Work Hardening Mechanisms Research Articles

Additively manufactured hierarchical stainless steels with high strength and ductility.

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

Effects of impact energy on the wear resistance and work hardening mechanism of medium manganese austenitic steel

Medium manganese austenitic steel (MMAS) fabricated through the hot rolling process has been used in the mining, military, and mechanical industries. In this paper, the abrasion performance and hardening mechanism were measured under a series of impact energies. The impact wear was tested at different impact energies from 0.5 J to 6 J using a dynamic load abrasive wear tester (MLD-10). Microstructure and surface morphologies were analyzed using scanning electron microscopy, X-Ray diffraction, and transmission electron microscopy. The results suggest that MMSA has the best wear resistance at 3.5 J and the worst wear resistance at 1.5 J. Furthermore, the wear mechanism and worn surface microstructure change with different impact energies. There are small differences between a large amount of martensite on the worn surfaces under different impact energies and the shapes of dislocation and twins change with different impact energies.

Wear behavior and subsurface layer work hardening mechanism of Fe-24.1Mn-1.21C-0.48Si steel

Abstract The wear tests of Fe-24.1Mn-1.21C-0.48Si austenitic ultra-high manganese steel under different impact energies were carried out by using MLD-10 impact abrasive wear tester. Results showed that after impact abrasive wear, severe plastic deformation occurred and it was inhomogeneous. Worn surface hardness and depth of hardening layer after impact wear increased with the increase of impact energy. The wear resistance increased with the impact energy increasing in the range of 0.5J ~ 1J and 2J ~ 4J. When the impact energy was 2J, the steel had the worst wear resistance. The work hardening mechanism also changed with the increase of impact energy. The work hardening mechanism included dislocation planner slip, high density dislocation tangle, stacking faults, crossed dislocation walls, dislocation cells and mechanical twins.

Ductility of Nanostructured Bainite

Nanostructured bainite is a novel ultra-high-strength steel-concept under intensive current research, in which the optimization of its mechanical properties can only come from a clear understanding of the parameters that control its ductility. This work reviews first the nature of this composite-like material as a product of heat treatment conditions. Subsequently, the premises of ductility behavior are presented, taking as a reference related microstructures: conventional bainitic steels, and TRIP-aided steels. The ductility of nanostructured bainite is then discussed in terms of work-hardening and fracture mechanisms, leading to an analysis of the three-fold correlation between ductility, mechanically-induced martensitic transformation, and mechanical partitioning between the phases. Results suggest that a highly stable/hard retained austenite, with mechanical properties close to the matrix of bainitic ferrite, is advantageous in order to enhance ductility.

Prediction for Flow Stress of 95CrMo Hollow Steel During Hot Compression

The compressive deformation behavior of 95CrMo hypereutectic steel was studied at temperatures ranging from 800 to 1050 °C and strain rates from 0.1 to 3 s−1 on a Gleeble-3500 thermo-simulation machine. The results showed that, with the decrease in deformation temperature and increase in strain rate, the fragmented retained austenite in finer and distributed more uniformly in the ferrite matrix as a result of the inhibited recovery. The recorded flow stress suggested that the stress level decreases with increasing temperature and decreasing strain rate. Based on the classical stress–dislocation relation, the constitutive equations of flow stress determined by work-hardening and softening mechanisms were established. A comparison between the experimental and calculated values confirmed the reliability of the model, and the predictability of the model was also quantified in terms of correlation coefficients and average absolute relative errors, which were found generally above 0.99 and below 2.50%, respectively. In the whole range of strain rate, the activation energy is 419.84 kJ/mol. By further identification based on Schöck’s model and Kocks–Argon–Ashby model, the rate-controlling mechanism is found to be dislocation cross-slip.

In-situ neutron diffraction during tension-compression cyclic deformation of a pearlite steel

Tension-compression cyclic deformation of a pearlite steel was investigated using in-situ neutron diffraction, in order to make clear the mechanisms of high work hardening, large Bauschinger effect and particularly cyclic softening. The high work hardening and large Bauschinger effect are mainly caused by phase stresses generated by plastic misfit strains between ferrite and cementite. Cyclic softening was observed in tension-compression test, the reason of which was presumably softening of ferrite phase due to reversible motion of dislocations. The changes in phase stress and Full-Width at Half Maximum shows good correspondence during cyclic deformation.

On the Mechanisms of Different Work-Hardening Stages in Twinning-Induced Plasticity Steels

The detailed work-hardening behaviors of two twinning-induced plasticity (TWIP) steels with and without Al addition are investigated. The work-hardening rate curves of both TWIP steels can be divided into three stages. The dominant work-hardening mechanism is different at different stages. Dynamic strain aging (DSA) is responsible for the high work-hardening rate at the very beginning of the first stage for the TWIP steel without Al, but the DSA’s contribution is not significant in the TWIP steel with Al. However, DSA may only play a dominant role at the early plastic deformation. For the strain higher than 3 pct in the first stage, the difference of work-hardening rate between the two TWIP steels becomes smaller. This suggests that the main work-hardening mechanism in the first stage changes to the multiplication of dislocations at strains higher than 3 pct in TWIP steel without Al. The increase of work-hardening rate in the second stage is mainly due to the formation of deformation twins in both TWIP steels. Nevertheless, comparing to the TWIP steel without Al, TWIP steel with Al shows a lower work-hardening rate at the second stage. This is due to the fact that the addition of Al increases the critical twinning stress, resulting in a lower twinning capability. Deformation twin plays a more and more important role on the work-hardening with the increase of strain in the second stage due to the increase of twin volume fraction with strain. It is found that, except being obstacles to the dislocation glide, deformation twins can also act as a new source of the emission of partial dislocations. Furthermore, it is also found that dislocations can transmit across the twin boundary and be stored in the twins, implying that deformation twins can also accommodate local strains.

Size effects and temperature dependence on strain-hardening mechanisms in some face centered cubic materials

The mechanical behavior of face centered cubic metals is deeply affected when specimen dimensions decrease from a few millimeters to a few micrometers. At room temperature, a critical thickness (t) to grain size (d) ratio (t/d)c was previously highlighted, under which the softening of mechanical properties became very pronounced both in terms of Hall–Petch relation and work hardening mechanisms. In this work, new experimental results are provided concerning the influence of temperature on this size effect for copper, nickel and Ni–20wt.%Cr, representative of a wide range of deformation mechanisms (i.e. dislocation slip character). It is shown that multicrystalline samples (t/d<(t/d)c) are not deeply affected by an increase in temperature, independently of the planar or wavy character of dislocation glide. For pronounced wavy slip character metals, surface effects in polycrystals (t/d>(t/d)c) are not significant enough to reduce the gap between polycrystal and multicrystal mechanical behavior when the temperature increases. However, a transition from wavy slip to planar glide mechanisms induces a modification of the polycrystalline behavior which tends toward multicrystalline one with a moderate increase in temperature. This work demonstrates that surface effects and grain size influence can be successfully disassociated for the three studied materials using an analysis supported by the Kocks–Mecking formalism. All these results are supported by microscopic investigations of dislocation substructures and compared to numerical simulations using a strain gradient plasticity model.

Development a dislocation density based model considering the effect of stacking fault energy: Severe plastic deformation

Considering the effect of an intrinsic material parameter, stacking fault energy (SFE), a model based on dislocation density is developed to investigate the evolutions of dislocation density, cell size and flow stress during severe plastic deformation of aluminum and nickel. In fact a model is presented considering the work hardening and different annihilation mechanisms for dislocation densities in cell interiors and cell walls. Annihilation terms are developed on the basis of SFE through the model. The calculations show that the total dislocation density, dislocation density in cell interiors, dislocation density in cell walls and flow stress are decreased with increasing SFE. To verify the model results, they are compared with the experimental data and a good agreement is observed.

Modeling and experimentation on multistage work-hardening mechanism in machining with nose-radiused tools and its influence on machined subsurface quality and tool wear

The paper reports on the modeling and respective experimental validation for the formation of the machined subsurface layer in turning with nose-radiused and round tools. An experimental work on the mechanisms of work-hardening of the machined surface and related wear of the cutting tools was conducted for high-speed turning of aged Inconel 718 with whisker-reinforced alumina tools. The model shows that multiple deformations of the machined surface occur when machining with small feeds and tools with large nose radius, thus changing the mechanics of surface formation. Experimental results confirm the localized increase in subsurface hardness in the vicinity of the tool tip. The variation in the degree of work-hardening and the extent of the area affected by it fully agree with the predictions of the model. The model also shows that a significant part of the cutting tool may cut through the extra work-hardened material. Tool wear tests show that the local increase in workpiece hardness results in a localized increase in the wear rate of the cutting tools.

Superior tensile ductility in bulk metallic glass with gradient amorphous structure.

Over centuries, structural glasses have been deemed as a strong yet inherently ‘brittle’ material due to their lack of tensile ductility. However, here we report bulk metallic glasses exhibiting both a high strength of ~2 GPa and an unprecedented tensile elongation of 2–4% at room temperature. Our experiments have demonstrated that intense structural evolution can be triggered in theses glasses by the carefully controlled surface mechanical attrition treatment, leading to the formation of gradient amorphous microstructures across the sample thickness. As a result, the engineered amorphous microstructures effectively promote multiple shear banding while delay cavitation in the bulk metallic glass, thus resulting in superior tensile ductility. The outcome of our research uncovers an unusual work-hardening mechanism in monolithic bulk metallic glasses and demonstrates a promising yet low-cost strategy suitable for producing large-sized, ultra-strong and stretchable structural glasses.

Effect of Cu addition on the microstructure and mechanical properties of Al–30 wt% Zn alloy

The effect of 0.5, 1, 1.5 and 2wt% Cu addition on the microstructure and mechanical properties of Al–30wt% Zn alloy has been investigated by stress–strain tests carried out in the temperature range from 508 to 608K. The work-hardening parameters of the test alloys decreased with increasing the deformation temperature and exhibited discontinuity at 548K, resulting two deformation temperature regions, the low-temperature region (below 548K) and high-temperature region (above 548K). The activation energy of fracture mechanisms has been calculated and found to be 19.6 and 33.2kJ/mol at the low and high temperature regions respectively. The operating mechanisms of work-hardening of the test alloys were confirmed by the analysis of X-ray diffraction patterns.

Prediction of hot flow plastic curves of ISO 5832-9 steel used as orthopedic implants

An austenitic stainless steel ISO 5832-9 used as a biomaterial was torsion-deformed over the temperature range of 1000-1200 °C and strain rates of 0.05, 0.1, 1.0 and 5.0 s- 1. The flow stress curves obtained showed two regions where firstly there is a rising on stress characterized as work hardening mechanism acting and secondly a decreasing in work-softening after a peak stress. The flow curves were modeled by adjusting the experimental data with Zener-Hollomom parameter to construct the constitutive equations that describe the plastic behavior in both regions. The first region was described until the peak stress, taking into consideration the competition between work hardening and recovery while the second one was described applying the softening time of 50% and the Avrami equation. In some hot deformation conditions the simulated curves showed good agreement with the experimental ones while in others conditions the simulated showed differences to experimental curves that was discussed and associated with other mechanisms that acted during hot deformation.

Some aspects on geometric and matrix work-hardening characteristics of sintered cold forged copper alloy preforms

Powder metallurgy (P/M) material subjected to plastic deformation results into densification, however the extended deformation would not only enhance the densification also supplements the strain hardening. Unlike fully dense material that would only undergo strain hardening while plastic deformation, the P/M material leads to pore closure as well; this phenomenon complicates the work hardening mechanism. The present study revealed that both strain and density configures strengthening of P/M preform, which respectively termed as matrix and geometric work hardening. An attempt has been made to delineate some aspects of work hardening behaviour with the influence of different aspect ratios of sintered and cold deformed copper alloy preforms. The preforms were initially prepared through conventional P/M route and finally subjected to cold upsetting under dry friction condition. A statistical analysis has also been introduced to study the quantitative impact of strain and density in the presence of aspect ratio on work hardening rate characteristics.

Work-Hardening and Deformation Mechanism of Cold Rolled Low Carbon Steel

The study reports the mechanical property and microstructure of cold rolled low carbon steel and its work-hardening behavior in the deformation process. The tensile test in room temperature of low carbon steel was implemented for the different cold rolling deformation, the stress-strain curve was draught according to the relationship between strength and deformation and fitted for the polynomial fitting, the strain hardening exponent (n) of test steel was calculated by the Hollomon method. In the whole cold deformation process, the work-hardening of cold rolled steel is significant, work-hardening rate has different degrees decreasewith the deformation increase. The strain hardening exponent is simple and dislocation strengthening is the major cause of hardening processing. The microstructure of test steel was observed after different deformation, the room temperature organization is the ferrite and few pearlite. The original grain is equiaxial and the average grain size is about 23.5 um, and pearlite distributes in ferrite grain boundaries. It was consequently established the cold deformation energy according to dislocation model, the cold deformation energy is main concerned on the plastic deformation to resistance and the initial stress.

Direct observation of Lomer-Cottrell locks during strain hardening in nanocrystalline nickel by in situ TEM.

Strain hardening capability is critical for metallic materials to achieve high ductility during plastic deformation. A majority of nanocrystalline metals, however, have inherently low work hardening capability with few exceptions. Interpretations on work hardening mechanisms in nanocrystalline metals are still controversial due to the lack of in situ experimental evidence. Here we report, by using an in situ transmission electron microscope nanoindentation tool, the direct observation of dynamic work hardening event in nanocrystalline nickel. During strain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. This study provides both the evidence to explain the roots of work hardening at small length scales and the insight for future design of ductile nanocrystalline metals.

Direct Observations of Confined Layer Slip in Cu/Nb Multilayers

In situ nanoindentation of a 30 nm Cu/20 nm Nb multilayer film in a transmission electron microscope revealed confined layer slip as the dominant deformation mechanism. Dislocations were observed to nucleate from the Cu-Nb interfaces in both layers. Dislocation glide was confined by interfaces to occur within each layer, without transmission across interfaces. Cu and Nb layers co-deformed to large plastic strains without cracking. These microscopy observations provide insights in the unit mechanisms of deformation, work hardening, and recovery in nanoscale metallic multilayers.

A new analysis of yielding and work hardening in AA1100 and AA5754 at low temperatures

Microstructural evolution determined by mechanical testing is usually modelled from perceived mechanisms of work hardening during dislocation passage and storage in complex arrays. The predicted stress–strain behaviour is then compared with the measured one. Recently a new functional constitutive relation was derived by Saimoto and Van Houtte based on Taylor slip analysis that is optimally fitted to replicate the measured data. The curve fitting parameters arise from the incremental strain Δγ producing a mean slip distance λ and the retained dislocation density ρ assessed from the flow stress τ. A method has been derived to precisely determine λ from the measured data. Moreover, the yield stress determined from the optimum curve fitting is derived from strains beyond the yield point and, hence, has the capacity to separate the solid solution and grain size effects from that of the yield phenomenon. Typically two fit curves are required, the intersection of which defines τ3, γ3 and λ3, akin to the stage II to stage III transition in single crystals. A tabulation of these parameters permits reproduction of the mechanical curves. Furthermore, a plot of flow stress τ vs. 1/λ results in a description of work hardening evolution onto which the inverse cell size 1/dC can be superimposed. Using the results from standard materials testing practice the temperature dependence and evolution of the pertinent parameters show that point defect drag effects are prominent below 273K for AA1100 but not for AA5754.

The effect of mechanical alloying on Al2O3 distribution and properties of Al2O3 particle reinforced Al-MMCs

AbstractThe properties of particle-reinforced aluminum composites depend on the microstructure and evolved uniformity distribution of particles in the matrix. The distribution of Al2O3 particles in the matrix and microstructure properties of varying volume fraction of particles up to 15% Al2O3 particle-reinforced Al metal matrix composites produced by the mechanical alloying technique was investigated. Alloying was performed in a planetary ball mill using a milling time varying from 0 h to 7 h, a ball-to-powder ratio of 10:1, and a ball mill velocity of 400 rpm. The produced compositions were cold-pressed at 700 MPa with a single action and sintered at 600°C for 3 h under Argon gas atmosphere. The relative density (both pressed and sintered), porosity and hardness of composites were also examined. The results of mechanical alloying processing were investigated with scanning electron microscopy (SEM), X-ray diffraction and laser particle size analyzer. SEM observations showed that relatively homogeneous distribution of Al2O3 reinforcement in the matrix could be obtained by mechanical alloying after 5 h. Moreover, the variation in hardness of the composites with alumina volume fraction and milling time was observed to be a strong function of the work hardening mechanism.

Quantitative Analysis of Work Hardening and Dynamic Softening Behavior of low carbon alloy Steel Based on the Flow Stress

In this study, the constitutive equation and DRX(Dynamic recrystallization) model of Nuclear Pressure Vessel Material 20MnNiMo steel were established to study the work hardening and dynamic softening behavior based on the flow behavior, which was investigated by hot compression experiment at temperature of 950°C, 1050°C, 1150°C and 1250°C with strain rate of 0.01s−1, 0.1s−1 and 10s−1 on a thermo-mechanical simulator THE RMECMASTOR-Z. The critical conditions for the occurence of dynamic recrystallization were determined based on the strain hardening rate curves of 20MnNiMo steel. Then the model of volume fraction of DRX was established to analyze the DRX behavior based on flow curves. At last, the strain rate sensitivity and activation volume V* of 20MnNiMo steel were calculated to discuss the mechanisms of work hardening and dynamic softening during the hot forming process. The results show that the volume fraction of DRX is lower with the higher value of Z (Zener–Hollomon parameter), which indicated that the DRX fraction curves can accurately predicte the DRX behavior of 20MnNiMo steel. The storage and annihilation of dislocation at off-equilibrium saturation situation is the main reason that the strain has significant effects on SRS(Strain rate sensitivity) at the low strain rate of 0.01s−1 and 0.1s−1. While, the effects of temperature on the SRS are caused by the uniformity of microstructure distribution. And the cross-slip caused by dislocation piled up which beyond the grain boundaries or obstacles is related to the low activation volume under the high Z deformation conditions. Otherwise, the coarsening of DRX grains is the main reason for the high activation volume at low Z under the same strain conditions.