Strain Hardening Mechanisms Research Articles

Microstructural evolution and strain hardening mechanism of a boron-containing metastable austenitic steel

Quantitative tensile tests were carried out to study the microstructural evolution and strain hardening mechanism of a boron-containing metastable austenitic steel with 13 wt-% Mn and 0.30 wt-% B. The results indicate that the great mechanical properties come from two-stage TRIP effect and the true stress–strain curve can be divided into three stages during the tensile process. In the initial stage, part of the initial austenite transformed to ϵ-martensite through the stacking faults. Then, α’-martensite was formed from austenite directly or through the ϵ-martensite formation, which led to a great rise on the strain hardening rate in this stage. In the last stage, decline of the strain hardening rate was observed due to the co-work between multiple phases and voids.

Understanding Mechanical Properties of Nano-Grained Bainitic Steels from Multiscale Structural Analysis

Steel components working in extreme conditions require materials presenting the highest performances. Nowadays, nanoengineering is being applied to the development of ultra-high strength steels as a key-enabling technology in the steel sector. The present article describes the multiscale structure of nano-grained steels designed using atomic transformation theory and processed by a simple heat treatment. Outstanding mechanical properties for these novel steels are reported, and strain-hardening mechanisms are discussed.

Changes in Mechanical Properties of Ultrahigh Strength Nanostructured Steel Resulting from Repeated High Strain Rate Deformation

The paper contains results of investigation of nanostructured bainitic steel subjected to repeated high-strain-rate deformations using split Hopkinson pressure bar method and uniaxial compression of cylindrical specimens in Gleeble simulator. Steel of chemical composition Fe-0.58%C-1.80%Si-1.95%Mn-1.3Cr-0.7Mo (weight %), after isothermal heat treatment at 210°C, is characterized by following mechanical properties determined at static tensile test: yield strength YS0.2 = 1.3 GPa; ultimate tensile strength UTS = 2.05 GPa; total elongation E = 12%, hardness 610 HV and Charpy-V impact toughness 24 J at +20℃ and 14 J at -40℃. Stress-strain curves obtained for pre-stressed material before the next dynamic compression and after repeated compressions were analysed. Microstructure of the deformed specimens in areas of the dynamic impact was investigated. The effects of the dynamic repeated impact on changes in characteristics of the investigated material, in that on strain hardening mechanism, were established. Critical strains of 5.3% at strain rate 910 s-1 and about 10% at strain rate 50 s-1 for the nanostructured bainite were determined. Exceeding the critical strain under uniaxial repeated high-strain-rate compression, resulted in decreasing of ability of the steel for further plastic deformation and strain hardening.

Strain hardening mechanisms during cold rolling of a high-Mn steel: Interplay between submicron defects and microtexture

The formation of submicron structural defects within austenite (γ), ε- and α′-martensite during cold rolling was followed in a 17.6 wt.% Mn steel. Several probes, including XRD, EBSD, and ECCI-imaging, were used to reveal the complex superposition of the strain hardening mechanisms of these phases. The maximum amount of ε-martensite is observed at a strain of ε = 0.11. At larger strains, the amount of ε decreases suggesting that it precedes the α′-formation (γ → ε → α′). Stacking faults and twins are the main planar defects noticed in ε-martensite. The remaining γ is finely subdivided by stacking faults and twins up to ε = 0.22. From ε = 0.51 on, twinning and multiplication of dislocations are the principal strain hardening mechanisms in austenite. Deformation is accommodated in α′ by the rearrangement of dislocation tangles into dislocation cells plus shear banding at ε = 1.56. During cold rolling, austenite develops a Brass-type texture component, which can be associated to mechanical twinning. ε-martensite presents its basal planes tilted ∼24° from the normal direction towards the rolling direction. The α′-martensite develops and strengthens both, the bcc α- and γ-texture fibers during cold rolling.

Effect of Shear Strain Rate on Microstructure and Properties of Austenitic Steel Processed by Cyclic Forward/Reverse Torsion.

In this work, commercial AISI 304 stainless steel rods were subjected to cyclic forward/reverse torsion (CFRT) treatments at low-speed and high-speed torsion at room temperature. Microstructures in the core and surface layers of the CFRT-treated samples were systematically characterized. Results show that the CFRT treatment can introduce martensite phase on the surface of the rods via strain-induced martensitic transformation. High-speed twisting is more effective in inducing martensite in the surface layer compared to low-speed twisting. During the stretching process, the overall strain-hardening behavior of the gradient material is related to the content of its gradient defects. Higher gradient martensite content results in a higher surface hardness of the material, but less overall tensile properties. The effect of twisting speed on torsion behavior and the strain-hardening mechanisms in tensile of the gradient structured steels was also addressed.

Generation of deformation-induced martensite when cryogenic turning various batches of the metastable austenitic steel AISI 347

Cryogenic turning of metastable austenitic steels allows for a surface layer hardening integrated into the machining process, which renders a separate hardening process obsolete. This surface layer hardening is the result of a superposition of strain hardening mechanisms and deformation-induced phase transformation from austenite to martensite. The activation energy required for the latter depends on the chemical composition of the metastable austenitic steel. It can hence be expected that the austenitic stability of the workpiece material varies depending on the batch and that differences in the metallurgical surface layer properties and thus also in the microhardness result after cryogenic turning. Therefore, in this paper, various batches of the metastable austenitic steel AISI 347 were turned utilizing cryogenic cooling with the same machining parameters. The thermomechanical load during the experiments was characterized and the resulting subsurface properties were investigated. The content of deformation-induced α′-martensite was quantified via magnetic sensor measurements and the distribution was examined using optical micrographs of etched cross-sections. It was found that similar amounts of deformation-induced α′-martensite were generated in the workpiece surface layer for all batches examined. Furthermore, the workpieces were analyzed with regard to the maximal hardness increase and the hardness penetration depth based on microhardness measurements. A significant surface layer hardening was achieved for all batches. This shows that surface layer hardening integrated in the manufacturing process is possible regardless of batch-dependent differences in the chemical composition and thus varying austenite stability of the metastable austenitic steel.

Investigation of grain boundary and orientation effects in polycrystalline metals by a dislocation-based crystal plasticity model

Polycrystalline metal with well-designed grain structure exhibits different strength and ductility compared with the conventional macro scale materials. In the present study, the mechanisms of strain hardening and inhomogeneous deformation in polycrystals are analyzed by considering the interaction between dislocations and grain boundaries. Firstly, a dislocation density based crystal plasticity model is developed and then used to quantitatively study the grain size and orientation effects in polycrystals. With the decrease of grain size, the accumulation and interaction of dislocations are promoted in the grain, resulting in back-stress hardening. Simultaneously, two opposite effects of the inhomogeneity in grain orientation are obtained by evaluating the local Schmidt factors in each grain and their standard deviation. The findings highlight the effect of grain boundary on the strength and provide quantitative insight into the effect of grain orientation on ductility.

On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions

We investigate the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with a nominal composition of Fe49.5Mn30Co10Cr10C0.5 (at. %). Extraordinary strain hardening of the iHEA at 77 K leads to a substantial increase in ultimate tensile strength (∼1300 MPa) with excellent ductility (∼50%) compared to that at room temperature. Prior to loading, iHEAs with coarse (∼100 μm) and fine (∼6 μm) grain sizes show nearly single face-centered cubic (FCC) structure, while the fraction of hexagonal close-packed (HCP) phase reaches up to ∼70% in the cryogenically tensile-fractured iHEAs. Such an unusually high fraction of deformation-induced phase transformation and the associated plasticity (TRIP effect) is caused by the strong driving force supported by the reduced stacking fault energy and increased flow stress at 77 K. The transformation mechanism from the FCC matrix to the HCP phase is revealed by transmission electron microscopy (TEM) observations. In addition to the deformation-induced phase transformation, stacking faults and dislocation slip contribute to the deformation of the FCC matrix phase at low strains and of the HCP phase at medium and large strains, suggesting dynamic strain partitioning among these two phases. The combination of TRIP and dynamic strain partitioning explain the striking strain hardening capability and resulting excellent combination of strength and ductility of iHEAs under cryogenic conditions. The current investigation thus offers guidance for the design of high-performance HEAs for cryogenic applications.

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes.

Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1-3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6-10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength-ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank-Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials.

Mechanical behaviour of Ti-Nb-Hf alloys

Ti-(24-26)Nb-(2-4)Hf at.% alloys were designed by assuming that hafnium has a similar effect to zirconium in the Ti-Nb-Zr system. Alloy specimens were produced using vacuum arc melting and subsequently hot-rolled. Uniaxial tensile testing was then performed both at ambient temperature and in liquid nitrogen at −196°C. While the alloys showed no obvious superelastic behaviour, they exhibited pronounced strain hardening and could achieve high elongations before failure (>30% engineering strain). Post-mortem examination revealed that the mechanism of strain hardening was extensive {332} and/or {211} deformation twinning. Twinning was found to be more prevalent in alloys with 2 at.% Hf compared to those with 4 at.%. The cryogenic temperature deformation also promoted deformation twinning when compared to ambient temperature results. As is the case with other metastable β-Ti alloys, maintaining control over the precipitation of ω phases was found to be crucial for attaining desirable mechanical behaviour. Further, microstructural engineering and alloying may be used to develop strong, lightweight alloys based on the Ti-Nb-Hf system with beneficial strain hardening characteristics for energy absorption, cryogenic and biomedical applications.

Comparing Morphological Evolution during Tensile Deformation of Two Precise Polyethylenes via 2D Fitting of in Situ X-ray Scattering

Some types of precise polyethylenes, or linear polyethylenes containing precisely periodic functional groups, exhibit significant strain hardening under tensile deformation. Here we perform in situ X-ray scattering during tensile deformation of two precise polyethylenes: one with pendant carboxylic acid groups every 21st carbon (p21AA) and another with pendant imidazolium bromide groups every 15th carbon (p15ImBr). p21AA exhibits significant strain hardening and its layered structure orients with the layer normal along the tensile axis, while p15ImBr does not exhibit strain hardening and its layer normal orients perpendicular to the tensile axis. Quantitative evaluation of the 2D fiber patterns shows that p21AA undergoes a morphological transition, from a semicrystalline structure to a nanofibrillar structure, while the morphology of p15ImBr reorients and the chain folding structure is preserved. This suggests that fibrillization plays an important role in the strain hardening mechanism.

Contributions of Various Mechanisms to the Hardening of Differentially Quenched Rails during Long-Term Operation

The contributions of the mechanisms of strain hardening of differentially quenched rails are quantitatively analyzed as functions of the distance from the roll surface. The carbon distribution in the structure of rails after ground tests is studied. The main contribution to the hardening of rail steel is shown to be made by long-range stress fields and the dislocation substructure that forms during deformation. Substantial carbon redistribution is detected in rails during operation.

Influence of Hydrogen-Charging Regime on Strain Hardening and Deformation Mechanism of Hot-Rolled High-Nitrogen Austenitic Steel

Influence of Hydrogen-Charging Regime on Strain Hardening and Deformation Mechanism of Hot-Rolled High-Nitrogen Austenitic Steel

Strain hardening behavior and mechanisms of friction stir welded dissimilar joints of aluminum alloys

Al-Cu alloy was friction stir welded (FSWed) with Al-Zn and Al-Mg alloys, respectively. Tensile properties, strain hardening behavior and fracture mechanisms of the FSWed dissimilar joints were evaluated. Results reveal that the strain-hardening rate of Al-Cu/Al-Zn dissimilar joint is higher than that of base metals due to the presence of finer recrystallized grains in the stir zone, while the strain-hardening rate of Al-Cu/Al-Mg dissimilar joint lies in-between those of base metals. The hardening level of Al-Cu/Al-Zn joint after tensile deformation, defined as HVf/HVw (a ratio of microhardness of fractured joints to that of as-welded joints), exhibits an ‘M’ shape across the welded joint.

Tensile properties of spark plasma sintered AISI 316L stainless steel with unimodal and bimodal grain size distributions

Powder metallurgy associated to spark plasma sintering was used to elaborate near full-dense samples of 316L austenitic stainless steel with unimodal or bimodal grain size distributions. To this aim, two different precursor powders were employed: a ball-milled one giving rise to ultrafine grains and a coarse one, as-received, for grains with conventional size. Sintered specimens were characterized in mechanical tension and their microstructure was revealed using transmission and scanning electron microscopy. Unimodal ultrafine grained samples show a large yield stress and a low ductility with a breakdown in the Hall-Petch relationship. For bimodal samples, a compromise between yield stress and ductility can be found. These features are then discussed in terms of strain mechanisms, grain size distribution and backstress. It is shown in particular that coarse grains contribute to enhance the ductility of the ultrafine grains matrix by modifying both the strain hardening mechanisms and the stress concentration areas.

GaSb and GaSb/AlSb Superlattice Buffer Layers for High-Quality Photodiodes Grown on Commercial GaAs and Si Substrates

The objective of this work is the integration of InGaAs/GaSb/GaAs heterostructures, with high indium content, on GaAs and Si commercial wafers. The design of an interfacial misfit dislocation array, either on GaAs or Si substrates, allowed growth of strain-free devices. The growth of purposely designed superlattices with their active region free of extended defects on both GaAs and Si substrates is demonstrated. Transmission electron microscopy technique is used for the structural characterization and plastic relaxation study. In the first case, on GaAs substrates, the presence of dopants was demonstrated to reduce several times the threading dislocation density through a strain-hardening mechanism avoiding dislocation interactions, while in the second case, on Si substrates, similar reduction of dislocation interactions is obtained using an AlSb/GaSb superlattice. The latter is shown to redistribute spatially the interfacial misfit dislocation array to reduce dislocation interactions.

In situ synchrotron X-ray diffraction investigations of the physical mechanism of ultra-low strain hardening in Ti-30Zr-10Nb alloy

A warm-rolled, metastable β-type Ti-30Zr-10Nb alloy exhibited a peculiar two-stage yielding behavior under uniaxial tensile loading, showing a first plastic stage with obvious strain hardening at 0.4%–10.4% strain and a second plastic stage with ultra-low strain hardening at 10.4%–23.5% strain. In situ high-energy X-ray diffraction (HE-XRD) was used to reveal the stress-induced martensitic transformation scenarios and physical mechanism of the different strain hardening rates. It was found that the deformation-induced phase transformation dominated the onset of the first plastic stage corresponding to the selection of favorable martensitic variants, and their elastic interaction contributed to the obvious strain hardening. HE-XRD experiments further verified that the ultra-low strain hardening rate in the second plastic stage was related to an interesting superelasticity of the martensite, which was characterized by the reversible, stress-induced reorientation of the martensite variants. This reorientation of the martensite variants was primarily due to the rigid lattice rotation of ∼23° about the [110]α″ axis toward the tensile direction. Our investigations provide in-depth understanding of the mechanism of the excellent plasticity with ultra-low strain hardening in β-type titanium alloys.

Microstructure and mechanical performance of welded joint between a novel heat-resistant steel and Inconel 617 weld metal

In the present study, the microstructure and mechanical performance of a tungsten inert gas welded joint between a novel heat-resistant austenitic steel and Inconel 617 weld metal were examined. Migrated grain boundaries were discovered more frequently in the lower portion of the weld metal that solidifies in austenite type. Ti-rich precipitates were observed in weld metal and Cr-rich and Nb-rich precipitates were observed in base metal. The fusion boundary area had the lowest impact toughness due to the large non-homogeneity of the microstructure. Microhardness profiles of the joint were obtained and the backing passes were found to produce the highest microhardness values, which was caused by the grain refinement effect. Ductile failure with slipping separation occurred in those joints ruptured after tensile tests at both room temperature and 923 K. In addition, multiscale precipitates in the joint after high-temperature tensile tests were inspected, where it was found that nanoscale particles contributed to the high-temperature resistance of the joint. Furthermore, microhardness changes in the joint after tensile tests were analysed according to the precipitation and strain-hardening mechanism.

Temperature dependent strain hardening and fracture behavior of TWIP steel

In this study, the strain-hardening and fracture behavior of high manganese austenitic twinning induced plasticity (TWIP) steel at temperatures ranging from 123 K to 773 K were investigated. Tensile tests were combined with electron backscatter diffraction (EBSD) and synchrotron X-ray diffraction (XRD) to study the evolution of microstructure during deformation. Twinning and strain induced ε-martensite transformation were identified as the governing strain hardening mechanisms. The temperature dependent stacking fault energy (SFE) plays a crucial role in determining the prevailing deformation mode. At temperatures below 298 K, the γ→ε-martensite transformation occurred and its volume fraction increased as the temperature decreased. Twinning was the dominant deformation mechanism at 298 K and the twin fraction increased with temperature until a transition temperature of about 473 K, above which, the deformation mode changed from twinning and dislocation glide to only dislocation glide. Serrated flow was observed at temperatures above 233 K and below 473 K, due to a dynamic strain aging (DSA) mechanism specific to TWIP steels, that include the interaction of solute atoms with both dislocations and twins. This plastic instability was related to the propagation of Portevin-Le Chatelier (PLC) bands during deformation. Furthermore, scanning electron microscopy (SEM) of the fractured specimens gave an insight into the change in dimple size and the presence of quasi-brittle islands which were a direct consequence of the various active deformation mechanisms. A dislocation density and temperature dependent crystal plasticity model incorporating both twinning-induced plasticity and transformation-induced plasticity (TRIP) was used to predict the strain hardening and deformation behavior. The predicted twin and ε-martensite fractions are in good agreement with the experimental observations. Furthermore, the proposed modeling strategy can aid in designing new TWIP/TRIP steels.

Development of a gall-resistant stainless-steel hardfacing alloy

This work details the development of a new cobalt-free stainless steel powder metallurgy hardfacing alloy designed to replace Stellite 6, a cobalt-based hardfacing alloy used in nuclear valve applications. The fundamental strategy centers on alloying stainless steels with up to 0.5 wt% nitrogen, which is shown to increase both the volume fraction of hard phase precipitates and the strain-hardening rate of the matrix. The resultant alloy, Nitromaxx, exhibits galling performance that is comparable to Stellite 6, up to 350 °C. This performance is attributed to the suppression of strain localization events associated with galling. In particular, transmission electron microscopy and diffraction measurements from tensile tests show that the nitrogen addition decreases the calculated matrix stacking fault energy and enhances both deformation-induced martensite transformation at room temperature and deformation twinning at elevated temperature. These strain-hardening mechanisms, coupled with the increase in precipitate volume fraction, effectively suppress localization and enhance galling resistance up to 350 °C. The enhanced galling resistance cannot be rationalized in terms of tensile stress-strain response alone.