Work-hardening Capacity Research Articles

Investigating the microstructure-property relationship in Mg matrix composites reinforced by particles of different sizes using experiments combined with finite element simulation

In this study, Ti particles (size of 45, 38, 15, and 5 μm) with a mass fraction of 5% reinforced the Mg-9Al-1Zn (AZ91) magnesium matrix composites are fabricated via spark plasma sintering method. Microstructure observation reveals the interfacial zone is composed of Mg17Al12 and Al-rich phase products. Mechanical testing demonstrates that the elongation of the AZ91-Ti (15 μm) composite reaches the peak value of 21.8%, the ultimate tensile strength can be maintained at 211 MPa, and the optimal elastic modulus is up to 47.85 GPa. The composites maintain high strength mainly through heterogeneous deformation induced (HDI) strengthening and thermal mismatch strengthening. From finite element simulation analysis, the relative small size of Ti particles can restrain strain concentration and facilitate stress conduction, making the strain and stress distributing uniformly. Ti particles with appropriate size can enhance the work hardening capacity, leading the AZ91-Ti composite to possess excellent comprehensive mechanical performance.

Investigation of enhanced strength anisotropy in an extruded Mg–Al–Ca–Mn alloy at cryogenic temperature

This study systematically compared the microstructural evolution and mechanical behavior of the extruded AXM10304 alloy during tensile deformation along both the extrusion direction (ED) and transverse direction (TD) at cryogenic temperature (CT) and room temperature (RT). The results indicate that the variations in work hardening mechanisms contribute to the observed strength anisotropy in both the ED and TD samples. The yield strength discrepancy between different orientations is ∼88 MPa at RT and ∼179 MPa at CT. It is evident that the alloy exhibits more pronounced strength anisotropy during cryogenic deformation. It is noteworthy that in the CT-ED samples, the ratio of the critical resolved shear stress (CRSS) for pyramidal I and II to that of basal <a> dislocations is relatively minimal, facilitating the activation of more pyramidal dislocations. Cross-slip in CT-ED provides a relatively facile path for dislocation slip, resulting in material softening during straining. The CT-TD sample exhibits the highest work hardening capacity and a consistent dislocation storage rate. Twinning significantly contributes to the hardening observed in the TD samples. This study also illustrates that tailoring twinning morphology to a refined needle-like shape through temperature manipulation can further enhance the work hardening of Mg alloys.

A novel multistage forming process with intermediate annealing for PEMFC titanium bipolar plates with fine flow channels

In this study, the preform annealing process is first introduced to improve the formability of titanium bipolar plates, comprising preforming, intermediate annealing and final shaping stages. Heat-assisted two-stage tensile tests are conducted to understand the mechanical properties evolution and the underlying mechanisms. Optimal annealing at 600 °C enables plastic recovery and recrystallization in pre-deformed titanium sheets without significant grain growth, leading to an enhancement in both work hardening capacity and forming limits. Feasibility of applying preform annealing to titanium bipolar plates is verified with three case studies, demonstrating its effectiveness in enhancing thickness uniformity and forming limits of microchannels compared to conventional room-temperature stamping. This facilitates the fabrication of titanium bipolar plates with narrow pitch (1.2 mm), high aspect ratio (0.79) and narrow wall angle (5°). Hence, preform annealing provides a potential solution for fabricating titanium bipolar plates with fine flow channels for high-power proton exchange membrane fuel cells.

Microstructural inhomogeneity and associated hetero mechanical behavior in advanced bainitic weathering steel weldments

A new filler metal was developed to join the new generation weathering steel Q690qNH. Microstructural evolutions and the associated mechanical behavior of weldments were comparatively studied in hot rolled and tempered thick Q690qNH plates with initial microstructure of lath bainite and granular bainite respectively. Phase transformations in the HAZ promote the work hardening capacity of the weld jointed tempered Q690qNH (Q690T) plate and enable the joints to exhibit a different mechanical response upon the tensile loading in comparison to the weld jointed hot rolled Q690qNH (Q690R) plate. Prior austenite reconstruction was carried out to understand the crack nucleation behavior in HAZ. The plastic incompatibility between martensitic-austenitic (M-A) and bainitic ferrite leads to the nucleation of plastic micro-voids from the prior austenite grain boundary or M-A boundary. The fracture of Q690R joint is caused by coalescing micro-voids in HAZ with a strength reduction of 10%–14% that caused by the welding process. The continuous yielding behavior regarding the retained austinite in HAZ results in a competition of the plastic deformation in each subzone of the Q690T joint, of which the catastrophic fracture is dominated by the initial microstructure comprising the ferrite and the granular bainite.

Strong and ductile niobium-based refractory alloy via deformable zirconia nanoparticles

The use of dispersed precipitates is an effective method for strengthening metals, but it typically results in a reduction in ductility. A potential way to overcome this strength-ductility trade-off is through the incorporation of coherent nanoscale deformable precipitates. Here, we present an approach of designing and fabricating a niobium-based (Nb-5W-2Mo-1Zr, Nb521) alloy with copiously and semi-coherently dispersed nanoscale zirconia (ZrO2) precipitates through laser powder bed fusion followed by heat treating. The intergranular ZrO2 precipitates at grain boundaries underwent tetragonal-to-monoclinic martensitic phase transformation and deformation twinning, while the intragranular ZrO2 precipitates within the matrix experienced deformation twinning and substantial plastic deformation during tension. The combination of transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) from the nanoscale ZrO2 precipitates provides sufficient capacity to accommodate strain and relieve stress concentration. Additionally, the intragranular ZrO2 precipitates effectively piled up incoming dislocations from the Nb matrix to form high-density dislocation networks during tension, which interacted with each other via the Orowan mechanism and finally resulted in its significant plastic deformation. These mechanisms contribute to the exceptional tensile properties of the Nb521 alloy, including an ultimate strength of 628.7 ± 2.3 MPa and a ductility of 17.8 ± 0.6% with a superior work-hardening capacity (∼230 MPa), achieving a record of strength and ductility combination for the Nb521 alloys fabricated by additive manufacturing to date. This strategy represents a promising approach for designing dispersion-strengthened refractory alloys with excellent synergy of strength and ductility.

Revealing extraordinary work-hardening capacity in a high-entropy alloy with homogeneous composite structures

High performance structural metal materials with high strength and certain ductility have a wide range of critical applications in fields such as aviation, aerospace, transportation. A vein-shaped structure composed of recrystallized ultrafine grains was introduced into a nano-precipitated high-entropy alloy (HEA) by inter-critical annealing after cold rolling. The vein-shaped structure coupled with nanoprecipitates contributes a remarkable improvement of mechanical properties, with yield strength enhanced from 1232 MPa to 1510 MPa, ultimate tensile strength from 1544 MPa to 1745 MPa, and uniform elongation from 1.84 % to ∼7 %, compared to the as-rolled matrix. The synchronized increase in strength and ductility is the outcome of controlling multiscale strengthening mechanisms from tailoring dislocations, nanoprecipitates, bimodal grains, and deformation microbands, which activate the potential work-hardening capacity in the present HEA.

Effect of SiC nanoparticle on microstructure and mechanical properties of graphene nanosheet (GNS) reinforced aluminum composites

Homogeneous dispersion of high-content GNS is the challenge and key to achieve high mechanical properties of graphene reinforced aluminum matrix composites (GNS/Al). In traditional methods, the homogeneous dispersion of GNS is often achieved by prolonging the ball milling time or increasing the ball milling energy, which is often accompanied by severe damage to GNS structure. Here, a two-step ball milling process was employed to introduce SiC nanoparticles (SiCnp) in GNS/Al composites, which improves the GNS distribution while also playing an additional strengthening role. The Al flakes with a large lateral to thickness ratio were produced by a first-step ball milling of SiCnp and Al powders. The obtained Al flakes effectively improved the dispersion efficiency of GNS and reduced the GNS structural damage. As a consequence, GNS and SiCnp were well dispersed and combined with Al matrix after hot extrusion. The nanograins were also obtained in the extruded (GNS+SiCnp)/Al composites, which were conducive to the strengthening of composites. Benefiting from the homogeneous dispersion of GNS, SiCnp and the hybrid enhancement effect between them, the extruded (GNS+SiCnp)/Al composites showed significant improvement in Young’s modulus, tensile strength and, in particular, work hardening capacity compared to GNS/Al composites by approximately 278%, yielding good comprehensive mechanical properties.

Revealing the influence of zirconium content on the cavitation erosion-corrosion of a wear-resistant steel in sodium chloride solution

Wear-resistant steels with different zirconium contents were prepared to investigate the influence of zirconium on the cavitation erosion-corrosion behaviour in sodium chloride solution. To better understand the behaviour of these steels, the microstructure, mechanical properties and corrosion behaviour were also evaluated. It was found that the addition of appropriate amounts of zirconium to current steel could effectively improve its cavitation erosion-corrosion resistance, mainly attributed to the improved corrosion resistance, impact toughness and work-hardening capacity. However, excessive addition of zirconium caused the precipitation of large-sized zirconium compounds. The interface between the compounds and the matrix could be the origin of damage.

Unraveling kinking: A plasticity enhancing failure mode in high strength nano metallic laminates

Kinking is an important and plasticity-enhancing deformation/failure mode in numerous mechanically anisotropic materials including high-strength nano metallic laminates (NMLs). However, our current limited understanding of the mechanics of kinking and its dependence on microstructural attributes is insufficient for thoroughly comprehending and eventually being able to control failure behaviors of materials. In this study, we investigate kinking dependencies on microstructural attributes in NMLs via in situ micropillar compression, multiscale microstructure characterization, dislocation dynamic simulations, and crystal plasticity modeling. By examining several NML systems (Cu/Fe, Ag/Fe, Al-4Mg/Fe), we demonstrate that the development of internal stresses during loading activates local layer-parallel glide triggering kinking in NMLs. Furthermore, this work reveals the effect of key microstructural features including layer thickness, layer waviness, interface barrier strength, and work hardening capacity on kink band formation in NMLs. More broadly, our efforts represent a generically applicable approach for probing large-strain deformation behavior of complex materials via synergetic modeling and experimental efforts.

Strengthening mechanisms and work hardening in a heterostructured cast aluminum alloy under compressive loading: Correlation with nanomechanical properties

The compressive deformation behavior of a high-pressure die-cast Silafont®-36 aluminum alloy was evaluated in relation to the microstructural changes during deformation. No strain rate sensitivity was observed for the alloy compressed at room temperature. The as-cast microstructure consisted of softer primary α-Al dendrites and harder Al-Si eutectic structure, which was well depicted by the difference in the nanohardness values using high-speed nanoindentation mapping. The load-bearing capacity of eutectic Si particles and thermal mismatch enhanced dislocation density strengthening were identified to be the predominant strengthening mechanisms in the alloy, plus a certain degree of Orowan strengthening mechanism. The work hardening capacity of the cast alloy was mainly attributed to the increase in the dislocation density in the softer α-Al matrix, which was further assisted by the presence of eutectic Si particles. The relative increase in the flow stresses also corresponded well to that in the nanohardness values of the α-Al grains with increasing strain levels.

Effects of added tungsten on the microstructure and mechanical properties of laser-directed energy deposited Inconel 625 alloys

To satisfy the extreme operational requirements of Inconel 625 alloys, the development of novel approaches for improving their strength and compensating for the corresponding loss in toughness/plasticity is critical. This study employed laser-directed energy deposition to fabricate Inconel 625 alloys containing different amounts of tungsten (W) (6.0–16.5 wt%) and investigated the effects of added W (based on the quantity) on the microstructure and mechanical properties of the samples. The moderate addition of W reduced the size of γ-Ni grains and increased the GND density, which can improve the sample strength. Adding W can increase the density of nano-stacking faults (nano-SFs) and promote the Lomer-Cottrell (L-C) locks production, thus improving the work-hardening capacity and balancing the strength and plasticity of the alloys. As the amount of W increased from 0.0 to 16.5 wt%, the elongation of the samples increased and then decreased, whereas the strength increased monotonously. In the prepared samples, the one containing 6.0 wt% W exhibited the highest elongation (51.8 ± 3.2%), 1.38 times higher than the sample without W. Furthermore, the sample containing 16.5 wt% W exhibited the highest strength: 1167.2 ± 16.5 MPa, 289 MPa greater than that without W. The theoretical yield strengths of all samples were also calculated considering the strengthening mechanisms of the grain boundary, dislocation, solid solution, and precipitation; the results agreed with the experimental data.

Re alloying-driven stacking fault energy decrease enabled strength-ductility synergy in NiCoCrFe high-entropy alloys

Metallic materials have long been enslaved in the dilemma of strength-ductility trade-off, which limits their potential applications. Here, we report on a new class of non-equiatomic Re-added Ni-Co-Cr-Fe face-centered cubic (FCC) high-entropy alloys (HEAs) with superior yield strength to those of Mo-added and other FCC HEAs without compromising ductility. The strengthening mechanisms of these HEAs were discussed in detail by experimental observation of the recrystallized microstructure, dislocation configuration and deformation substructure combined with theoretical evaluation of yield stress and stacking fault energy (SFE). It has been clarified that the unique work-hardening capacity and deformability of these HEAs is attributed to solid solution strengthening and the grain boundary strengthening caused by refined crystalline on the one hand. And what's more important, the appropriate addition of Re greatly reduces SFE so as to modulate the dislocation slip and dissociation behavior and promote phase transformation. It has been confirmed that the plastic deformation process was controlled by abundant microbands, multiple nanoscale deformation twins and phase transformation in the Re-added HEAs.

A 4340 Steel with Superior Strength and Toughness Achieved by Heterostructure via Intercritical Quenching and Tempering

The conventional 4340 steel was used after quenching and tempering, strengthened by the classical pearlitic structure where cementite particles are dispersed through the ferrite matrix. In the present study, a heterostructure microstructure consisting of micro-sized residual ferrite zones and pearlitic zones was introduced by an optimized process of intercritical quenching and tempering, resulting in a steel with higher strength and better toughness. The pearlite steel has a tensile strength of 1233 MPa, yield strength of 1156 MPa, and toughness of 121.5 MJ/m3. Compared with the pearlite steel, the tensile strength and yield strength of the heterostructure steel have been improved by 67 MPa and 74 MPa, respectively, while the toughness has been increased by 52.5 MJ/m3. In this heterostructure, the micro-sized ferrite bulks serve as the soft zones surrounded by the hard zones of the pearlite structure to achieve a remarkable work-hardening capacity. Statistical analysis shows that the heterostructure has the best hetero-deformation-induced (HDI) hardening capability when the residual ferrite bulk contributes ~31% by volume fraction, and the quenching temperature is around 780 °C. This study opens new ways of thinking about the strengthening and toughening mechanism of heat treatment of medium carbon steels.

Outstanding fracture toughness combines gigapascal yield strength in an N-doped heterostructured medium-entropy alloy

The superior strength and toughness balance prevails in the face-centered-cubic-structured high/medium-entropy alloys (H/MEAs) of low strength, while a gain in yield strength is normally accompanied by a sacrifice in toughness, leading to the strength and toughness trade-off particularly when yield strength increases to the gigapascal levels. Here, we showed the superior fracture toughness by heterostructuring an N-doped CrCoNi MEA having different levels of yield strength higher than 1 GPa. The fracture toughness was 91 MPa·m1/2 at yield strength of 1.3 GPa in the heterogeneous lamella structure and 168 MPa·m1/2 at yield strength of 1.0 GPa in the heterogeneous grain structure. The fracture toughness was attributed to forest hardening plus an extra hetero-deformation induced hardening. The pile-ups of geometrically necessary dislocations were observed to be formed at the domain boundaries to accommodate strain gradient at the plastic zone of the crack tip. The chemical short-range orders were found for the enhanced strain hardening near the crack tip by the interaction with dislocations. Moreover, a new parameter was proposed to characterize the work hardening capacity at the crack tip by the integral of hardness increment in the plastic zone which shows a linear relation-ship with the J-integral value during the crack initiation.

Evolution mechanism for a surface gradient nanostructure in GH4169 superalloy induced by an ultrasonic surface rolling process

In this study, the evolution mechanism of surface gradient nanostructure induced by the ultrasonic surface rolling process (USRP) in GH4169 superalloy was investigated. The gradient nanostructure, which was characterized using transmission electron microscopy, revealed that the repeated impacts induced by USRP produced a gradient nanostructure with a thickness of ∼330 μm at the surface of the material. Equiaxed nanograins with an average grain size of ∼30 nm were observed on the topmost surface, and the grain size gradually increased with depth. In the nanocrystallization mechanism, dislocations first formed as a result of the initial plastic strain introduced by USRP. Next, dislocation tangles and dislocation walls were formed and gave rise to the formation of low-angle and high-angle grain boundaries, which resulted in grain refinement. As plastic strain accumulated, nanoscale deformation twins were formed. The interaction between dislocations and deformation twins further refined the parent grains to produce equiaxed nanograins with high-angle grain boundaries. In addition, stacking faults were generated when the interaction between dislocations and deformation twins was initiated. A 9R structure was observed inside the nanograins at the topmost surface that significantly improved the work-hardening capacity of the superalloy by acting both as dislocation blockers and dislocation storage sites. The microhardness of the nanostructured GH4169 superalloy was improved by 67% as compared to the untreated material.

Design of ultra-strong but ductile iron-based alloys with low supersaturations

High-performance but low-cost structural materials with nanoscale precipitations are vitally important for engineering applications in advanced industry systems. Thus far, the classical nucleation mechanism assumes a critical nucleus of a homogeneous composition up to a sharp boundary with a large energy barrier of nucleation. A fine dispersion of nanoscale precipitates often requires strong solute supersaturations with expensive alloying additions or otherwise deteriorated strength-ductility, weldability, and fabricability. The simultaneous satisfaction of all these mutually exclusive requirements is the longstanding issue for the design of advanced materials for structural applications. In this alloy design, we have adopted a revolutionary approach of ultra-strong iron-based alloys that have resolved all these key issues through non-classical nanoscale precipitations and multi-elemental partitioning. Such an alloy design strategy offers a unique approach to control nanoscale precipitates with low solute supersaturations. Fine precipitations so designed lead to the strong enhancement of both the precipitate-dislocation interactions and work-hardening capacity, resulting with a sharp increase of the yield strength (YS) to as high as 1500 MPa and tensile ductility up to 15%. Furthermore, our design assures outstanding fabricability together with superior post-weld properties through dense nanoscale reprecipitations (∼1024m−3) in a well economical manner for production. The non-classical actions unfold brand new pathways for precipitation of various intermediate structures and chemistries with low energy barriers, especially useful for the design of sustainable and economical modern alloy systems.

Effect of Cold Rolling and Flash Annealing on Microstructure and Mechanical Behaviour of Austenitic Manganese Steel (Fe-1.15C-13Mn)

Austenitic manganese steel (AMS) is widely used in the mineral and mining industry under high workload condition due to its remarkable work-hardening capacity under impact. However, AMS typically has an austenitic matrix with carbides precipitated resulting in low hardness and brittle properties, which impaired its wear performance. In this study, cold deformation followed by flash annealing were conducted to improve the mechanical behavior of AMS. The experimental was carried out by solution treatment (ST), cold rolling (CR) with deformation degrees of 10% and 20%, and flash annealing (FA) at 915°C with holding times of 90 and 150 seconds. The microstructure evolution and mechanical behavior were studied. The ST produced a completely austenitic microstructure which was free of carbide, resulting in a decrease in strength and hardness, as well as improved ductility. After cold deformation, both of strength and hardness were substantially improved, followed by loss in ductility. Optical microstructures reveal the formation of deformation twin (DT) and annealing twin (AT) after cold deformed. Higher intensity of DT results in AMS with considerable strength and hardness but decrease in ductility. After FA process, cold deformed structures undergo microstructural restoration, which manifests by recovery stage at 90 s and recrystallisation stage at 150 s annealing time. At the same FA time, higher degree of deformation led to increase the hardness, while at the same degree of strain, longer annealing time led to a decrease of hardness. Moreover, both of higher degree of strain and longer annealing time during flash annealing were contributed to grain refinement, even though did not affected to the increase of tensile and hardness. In addition, 20% cold rolling followed by 150 s annealing time (CR20FA150) could be considered as an effective method to obtain the most optimum combination of strength and ductility with finer grain.

Application of ultrafast heating and tempering to plate steel

The microstructure and tensile properties were studied in a low-alloy steel plate treated under ultra-fast heating, quenching, and tempering. The microstructure after heat-treatments was predominantly martensite, with low fraction of bainite and ferrite. The tensile tests showed a 100 MPa improvement in strength and a doubling of the total elongation with the increase in heating rate. The tempering process considerably enhanced the ductility from 1–4% to up to 14% in the samples treated at fast heating rates..Furthermore, the work hardening capacity of ultra-fast heated steel was superior compared to steel treated under conventional quenching and tempering. The results suggest that the tempering stage after ultra-fast heat-treatments and quenching further improves the properties compared to the standard quenched and tempered condition.

Simultaneously improving yield strength and formability potential of Mg alloy via introducing nano precipitates and texture-weakened heterogeneous grains

Simultaneously improving the yield strength (YS) and formability at room temperature of magnesium (Mg) alloys is one of the key issues to broaden the application further. In this work, we report a new hot-extruded + annealed alloy (i.e., Mg–2Sm–0.8Mn–0.6Ca–0.5Zn) having good combination of yield strength (YS: 167 MPa) and formability potential (UTS−YS: 115 MPa and elongation: 25%), mainly due to formation of nano precipitates and texture-weakened heterogeneous grains. The results indicate that introduction of a certain amount of nano precipitates can enhance the strength of the alloy with little detrimental effect on ductility. The formation of heterogeneous-grained structure with a weak basal texture is mainly related to Sm/Zn/Ca elements grain boundary (GB) segregation. Geometrically necessary dislocations would nucleate at grain boundaries due to uneven deformation behavior caused by heterogeneous-grained structure, obtaining additional hetero-deformation induced hardening and large work-hardening capacity. Formation of heterogeneous-grained structure and solid solution of Sm/Zn/Ca promote the activation of considerable non-basal slips and the suppression of twinning during the deformation process. Meanwhile, GB segregation is conducive to dislocation nucleation from GBs and suppression of the GB cracking. All of these contribute to the high elongation. Thus, we put forwards a new strategy for improving the YS and formability potential via introducing nano precipitates and texture-weakened heterogeneous grains.

Enhancing the hydrogen embrittlement resistance with cementite/VC multiple precipitates in high-strength steel

The superior hydrogen embrittlement (HE) resistance and satisfied work hardening capacity are achieved in high-strength spring steel after hydrogen pre-charging with dispersed multiple precipitates via the multi-microalloying of 1.04 wt% Cr and 0.14 wt% V. The precipitates are determined to be the triplex microstructures, i.e., the isolated (Mn, Cr, V)-enriched cementite, isolated V-enriched VC, and the co-precipitation of cementite and VC. These multiple precipitates could act as the effective hydrogen traps to achieve more potent irreversible hydrogen trapping capacity, thus inhibiting the HE susceptibility by ∼23%. Hence, the strategy of cementite/VC multiple precipitates is beneficial in designing the high-strength and HE-resistant automotive steels.