multi-physics hydrogen embrittlement phenomena in metallic materials. For nano-scale materials, the initiation of dislocation nucleation marks the incipient plastic deformation, making the study of hydrogen’s role on nucleation processes pivotal. Nanoindentation experiments provide evidences of the hydrogen-reduced critical load, i.e., pop-in load, for homogeneous dislocation nucleation (HDN). However, there is no consensus to be reached on the underlying mechanism. Here, by directly calculating the activation energies of HDN through the nudged elastic method (NEB), we reveal that the hydrogen atoms would decrease the energy barrier of HDN and then reduce the corresponding pop-in load. Molecular dynamics simulations also capture the hydrogen-reduced critical resolved shear stress under varying hydrogen background fractions and temperatures, qualitatively verifying the theoretical prediction of NEB calculations. These findings at the atomistic scale will inform the understanding of hydrogen embrittlement, especially on the hydrogen effect on the dislocation multiplication, tangled dislocation, and dislocation wall.

Probing pre-serration deformation in Zr-based bulk metallic glass via nanoindentation testing

Bulk metallic glasses (BMG) are promising candidates for structural applications. However, they exhibit catastrophic failure, and the detection of their incipient plastic response remains unclear. In this study, we investigated the incipient plastic deformation behavior in Zr50Cu40Al10 at% bulk metallic glass via nanoindentation testing. An original and validated analytical layout, which is based on the load-over-displacement versus the displacement P/h-h plot, facilitates the analysis of any unstable deformation mode in the loading segment of the nanoindentation test. A certain anelastic deformation was detected definitely prior to the first serration that is conventionally recognized as a plasticity initiation. This result clearly indicates the presence of a precursor phenomenon to incipient plasticity before the first serration in BMG.

Quantifying chemical fluctuations around medium-range orders and its impact on dislocation interactions in equiatomic CrCoNi medium entropy alloy

Equiatomic multi-principal element alloys manifest unique mechanical properties derived from the single-phase solid solutions with compositional disorders. Recently discovered medium-range orders (MROs) with a size of 1–5 nm in CrCoNi medium entropy alloy acclaimed measurable impact on mechanical properties, which, however has been questioned by density functional theory calculations. Here we report that the formation of MROs is accompanied by the redistribution of constituent elements, and the subtle chemical composition fluctuations could by directly probed and quantified by using a state-of-the-art aberration-corrected transmission electron microscope. The presence of such MROs contributes to an appreciable increase in yield strength (∼40 MPa) with a higher work hardening rate, originating from the strong dislocation interactions with MROs at the incipient plastic deformation. These findings demonstrate that MROs have a significant impact on reducing the mean free path of full/partial dislocations along a specific slipping plane, offering a new avenue for strengthening of equiatomic element alloys by tuning local composition and atomic configurations.

Basal dislocations in MAX phases: Core structure and mobility

The MAX phases, layered ternary carbides and nitrides, plastically deform via exclusively basal slip systems. This constraint leads to their well-documented plastic anisotropy and deformation mechanisms such as kinking. Basal dislocations govern the incipient plastic deformation, and therefore understanding them is a key step to utilizing the unique properties of MAX phases. As such, we performed the first fully atomistic calculations of the core structures and mobilities of basal dislocations using a bond-order interatomic potential. Calculations show that the weakly bonded A-layer can have a large impact on core structure, with several configurations exhibiting non-planar cores and being an order of magnitude less mobile than their planar counterparts. Additionally, due to the unique stacking sequence and stacking faults in MAX phases, basal dislocations preferentially nucleate and glide as same-signed pairs on adjacent slip planes. These pairs dissociate as zonal dislocations over much larger distances, enabling greater core spreading and therefore increased mobility. Edge pairs have Peierls stresses more closely resembling those in metals than in ceramics, suggesting they control the incipient plasticity of MAX phases at lower temperatures. Overall, these results provide a foundation for better understanding MAX phase incipient plasticity through basal dislocations and thus enabling them to be better engineered for specific applications.

Tension-compression mechanical behavior and corresponding microstructure evolution of cast A356-T6 aluminum alloy

It is crucial to clarify the intrinsic deformation and damage behaviors of casting materials under different loading conditions for establishing the material damage model and accurately predicting the security performance of cast components. In this study, uniaxial tensile and compressive tests were performed to investigate the complete mechanical behavior of cast A356-T6 aluminum alloy at room temperature, and the corresponding microstructure evolution were analyzed. The tensile and compressive specimens were prepared at porosity-free locations on a low pressure die cast A356 wheel subjected to T6 heat treatment to avoid premature fracture caused by initial cast pores. The results show that negligible asymmetry in yielding and hardening behaviors is observed between tension and compression, but obvious differences in ductility exhibit. Specifically, the elongation for tension is limited to 15%, but reaches over a strain of 1.2 for compression. Additionally, microcracks during tension are prone to be initiated at the interface between Si and Al matrix, which mainly propagates in the Al dendrites, and finally leads to the fracture. While the dominant strain accommodated mechanism shifts with strain levels in compression. It evolves from the incipient plastic deformation inside grains at initial strain stage to the sliding of fragmented dendrite assisted by eutectic silicon particles at later stage, which may be the main reason for the superior ductility during compression.

Nanoscale to microscale reversal in room-temperature plasticity in SrTiO3 by tuning defect concentration

Incipient room-temperature plastic deformation in single-crystal SrTiO3 at the nanoscale and the microscale is contrasted by applying a scale-dependent indentation technique. Using nanoindentation pop-in, nanoindentation creep, and an evaluation of dislocation spacing via etch pit study, we demonstrate a reversal of yield stress for crystals with different vacancy concentrations and pre-existing dislocation densities. A competing mechanism between dislocation nucleation and dislocation motion on crystal plasticity at different length scales is highlighted. This finding enables us to complete the understanding of dislocation-mediated plasticity for ceramic oxides at the nanoscale as compared to the microscale and macroscale.

The effect of grain boundary on the local incipient plastic deformation of fcc metals during nanoindentation

The effect of grain boundaries (GBs) on deformation mechanisms becomes increasingly important as the volume of deformation reaches the submicrometer and nanoscale. The current work investigates the impact of grain boundaries on the incipient plasticity of small-scale deformations of fcc metals. For this purpose, the behavior of single and bi-crystal Au thin films during nanoindentation are studied, using large-scale atomistic simulations. Various symmetric ⟨110⟩ tilt GBs with a wide range of misorientation angles are included to analyze the effect of GB geometry on the nanoscale plasticity mechanisms. Potentially, GBs can act as a source, sink, or obstacle for lattice dislocation, depending on their geometry, energy level, and distance from the deformation zone. The role of the heterogeneous nucleation and emission of dislocations from GBs on the plasticity and hardness of bicrystals is analyzed. According to our results, the intrinsic free volume involved in the GB region is associated with dislocation nucleation at the GB. The volume of the plastic zone generated beneath the tip and the way it grows is strongly dependent on the GB structure. Dislocation nucleation occurs predominantly in the early stages of indentation at GBs with a dissociated interface structural unit, before the interaction of lattice dislocation and GB. Coherent twin boundaries display the lowest effect on the hardness. Based on our results, there is a strong correlation between the interfacial boundary energy and its effect on the bicrystal hardness. GBs with lower interfacial energy offer a higher barrier against slip transmission and nucleation at the GB.

An algorithm for resolving intragranular orientation fields using coupled far-field and near-field high energy X-ray diffraction microscopy

We present a novel algorithm for reconstructing spatial intragranular lattice orientation fields using both far-field and near-field high energy X-ray diffraction microscopy (HEDM) measurements. An established far-field indexing algorithm is modified to include lattice orientation distribution information (grain orientation envelopes) in addition to average grain orientations. The near-field data reduction algorithm utilizes this enriched far-field orientation data as a seed for reconstructing intragranular spatial maps of lattice orientation from the diffraction images. The primary benefit of the new algorithm is a significant decrease in the number of trial calculations that must be performed in the reconstruction process compared to existing methodologies while maintaining scalability. The resulting gains in efficiency facilitate the use of relatively modest computational resources and improve throughput at the point of measurement. We provide two example applications: volumetric orientation field reconstructions for a Ti-Al alloy both before and after the application of 3% uniaxial strain. The results showcase the efficiency of the new method and the ability to resolve subtle changes in microstructure, which are associated with incipient plastic deformation.

On the delayed incipient plastic deformation in a LiTaO3 single crystal by nanoindentation

Relying on nanoindentation technology, the process of elastic-to-elastoplastic transition can be traced at the nanoscale via the sudden excursion in depth, namely the first pop-in event. In the current work, we investigated the delayed incipient plastic deformation at the holding stage in a LiTaO3 single crystal under a 9.8 µm spherical tip. The critical loads for spontaneous incipient plasticity fall in a wide range on the X-112° ( 1 0 2) and Y-42° (0 1 2) planes. The delayed incipient plasticity partially occurs in those elastic holding tests. The effects of atomic orientation, holding load and loading rate on the occurrence probability and length scale of delayed incipient plasticity were revealed. The delaying mechanism of incipient plasticity was qualitatively discussed based on the evolution of the plastic unit beneath nanoindentation.

Bending Nanoindentation and Plasticity Noise in FCC Single and Polycrystals

We present a high-throughput nanoindentation study of in situ bending effects on incipient plastic deformation behavior of polycrystalline and single-crystalline pure aluminum and pure copper at ultranano depths (&lt; 200 nm). We find that hardness displays a statistically inverse dependence on in-plane stress for indentation depths smaller than 10 nm, and the dependence disappears for larger indentation depths. In contrast, plastic noise in the nanoindentation force and displacement displays statistically robust noise features, independently of applied stresses. Our experimental results suggest the existence of a regime in Face Centered Cubic (FCC) crystals where ultranano hardness is sensitive to residual applied stresses, but plasticity pop-in noise is insensitive to it.

Stochastic deformation and shear transformation zones of the glassy matrix in CuZr-based metallic-glass composites

The incipient plasticity of an amorphous solid represents the onset of shear deformation, which is a stochastic progress closely related to microstructure evolution. It is well known that the cooling rate plays a crucial role in the microstructure evolution of a glass. Nevertheless, how the cooling rate affects the incipient plasticity is still unclear. In this work, the incipient plastic deformation of the amorphous phase in Cu47.5Zr48Al4Nb0.5 bulk metallic glass composites (BMGCs) cast with different cooling rates is systematically studied. The incipient plasticity of the glassy matrix of the BMGCs is found to occur via a first displacement burst, the occurrence of which is more stochastic as the cooling rate decreases. According to the auto-correlation functions and cooperative shear model, the stochastic behavior is attributed to the effects of the cooling rate on the initial free volume and subsequent activated shear transformation zones (STZs). A higher content of free volume is present in the glassy matrix of the studied samples cooled at a faster rate, which facilitates the operation of STZs. Moreover, the larger incipient burst size is correlated with the higher content of free volume and larger STZs in the glassy matrix. The larger STZs result in more preferable propagation of multiple shear bands, leading to less stochastic deformation and more obvious plastic deformation. This study provides further understanding on the incipient plasticity of glassy matrix in BMGCs.

The deformation behaviour of silver nanowires with kinked twin boundaries under tensile loading

ABSTRACTAtomistic simulations are used to investigate the mechanical properties of silver nanowires (NWs) with kinked twin boundaries (TBs) under tensile loading. For comparison, a different ledge width of twinned NWs with both square and circular kink-steps are considered in this study. The embedded-atom-method potential is employed to describe the atomic interactions. To identify the defect evolution and incipient plastic deformation mechanism, the centrosymmetry parameter is implemented in our self-developed programme. Twinned NWs with both square and circular ledges are shown to have a reduced impact on yield stress as compared to their perfect TBs counterpart models in elastic deformation. In twinned NWs with rectangular ledges, a strain-hardening effect was observed in defective NWs. The incipient plastic deformation is influenced by the ledge width. While in twinned NWs with circular ledges, the ledges rather than the surface effect are the only dislocation source in their incipient plastic deformation. Our findings offer a view of imperfection in twinned NWs, and it is believed that the attention being paid to defective TBs will be helpful to further understanding of the mechanical properties of TB-strengthened NWs.

Rebuilding the Strain Hardening at a Large Strain in Twinned Au Nanowires.

Metallic nanowires usually exhibit ultrahigh strength but low tensile ductility, owing to their limited strain hardening capability. Here, our larger scale molecular dynamics simulations demonstrated that we could rebuild the highly desirable strain hardening behavior at a large strain (0.21 to 0.31) in twinned Au nanowires by changing twin orientation, which strongly contrasts with the strain hardening at the incipient plastic deformation in low stacking-fault energy metals nanowires. Because of this strain hardening, an improved ductility is achieved. With the change of twin orientation, a competing effect between partial dislocation propagation and twin migration is observed in nanowires with slant twin boundaries. When twin migration gains the upper hand, the strain hardening occurs. Otherwise, the strain softening occurs. As the twin orientation increases from 0° to 90°, the dominating deformation mechanism shifts from slip-twin boundary interaction to dislocation slip, twin migration, and slip transmission in sequence. Our work could not only deepen our understanding of the mechanical behavior and deformation mechanism of twinned Au nanowires, but also provide new insights into enhancing the strength and ductility of nanowires by engineering the nanoscale twins.

Birefringence and incipient plastic deformation in elastically overdriven [100] CaF2 under shock compression

[100] CaF2 single crystals are shock-compressed via symmetric planar impact, and the flyer plate–target interface velocity histories are measured with a laser displacement interferometry. The shock loading is slightly above the Hugoniot elastic limit to investigate incipient plasticity and its kinetics, and its effects on optical properties and deformation inhomogeneity. Fringe patterns demonstrate different features in modulation of fringe amplitude, including birefringence and complicated modulations. The birefringence is attributed to local lattice rotation accompanying incipient plasticity. Spatially resolved measurements show inhomogeneity in deformation, birefringence, and fringe pattern evolutions, most likely caused by the inhomogeneity associated with lattice rotation and dislocation slip. Transiently overdriven elastic states are observed, and the incubation time for incipient plasticity decreases inversely with increasing overdrive by the elastic shock.

Effect of a Surface Constraining Layer on the Plastic Deformation of Au Microspheres

ABSTRACTSingle crystal Au microspheres, of 3 μm diameter, with sputter-deposited Ni surface layers, of 40 or 80 nm thickness, were tested in compression at three loading rates to investigate the role of thin passive layers on the mechanisms of plastic deformation of small-volume FCC ductile metal samples. The Ni layer resulted in an increase in the incipient yield force by about 10%. Micro-cracking of the Ni layer was observed to occur with incipient yielding. The estimated apparent activation volume of the incipient plastic deformation process was found to be nearly identical for the Ni-coated and the uncoated Au microspheres. This suggests that, while the stress required to initiate incipient plastic deformation was increased by the constraint imposed by the Ni layer, the subsequent plastic flow occurred by a dislocation nucleation and glide mechanism that is essentially the same as that occurring in an unconstrained Au microsphere.

Effects of annealing and swelling to initial plastic deformation of polyethylene probed by nonlinear ultrasonic guided waves

Abstact Identification of precursor events related to incipient plastic deformation in polyethylene parts using a nondestructive technique is investigated in this study. A pair of ultrasonic transducers mounted on the surface of a test sample were used to propagate ultrasonic pulses of varying frequency while progressively small flexural deformations below yielding were applied. The evolution of higher order harmonics were observed in association with increasing micro-structural modification. Three different polyethylene grades were molded using different thermal treatments. Results showed that different crystalline networks could be correlated to different mechanisms of plastic deformation that were observed by a defined ultrasonic parameter under proposed method. Variation of the ultrasonic parameter was similarly observed with the residual stresses associated with solvent swelling, as studied by penetration of toluene into the bimodal and copolymer grades. Results and discussion presented in this study connect this non-destructive characterization method with mechanisms of incipient plastic deformation in polyethylene.

Molecular dynamics study of plastic deformation mechanism in Cu/Ag multilayers**Project supported by the National Natural Science Foundation of China (Grant Nos. 11572118 and 11372103), the Hunan Provincial Science Fund for Distinguished Young Scholars, China (Grant No. 2015JJ1006), and the National Key Research and Development Program of China (Grant No. 2016YFB0700300).

The plastic deformation mechanism of Cu/Ag multilayers is investigated by molecular dynamics (MD) simulation in a nanoindentation process. The result shows that due to the interface barrier, the dislocations pile-up at the interface and then the plastic deformation of the Ag matrix occurs due to the nucleation and emission of dislocations from the interface and the dislocation propagation through the interface. In addition, it is found that the incipient plastic deformation of Cu/Ag multilayers is postponed, compared with that of bulk single-crystal Cu. The plastic deformation of Cu/Ag multilayers is affected by the lattice mismatch more than by the difference in stacking fault energy (SFE) between Cu and Ag. The dislocation pile-up at the interface is determined by the obstruction of the mismatch dislocation network and the attraction of the image force. Furthermore, this work provides a basis for further understanding and tailoring metal multilayers with good mechanical properties, which may facilitate the design and development of multilayer materials with low cost production strategies.