Plastic Flow Stress Research Articles

Theoretical study including physic-based material model to identify underlying effect of vibration amplitude on residual stress distribution of ultrasonic burnishing process

The potential of distribution of compressive residual stress at deep surface layers caused by ultrasonic-based surface sever plastic deformation (SSPD) has attracted researchers' attention to deeply study the mechanism of the process by simulation models. In majority of the researches, macro-mechanical Johnson-Cook (JC) model was utilized to govern material constitutive behavior. However, the large differences between measured and predicted values of residual stress of some materials imply that the underlying mechanism is still unclear and merits further studies. In the present work, a new physics-based constitutive equation incorporating micromechanics of initial grain size, dislocation evolution, dislocation drag and acoustic softening has been developed to obtain plastic flow stress. Then, the mechanic of the ultrasonic burnishing process incorporating superposition of static and dynamic loads was simulated using concept of expanding cavity model (ECM). The residual stress has been further calculated through combination of developed models aiming to discover the undying mechanism of vibration amplitude effect on stress fields. In order to confirm the obtained results, series of ultrasonic burnishing experiments have been carried out on AA6061-T6 under different vibration amplitude. The obtained results revealed that the developed analytical models of residual stress field are considerably more accurate than those previously established that took the macro-mechanical constitutive models to calculate plastic flow stress. Through the developed simulation model, it was studied in depth that how the vibration amplitude as the most adjustable ultrasonic vibration parameter determine variation of residual stress field. It has been demonstrated that the acoustic softening because of presence of ultrasonic energy field has the greatest impact in generation of compressive residual stress in surface and beneath layers.

IDENTIFICATION OF PARAMETERS FOR CHARACTERIZATION AT HIGH-TEMPERATURE OF 38MnVS6 STEEL USED IN HOT FORGING PROCESSES

Finite element reliability is highly sensitive to the input data quality and constitutive equations used. In this article, two constitutive equations for modeling the plastic flow of metals are used. Two equations commonly employed in metal forming finite element analysis were selected, e.g., the Hensel-Spittel and Johnson-Cook. Compression tests were conducted at various temperatures and strain rates to obtain the plastic flow stress curves. These experimental results were used as a target to fit the constitutive equation parameters. The parameter identification procedure was conducted by solving the so-called inverse problem. An objective function gives a value (error) that indicates how well a parameter set makes the equation fits the experimental results. This error value is minimized by applying optimization techniques; in this case, by using a Genetic Algorithm followed by a gradient-based strategy. Two objective functions are used; the first is based on the well-established least square approach, and the second is proposed. The proposed objective function shows a faster performance than the least square. In addition, the proposed objective function also gives the best fit for the Hensel-Spittel equation. The Johnson-Cook equation is fitted using only the proposed function. The Hensel-Spittel equation shows several local minimums, whereas the Johnson-Cook shows a global one. Keywords: Hot forging, Parameter identification, constitutive equations, finite element analysis.

A phenomenological model for plastic flow behavior of rotating band material with a large temperature range

The plastic flow behavior of the rotating band material is investigated in this paper. The rotating band material is processed from H96 brass alloy, which is hardened to a much higher yield strength compared to the annealed one. The dynamically uniaxial compression behavior of the material is tested using the split Hopkinson pressure bar (SHPB) with temperature and strain rate ranging from 297 to 1073 K and 500 to 3000 s−1, respectively, and a phenomenological plastic flow stress model is developed to describe the mechanical behavior of the material. The material is found to present noticeable temperature sensitivity and weak strain-rate sensitivity. The construction of the plastic flow stress model has two steps. Firstly, three univariate stress functions, taking plastic strain, plastic strain rate and temperature as independent variable, respectively, are proposed by fixing the other two variables. Then, as the three univariate functions describe the special cases of flow stress behavior under various conditions, the principle of stress compatibility is adopted to obtain the complete flow stress function. The numerical results show that the proposed plastic flow stress model is more suitable for the rotating band material than the existing well-known models.

Mechanical Properties and Microstructural Evolution of TiNi-Based Intermetallic Alloy with Nb Addition.

TiNi intermetallic alloys were prepared with 2, 4 and 6 at.% niobium (Nb) addition. The mechanical properties and microstructures of the alloys were investigated under both static (1 × 10−1 to 1 × 10−3 s−1) and dynamic (4 × 103 to 6 × 103 s−1) loading conditions. The intermetallic alloy structures and surface morphologies of the alloys were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. In addition, the fracture morphologies were observed by optical microscopy (OM). It was shown that the addition of 2 to 4 at.% Nb increased the strength of the TiNi alloy. However, as the level of Nb addition was further increased to 6 at.%, a significant reduction in strength occurred. For a constant Nb addition, the plastic flow stress and strain rate sensitivity increased with increasing strain rate under both loading conditions (static and dynamic). The XRD and SEM results showed that the original surface morphologies were composed primarily of dendritic structures and fine β-Nb + TiNi eutectic systems. Moreover, the OM results showed that the alloys underwent a transition from a brittle fracture mode to a ductile fracture mode as the level of Nb addition increased.

Fracture Analysis of Ultrahigh-Strength Steel Based on Split Hopkinson Pressure Bar Test

Impact loading is an important cause of fracture failure of ultrahigh-strength steel parts during service. Revealing the fracture mechanism of ultrahigh-strength steel under impact loading has important reference significance for the material preparation, part design, and manufacturing of such steel. Based on the split Hopkinson pressure bar (SHPB) test, the mechanical response characteristics of 45CrNiMoVA steel under impact loading were analyzed, and the true stress–true strain curves under a high strain rate (103 s−1) were obtained. It was found that under the simultaneous action of forward and tangential loading forces, a severe plastic deformation layer with a thickness of 20–30 μm was generated in the near impact-loading end face, which is the main cause for crack initiation and propagation. Under the condition of a high strain rate, the plastic flow stress of 45CrNiMoVA steel was characterized by the equilibrium of strain hardening and strain softening, and its impact fracture toughness decreased by 43.6%, resulting in increased quasi-cleavage fracture. Hence, severe surface plastic deformation during 45CrNiMoVA steel machining should be avoided, as it may lead to early failure.

Synchronous involvement of topology and microstructure to design additively manufactured lattice structures

This article presents a methodical approach to optimize microstructure (e.g., the crystallographic texture) and topology (e.g., unit cell and struts) concurrently to improve the mechanical properties of additively manufactured metallic lattice structures (AMLS), i.e., yield strength and plastic flow stress. Full-field elasto-viscoplastic Fast Fourier Transform (EVP-FFT) crystal plasticity (CP) simulations are employed to determine the optimal microstructure. The CP model parameters were calibrated to measured macroscopic stress–strain response and microstructural data for polycrystalline samples of additively manufactured (AM) Inconel 718 with solution treated and aged (STA) microstructure. Since the crystallographic orientation of the constituent single-crystal grains with respect to the loading direction has a significant impact on the mechanical behavior of the material, stress projection factor analysis was used to determine four candidate textures to explore in for a given unit cell topology. Full-field crystal plasticity simulations were used to determine macroscale yield surface parameters for each of the considered textures, thereby enabling macroscale lattice unit cell simulations that account for the underlying microstructure. The calibrated microstructure-dependent yield surfaces are used to investigate the effect of different microstructures on the mechanical response of different LS topologies with the same relative density. The results show that in a texture with <111> crystallographic direction, parallel to the loading direction, the tensile and compressive yield strength are 20% and 58% larger, respectively compared to the AM STA IN718 texture. Furthermore, when this texture is used in conjunction with the Rhoctan topology, the results demonstrate 50% improvement in both the yield strength and modulus of elasticity relative to previously optimized AMLS designs that did not directly account for microstructure. This simultaneous consideration of microstructure and topology during optimization, thus, significantly enhances the structural integrity of the AMLS.

Plastic deformation and strengthening mechanism of FCC/HCP nano-laminated dual-phase CoCrFeMnNi high entropy alloy

FCC-structured CoCrFeMnNi high entropy alloy (HEA) has attracted abroad interests for years because of its excellent mechanical properties, except for strength. Recent experiments have reported a kind of nano-laminated dual-phase (NLDP) FCC/HCP structure that can strengthen the HEA. However, it is still unknown why the HEA can be strengthened by this kind of NLDP structure. Here, we employ molecular dynamics simulations to study the atomistic strengthening mechanism of the NLDP HEA. Dislocation-assisted multiple plastic deformation mechanisms in both FCC and HCP single phase HEAs are observed, and amorphization is also found in the plasticity of HCP phase, which are consistent with the previous experimental characterizations. The HCP phase possesses higher strength because of its higher stacking fault energy, higher Peierls–Nabarro stress and less active dislocation slip systems. It is also found that the introduction of HCP phase can enhance the mechanical properties, including yield stress, yield strain and plastic flow stress, of the NLDP HEAs, which also show volume fraction dependence. And the phase boundary plays crucial roles in the deformation and strengthening of the NLDP HEAs. The plastic deformation of the NLDP HEAs can be divided into two stages, i.e. stage I (plasticity only appears in FCC lamella) and stage II (plasticity in both FCC and HCP lamellas). With the increase of volume fraction, the lamella thickness of FCC matrix phase decreases, leading to continuous strengthening of yield properties and flow stress of stage I because of suppressed dislocation nucleation and confined dislocation motion in FCC matrix phase by the phase boundary. While there is no monotonous relationship between the flow stresses of stage II and the increasing volume fraction of HCP phase, which can be attributed to the competitive mechanisms between strengthening effect of phase boundary on the dislocation motion in FCC phase and softening effect of phase boundary on the dislocation motion in HCP phase. The results should be helpful for understanding the underlying physical mechanism of strengthening of HEAs with NLDP structure.

Damage constitutive model of pure copper at different annealing temperatures

Aiming at the problem of damage evolution of pure copper during the plastic deformation, the normalized shape factor is introduced based on the RO model (Ramberg-Osgood model). The mesoscopic damage constitutive model of pure copper at different annealing temperatures is established and the tensile deformation of industrial pure copper at different annealing temperatures is analyzed. The results show that the error between the calculated value and the experimental value of the damage constitutive model, based on normalized shape factor, at different annealing temperatures, is less than 10%. The model can effectively reveal the tensile damage evolution behavior of industrial pure copper and accurately predict the plastic tensile flow stress of industrial pure copper at different annealing temperatures. The hardening coefficient and hardening exponent in the model are closely related to the annealing temperature of the material. The annealing temperature has little effect on the hardening exponent and has a significant effect on the hardening coefficient and the hardening coefficient decreases with the increase in annealing temperature.

Constitutive modeling of the hot deformation behavior of CoCrFeMnNi high-entropy alloy

The unfolding characteristics of high-entropy alloys (HEAs) are challenging traditional materials science field with promise for innovative applications, including aerospace and defence. This makes them strong candidates for future applications in the aerospace and defense industries. This research focuses on the hot compressive behavior of equiatomic CoCrFeMnNi alloy at strain rates of 0.01 s−1 and 1 s−1 and temperatures ranging between 200 and 800 °C. The experimental results were used to develop plastic flow stress models for a variety of existing constitutive models, namely, the Johnson-Cook, modified Johnson-Cook, Zerilli-Armstrong, modified Zerilli-Armstrong, Zener-Hollomon, Hensel-Spittel, and modified Hensel-Spittel. The models were then compared using the correlation coefficient (R) and average absolute relative error (AARE) to determine their suitability for predicting the deformation behavior of this alloy. The results show that the modified Johnson-Cook, Zener-Hollomon, Hensel-Spittel, and modified Hensel-Spittel models are all found to provide reasonable predictive accuracy for the studied alloy. Microstructural analysis was also conducted to compare the samples' microstructures before and after deformation to confirm the occurrence of discontinuous dynamic recrystallization when strained at 800 °C. This phenomenon explains the material's stress behavior at a lower strain rate, which affects the modeling results.

Quasi-Static and Dynamic Compressive Behavior of Gum Metal: Experiment and Constitutive Model

The quasi-static and high strain rate compressive behavior of Gum Metal with composition Ti-36Nb-2Ta-3Zr-0.3O (wt pct) has been investigated using an electromechanical testing machine and a split Hopkinson pressure bar, respectively. The stress–strain curves obtained for Gum Metal tested under monotonic and dynamic loadings revealed a strain-softening effect which intensified with increasing strain rate. Moreover, the plastic flow stress was observed to increase for both static and dynamic loading conditions with increasing strain rate. The microstructural characterization of the tested Gum Metal specimens showed particular deformation mechanisms regulating the phenomena of strain hardening and strain softening, namely an adiabatic shear band formed at ~ 45 deg with respect to the loading direction as well as widely spaced deformation bands (kink bands). Dislocations within the channels intersecting with twins may cause strain hardening while recrystallized grains and kink bands with crystal rotation inside the grains may lead to strain softening. A constitutive description of the compressive behavior of Gum Metal was proposed using a modified Johnson–Cook model. Good agreement between the experimental and the numerical data obtained in the work was achieved.

High-Rate Deformation of Titanium in Shock Waves at Normal and Elevated Temperatures

The elastoplastic shock compression, unloading, and steplike shock compression of titanium at room temperature and in the vicinity of 600°C are measured. The dependences of the dynamic yield strength on the pressure of shock compression and temperature are determined, and the dependences of the strain rate in shock waves in the range 105–107 s–1 on the shear stress and temperature are obtained. In contrast to the data obtained for fcc metals, the yield strength of titanium behind a shock wave is found to decrease. The decrease in the yield strength is thought to be caused by a decrease in the plastic flow stress during the growth of twins and their reversibility.

Numerical investigation of plastic flow and residual stresses generated in hydroformed tubes

During tube hydroforming process, the friction conditions between the tube and the die have a great importance on the material plastic flow and the distribution of residual stresses of the final component. Indeed, a three-dimensional finite element model of a tube hydroforming process in the case of square section die has been performed, using dynamic and static approaches, to study the effect of the friction conditions on both plastic flow and residual stresses induced by the process. First, a comparative study between numerical and experimental results has been carried out to validate the finite element model. After that, various coefficients of friction were considered to study their effect on the thinning phenomenon and the residual stresses distribution. Different points have been retained from this study. The thinning is located in the transition zone cited between the straight wall and the corner zones of hydroformed tube due to the die–tube contact conditions changes during the process. In addition, it is clear that both die–tube friction conditions and the tube bending effects, which occurs respectively in the tube straight wall and corner zones, are the principal causes of the obtained residual stresses distribution along the tube cross-section.

Effect of stable stacking fault energy and crystal orientation on fracture behaviour of thin metallic single crystals

ABSTRACT Understanding the evolution of dislocations and twinning at the crack front is critical for designing micro-mechanical systems with improved performance. In this work, the dislocation evolution at the crack front in thin pre-cracked FCC single crystals is correlated with the associated fracture toughness, which is shown to be dependent on material specific properties such as stable stacking fault energy and crystal orientation using atomistic simulations. For materials with high value, sessile dislocations form at the crack front causing increased localised plastic flow stress that leads to low fracture toughness. Whereas the fracture process in materials with low value is governed by the motion of glissile dislocations and stress-induced twinning leads to high fracture toughness. For this case, twinning occurs at high stress levels followed by un-twinning due to stress relaxation at crack front by twinning. The crystal orientation influences the type of dislocations emitted (screw/edge) from the crack front which governs the mode of crack propagation. The Mode-III crack propagation by the emission of screw type dislocations causes significant decrease in the fracture toughness compared to Mode I crack propagation which is caused by simultaneous emission of edge type dislocations on the two symmetrically inclined slip planes at the crack front. For certain pre-cracked crystal orientations, twinning is seen during the early stages of plastic deformation in materials with high value. However, un-twinning is not observed in crystal orientation-based twinning at the crack front.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ СТЫКОВЫХ ДИСКЛИНАЦИЙ НА ДЕФОРМИРУЮЩЕЕ НАПРЯЖЕНИЕ ПОЛИКРИСТАЛЛОВ

Plastic deformation of polycrystalline solids is accompanied by the appearance of linear rotational-type mesodefects at grain boundary ledges and triple junction of grains, such as starin induced junction disclinations. Junction disclinations generate long-range spatially inhomogeneous fields of elastic stresses, which significantly influence on the structure formation, strain hardening and fracture of materials. In present work a comparative analysis of the contributions of junction disclinations of different sign and strength to the plastic flow stress of a polycrystal is performed. The results of calculationsshow, that when a pile-up of lattice dislocations passes through the elastic field of disclinations, general regularities are observed.Regardless of the sign of disclination, it has a retarding effect on the plastic shear. The equilibrium distributions of the linear density and the density of the Burgers vector of dislocations pile-upretarded by the elastic field of disclination are calculated.It is shown that the largest number of dislocations is concentrated not in the pile-up head, as in classical dislocation pile-upsstoped near impenetrable barriers, but in its central part. The dependences of the critical stress of the passage of the head dislocation of the pile-up through the force barrier of disclination are calculated depending on the strength and sign of disclination, the number of dislocations in the pile-up, and the distance between the disclination and the slip plane of lattice dislocations.It is shown that the change in the sign of disclination significantly influences on the form of the equilibrium distribution of dislocations along the length of the pile-up, but practically does not affect the value of the critical shear stress. It is shown that for a fixed number of dislocations in the pile-up, the critical shear stress increases with the distance between the slip plane and disclination. Thus, when plastic deformation is localized, the greatest strengthening effect from the elastic field of junction disclination is achieved not near the boundary, but far from it.

Material Model for Predicting Dynamic Response of Copper and Nickel at Very High Strain Rates Under Cold Spray Conditions

Numerous material models have been proposed to describe material properties under impact-induced extreme conditions. All these models suffer from the fact that the tests used to provide the required empirical parameters were performed on bulk material at limited strain rates, much lower than those found in the cold spray process. In this work, recent published results available in the literature that were obtained using advanced imaging techniques are used to modify material plasticity models to accurately predict the material plastic flow stress under cold spray conditions. The capability of two frequently used material models, Johnson–Cook and Preston–Tonks–Wallace models, to predict real-time experimental data for copper and nickel particles is evaluated. A series of finite element simulations are conducted and the comparison is made with the coefficient of restitution from laser-induced projectile impact test data. The original empirical parameters in Preston–Tonks–Wallace model are re-calibrated so that the coefficient of restitution matches the laser-induced projectile impact test data, providing an enhanced quality in the particle impact behavior predictions. Additionally, experiments and modeling are conducted to explore the bonding mechanism and rebound phenomena of copper powder cold sprayed on low carbon steel substrate.

Effect of Reactive SPS on the Microstructure and Properties of a Dual-Phase Ni-Al Intermetallic Compound and Ni-Al-TiB2 Composite.

As part of the tests, a two-phase NiAl/Ni3Al alloy and a composite based on this alloy with 4 vol% addition of TiB2 were produced by the reactive FAST/SPS (Field Assisted Sintering Technology/Spark Plasma Sintering) sintering method. The sintering process was carried out at 1273 K for 30 s under an argon atmosphere. The effect of reactive SPS on the density, microstructure, and mechanical and tribological properties of a dual-phase Ni-Al intermetallic compound and Ni-Al-TiB2 composite was investigated. Products obtained were characterized by a high degree of sintering (over 99% of the theoretical density). The microstructure of sinters was characterized by a large diversity, mainly in regard to the structure of the dual-phase alloy (matrix). Compression tests showed satisfactory plastic properties of the manufactured materials, especially at high temperature (1073 K). For both materials at room temperature, the compressive strength was over 3 GPa. The stress–strain curves were observed to assume a different course for the matrix material and composite material, including differences in the maximum plastic flow stress depending on the test temperature. The brittle-to-ductile transition temperature was determined to be above 873 K. The research has revealed differences in the physical, mechanical and tribological properties of the produced sinters. However, the differences favourable for the composite were mostly the result of the addition of TiB2 ceramic particles uniformly distributed on grain boundaries.

Microstructure Evolution of 316L Steel Prepared with the Use of Additive and Conventional Methods and Subjected to Dynamic Loads: A Comparative Study.

The mechanical properties and microstructure evolution caused by dynamic loads of 316L stainless steel, fabricated using the Laser Engineered Net Shaping (LENS) technique and hot forging method were studied. Full-density samples, without cracks made of 316L stainless steel alloy powder by using the LENS technique, are characterized by an untypical bi-modal microstructure consisting of macro-grains, which form sub-grains with a similar crystallographic orientation. Wrought stainless steel 316L has an initial equiaxed and one-phase structure, which is formed by austenite grains. The electron backscattered diffraction (EBSD) technique was used to illustrate changes in the microstructure of SS316L after it was subjected to dynamic loads, and it was revealed that for both samples, the grain refinement increases as the deformation rate increases. However, in the case of SS316L samples made by LENS, the share of low-angle boundaries (sub-grains) decreases, and the share of high-angle boundaries (grains of austenite) increases. Dynamically deformed wrought SS316L is characterized by the reverse trend: a decrease in the share of high-angle boundaries and an increase in the share of low-angle boundaries. Moreover, additively manufactured SS316L is characterized by lower plastic flow stresses compared with hot-forged steel, which is caused by the finer microstructure of wrought samples relative to that of additive samples. In the case of additively manufactured 316L steel samples subjected to a dynamic load, plastic deformation occurs predominantly through dislocation slip, in contrast to the wrought samples, in which the dominant mechanism of deformation is twinning, which is favored by a high deformation speed and low stacking fault energy (SFE) for austenite.

Effect of Strain Rate on Microstructure Evolution and Mechanical Behavior of Titanium-Based Materials

The goal of the present work is a systematic study on an influence of a strain rate on the mechanical response and microstructure evolution of the selected titanium-based materials, i.e., commercial pure titanium, Ti-6Al-4V alloy with lamellar and globular microstructures produced via a conventional cast and wrought technology, as well as Ti-6Al-4V fabricated using blended elemental powder metallurgy (BEPM). The quasi-static and high-strain-rate compression tests using the split Hopkinson pressure bar (SHPB) technique were performed and microstructures of the specimens were characterized before and after compression testing. The strain rate effect was analyzed from the viewpoint of its influence on the stress–strain response, including the strain energy, and a microstructure of the samples after compressive loading. It was found out that the Ti-6Al-4V with a globular microstructure is characterized by high strength and high plasticity (ensuring the highest strain energy) in comparison to alloy with a lamellar microstructure, whereas Ti6-Al-4V obtained with BEPM reveals the highest plastic flow stress with good plasticity at the same time. The microstructure observations reveal that a principal difference in high-strain-rate behavior of the tested materials could be explained by the nature of the boundaries between the structural components through which plastic deformation is transmitted: α/α boundaries prevail in the globular microstructure, while α/β boundaries prevail in the lamellar microstructure. The Ti-6Al-4V alloy obtained with BEPM due to a finer microstructure has a significantly better balance of strength and plasticity as compared with conventional Ti-6Al-4V alloy with a similar type of the lamellar microstructure.

Pressure-induced maximum shear strength and transition from shear banding to uniform plasticity in metallic glass

The effect of pressure on the mechanical and physical properties of metallic glasses (MGs) has been widely reported. In experiments, it remains challenging to mechanically loading the materials under extreme hydrostatic pressures and carrying out the corresponding measurements. Here, we investigate the pressure dependence of shear response of a typical brittle FeP MG by using molecular dynamics (MD) simulations. We show that the plastic deformation of this MG loaded at a pressure below 10 GPa is highly localized in the form of a dominant shear band. The plastic flow stress during shear banding is measured to increase with the applied pressure, leading to a pressure-induced strengthening behavior at pressures in the range of 0–10 GPa. As the pressure is increased to a critical pressure of approximately 15 GPa, there occurs a transition in the deformation mechanism from localized shear banding to a delocalized shear deformation mechanism governed by the uniform activation of individual shearing events in the material. At this critical pressure, both the yield strength and the average flow stress are maximized. Further increasing the pressure lowers the strength of the material. The pressure effect on the evolution of different types of atomic clusters is analyzed, revealing that the pressure-sensitivity of atomic clusters in MGs is related to their structural symmetry. Under high hydrostatic pressures, low-symmetry clusters become sluggish due to their larger pressure sensitivity. In such circumstances, high-symmetry clusters with lower pressure sensitivity are relatively more active during shear deformation and evenly distributed in the whole sample, leading to the uniform plastic deformation at high pressures. Our findings calls for further experimental investigations of the effect of pressure on mechanical properties and deformation mechanisms of MGs.

Stacking fault energy-dependent plastic deformation of face-centered-cubic metal nanowires under torsional loading

Using a series of molecular dynamics simulations, we study the plastic deformation of 110-oriented single crystalline face-centered-cubic (FCC) metallic nanowires (NWs) under torsional loading for two representative materials with different stacking fault energies, i.e., gold (Au) and aluminum (Al). A torsional periodic boundary condition is employed to avoid artifacts from otherwise fixed regions at the ends. We analyze the correlation among the surface tension, dislocation structure, and applied torque for Au and Al NWs with different radii to understand the effect of stacking fault energy on the hardening behavior. We discover that, contrary to uniaxial loading, smaller NWs are weaker and less stiff in the elastic regime under torsion; subsequently, we explain the origin by using a continuum model and computing the resolved shear stress from the surface tension. Unlike the elastic response, smaller NWs indicate a higher plastic flow stress during plastic deformation. Finally, we demonstrate that the flow stress increases with the ratio of stacking fault width to NW radius due to the higher density of planar defects such as stacking faults and twin boundaries, which impede dislocation motion.