Hardenability Research Articles

A strong ferritic high‐resistivity multicomponent alloy with tunable ordered coherent multicomponent nanoprecipitates

We design and investigate a ferritic multicomponent alloy with high electrical resistivity, low temperature coefficient of resistivity (TCR) in a wide-temperature range, high strength, and moderate deformability, aiming to overcome the bottlenecks limiting the development of cost-efficient high-quality resistors. The developed bulk alloy with nominal composition Fe55Cr28Al12Ti3Si2 (at. %) shows a body centered cubic (BCC) matrix phase embedded with ordered coherent multicomponent nanoprecipitates in as-cast condition, which enables a high resistivity of 183 µΩ·cm, a low TCR of -35 ppm/K (from 298 to 673 K), a high yield strength of 1055 MPa, a high ultimate compressive strength of 1980 MPa and a moderate uniform compressive strain of 33% (at 673 K). The fully coherent and ordered multicomponent L21 nanoprecipitates in the BCC-α matrix enhance the stress required for dislocation propagation, leading to the improved strength and work hardening ability, according to the comparative study on the alloy variants with and without nanoprecipitates. Also, the multicomponent L21 nanoprecipitates enable the effective tuning of Kondo-like scattering and contribute to the low TCR of the material. The work thus demonstrates a design strategy for low-cost precision resistance alloys to achieve high and stable resistivity in wide temperature ranges combined with high strength and moderate compressive deformability by introducing well-tuned ordered coherent multicomponent precipitates in multicomponent alloy systems.

Decreased dislocation density as an origin for the quench sensitivity of the Al-Si-Mg alloys with high Si content

The effect of Si content on the quench sensitivity of the Al-Si-Mg alloys was investigated by conducting the age hardening test of the alloys with different cooling rates after solution heat treatment. The quench sensitivity increases with increasing Si content in the range of 2–12%. The microstructure shows that, apart from the solute loss to the dynamic precipitates with low hardening ability, the decrease in the dislocation density induced by the thermal stress at a low cooling rate contributes to enhanced quench sensitivity by lowering the density of heterogeneous nucleation sites for precipitates. The quench sensitivity map is constructed, it relates the decrease in the peak hardness to the average cooling rate and the strain energy stored in the aluminum matrix due to the mismatch of thermal expansion under the assumption of pure elastic deformation. The effect of the microstructure characteristics, including the size, fraction and shape of Si on the quench sensitivity can be accounted for by the latter parameter. • The quench sensitivity of the Al-Si-Mg alloy increases with increasing Si content. • The decreased dislocation density is an origin of the quench sensitivity. • Quench sensitivity map considering the dislocation density change is constructed.

On the microstructure and mechanical behaviour of the Cr2Fe2NiCo0.67Cu0.67Ti0.33 alloy during homogenisation

The present study investigates the microstructure and mechanical properties of the Cr2Fe2NiCo0.67Cu0.67Ti0.33 high-entropy alloy after homogenisation. The results show that the as-cast microstructure consisted mainly of a dual-phase FCC + σ structure. By heat-treating, the microstructure changed from the FCC + σ to the mixed FCC + BCC solid solutions. The EDS and XRD analysis show that despite different crystal structures of the σ and BCC phases, their chemical composition is the same. In addition, two kinds of precipitations, enriched by Ti and Cu, are found in the microstructure. Formation of these precipitates beside hard σ-phase resulted in an increase in hardness. Additionally, higher elongation and strain hardening ability achieved in the specimen that σ → BCC transformation occurred.

Architecture dependent strengthening mechanisms in graphene/Al heterogeneous lamellar composites

An architecture-based model revealing the architecture-strengthening mechanism correlations of Graphene/aluminum (GR/Al) composites with heterogeneous lamellar (HL) architecture is proposed. The microstructural features such as matrix grain size, shape, and orientation in HL GR/Al composites are drivers for the dislocation storage and strain hardening ability. The proposed model allows to describe the role of the HL design factors affecting the microstructural architecture in GR/Al composite and the strengthening mechanisms of the material. The improved mechanical properties are linked to strain hardening ability, the high dislocation storage capability of the coarse grain bands (CGBs) as well as the generation of further geometrically necessary dislocations (GNDs) in the GR/Al interfaces. Moreover, the optimal-alignment of the structural elements and GRs not only causes a combined effect of isotropic and kinematic strain hardening but also leads to higher levels of back stress and back stress hardening, respectively. Actually, the significant effect of the interfacial dislocations including GNDs on the internal stresses especially back stress, which leads to the generation of extra strain hardening is underlined. In a word, the proposed model, which is based on the dislocation induced strengthening mechanisms, enables to guide the processing-microstructure design.

Analysis of work hardening mechanisms in Quenching and Partitioning steels combining experiments with a 3D micro-mechanical model

Quenching & Partitioning (Q&P) steels owe their good strength-ductility combinations to the martensite/austenite (α’/γ) mechanical interactions and to the formation of mechanically-induced martensite (α′mech) through the transformation-induced plasticity (TRIP) effect. An essential role is played by carbon, whose distribution among the phases can be modified through the Q&P route. This study presents a methodology to systematically and quantitatively examine the influence of the α’/γ mechanical interactions on the overall work hardening of the steel with respect to the role of carbon in the martensite. The methodology rests on the generation of a 3D micro-mechanical model that allows to derive, by crystal plasticity simulations, the overall response of a mechanically-stable α’/γ virtual microstructure. In combination with theoretical knowledge on hardening, the comparison between the experimental and simulated mechanical responses enables the quantification of the influence of the martensite carbon content and distribution on the overall TRIP strengthening contribution of the steel. The approach is applied to two low carbon Q&P-processed α’/γ microstructures of similar initial volume fractions of austenite and α′mech formation kinetics with strain, but one containing a Nb-microaddition and displaying improved strength-ductility values. It is shown that the martensite strength and work hardening ability might additionally enhance or partially counteract the strengthening contribution from the austenite-to-α′mech transformation during uniaxial loading. The results of this study highlight that the processing-dependent properties of the carbon-depleted martensite should be considered in the optimization of Q&P processed steels.

Prediction of the overall mechanical response of gradient nano-grained materials based on grain size distribution profile

Inspired by the gradient structure of biological materials, various gradient nano-grained (GNG) materials, inside which grain size spans several orders of magnitude, have been manufactured in recent years. This kind of material often exhibits extra hardening ability and ultimately achieves synergetic strength and ductility. The extra hardening of GNG materials is closely related to the strain gradient formed during deformation. However, the analytical relation between the structural gradient, strain gradient is still unexplored. Besides, the previous works focused on only one type of GNG material, and the understanding of the effect of different kinds of microstructures is limited. This paper explores the analytical relations between the strain gradient and the average grain size, as well as the grain size gradient in GNG materials. The overall mechanical responses of GNG interstitial-free (IF) steel and GNG Ni are well predicted. The results verify that the extra hardening of GNG materials is dependent on their unique microstructures. The strength and ductility of GNG materials can be improved by adjusting the composition and distribution of coarse grains and nano-grains.

Cryogenic deformation behavior of 6061 aluminum alloy tube under biaxial tension condition

Cryogenic forming has been proposed for the fabrication of complex aluminum alloys tubular components to overcome the shortcomings of traditional forming processes. It is difficult to evaluate the cryogenic biaxial deformation behavior. In this study, a novel cryogenic hydro-bulging device was established to evaluate cryogenic deformation behavior and verify the feasibility of the new forming process. The cryogenic biaxial stress-strain relations were determined using a new analytical model to quantitatively characterize the hardening behavior. The cryogenic deformation mechanism was revealed by microstructure characterization. It is found that the maximum expansion rate of 6061 aluminum alloy tube at − 196 ℃ increases to 34.0% ± 0.6%, being 99.8% higher than that at room temperature. The obtained cryogenic biaxial flow curve exhibits the significantly improved plasticity and hardening ability with a strain hardening exponent of 0.43 at − 196 ℃. The high cryogenic strain hardening rate contributes to the stable uniform deformation. The improved cryogenic plasticity is associated with the diminished dislocations accumulation at grain boundaries. The enhanced cryogenic hardening ability is attributed to the impeded movable dislocations slip. The research demonstrates that cryogenic medium pressure forming has great potential for the fabrication of complex aluminum alloy tubular components. • A novel cryogenic hydro-bulging device was established for evaluating the cryogenic biaxial deformation behavior oftubes. • The cryogenic biaxial mechanical properties and hardening behavior were quantitatively evaluated by a new analytical model. • Excellent cryogenic biaxial formability can be obtained, contributing to the fabrication of complex tubular parts. • Microstructure was characterized to reveal the cryogenic deformation mechanism.

Multiple-mechanism and microstructure-based crystal plasticity modeling for cyclic shear deformation of TRIP steel

The transformation-induced plasticity (TRIP) steel has an excellent synergy between strength and ductility and is widely used in industry. Cyclic loading is often seen in its industrial application. Therefore, it is of great significance to study cyclic plastic behavior of the TRIP steel. The TRIP steel's cyclic shear behavior and microstructure evolutions were investigated by mechanical testing, electron backscatter diffraction (EBSD) characterization, and in-situ electron channeling contrast imaging (in-situ ECCI) observation. Then, a multiple-mechanism crystal plasticity constitutive model was developed, considering both dislocation slip and martensitic transformation mechanisms. Furthermore, an isotropic hardening law and a modified kinematic hardening rule were taken into account. The crystal plasticity model was implemented into DAMASK with the spectral method. A polycrystalline RVE with realistic grain morphology was constructed from the EBSD data. The simulations of the TRIP steel under cyclic shear loading showed that the samples under higher strain amplitude exhibit stronger cyclic shear hardening. The activation of the martensitic transformation mechanism promotes cyclic hardening. In grain level, the grains with Taylor factor (related to orientation) between 3.3 and 3.9 can activate more slip systems and have a better cyclic hardening ability. So, the Taylor factor is defined as an indicator describing the cyclic response of individual grains in polycrystalline materials. Considering the capability of the established modeling framework, it is of great significance to guide the TRIP steel to serve safely and help the microstructural design in grain scale.

Micro-mechanical characterisation of slip behaviour and precipitation strengthening in CoCrFeNiTiMo alloy additively manufactured by laser powder bed fusion

Micro-tensile tests on CoCrFeNiTiMo multicomponent alloys additively manufactured by laser powder bed fusion revealed that the slip system activated at the yielding point was octahedral and cubic slips depending on the loading direction. Ordered precipitation by heat treatment inhibited the cubic slip and enhanced the strength and strain hardening ability.

Abnormal work hardening in a TRIP-assisted metastable β titanium alloy under high strain rate loading

In this study, the dynamic tensile deformation behavior and microstructural evolution of a metastable β titanium alloy (Ti–10 V–2Fe–3Al, Ti-1023) with a single β phase structure were investigated at ambient temperature and high strain rates ranging from 1000 s−1 to 4000 s−1 on a split Hopkinson tension bar (SHTB) system. The dynamic tensile properties and work hardening ability exhibited a significant strain rate dependence. The calculated strain rate sensitivity (0.026–0.090) showed a strong strain and strain rate hardening ability. Microstructural analyses indicated that the dynamic deformation mechanisms were dislocation slip and deformation-induced α" martensite at all strain rates, while deformation-induced ω phase was activated beyond a strain rate of 2000 s−1. Deformation-induced phase transformations during dynamic tensile deformation led to a significant transformation-induced plasticity (TRIP) effect and a double-yielding phenomenon. As strain rate increased, deformation-induced martensitic transformation and the corresponding TRIP effect were suppressed, while work hardening rate first increased and then rapidly decreased. This abnormal TRIP effect on work hardening behavior was mainly due to the deformation-induced ω phase, promoting the dislocation plasticity in the β matrix and contributing to extra work hardening ability.

On the enhanced hardening ability and plasticity mechanisms in a novel Mn-added CoCrNi medium entropy alloy during high-pressure torsion

Microstructure and texture evolution during high-pressure torsion (HPT) of a novel Mn-added CoCrNi medium entropy alloy (Co33Ni33Cr19Mn15) is investigated for the first time. The alloy exhibited a rapid rise in hardness at relatively low shear strains (γ≤20). It is attributed to an extensive dislocation activity to achieve saturation in dislocation density of ~1016 m−2, combined TWIP and TRIP effects and microstructural refinement. At higher shear strain, hardness increased at much reduced rates owing to saturation of dislocation density, twin fault probability and the TRIP effect, besides continued grain refinement for severe nano-structuring led to subsequent strengthening. The FCC phase showed remarkable stability except a small degree of initial deformation-induced HCP martensitic transformation in an early stage of HPT. The ideal shear texture components were observed at low shear strain, and these continued to evolve up to 5 turns of HPT processing. For similar HPT processing conditions, the studied alloy showed superior hardness (~650 Hv) compared to a wide spectrum of FCC materials, which is ascribed to a combination of the strengthening mechanisms of Taylor hardening, the TRIP and TWIP effects and Hall-Petch strengthening resulting from the nano-structured grains having an average size of ~35 nm.

A Prospective Way to Achieve Ballistic Impact Resistance of Lightweight Magnesium Alloys

The ballistic impact resistance of lightweight magnesium alloys is an eye-catching material for the military and aerospace industries, which can decrease the cost of a project and the fuel consumption. The shockwave mitigation ability of a magnesium alloy is 100 times stronger than an aluminum alloy; nonetheless, ballistic impact resistance has still not been achieved against blunt and API projectiles. The major obstacles are the low hardness, low mechanical strength, basal texture and strain hardening ability under loading along the normal direction of the sheet. The high yield strength and ultimate strength can be achieved for a specific loading condition (tensile or compression) by adjusting the texture in magnesium alloys. The projectile impact along the normal direction in a strong basal-textured magnesium alloy can only produce a slip-induced deformation or minor twinning activity. Here, we propose a practical technique that can be valuable for altering the texture from c-axes//ND to c-axes//ED or TD, and can produce high strain hardening and high strength through a twinning and de-twinning activity. Subsequently, it can improve the ballistic impact resistance of magnesium alloys. The effect of the technique on the evolution of the microstructure and possible anticipated deformation mechanisms after ballistic impact is proposed and discussed.

Developing harmonic structure in CoCrFeMnNi high entropy alloy to enhance mechanical properties via powder metallurgy approach

The FCC structured CoCrFeMnNi high entropy alloy (HEA) has attracted great attention due to its excellent ductility and strain hardening ability, but the lowest yield strength is insufficient for structural applications. To achieve high strength and ductility, we developed a new strategy called harmonic structure (HS) consisting of soft coarse and hard fine grains via controlled mechanical milling time. The SEM and EBSD microscopic characterizations reveal that the formation of HS from surface to the interior of powder is due to a difference in the degree of plastic deformation. The hardness of HS HEAs increased with increasing distance from center to surface regions due to a reduction in grain size. Compared with the homogeneous HEA, the yield strength of HS HEAs increased from 370 MPa to 730 MPa and 760 MPa due to grain boundary strengthening, dislocation strengthening, and twin boundary strengthening effects. While HS HEAs exhibited enhanced strain hardening ability due to gradient structure with their mechanical incompatibility provide back stress strengthening. Furthermore, the compressive deformation behavior of homogeneous and HS HEAs was analyzed by microstructural evolution.

PENGARUH VARIASI TEMPERATUR AUSTENIT PENGUJIAN JOMINY TEST PADA SIFAT HARDENABILITY BAJA ST 90

The purpose of this research is to obtain information about the hardness value (hardening) and changes in microstructure as well as changes in the properties of the material at a temperature variation of 600℃ to 900℃. The method used in this study is an experimental method, where the material used is ST.90 steel, and Jominy testing. 5 samples were made at each temperature 22℃, 600℃, 700℃, 800℃, and 900℃ with the type of heat treatment in the hardening process. The test results for Vickers hardness decreased by an average of 33.99 HV or 31% and the carbon content decreased by an average of 0.19. or 48% of the normalizing process is not done. Based on the data above, it shows that the results of the Vickers hardness test and chemical composition with the hardening process of ST.90 steel, which are heated at 600℃, 700℃, 800℃, and 900℃ respectively, each temperature is reached and stopped with 60 minutes. In order for phase transformation to occur in all parts of the material, then the cooling process is carried out using the Jominy test, showing the mechanical strength, namely the Vickers hardness test decreases by 23% and the carbon content decreases at 37%.

Microstructure and mechanical properties of an Fe–Mn–Al–C lightweight steel after dynamic plastic deformation processing and subsequent aging

An austenitic low-density steel was processed by dynamic plastic deformation (DPD) over the strain range from 0.25 to 0.75 followed by aging at 450 °C and then it was subjected to compressive testing at strain rates of 1.0 × 10−3 − 2.0 × 103 s−1. The results show that fine grain structures with high density dislocations are achieved after DPD processing. After aging, the grain size increased slightly together and there was an additional marginal decrease in the dislocation density. κ-carbides only appeared in the samples after DPD processing at the strain of 0.75 and after subsequent aging. Submicron-sized (Nb, Mo)C particles existed in the matrix before DPD and there was no change in size and distribution during DPD processing and post-DPD aging. The yield strengths of the steels after DPD at different strain rates increased significantly by ∼120–190% compared with the as-received sample, where this is mainly due to a combination of dislocation strengthening and grain boundary strengthening. For the steel processed by DPD at strain of 0.75, there was an additional precipitation strengthening of κ-carbides besides the dislocation strengthening and grain boundary strengthening, and this produced an increase of over ∼900 MPa in yield strength by comparison with the as-received steel. After aging, the yield strength decreased slightly due to a reduction in the dislocation density and a slight coarsening of the grains, except for samples after DPD at a strain of 0.75 which showed a slight increase in strength due to further κ precipitation. The strain rate strengthening effect and strain hardening ability were also analyzed.

Effect of nickel addition on enhancing nano-structuring and suppressing TRIP effect in Fe40Mn40Co10Cr10 high entropy alloy during high-pressure torsion

The present work unravels the effect of nickel (Ni) addition on the deformation mechanism and hardness evolution in a Fe40Mn40Co10Cr10 high entropy alloy (HEA) during high-pressure torsion (HPT) processing. For this purpose, two variants of the high entropy Cantor alloy, with compositions (atomic%) Fe40Mn40Co10Cr10 (Ni0 alloy) and Fe35Mn35Co10Cr10Ni10 (Ni10 alloy) were selected. The study revealed a transition in the predominant plasticity mechanism with addition of Ni from TRIP in Ni0 to dislocation slip in Ni10 alloy. Such transition of plasticity mechanism was the direct consequence of an increase in the free energy of phase transformation, ΔGγ→ε towards a more positive value with Ni addition. Interestingly, the Ni10 alloy showed a greater extent of nano-structuring than the Ni0 alloy with nearly three-fold refined grain sizes, that is, lesser than 30 nm in Ni10 alloy and ∼90 nm in Ni0 alloy. Furthermore, a 3–4 times higher dislocation density was observed in the FCC phase of the Ni10 alloy compared to that in the transformed HCP phase in the Ni0 alloy for any given HPT processing conditions. These differences in mechanism(s) of deformation and the extent of nano-structuring manifested as a greater ability of Ni added Ni10 alloy to harden itself during HPT. The present study suggests that a large fraction of hard HCP phase originating from TRIP effect in the Ni0 alloy has a lower hardening ability than the high dislocation density and nano-structuring in the Ni10 alloy.

Effect of Bimodal Grain Structure on the Microstructural and Mechanical Evolution of Al-Mg/CNTs Composite

The Al-Mg alloy structure reinforced with carbon nanotubes was evaluated after the composites production through a modified flake metallurgy technique followed by hot extrusion. The obtained bimodal microstructure of the matrix allowed to identify the microstructural mechanisms leading to high strength; uniform elongation and strain hardening ability of the produced composites. The presence of Mg transformed the native Al2O3 layer into spinel MgAl2O4 nano-phases dispersed both inside CG and UFGs and on the interfaces, improving the interfacial bonding of Al-Al as well as Al-CNT. The effect of the reinforcing phases percentages on the dislocations mechanisms evolution was evaluated through stress relaxation tests leading to the underlying of the effect of reinforcing phases on the modification of the interphase influence zone

Strengthening and strain hardening mechanisms of a plain medium carbon steel by multiscale lamellar structures

Plain medium carbon steel with the multiscale lamellar structure was produced through quenching and subsequent warm rolling. The microstructure characteristic was conducted based on statistical parameters and physical metallurgy by electron backscattered diffraction (EBSD), the mechanical properties were investigated using uniaxial tensile tests at room temperature. The heterogeneous structure is characterized with multiscale lamellar distribution with three-level grain size spanning the nano-to micro-meter range, which is defined as ultrafine-grained (UFG), fine-grained (FG) and coarse-grained (CG). The results of kernel average misorientation (KAM) and Taylor factor (TF) revealed that the existence of interface effects. The nucleation and accumulation of geometrically necessary dislocation (GND) were activated by multiscale lamellae, to cause the formation of sharp up-turn in strain hardening curves. The results of multiple model fits showed that the Swift model was the most appropriate for the studied steel, and the instantaneous strain hardening values were obtained on its basis. The existence of an extra strengthening mechanism (hetero-deformation induced strengthening) was confirmed by calculating. The results showed WR84 had outstanding mechanical properties and strain hardening ability. The improved toughness of WR84 is mainly attributed to ultrafine grain strengthening, and the reasonable ductility is owing to hierarchical hetero-structured of the lamellae, particle dispersion distribution and the three-level coordinated control of grain size as they can increase the strain hardening ability by the interaction constraint.

Improvement of ductility and work hardening ability in a high strength Zn-Mg-Y alloy via micron-sized and submicron-sized YZn12 particles

In this study, the effects of Y addition on microstructure and mechanical properties of Zn-Mg alloy during multi-pass equal channel angular pressing (ECAP) were systematically investigated. The results show that apart from α-Zn matrix and network-shaped eutectic Zn+Mg2Zn11 structure, micron-sized and submicron-sized YZn12 particles were formed in as-cast alloy. During multi-pass ECAP, the α-Zn matrix was refined via dynamic recrystallization (DRX), and the eutectic structure was fragmented into fine particles gradually, while the YZn12 particles became further refined (~100 nm) only from 8p to 12p ECAP. Tensile tests at room temperature indicated that the 12p-ECAP alloy exhibits the optimal mechanical properties with ultimate tensile strength of 465 MPa and fracture elongation of 11%. The fine and uniform microstructure contributes to the enhancement of ductility. Furthermore, the improved work hardening ability of 12p alloy was mainly ascribed from the generation of submicron-sized YZn12 particles with the size of ~100 nm, which could induce twinning with them and increase the density of geometrically necessary dislocations. The results suggest that the introduction of second phase particles with twinning ability may be effective to improve the work hardening ability of Zn alloys.

Effect of Vanadium on Superhardenability of 23MnNiMoCr54V Steel for Round-Link Chain in Mining Applications

The effect of vanadium on the hardenability and microstructure of 23MnNiMoCr54V steel was investigated after austenitizing at 950 and 1050 °C by dilatometry and end-quenching Jominy tests, combined with nitrogen removal with aluminum. Electron probe micro-analysis, scanning electron microscopy and transmission electron microscopy techniques were used to characterize the microstructure and existence state of vanadium atoms in the steel. The presence of aluminum-stabilized nitrogen by aluminum nitride production, which promoted the segregation of solid solution vanadium atoms at grain boundaries, delayed the transformation of ferrite and improved the austenite stability. Thus, vanadium addition could improve the hardenability, and an excellent steel performance could be obtained during slower cooling. This process could be used to produce heavy section steel components, such as round-link chains for mining applications.