Plastic Deformation Mechanisms Research Articles

Grain size gradient effect on grain boundary mediated deformation of nano-grained metals

The extraordinary mechanical performance of gradient nano-grained (GNG) structures is characterized by the heterogeneity of dislocation-based and grain boundary (GB)-based plasticity, where the grain size gradient effect on these deformation mechanisms even fracture process still remains unclear. Herein, molecular dynamics simulation is conducted on the GNG α-Fe structures to investigate the grain size gradient effect on the GB-mediated hetero-deformation mechanisms. The plastic deformation mechanism changes from GB motion (grain coarsening and merging) to GB-dislocation interaction when the grain size distribution extends from the Hall-Petch softening region to the strengthening region. With increasing grain size gradient, the ultimate tensile stress and flow stress increase in the GB motion-controlled samples, and strain partitioning between hard and soft zones is found to increase in the dislocation-GB interaction- controlled samples from an atomistic perspective. The increased strain partitioning and its accommodation process further contribute to the fracture resistance enhancement, which is demonstrated by the uniform hetero-deformation between coarse and fine grains of the GNG structure, and further avoids localized plastic deformation.

Texture Evolution and Plastic Deformation Mechanism of Cold-Drawn Co-Cr-Ni-Mo Alloy

The plastic deformation behavior and mechanisms of Co-Cr-Ni-Mo alloy were investigated. The wires were subjected to different reductions using a multi-pass drawing approach and the resulting microstructures were characterized by EBSD and TEM. It was found that the alloy cold-drawn from surface to center exhibited non-uniform radial strain, with decreasing strain from surface to center. As the strain increased, the transverse texture of the alloy evolved from the initial bimodal texture consisting of strong {100}<110> and weak {110}<001> components to bimodal texture with {110}<233> and {112}<111> components, with significant twinning and mirror orientation between twin and matrix. The longitudinal texture evolution of the alloy mainly occurred on the α-fiber line, and ultimately did not form a significant texture due to grain elongation and crystal rotation. The plastic deformation mechanism of the Co-Cr-Ni-Mo alloy was dominated by dislocation slip at lower strain levels, which gradually transitioned to a combination of dislocation slip and twinning at higher strain levels. The deformation twins were typically distributed in high-density dislocation regions, and the twin boundaries transformed into high-angle sub-grain boundaries, hindering the extension of dislocation slip and deformation twin. With the increase in strain, work hardening results in a significant increase in strength and microhardness.

Size dependent scaling law of plastic flow in FCC Nanolattices: A Dislocation Dynamics Study

Micro- and nanolattices can achieve simultaneously both lightweight and ultra-high strength due to a combination of resilient architecture and enhanced mechanical properties of small-scale unit constituents. However, understanding the fundamental deformation mechanism for these novel materials remains intriguing due to the intricate interplay of various geometric characteristics and intrinsic microscopic defects. Here, we investigate the fundamental mechanism of plastic deformation in terms of dynamic evolution of defects and corresponding mechanical responses in aluminum nanolattices using a mesoscale defect dynamics model which couples three-dimensional dislocation dynamics and finite element method. Our concurrently coupled model could capture detailed dislocation motion under complex loading conditions and predict the plastic flow stress of the nanolattices. We demonstrate that the size of individual constituent beam plays a critical role in the properties of nanolattices through small-scale strengthening, showing the strength of the nanolattices increases with decreasing beam size with the scaling law of σ∝db−0.89. As a result, the scaling law of plastic yielding with material density drastically increases from the continuum prediction. In addition, our modeling could allow us to study the geometric effect with various nanolattices. These findings establish a foundation for the fundamental understanding of the governing mechanism of plastic deformation, allowing access to a new regime in designing optimal structures using nanolattices.

Atomic scale mechanism of reorientation induced plasticity in titanium alloys - experimental and first principles investigation of the mobile [formula omitted] interfaces

Recent experiments identified a new type of stress induced structural transformation allowing to combine high strength, great work hardening and good ductility in multiphase Ti alloys. These properties are achieved through reorientation of the α′ martensite plates being in specific self accommodating 〈54‾1‾3〉 type II twin relation, i.e. under applied load one martensite variant reconfigure to its twinned configuration with visible motion of the {134‾1} twin boundary. This mechanism of plastic deformation was never observed before thus, its current understanding is fragmentary. In this article we present the results of experimental observations and ab initio calculations of 〈54‾1‾3〉 type II twins determining the crystallography of twin formation, structure and energy of the {134‾1} interface as well as mobility of the corresponding twinning disconnections. It was found that the investigated boundary has one of the lowest energies among known twinning modes in hexagonal Ti. Moreover, the 〈54‾1‾3〉{134‾1} disconnections have the smallest Burgers vector and step height in comparison to other active twinning systems. As a result, these disconnections are highly mobile which rationalize migration of the {134‾1} twin boundaries at straining. Furthermore, energy of the twin boundary and mobility of disconnections can be adjusted by particular alloying elements enabling the conscious development of new alloys exhibiting reorientation induced plasticity.

Enhancing strength-ductility properties of immiscible W–Cu composite by creating low-energy phase interfaces

The immiscible bimetallic composites play an important role in civilian industries and aerospace fields owing to their integrated mechanical and physical properties. However, metallic composites of immiscible constituents suffer from one critical drawback of weak interface bonding, which makes the composites particularly vulnerable at elevated service temperatures. Here we proposed a novel strategy to resolve the challenge by creating low-energy phase interfaces in a typical immiscible bimetallic W–Cu composite. This was achieved by in-situ reactions of WO3 and CuO with WC, which acts in a dual function as the reducing agent for the reactions and an interface forming agent. The sintered W-WC-Cu composite possessed a significant number of low-energy WC/W and WC/Cu interfaces and exhibited a compressive strength of 609 MPa and a compressive ductility of 22.6 % at 600 °C, which ranks among the highest in similar composites. The characteristics of WC/W and WC/Cu interfaces and their effects on the mechanical behavior of the composite were demonstrated by the timely and atomic-resolved high-temperature mechanical investigations. The introduced stable interfaces were also found to induce [1‾ 11]BCC→ [011]FCC phase transformation of W as another mechanism of plastic deformation in addition to dislocation activities in W and Cu. This study provides a new approach to greatly enhance the mechanical properties of the immiscible metallic composites.

Molecular dynamics study on the effect of hydrogen on the plastic behavior of α-Fe crack tip

In this paper, the unilateral tension of α-Fe in the local region of the crack tip are simulated by molecular dynamics method, the plastic deformation behaviors and hydrogen embrittlement mechanism of the crack tip under different hydrogen concentration and hydrogen positions are analyzed. The results show that for the hydrogen-containing models, there exists a critical hydrogen concentration c0 that can make the plasticity of the α-Fe material reduce greatly. For the crack-free model, the main hydrogen embrittlement mechanism is adsorption-induced dislocation emission (AIDE), and the critical hydrogen concentration c0 is 0.3%. When the hydrogen concentration cH<c0, the plastic deformation mechanism is mainly dominated by phase transformation; when cH≥c0, the plastic deformation mechanism is phase transformation at low strains and dislocation motion at high strains. For the cracked model, the main mechanism of hydrogen embrittlement is hydrogen-enhanced local plasticity at the crack tip, with a critical c0 of 1.7%. When cH<c0, the plastic deformation mechanism of the model is a combination of phase transformation and dislocation; when cH≥c0, the plastic deformation mechanism is controlled by phase transformation. When the hydrogen is far away from the crack tip, the local mechanical behavior of the crack tip does not change significantly, while when hydrogen gathered at the crack tip, the addition of a small amount of hydrogen atoms will make the fracture strain decrease rapidly, which shows a mechanism of hydrogen-enhanced strain-induced vacancies (HESIV). A saturation value of hydrogen concentration at the crack tip is confirmed in this paper, when the local hydrogen concentration exceeds this saturation value, the plastic behaviour tends to be stable. The influence of hydrogen atoms distribution on the crack initiation behavior was determined by atomic configuration analysis.

Wear behaviors and plastic deformation mechanisms induced by nano-grinding of indium-doped gallium nitride single crystal

The wear behaviors and plastic deformation mechanisms of indium (In)-doped gallium nitride (GaN) induced by nano-grinding are investigated in this work, which is of great significance for improving the quality and precision of the machined surface of the nitride semiconductor, as well as for elucidating their mechanical properties during the nano-grinding process. A numerical simulation approach based on molecular dynamics (MD) is used to carry out a comparative analysis of the nano-grinding process for the undoped and In-doped GaN by analyzing the machined surface morphology, formation mechanisms of the subsurface damage (SSD) layer, dislocation behaviors, and residual stress. In addition, the effects of material temperature and grinding speed on the nano-grinding process of the In-doped GaN are investigated. Compared to the undoped GaN, indium doping increases the pile-ups and chips on both sides of the abrasive, the roughness of the machined surface, and the coefficient of friction. Moreover, it reduces the thickness of the SSD layer and restricts the occurrence of phase transformations. Indium doping also weakens the dislocation interactions and reduces the generation of dislocation entanglements as well as wall-like dislocations. Furthermore, it decreases the maximum value of the residual stress and increases the area of high residual stress regions. The wear behaviors and plastic deformation mechanisms of In-doped GaN are quite sensitive to material temperature, but not much influenced by grinding speed.

Shock compression of single-crystal stainless steel

We studied the orientation-dependent dynamic properties and plastic deformation mechanisms in single-crystal austenitic FeCr18Ni12.5 stainless steel shocked to peak stresses ranging 3.8-12.0 GPa. The 〈110〉 and 〈111〉 principal Hugoniots generally match that of polycrystalline 304L stainless steel, but the 〈100〉 Hugoniot exhibits lower shock velocities and comprises two distinct regions. The Hugoniot elastic limits (HELs) and spallation strengths range 0.5-1.2 GPa and 2.2-3.8 GPa, respectively, and exhibit trends previously observed in polycrystalline steels. We generally observed the largest HELs along 〈100〉 and the largest spallation strengths along 〈111〉. Microscopy revealed larger twin and martensite fractions in the 〈100〉 samples, correlations in twinning and martensite prevalence with dynamic response, and orientation-dependent differences in dislocation behavior near the spallation plane.

Increase in interplanar distances and formation of amorphous shear bands in deformed diamond

Amorphous transformation-deformation bands have been observed in diamond under cyclic loading (at planetary ball milling) near the graphite-diamond equilibrium line at temperatures below the Debye temperature. The maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K, respectively. These bands add to the set of plastic deformation mechanisms in diamond. TEM methods and conventional X-rays were used to investigate the mechanisms of plastic deformation of diamond. The observed mechanisms include amorphous deformation bands, bond breaking, twin formation, and phase transitions of diamond-lonsdaleite and diamond-onions. The increased interplanar distances provide evidence of the plastic deformation mechanism due to bond breaking. The distances of 0.222 and 0.255 nm are significantly larger than the initial value of 0.206 nm, which is characteristic of d111 diamond. This increase in distance cannot be attributed to lattice expansion caused by point defects, but rather to the breakage of interatomic bonds.

Surface characteristics and material removal mechanisms during nanogrinding on C-face and Si-face of 4H-SiC crystals: Experimental and molecular dynamics insights

4H-SiC wafers hold significant promise for applications in power devices and radiofrequency devices. In order to develop a cost-effective and efficient grinding technology for 4H-SiC wafers, it is imperative to thoroughly investigate the mechanisms underlying damage evolution on different crystal faces of 4H-SiC. Our approach involved employing a range of experimental techniques including nanoindentation, scratching, and grinding, coupled with molecular dynamics (MD) simulation to systematically analyze the material removal mechanisms on both the C-face and Si-face. Through a comprehensive comparison of mechanical properties, surface morphology, and subsurface damage distribution between the C-face and Si-face, we uncovered the plastic deformation mechanisms operating on each face. Our research findings emphasize that, compared to the Si-face, the C-face exhibits higher elastic modulus and hardness, but lower fracture toughness. Additionally, due to the C-face’s greater propensity for high-density dislocation formation compared to the Si-plane, stress release occurs in the machining area, resulting in smaller normal forces and aiding in reducing surface roughness after grinding. This study provides important guidance for the grinding of the two critical basal planes of single-crystal 4H-SiC.

Size effect and plastic deformation mechanisms of AlON micro/nanopillars with different crystallographic orientations

In this work, the plastic deformation of micro/nanoscale AlON ceramic pillars with different crystallographic orientations was investigated by in situ compression on scanning electron microscopy (SEM) system. The experimental results demonstrated a "smaller is stronger" size dependence of the yield strength of <100>-, <110>- and <111>-oriented AlON micro/nanopillars with scaling exponents of -0.19, -0.24 and -0.33, respectively. Transmission electron microscopy (TEM) illustrated that the Lomer-Cottrell dislocation reaction dominated the plastic deformation of <111>-oriented pillars. The plastic deformation of <110>-oriented AlON pillars was driven by rotation of the slip region, whereas the stress of <001>-oriented pillars were released through microcracks and plastic deformation.

Preparation of the gradient nanostructured layer in uranium by surface shot peening treatment

The gradient nanostructured layer in uranium prepared by surface shot peening treatment has been systematically characterized. The evolution of the ultrafine grains within the surface layer of uranium was derived. The results show that the gradient microstructures from the surface to the matrix could be divided into four zones including ultrafine grains, high density deformation twins, low density deformation twins, and coarse grain matrix. The deformed microstructures in the four zones were characterized in detail. Twinning and dislocation activities were found to be the main plastic deformation mechanisms for grain refinement in uranium. Four types of twinning and dislocation slip modes were distinguished. The formation of fine equiaxed grains on uranium surface should be attributed to the combined action of the twinning, dislocation activities, and the subgrain rotation driven by mechanical-thermal coupling. The surface microhardness and the natural oxidation corrosion performance were improved in uranium prepared by shot peening treatment.

An atomistic simulation study on ductility of amorphous aluminum oxide

Aluminum oxide is a widely recognized structural ceramic material known for electrical insulation and wear/corrosion resistance. Recent studies have challenged the conventional perception of brittleness by uncovering remarkable plasticity in amorphous aluminum oxides. To further enhance the fracture toughness of aluminum oxides and expand their applications in fields requiring resistance, it is crucial to analyze the factors influencing the tensile deformation process and investigate the underlying origins of ductility. In this study, molecular dynamics simulations were employed to compare the ductility of amorphous aluminum oxides with crystalline aluminum oxides and other amorphous ceramic oxides. Our findings indicate that the degree of crystallinity significantly influences ductility. Moreover, the simulations revealed that amorphous aluminum oxides exhibit the highest total elongation among binary structural ceramic oxides such as TiO2 and SiO2, underscoring the intrinsic plasticity of aluminum oxide. Furthermore, we discovered that aluminum oxides possess a low bond-dissociation energy (EBD) compared to other structural ceramic oxides, leading to a higher occurrence frequency of plastic deformation mechanisms. Based on these observations, low EBD could be proposed as the probable indicator that enables the estimation of ductility of structural amorphous oxides without resorting to complex atomistic simulations. This quantitative criterion, based on an easily accessible material property, provides valuable guidance for predicting the ductility of newly-designed structural ceramic materials.

Densification mechanisms during high-pressure sintering of nanocrystalline Gd2Zr2O7 ceramic

High-pressure sintering (HPS) is a promising technique for producing nanocrystalline ceramics with unique properties. However, the densification mechanisms of HPS for nanocrystalline ceramics at different sintering stages remain controversial. This study focuses on Gd2Zr2O7 (GZO) nanocrystalline ceramics, investigating their microstructure evolution and densification behavior under varying pressures, temperatures, and dwelling times. The HPS process involves distinct stages: cold compaction, hot compaction, and isothermal progression. During cold compaction, densification is driven by the breakdown of aggregates, particle rearrangement, and local plastic deformation, resulting in a relative density of 84.8 %. The intermediate stage achieves an increase to ∼93.7 % in relative density by grain boundary-mediated plastic deformation mechanism. The final isothermal stage, held at 500 °C/5 GPa, achieves nearly full density (∼99.2 %) through diffusion creep, eliminating microstructural defects. This comprehensive understanding of HPS densification mechanisms, exemplified by GZO ceramics, contributes to advancing nanocrystalline ceramic applications.

Nanomechanical characterization of BiFeO3 ferroelectric ceramics

Ferroelectric perovskites are pivotal in diverse technological applications; however, a gap persists in our comprehension of their mechanical properties, emphasizing the need for additional exploration. In the present study, we report on the nanomechanical behavior of BiFeO3 ceramics in the force range between 200 μN and 2 mN, including the evolution of the hardness and the reduced Young’s modulus from (9.9 ± 0.4) GPa and (113.7 ± 6.9) GPa, respectively, with increasing force. A sequence of plastic-deformation mechanisms under the cube-corner probe was revealed through a first pop-in analysis in combination with a variety of electron microscopy techniques, starting with a homogeneous dislocation nucleation, multiplication and rearrangement leading to sub-grain formation, a phenomenon observed in metals but not in ceramics.

Theoretical study on the influence of grain size on the strength, toughness and plastic deformation mechanism of pre-cracked polycrystalline high entropy alloys

High entropy alloys (HEAs) is a kind of structural material with excellent mechanical properties. The material exhibits excellent damage tolerance at low temperature, which is closely related to its potential crack tip plastic mechanism and overall fracture mode. In the research, the plastic deformation and fracture mechanical properties of polycrystalline CrMnFeCoNi HEAs with pre-crack under type I tensile load were investigated by molecular dynamics (MD) simulation. The effects of grain size and crack length on the strength, toughness and microstructural deformation of the alloy were analyzed. It is found that with the increase of grain size, the strength of the material increases and the toughness decreases. With the increase of crack length, the strength and toughness decrease, and the influence of grain size on the strength is weakened. The plastic mechanism of crack initiation in nanocrystalline HEAs is ductile cracking under the joint action of dislocations, amorphization and grain boundary plasticity, which provides high damage tolerance. As the grain size increases, additional cavities appear near the crack tip or at the grain boundary, resulting in brittle fracture mode along the grain boundary or at the end of the cavity, which greatly affects the plastic properties of the material. The initial crack length has a significant effect on the transformation of the fracture mode. With the increase of the initial crack size, the critical grain size of the brittle fracture mode increases, and its proportion in the whole fracture mode decreases. This study provides a theoretical reference for the design and application of nanocrystalline HEAs.

Shear localization-induced amorphization in nanocrystals during high strain rate deformation

Shear localization is the predominant form of failure of metals at high strain rates where amorphization often occurs. However, the mechanism and role of amorphization in shear localization are not yet clearly understood. In this work, a series of shear deformations of nanocrystals with different grain sizes at high strain rates are investigated using molecular dynamics simulations. The results show that amorphization is not the result of thermal melting, but a new plastic deformation mechanism in the process of shear localization, which serves as a complement to dislocation and grain boundary to support plastic deformation at a high strain rate. The critical shear strain at which shear localization and amorphization occur is greater for small-grained specimens than for large-grained specimens. This can be attributed to the fact that small-grained specimens have more defects and slip systems that can be activated, which cause amorphization to occur later. The strain rate has a significant influence on shear localization and amorphization. The processes of shear localization and amorphization become more complicated at extremely high strain rates than at relatively low strain rates.

Influence of Interface Type on Dynamic Deformation Behavior of 3D-Printed Heterogeneous Titanium Alloy Materials.

Using the Split Hopkinson Pressure Bar technique, strain-limited dynamic compressive loading experiments were performed on TA1/TA15 heterostructure (HS) materials. The plastic deformation mechanisms, fracture forms, and energy absorption properties of an HS material with a metallurgical bonding interface (MB) and an HS material without a metallurgical bonding interface (NMB) are compared and analyzed. The results show that there is no significant difference between the two deformation mechanisms. The fracture forms are all "V-shaped" fractures within the TA1 part. The NMB was carried for 57 μs before failure and absorbed 441 J/cm3 of energy. The MB was carried for 72 μs before failure and absorbed 495 J/cm3 of energy. Microstructure observations show that there is a coordinated deformation effect near the MB interface compared to the NMB, with both TA1 and TA15 near the interface carrying stresses. This causes an enhancement of the MB load-bearing time and a 12% increase in energy absorption.

Ultra strong FCC structured Ni8Cr4Co4Fe6W2 high entropy alloys with high strength and ductility by laser powder bed fusion

The high entropy alloys (HEAs) are widely used in high temperature service environment (such as aviation engine blades) with bright application potential in additive manufacturing. Laser powder bed fusion (LPBF) is technology currently offers the best reproducibility and dimensional accuracy for part production. The unique atomic lattice structure of HEA makes its plastic deformation mechanism different from that of traditional dilution alloy, resulting in the strengthening effect of face-centered cubic high-entropy alloy different from that of traditional metal alloy materials. In this paper, Ni8Cr4Co4Fe6W2 was designed by means of composition control. Its spherical powders with uniform composition were successfully prepared by high temperature remelting spheroidization (PHTR) method, compared with gas atomization (GA) method. The strain rate sensitivity m values of GA-Selective Laser Melting and PHTR-Selective Laser Melting samples were 0.030 and 0.027, respectively. The hardness of Ni8Cr4Co4Fe6W2 high-entropy alloy showed a trend of continuous increase with the decrease of grain size, while the elastic modulus didn’t change much (PHTR-SLM is 193 Gpa and GA-SLM is 189 Gpa). The simulated value of the contact stress decreased by 23 % compared with the theoretical value. The contact radius increased with the increase of plastic deformation in addition to the reduction of contact pressure. The printing sample value of the elongation, yield strength (YS) and Ultimate Tensile Strength (UTS) on PHTR-SLM has increased by 39.6 %, 33.7 % and 25.9 %, respectively in additive manufacturing, compared to GA-SLM. The ability to produce refractory HEAs through PHTR method will make it possible to manufacture complex geometry shapes at a reasonable cost.

Dynamic mechanical behavior of ultra-high specific strength lightweight Ti61Al16Cr10Nb8V5 multi-principal element alloy

This study investigated the dynamic mechanical property and microstructure evolution of the Ti61Al16Cr10Nb8V5 (Ti61) lightweight multi-principal element alloy(MPEA) with a density of 4.83 g/cm3. After melting and rolling, the Ti61 alloy was composed of a BCC matrix phase and dispersed B2 phase, and also contained a small amount of fine needle-like FCC phase and spherical HCP phase. Mechanical testing of Ti61 alloy revealed that the ultimate tensile strength is about 1223 MPa, and the tensile elongation is about 13.2 %, with a specific strength of around 253 MPa*cm3/g. Dynamic compression testing of Ti61 alloy showed a significant strain rate strengthening effect and excellent fracture strain greater than 50 %. Calculation and microstructural observations indicated that at low strain rates ranging from 10−3/s to 10/s, the strain rate sensitivity of Ti61 alloy was 0.0087, with the main plastic deformation mechanism of dislocation nucleation, dislocation slip, and stacking faults. At high strain rates ranging from 500/s to 5000/s, the strain rate sensitivity raises to 0.0963 and the main deformation mechanism of the Ti61 alloy involved higher-density dislocation nucleation, dislocation slip, stacking faults and twinning. At strain rates of 5000/s, HCP-FCC phase transition accompanied by FCC microtwins and BCC-α2HCP phase transition occurred. Furthermore, dynamic recrystallization also occurred in Ti61 alloy. The HCP-FCC has the orientation of (0001)HCP//(11−1)FCC, [1−210]HCP//[011]FCC. The BCC-α2HCP has the orientation of {101}BCC//{0001}HCP and [111]BCC//[2−1−10]HCP. Twining and phase transition strengthen the yield stress of Ti61 alloy, while dynamic recrystallization reduced the flow stress of Ti61 alloy. Ti61 alloy has high strength, high compression fracture strain, and ultra-high specific strength, which is a new LWMPEA with great application potential in military armor.