Abstract
Precipitation strengthening has been the basis of physical metallurgy since more than 100 years owing to its excellent strengthening effects. This approach generally employs coherent and ...nano-sized precipitates, as incoherent precipitates energetically become coarse due to their incompatibility with matrix and provide a negligible strengthening effect or even cause brittleness. Here we propose a shear band-driven dispersion of nano-sized and semicoherent precipitates, which show significant strengthening effects. We add aluminum to a model CoNiV medium-entropy alloy with a face-centered cubic structure to form the L2
1
Heusler phase with an ordered body-centered cubic structure, as predicted by ab initio calculations. Micro-shear bands act as heterogeneous nucleation sites and generate finely dispersed intragranular precipitates with a semicoherent interface, which leads to a remarkable strength-ductility balance. This work suggests that the structurally dissimilar precipitates, which are generally avoided in conventional alloys, can be a useful design concept in developing high-strength ductile structural materials.
Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based ...industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10
s
, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy's surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm
. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices.
Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength ...values of medium‐ and high‐entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium‐entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid‐solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation‐mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra‐strong materials for structural engineering applications.
Ultrastrong VCoNi equiatomic medium‐entropy alloys are developed via severe lattice distortion. The distortion enhances the lattice friction stress and also increases the sensitivity of the yield stress on grain size changes. These design ideas lead to a new alloy with near 1 GPa yield strength despite its precipitation‐free and fully recrystallized fcc structure.
Demands for ultrahigh strength in structural materials have been steadily increasing in response to environmental issues. Maraging alloys offer a high tensile strength and fracture toughness through ...a reduction of lattice defects and formation of intermetallic precipitates. The semi-coherent precipitates are crucial for exhibiting ultrahigh strength; however, they still result in limited work hardening and uniform ductility. Here, we demonstrate a strategy involving deformable semi-coherent precipitates and their dynamic phase transformation based on a narrow stability gap between two kinds of ordered phases. In a model medium-entropy alloy, the matrix precipitate acts as a dislocation barrier and also dislocation glide media; the grain-boundary precipitate further contributes to a significant work-hardening via dynamic precipitate transformation into the type of matrix precipitate. This combination results in a twofold enhancement of strength and uniform ductility, thus suggesting a promising alloy design concept for enhanced mechanical properties in developing various ultrastrong metallic materials.
Representative face-centered-cubic (FCC) high-entropy alloys (HEAs) or medium-entropy alloys (MEAs), e.g., equi-atomic CoCrFeMnNi or CrCoNi alloys, have drawn many attentions due to the excellent ...damage-tolerance at cryogenic temperature. The investigation of fracture toughness at 77 K is basically required for the reliable evaluation of high-performance alloys used for cryogenic applications; however, it has been rarely carried out for the non-equi-atomic FCC HEAs yet. In this study, tensile and fracture toughness tests were conducted on the non-equi-atomic V10Cr10Fe45Co20Ni15 alloy, and the results were compared with those of the equi-atomic CoCrFeMnNi and CrCoNi alloys. The present alloy shows a good damage tolerance at cryogenic temperature with tensile strength of 1 GPa and elongation of ∼60%. The KJIc fracture toughness values are 219 and 232 MPa m1/2 at 298 and 77 K, respectively, showing the increase in toughness with decreasing temperature. This increase results from the absence of twins at 298 K and the increased propensity to twin formation at 77 K, which is well confirmed by the variation of stacking fault energies (SFEs) by using Ab-initio calculations. The mechanical properties of the present alloy are actually similar or slightly lower than those of the other CoNiCr or FeMnCoNiCr alloy; instead, this study provides that neither composition nor certain elements are the most important factors dictating damage-tolerance of HEAs or MEAs.
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•The increase in fracture toughness with decreasing temperature for the non-equi-atomic HEA is achieved for the first time.•The SFE decreases in the order of the V10Cr10Fe45Co20Ni15, CrMnFeCoNi, and CrCoNi alloys.•Increase in twins acts as a key parameter contributing to the relief of detrimental cryogenic-temperature effects.•Neither composition nor certain elements are the most important factors dictating damage-tolerance of HEAs or MEAs.
Effects of subzero martensitic transformation on tensile properties were investigated in an Fe-0.3C-9Mn-5Al-1Si (wt.%) lightweight steel. The microstructure for the hot-rolled state consists of ...δ-ferrite, bainite, and γ-austenite, while the retained austenite transformed to the lenticular martensite after the subzero-treatment between -30 and -80 °C. Then, nano-sized austenite was reverted inside the lenticular martensite after tempering at 300 °C for 2 h. The fraction of lenticular martensite increased as the subzero-treatment temperature decreases, resulting in the enhanced yield and tensile strengths but the almost maintained tensile elongation. The main fracture mechanism including fracture modes and secondary cracking was almost identical even after the subzero-treatment because it is not directly affected by the lenticular martensite formed during the subzero-treatment. As a result, yield and tensile strengths were simultaneously enhanced without the elongation loss, which provides promising ideas for overcoming the strength-ductility trade-off and for widening their automotive applications.
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Abstract
Chemical short-range order in disordered solid solutions often emerges with specific heat treatments. Unlike thermally activated ordering, mechanically derived short-range order (MSRO) in a ...multi-principal-element Fe
40
Mn
40
Cr
10
Co
10
(at%) alloy originates from tensile deformation at 77 K, and its degree/extent can be tailored by adjusting the loading rates under quasistatic conditions. The mechanical response and multi-length-scale characterisation pointed to the minor contribution of MSRO formation to yield strength, mechanical twinning, and deformation-induced displacive transformation. Scanning and high-resolution transmission electron microscopy and the anlaysis of electron diffraction patterns revealed the microstructural features responsible for MSRO and the dependence of the ordering degree/extent on the applied strain rates. Here, we show that underpinned by molecular dynamics, MSRO in the alloys with low stacking-fault energies forms when loaded at 77 K, and these systems that offer different perspectives on the process of strain-induced ordering transition are driven by crystalline lattice defects (dislocations and stacking faults).
In order to understand and improve fracture toughness of heat affected zones (HAZs) of high-strength low alloy (HSLA) steels, complex microstructures including quasi-polygonal ferrite (QPF), acicular ...ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent should be identified, quantified, and then correlated with critical crack tip opening displacement (CTOD). In this study, microscopic analysis methods were achieved for identification and quantitation of microstructures in the HAZs of three HSLA steels. The coarse-grained HAZ (CGHAZ) consisted of AF, GB, and BF together with a small amount of MA, while the inter-critically heated HAZ (ICHAZ) consisted of QPF, GB, and MA. In the CGHAZ, Ni promoted the formation of AF, while it prevented the formation of GB, and the addition of Ni resulted in very high critical CTOD. In the CGHAZ, both Ni and Mn promoted the formation of AF and prevented the formation of GB, while Ni was more effective than Mn. Thus, the addition of Ni resulted in very high critical CTOD. In the ICHAZ, both Ni and Mn promoted the formation MA. However, in the high-Ni-containing steel, a number of MAs were densified along Ni-segregated bands, and thus readily provided void initiation sites. This played an important role in reducing the mean free path for coalescence of voids and crack propagation, which easily led to the serious deterioration of critical CTOD.
The existing deformation-induced martensitic transformation mostly focuses on overcoming the trade-off of cryogenic strength-ductility; however, an enhancement of cryogenic strength further is still ...challenging. We present a concept to yield a cryogenic strength of 2 GPa in a duplex V10Cr10Co30Fe50 alloy. We adopt a thermodynamic calculation to reduce the stability of metastable face-centered-cubic (FCC) matrix, significantly promoting the martensitic transformation. In conjunction with the chemically driven promotion, the duplex structure including athermal body-centered-cubic (BCC) martensite enables mechanical strain partitioning to accelerate the transformation further. This finding could be an appropriate design strategy to develop new ultrastrong alloys for cryogenic applications.
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Austenitic high-Mn steels present a dominant deformation mechanism of TWinning Induced Plasticity (TWIP) or TRansformation Induced Plasticity (TRIP), which effectively enables be used for various ...cryogenic applications. This mechanism affects significantly tensile or impact properties, and can be carefully tuned by adjusting alloying compositions; however, the alloying effects of Ni and Cu have not been studied yet. In the present study, the steels were fabricated by adding Ni or Cu and their microstructural evolutions were examined for the quasi-statically-tensioned and dynamically-Charpy-impacted specimens. In the room-temperature tensile deformation, many twins were populated without any martensite. At cryogenic temperature, however, ε- and α′-martensite were formed together with twins in the 22Mn-0.45C–1Al (Base) and 1-wt.%-Ni-added (1Ni) steels, whereas they were not in the 1-wt.%-Cu-added (1Cu) steel. This TWIP or TRIP amount showed a good correlation with the stacking fault energy (SFE) calculated by considering Mn-segregation bands, and affected the tensile ductility. At cryogenic temperature, the TRIP occurred in the Base and 1Ni steels as SFEs of low-Mn bands decreased down to the TRIP range, whereas it did not in the 1Cu steel, and thus the Base and 1Ni steels showed the lower ductility than the 1Cu steel. For the cryogenic-temperature Charpy impact test, the martensite was not formed even at the heavily-deformed area near the notch tip as the time was not enough for inducing the martensitic transformation, while twins were populated. Since the twin fraction near the notch-tip area increased in the order of the 1Cu, 1Ni, and Base steels, it was expected that the Charpy impact energy increased in this order, but the Base steel showed the lowest energy because of the precipitation of fine M23C6-type carbides on grain boundaries.
•Effects of Ni and Cu addition on deformation mechanisms was studied.•The range of stacking fault energies (SFEs) was affected by Mn segregation bands.•The SFEs increase in the order of Base, 1Ni, and 1Cu steels.•Fine M23C6-type carbides seriously deteriorate the Charpy impact properties.
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