Strain Hardening Mechanisms Research Articles

Experimental study of forming limit diagram and mechanical properties of aluminum foils processed by the accumulative roll bonding

In this article, for the first time, the forming limit diagram (FLD) and mechanical properties of aluminum foil samples processed by the accumulative roll bonding (ARB) process have been studied experimentally. For this purpose, thin aluminum foils with a thickness of 200 microns have been produced using ARB in five passes at ambient temperature. By rising the number of ARB passes, the ultimate tensile strength (UTS) enhanced drastically, and at the end pass of ARB, it reached 393 MPa, about 5.9 times larger than the initial sample. Also, during the ARB process, the applied strain increased, and the thickness of the layers decreased, and the bonding quality between layers improved. SEM images of tensile fracture surface after five cycles showed the mechanism of fracture retained ductile. However, due to the unevenly applied strain, the dimples were drawn in different directions, and their depth and number were reduced relative to the raw material. The area under the FLDs, a criterion of formability, declined sharply after the first pass and then increased at a low rate until the final pass. The trend of similar changes of formability in the tensile (elongation) and Nakazima tests (FLDs) was reported. Responsibility for all mechanical properties and ductility changes is related to the ARB process’s nature and the two dominant mechanisms of strain hardening and grain refinement.

Effects of grain size on body-centered-cubic martensitic transformation in metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy

Systematic investigation of microstructure characterization and mechanical property was performed on metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy (HEA) with varying the grain size. The grain coarsening led to the decrease in stability of face-centered-cubic (FCC) phase, and consequently generated the formation of laminate-morphology hexagonal-close-packed (HCP) and butterfly-morphology body-centered-cubic (BCC) martensite. During the tensile test, a transformation-induced plasticity (TRIP) from the FCC to BCC via the intermediate HCP occurred, and the accelerated transformation rate of BCC increased the strain-hardening rate. Our results suggest that microstructural evolutions related to thermally-induced martensitic transformation and strain-hardening mechanisms can be well interpreted by understanding the FCC to HCP to BCC martensitic transformation.

Evaluation of novel jointless engineered cementitious composite ultrathin whitetopping (ECC-UTW) overlay

The objective of this study was to evaluate the construction, material characterization, initial structural assessment, and cost of a novel jointless Engineered Cementitious Composite Ultra-Thin Whitetopping (ECC-UTW) overlay. After construction, early-age cracks (from less than 0.25 mm to up to 2.54 mm width) were observed in the 101.6-mm (4 in.) jointless ECC-UTW section; yet, the majority of the cracks coincided with areas where fiber clumps were observed during construction and places of heavy instrumentation. On the other hand, no cracks were observed in the 63.5-mm (2.5 in.) jointless ECC-UTW at early ages; yet, when winter arrived, micro-cracks (<0.25 mm in width) developed. These micro-cracks were attributed to the normal functioning of the ductile pseudo strain-hardening (PSH) mechanism of ECC materials. Falling Weight Deflectometer (FWD) evaluation conducted before and after the construction of the UTWs showed a significant reduction in pavement deflection after construction. This deflection reduction was much more substantial for thicker 101.6-mm UTWs; yet, no major difference in deflection reduction was found between the 101.6-mm ECC and the 101.6-mm regular concrete UTW sections. The 28-day compressive and flexural strength of ECC evaluated from field specimens underperformed that of previous studies by 9.6% and 14.8%, respectively; however, deflection capacity slightly outperformed that of previous studies by 4.1%. The flexural performance of the ECC material was vastly superior to that of regular concrete exceeding the flexural strength by 92%. All specimens evaluated in uniaxial tension exhibited a PSH behavior after first-cracking; however, the tensile strength and tensile ductility values obtained underperformed expected values for ECC M3-15% and exhibited substantial variability. Uniaxial tensile test results from field specimens were deemed unreliable. Cost analysis results showed that a cost reduction of 14.8% per lane kilometer is expected by implementing the proposed jointless ECC-UTW system compared to that of the regular jointed concrete UTW. The greatest construction cost savings for utilizing ECC originates from its potential jointless attribute and its reduced thickness.

Current Challenges and Opportunities in Microstructure-Related Properties of Advanced High-Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure–property relations of advanced high-strength steels (AHSS). These alloys constitute a class of high-strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels, etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation, and twinning effects. Recent developments on microalloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modeling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation, and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.

High-energy synchrotron x-ray study of deformation-induced martensitic transformation in a neutron-irradiated Type 316 stainless steel

An unusual tensile deformation behaviour in the form of a propagating band along the sample gauge was observed in two neutron-irradiated 316 stainless steel samples during room-temperature tests, leading to a combination of high strength and high ductility. These bands were not observed in an unirradiated counterpart. With the help of in situ high-energy synchrotron x-ray diffraction, the phase-specific crystal information was tracked at different deformation levels in each sample. Post-irradiation and post-deformation samples were examined using electron microscopy to characterize various microstructural features. All samples displayed a deformation-induced martensitic phase transformation, which was identified as a second strain-hardening mechanism accompanying the dislocation hardening. The deformation-induced martensitic transformation was rationalized by the effect of applied stress on the effective martensite start temperature. The results showed that the irradiation did not alter the dislocation hardening and the martensitic transformation mechanisms, but the increased yield strength in irradiated materials facilitated the localized phase transformation at the onset of plastic deformation, in contrast to the unirradiated material which required pre-straining. The hardening effect of the martensitic transformation reduced the tendency towards necking and mitigated the loss of ductility in the irradiated material by carrying the deformation in the form of a propagating band. Despite the beneficial effect from the martensitic transformation, this study indicates that this mechanism cannot not be activated at typical operating temperatures of nuclear reactors.

Calibration Method for Monitoring Hygro-Mechanical Reactions of Pine and Oak Wood by Acoustic Emission Nondestructive Testing.

The main issue of wood is its sensitivity to Relative Humidity (RH) variations, affecting its dimensional stability, and thus leading to crack formations and propagations. In situ structural health monitoring campaigns imply the use of portable noninvasive techniques such as acoustic emission, used for real-time detection of energy released when cracks form and grow. This paper proposes a calibration method, i.e., acoustic emission, as an early warning tool for estimating the length of new formed cracks. The predictability of ductile and brittle fracture mechanisms based on acoustic emission features was investigated, as well as climate-induced damage effect, leading to a strain-hardening mechanism. Tensile tests were performed on specimens submitted to a 50% RH variation and coated with chemicals to limit moisture penetration through the radial surfaces. Samples were monitored for acoustic emission using a digital camera to individuate calibration curves that correlated the total emitted energy with the crack propagation, specifically during brittle fracture mechanism, since equations provide the energy to create a new surface as the crack propagates. The dynamic surface energy value was also evaluated and used to define a Locus of Equilibrium of the energy surface rate for crack initiation and arrest, as well as to experimentally demonstrate the proven fluctuation concept.

Dynamics of edge dislocation in Cu–Ni solid solution alloys at atomic scale

The mobility of edge dislocation in Cu–Ni solid solution alloys within a wide range of temperatures 100–1100 K and Ni concentrations 0–30 at.% is studied by molecular dynamics simulation. Two different regimes of the influence of substitutional Ni atoms on the motion of edge dislocation are observed. In the first regime, in which the velocity of edge dislocation is much less than the shear sound speed, Ni atoms act as barriers and slow the dislocation down. In this regime, the dependence of the edge dislocation velocity on Ni concentration weakens with increasing temperature. In the second regime, in which the dislocation moves faster, with a speed closer to the shear sound speed, Ni atoms accelerate its motion. The latter result is a consequence of higher shear sound speed along the [110] direction of Cu–Ni solid solution alloy associated with higher Ni content. The distance between Shockley partial dislocations representing the core of edge dislocation is analyzed as a function of temperature and Ni concentration. The partial splitting increases with temperature and decreases with Ni concentration. The latter result might be important for predicting strain hardening mechanisms in Cu–Ni alloys.

Modified Hadfield Steel for Castings of High and Low Gouging Applications

Modified Hadfield steel represents as a new grade of high manganese steel, that is ladle-treated instead of long-term heat treatment process. It has an odd structure, in which austenite is reinforced by new eutectic constituent. Regarding the segregation coefficient of alloying elements, in particular Mn, and C, alteration in SFE is significantly affected. Reduction in stacking fault energy (SFE) value promotes the strain hardening mechanism of austenite structure and consequently on the wear performance of Hadfield steel. Based on SFE of austenite structure, the wear abrasion resistance of conventional heat-treated Hadfield steel and ladle-treated Hadfield steel was detected under either low or high gouging wear abrasion conditions. OM and SEM observations show that micro-plowing mechanism is dominant at ladle-treated Hadfield surface, while micro-cutting mechanism is strongly observed at the surface of conventional Hadfield steel. By using XRD and colored etching technique, it was observed that e-lath martensite and slip bands are strongly generated through the strain hardening mechanism in a ladle-treated Hadfield steel. As a result of micro-plowing and e-martensite, ladle-treated Hadfield steel performs better than conventional Hadfield steel either at low or high gouging applications.

The impact of grain-scale strain localization on strain hardening of a high-Mn steel: Real-time tracking of the transition from the γ → ε → α’ transformation to twinning

Strain partitioning and localization were investigated in a high-Mn steel (17.1 wt.% Mn) during tensile testing by a correlative probing approach including in-situ synchrotron X-ray diffraction, micro- digital image correlation (μ-DIC) and electron microscopy. By combining Warren's theory with the μ-DIC analysis, we monitored the formation of planar faults (stacking faults and mechanical twins) and correlated them with the local strain partitioning behavior within the microstructure. Starting with an initial microstructure of austenite (γ) and athermally formed ε- and α’-martensite, strain accumulates preferentially near the γ/ε interfaces during tensile straining. The local microscopic von Mises strain (εvM) maps obtained from μ-DIC probing show that these local strain gradients produce local strain peaks approximately twice as high as the imposed macroscopic engineering strain (ε), thus locally triggering formation of ε-martensite already at early yielding. The interior of the remaining austenite, without such interfacial strain peaks, remained nearly devoid of planar faults. The local strain-driven growth of the ε-domains occurs concomitantly with the α’-martensite formation. At intermediate macroscopic applied strains, austenite grain size is considerably reduced to a few nanometers and the associated γ/ε interfacial microscopic strain peaks increase in magnitude. This scenario favors twinning to emerge as a competing strain hardening mechanism at engineering strain levels from ε = 0.075 onwards. At large tensile strains, the γ → ε → α’ transformation rates tend to cease making both twinning and SFs formation to operate as the main strain hardening mechanisms. The findings shed light on the transformation micro-mechanisms in multiphase Mn-TRIP steels by revealing how strain localization among the constituents can directly influence the kinetics of the competing strain hardening mechanisms.

The effects of double notches on the mechanical properties of a high-strength pipeline steel under hydrogen atmosphere

Tensile tests and fatigue life tests are performed on double-notched specimens in hydrogen and nitrogen atmospheres to investigate the effects of double notches on the mechanical properties of a high strength pipeline steel. The results show that the fracture occurs at the notch with a lower stress concentration factor (Kt), which is governed by the combination of the stress concentration and the strain hardening caused by plastic deformation in the tensile process. Hydrogen gas accelerates the crack initiation and growth, but it doesn't affect the competitive mechanism of stress concentration and strain hardening.

Effect of Structure on the Critical Stresses and Strains in Titanium Nickelide-Based Alloys

The effect of the test temperature on the mechanical behavior of a titanium nickelide–based alloy is considered. The behavior of a titanium nickelide–based alloy is shown to be substantially dependent on the level of stresses and strains of the material. The critical stresses are determined by the solid-solution, grain-boundary, strain, and precipitation hardening mechanisms. The maximum critical strain can be achieved by a combination of several hardening mechanisms.

Cocktail effects in understanding the stability and properties of face-centered-cubic high-entropy alloys at ambient and cryogenic temperatures

The multimetallic cocktails provide vast compositional space with unlimited elemental combinations and structural possibilities for alloy design. The crystallographic metastability and thermodynamic instability offer opportunities for the activation of athermal transformations and intermetallic phase formation, particularly when exposed to mechanical perturbations and thermal input. Elemental interactions between the constituent and/or interstitial elements attribute to the remarkable interstitial strengthening effect. The associated strengthening mechanisms and strain-hardening mechanisms are also discussed. Here, we attribute the above-mentioned properties to cocktail effects, which is originated from the synergistic multicomponent alloying. These results provide the insights for the development of structural materials via cocktail effects.

Tensile properties and strain hardening mechanism of Cr-Mn-Si-Ni alloyed ultra-strength steel at different temperatures and strain rates

To reveal the relationship between the microstructural morphologies, tensile behaviors, and strain hardening mechanism of Cr-Mn-Si-Ni alloyed ultra-strength steel, uniaxial tensile tests of a Cr-Mn-Si-Ni alloyed steel were conducted at room temperature (RT) and elevated temperatures with different strain rates in this study. When the tensile test was carried out at RT, increased strain rate led to increased dislocation density of the tensed Cr-Mn-Si-Ni alloyed steel specimen and resulted in increased value of yield strength. Because the dislocation entanglement and strain hardening of tensed Cr-Mn-Si-Ni alloyed steel were inhibited by increased strain rate, decreased value of ultimate strength was measured. During tensile test at RT, the dislocations rearranged and transformed to low-angle grain boundaries and high-angle grain boundaries sequentially. Above microstructural evolution resulted in microstructural refinement of Cr-Mn-Si-Ni alloyed steel. The strain hardening of Cr-Mn-Si-Ni alloyed steel during tensile test at RT was divided into four stages according to the calculated values of strain hardening rate. The microstructural evolution behaviors including accumulation of dislocations, generation and transformation of slips took place in different stages of strain hardening were affected significantly by the strain rate. Based on the true stress-true strain curves, the effects of the tensile temperature and strain rate on the values of the yield strain, fracture strain, yield strength, and ultimate strength of Cr-Mn-Si-Ni alloyed steel tensed under different experimental conditions were studied. A two-stage constitutive model of Cr-Mn-Si-Ni alloyed steel associated with dislocation density was established based on the experimental results.

Recrystallization and Strengthening Mechanism in Friction-Stir-Processed Al Powder Compacts

Green Al powder compacts were subjected to densification by friction stir processing (FSP). Prior to FSP, the Al powder was cold compacted at three different pressures of 50, 200 and 380 MPa to achieve green densities of 2.1, 2.3 and 2.5 g/cm3, respectively. After FSP density increased to 2.7 g/cm3 (100% densification), green density had effect on microstructure and properties obtained after FSP. The size and shape of the nugget area was found to depend on compaction pressure though the FSP parameters were unchanged. After FSP, the sample compacted at highest compaction pressure showed fine and recrystallized grains, whereas the sample compacted at lowest compaction pressure showed elongated grains with higher dislocation density. An extensive microstructural investigation suggested that grain refinement occurred by continuous dynamic recrystallization. Strengthening in sample with fine, recrystallized microstructure was dominated by grain boundary strengthening mechanism, whereas, in unrecrystallized microstructure, strengthening was due to strain hardening mechanism.

Microstructural and mechanical properties of a novel Al-based hybrid composite reinforced with metallic glass and ceramic particles

A new type of Fe-based metallic glass (FMG) and SiC reinforced hybrid composite was successfully developed. The current work was set to evaluate the microstructure and mechanical properties of the hybrid composite consolidated through the spark plasma sintering (SPS) process. The densities of the samples were determined in order to examine the performance of the sintering process. The experimental results indicated that, in the composites having larger amounts of FMG particles, the reinforcements/matrix interfaces were free from any discontinuities. The quantitative analysis of microstructural features revealed that the more homogenous distribution of reinforcing particles was occurred for FMG rich composites. The microstructural analysis of the samples pointed that the amorphous structure of the FMG reinforcement remained unchanged and no interfacial chemical products were created in the consolidated samples. It was also found that using FMG along with SiC particles in the samples led to the more strengthening of the Al matrix compared to using just one type of reinforcement particles. In the current study, the hybrid composite reinforced with 7 vol% of FMG and 3 vol% of SiC particles demonstrated the optimum combination of compressive yield strength (98 MPa) and strain to fracture (62.8%). The strengthening mechanisms in all the consolidated samples were calculated quantitatively considering the load bearing of reinforcing particles, grain boundary and strain hardening mechanisms. The strain hardening with the contribution of about 30% in the yield strength was the predominant strengthening mechanism in the composite samples. Also, there was a reasonable agreement between the calculated and experimentally obtained yield strength for hybrid composite samples.

An experimental study on lap joining of multiple sheets of aluminium alloy (AA 5754) using friction stir spot welding

Friction stir spot welding (FSSW) process is widely used in the automotive industry for a range of applications such as battery components, standard wire connectors and terminals. This manuscript addresses two grand challenges in the arena of FSSW, hitherto, unaddressed in the extant literature: (i) lap joining of thin sheets (0.3 mm thickness) of AA 5754 alloy and (ii) lap joining of more than two sheets using FSSW. To accomplish this task, a novel pinless convex-shaped tool was designed to alter the stress state while gradually advancing the tool which led to achieving stress state necessary for obtaining defect-free lap joints. The weld joints were inspected by optical microscopy, SEM imaging and analysed by nanoindentation tests and Vickers microindentation tests for assessment of the quality of the weld interface (WI). Process parameters of FSSW such as torque on the tool and axially applied load were used to analytically obtain the average local measure of peak normal and axial stresses as well as the coefficient of friction in the contact zone. In samples welded at low rotational speeds, the strain-hardening mechanism was seen dominating in contrast to samples welded at higher rotational speeds, which showed thermal softening. As a direct consequence of this, the samples welded at low rotational speeds showed much higher hardness at the weld surface than the samples welded at higher speeds. A strong transition of strain hardening to thermal softening was noticeable beyond an applied strain rate of 400 s−1.

Enhancement of mechanical properties for Mg-9Li-1Zn alloy by accumulative roll bonding

The microstructures and mechanical properties of LZ91 alloys by accumulative roll bonding (ARB) were firstly investigated. The results show that the cycle numbers of ARB process and preheating temperatures before ARB process have an impact on the microstructures and mechanical properties of LZ91 alloys. The microstructures of LZ91 sheets after ARB perform two main phases of α and β in the fibrous form. ARB process can drive more numbers of crystallographic planes for α and β phases in LZ91 alloys to proof the grain refinement after ARB. More cycle numbers of ARB on LZ91 sheets makes the fibrous microstructures finer and increases the mechanical properties such as hardness and ultimate tensile strength due to the mechanism of strain hardening and grain refinement.

Characterization of the microstructure and deformation substructure evolution in a hierarchal high-entropy alloy by correlative EBSD and ECCI

We investigate the microstructure and deformation substructure evolution in an additively manufactured 1.8 at.% N-doped FeCoNiCr high-entropy alloy (HEA) by means of correlative electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The combinational use of EBSD and ECCI uncovers hierarchal structures of the HEA at multiple length scales, which consist of a bimodal grain structure, low angle boundaries, and dislocation networks, along with the quantitative evolution of geometrically necessary dislocations (GNDs) distribution. The ECCI technique shows comparable resolution to transmission electron microscopy (TEM) in terms of characterizing the dislocation substructures. We further clarify the strain hardening mechanisms by investigating the deformation microstructures. The excellent strain hardening rate of the HEA even at large strain is due to the jointly activation of slip bands and mechanical twining. The deformation induced twining shows strong orientation dependence.

Strain hardening behaviors and mechanisms of polyurethane under various strain rate loading

AbstractThe uniaxial tension experiments are performed on thermoplastic polyurethane to investigate its mechanical behaviors and related potential mechanisms, and the loading strain rate is designing to be wide ranging from 0.0001 to 1 s−1. It is found that the polyurethane presents an obvious rate‐dependence, and the stress strain curves share distinct strain hardening characteristics under the investigated strain rates. Furthermore, the strain hardening ratios are sharing nearly same trends and appear to be influenced by both strain rate and the induced adiabatic heating. Besides, the ratio is also strain‐dependent on previous loading history. Then, a two‐dimension unit cell model is built to investigate potential equivalent mechanisms, of which the hard phase as inclusion is equivalent with crystallization zone, crosslinking sites, and so forth. The simulation results facilitate to explain the distinct strain hardening ratios, even for the matrix from the extrapolated curves under super‐low strain rate loading. Finally, the analogic mechanisms of equivalent hard inclusions are proposed, which can reasonably explain the strain rate‐ and strain‐dependence characteristics of polyurethane mechanical behaviors.

A novel technique to form gradient microstructure in AA5052 alloy

In the present research, the microstructure and mechanical properties of AA5052 alloy with gradient microstructure produced by a novel technique based on hot compression bonding were investigated. It was found that a gradient microstructure with a strain-hardened core and a grain-refined shell formed by hot compression bonding. Some recrystallization mechanisms occurred near the surface of the processed samples including discontinuous dynamic recrystallization (DDRX), geometric dynamic recrystallization (GDRX), and particle stimulated nucleation (PSN). The ultimate tensile strength (UTS) strength of the AA5052 significantly increased after the hot compression bonding (280.7 MPa) without too much loss in ductility (18.9%). This was owing to the gradient microstructure (both grain boundary strengthening and strain hardening mechanisms). By comparison, the good combination of strength and ductility of the present AA5052 alloy distinguished it from the previously reported AA5052 alloy processed by other techniques. The Portevin-Le Chatelier (PLC) effect was clearly visible in the stress-strain curves of the initial and processed AA5052. There was an increase in the critical strain and stress with the increase in the number of passes. The formation of the gradient microstructure after the third, fourth, and fifth passes decreased the severity of the PLC effect. In this study, the recovery of strain hardening rate (RSHR) observed for the first time in the AA5052 alloy after the third, fourth, and fifth passes resulting in a good combination of strength and ductility. The fracture surfaces indicated that as the distance from the surface increased, the dimple depth in the fracture surface gradually increased. On the other hand, with increasing the number of passes, the size of dimples decreased owing to the formation of more recrystallized fine grains at the higher number of passes.