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Showing papers on "Hardening (metallurgy) published in 2018"


Journal ArticleDOI
TL;DR: In this paper, a near-fully dense CoCrFeNiMn high entropy alloy was additively manufactured by selective laser melting successfully, and the as-built samples exhibit a hierarchical structure, including melt pools, columnar grains, sub-micron cellular structures, and dislocations.

369 citations


Journal ArticleDOI
TL;DR: In this article, a scanning electron microscope and electron backscattered diffraction (EBSD) analysis revealed a fine cellular-dendritic (0.5 to 2.μm) substructure inside large irregularly shaped grains.
Abstract: Structure–property relationships of an additively manufactured 316L stainless steel were explored. A scanning electron microscope and electron backscattered diffraction (EBSD) analysis revealed a fine cellular-dendritic (0.5 to 2 μm) substructure inside large irregularly shaped grains (~ 100 μm). The cellular structure grows along the 〈100〉 crystallographic directions. However, texture analysis revealed that the main 〈100〉 texture component is inclined by ~15 deg from the building direction. X-ray diffraction line profile analysis indicated a high dislocation density of ~1 × 1015 m−2 in the as-built material, which correlates well with the observed EBSD microstructure and high-yield strength, via the traditional Taylor hardening equation. Significant variations in strain hardening behavior and ductility were observed for the horizontal (HB) and vertical (VB) built samples. Ductility of HB and VB samples measured 49 and 77 pct, respectively. The initial growth texture and subsequent texture evolution during tensile deformation are held responsible for the observed anisotropy. Notably, EBSD analysis of deformed samples showed deformation twins, which predominately form in the grains with 〈111〉 aligned parallel to the loading direction. The VB samples showed higher twinning activity, higher strain hardening rates at high strain, and therefore, higher ductility. Analysis of annealed samples revealed that the observed microstructures and properties are thermally stable, with only a moderate decrease in strength and very similar levels of ductility and anisotropy, compared with the as-built condition.

178 citations


Journal ArticleDOI
TL;DR: In this paper, the deformation mechanisms were systematically investigated using EBSD and TEM for samples strained to 1.3, 5% and 16% and a substantial 33.3% uniform elongation was achieved.

174 citations


Journal ArticleDOI
TL;DR: In this paper, structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy.

158 citations


Journal ArticleDOI
TL;DR: In this article, microstructure evolution in P91 steel and their weldments are reviewed in as-virgin and heat treatment and creep exposure condition, and the role of grain coarsening, Cr/Fe ratio, lath widening and dislocation density on creep rupture life of base metal and weldments is discussed.

155 citations


Journal ArticleDOI
TL;DR: In this article, the deformation behavior of a harmonic structured material (HSM), core-shell 304L stainless steel, was investigated using micro-digital image correlation (micro-DIC).
Abstract: Deformation behavior of a harmonic structured material (HSM), core–shell 304L stainless steel, is investigated using micro-digital image correlation (micro-DIC). High strain-partitioning between co...

150 citations


Journal ArticleDOI
TL;DR: Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy as discussed by the authors.
Abstract: Rod shaped samples of AlSi10Mg additively manufactured using recycled powder through direct metal laser sintering (DMLS) process showed higher quasi-static uniaxial tensile strength in both horizontal and vertical build directions than those of cast counterpart alloy. In addition, they offered mechanical properties within the range of other additively manufactured counterparts. TEM showed that the microstructure of the as-built samples comprised of cell-like structures featured by dislocation networks and Si precipitates. HRTEM studies revealed the semi-coherency characteristics of the Si precipitates. After deformation, the dislocation density increased as a result of generation of new dislocations due to dislocation motion. The dislocations bypassed the precipitates by bowing around them and penetrating the semi-coherent precipitates. Strengthening of recycled DMLS-AlSi10Mg alloys manufactured in both directions was attributed to Orowan mechanism (due to existence of Si precipitates), Hall-Petch effect (due to eutectic cell walls), and dislocation hardening (due to pre-existing dislocation networks). Due to the slightly different microstructure, the contribution of each strengthening mechanism was slightly different in identical samples made with virgin powder.

145 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated.
Abstract: In this work, the effects of strain rate on mechanical deformation and microstructural evolution of CoCrFeMnNi high-entropy alloy (HEA) under quasi-static and dynamic compression were investigated. The HEA exhibited high strain-rate sensitivity values (m = 0.028) of yield strength under quasi-static conditions. In particular, due to the viscous drag effect, the variation of yield strength with strain rate under dynamic compression was much larger than that under quasi-static compression. Microstructural analysis using electron backscatter diffraction shows profuse twinning under both conditions. The dynamically deformed specimens exhibited strongly localized deformation regions (i.e., adiabatic shear bands). The process of dynamic compressive behavior in this HEA is competitive between hardening by dislocation and twinning, and thermal softening. To analyze numerically the flow behavior of the HEA under dynamic conditions, the modified Johnson-Cook model considering adiabatic temperature rise was employed. The modified Johnson-Cook model offered good agreement with experimental results regarding dynamic flow curves of this HEA.

137 citations


Journal ArticleDOI
TL;DR: In this paper, the activation volumes in CrMnFeCoNi have been measured as a function of plastic strain and temperature between 77 K and 423 K using repeated load relaxation experiments.

129 citations


Journal ArticleDOI
TL;DR: In this paper, laser shock peening is combined with laser additive manufacturing to modify the surface microstructures and mechanical properties of as manufactured Ti6Al4V titanium alloy, which is examined before and after LSP.
Abstract: Laser shock peening is combined with laser additive manufacturing to modify the surface microstructures and mechanical properties of as manufactured Ti6Al4V titanium alloy. Microstructural evolution, microhardness distribution, residual stress distribution and mechanical properties are examined before and after LSP. After peening, the interplanar spacing of lattices of both α and β phases decreases without any new phase formation. Grain refinement is achieved with average grain size of α phase decreasing from 33.6 to 24.3 μm. High density of dislocation lines, tangles, and multi-directional mechanical twins are observed. Residual stress is turned from tensile to compressive state with an affected depth of around 700 μm. The hardening layer reveled by microhardness is around 900 μm in depth. Grain refinement accounts for the yield strength, ultimate tensile strength, and elongation enhancements after peening.

120 citations


Journal ArticleDOI
TL;DR: In this paper, the ageing behavior of a nanostructured Mg-8.2Gd-3.8Y-1.4Zr (wt.%) alloy produced by solution treatment followed by high pressure torsion (HPT) was systematically investigated using hardness testing, high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, X-ray diffraction (XRD) and XRD line broadening analysis.

Journal ArticleDOI
TL;DR: It is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned columnar grains with high-density 9R phase in Al(Fe) solid solutions.
Abstract: Light-weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high-density growth twins into Al remains difficult due to its ultrahigh stacking-fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high-density 9R phase in Al(Fe) solid solutions. The nt Al-Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al-Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high-strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al-Fe alloys arise mainly from the high-density 9R phase and nanoscale grain sizes.

Journal ArticleDOI
TL;DR: In this paper, the authors measured geometrically necessary dislocation densities (GND) associated with non-uniform plastic deformation and calculated the statistically stored dislocation (SSD) density by subtracting the GND density from the total dislocation density.

Journal ArticleDOI
TL;DR: In this paper, the effects of Sn content on strain hardening behavior of as-extruded Mg-xSn (x = 1.3, 2.4, 3.6 and 4.7) binary alloys were investigated by uniaxial tensile tests at room temperature.
Abstract: The effects of Sn content on strain hardening behavior of as-extruded Mg-xSn (x = 1.3, 2.4, 3.6 and 4.7 wt%) binary alloys were investigated by uniaxial tensile tests at room temperature. Strain hardening rate, strain hardening exponent and hardening capacity were obtained from the true plastic stress-strain curves. After hot extrusion, the as-extruded Mg-Sn alloys are mainly composed of α-Mg matrix and second phase Mg2Sn, which only exists in Mg-3Sn and Mg-4Sn. Average grain size decreases from 15.6 μm to 3.6 µm with Sn content increases from 1.3 to 4.7 wt%. The experimental results show that Sn content decreases strain hardening ability of as-extruded Mg-Sn alloys, but gives rise to an obvious elevation in tensile strength, yield strength and elongation of them. With increasing Sn content, strain hardening rate decreases from 3527 MPa to 1211 MPa at (σ-σ0.2) = 50 MPa, strain hardening exponent decreases from 0.21 to 0.13 and hardening capacity decreases from 1.66 to 0.63. The variation in strain hardening behavior of Mg-Sn alloys with Sn content is discussed in terms of the influences of grain size and distribution of grain orientation.

Journal ArticleDOI
TL;DR: In this article, the effect of Si content on the activation and kinetics of transformation-induced plasticity (TRIP) effect, the formation of deformation twins, as well as the strain partitioning between phase constituents during deformation was studied in detail.

Journal ArticleDOI
TL;DR: In this article, the tensile properties of an as-cast CoCrFeMnNi high-entropy alloy were investigated at various temperatures ranging from −160 to 1000°C.
Abstract: Tensile properties of an as-cast CoCrFeMnNi high-entropy alloy were investigated at various temperatures ranging from −160 to 1000 °C. The tensile strength and ductility did not vary significantly with loading direction, despite the alloy’s strongly preferred crystallographic orientation. The impact toughness values of the as-cast high-entropy alloy were much higher than those of many traditional alloys, particularly at low temperatures. The mechanical properties of the as-cast high-entropy alloy were compared with those of wrought high-entropy alloy and noticeable differences between the two alloys were found. The maximum tensile ductility and three different strain hardening stages were observed at 500 °C in the as-cast structure. Transmission electron microscopy observations demonstrated that the initiation of deformation twinning was very active even at 500 °C. A simple calculation suggests that very large grains of the as-cast structure induced a reduction in twinning stress, retarding the onset of strain localization.

Journal ArticleDOI
TL;DR: The fundamental role of junction formation in connecting dislocation microstructure evolution and strain hardening in face-centered cubic (fcc) Cu is demonstrated and it is found that glissile junctions make the dominant contribution to strainhardening.
Abstract: When metals plastically deform, the density of line defects called dislocations increases and the microstructure is continuously refined, leading to the strain hardening behavior. Using discrete dislocation dynamics simulations, we demonstrate the fundamental role of junction formation in connecting dislocation microstructure evolution and strain hardening in face-centered cubic (fcc) Cu. The dislocation network formed consists of line segments whose lengths closely follow an exponential distribution. This exponential distribution is a consequence of junction formation, which can be modeled as a one-dimensional Poisson process. According to the exponential distribution, two non-dimensional parameters control microstructure evolution, with the hardening rate dictated by the rate of stable junction formation. Among the types of junctions in fcc crystals, we find that glissile junctions make the dominant contribution to strain hardening.

Journal ArticleDOI
Christian C. Roth1, Dirk Mohr1
TL;DR: In this article, a parametric study evaluating more than 600 distinct specimen geometries is performed in an attempt to identify a universal specimen geometry for all types of materials, but no single geometry can be found for different hardening behaviors, it seems to be necessary to use several distinct geometry to cover all levels of ductility, and the authors recommend testing three different types of specimen per material, reporting the highest measured strain among all specimens as strain to fracture for simple shear.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the influence of precipitates on twinning mediated plasticity and the development of corresponding hardening models that account for twin-precipitate interactions using molecular dynamics simulations.

Journal ArticleDOI
TL;DR: In this paper, a nonlinear kinematic hardening rule is proposed based on the classical decomposed Armstrong and Frederick's description, by incorporating the hardening/softening effect in the back stress.

Journal ArticleDOI
Yu Li1, Yufei Lu1, Wei Li1, Mahmoud Khedr1, Huibin Liu, Xuejun Jin1 
TL;DR: Combined nanoprecipitation and grain refinement were introduced in a bimodal grained (BG) twinning-induced plasticity (TWIP) high manganese steel to achieve high strength-ductility combinations as discussed by the authors.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the use of ultrasonic nanocrystal surface modification (UNSM) to improve the fatigue life of ATI 718 Plus (718Plus), which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions.
Abstract: Ultrasonic Nanocrystal Surface Modification (UNSM) is a mechanical surface treatment that induces large compressive residual stresses and near-surface microstructural changes in the material using repetitive impacts at ultrasonic frequencies with a WC tip. In this study, we investigate the use of UNSM to improve the fatigue life of ATI 718 Plus (718Plus). UNSM induced severe surface plastic deformation in 718Plus, which led to nano-sized crystallites, twins, and high dislocation density in the near surface regions, coupled with an increase in surface hardness by 2.3 GPa and high magnitude of compressive residual stresses. These changes increased the endurance limit by ~ 13% (~ 100 MPa) in room temperature fatigue tests. This improvement in fatigue life was attributed to near-surface microstructural changes, material hardening and high compressive residual stress induced by UNSM. The crack propagation rates were 66% lower for UNSM-treated 718Plus as compared with untreated material. The residual stress relaxation after cyclic loading indicates the effectiveness of UNSM in improving the fatigue life of 718Plus.

Journal ArticleDOI
TL;DR: In this paper, a comprehensive combined hybrid numerical-experimental program is executed on three advanced high strength steels (DP980, CP980 and CP1180) with the objective of validating the hypothesis of a positive strain rate effect on the ductility under biaxial tension, as well as the Hosford-Coulomb model assumptions with regards to the Lode parameter and stress triaxiality dependency of fracture initiation.

Journal ArticleDOI
TL;DR: In this article, a self-propagating high-temperature synthesis (SHS) was applied to a CoCrFeNiMn-type high entropy alloy for fiber laser beam welding.

Journal ArticleDOI
TL;DR: In this article, phase transformations occurring after welding and subsequent post weld heat treatments in simulated sub-heat affected zones (HAZ) of P91B steel are evaluated. But, after PWHT, same phase transformation mechanism (interface diffusion) was observed.
Abstract: This paper critically assesses phase transformations occurring after welding and subsequent post weld heat treatments in simulated sub-heat affected zones (HAZ) of P91B steel. Samples for weld-HAZ simulation were produced corresponding to grain-coarsened HAZ, grain-refined HAZ and inter-critical HAZ. Analyses revealed diverse phase transformation mechanisms (for GCHAZ = pipe-diffusion and for GR/ICHAZ = GB-diffusion) owing to manipulation in grain size and boron-enriched nanosized particles as regards virgin steel after welding. However, after PWHT, same phase transformation mechanism (interface diffusion) in all simulated sub-HAZs is observed. Hardness evaluations and prior austenite grain boundaries dissolution confirm GB embrittlement after welding. Boron segregation, the presence of borides and boron-enriched particles heads to ~ 50% drop in hardness deviations enhancing GB hardening after PWHT. Particle refinement is observed after PWHT which is further validated by numerical modelling. In addition, particle evolution during cooling from peak temperature of weld thermal cycle and isothermal holding of PWHT is analysed. Apparent activation energy of nucleation/growth follows descending order, i.e. GC/GR/ICHAZ for nanosized particles during welding.

Journal ArticleDOI
TL;DR: In this article, a detailed investigation has been performed on the creep deformation and age hardening during creep ageing of an aluminum-copper alloy AA2219-T4 under various stress levels that lead to elastic or initial plastic deformation.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the decomposition of supersaturated solid solution in the as-cast Al-3%Mg-0.25%Zr alloy for different annealing modes to understand the precipitation kinetics of the Al3Zr phase in the presence of Mg.

Journal ArticleDOI
TL;DR: In this article, NiCrBSi alloy coatings deposited by atmospheric plasma spray technology were heated at 440°C, 460°C and 480°C for a short-time respectively to achieve a better understanding of the relationship between their microstructures and properties.
Abstract: NiCrBSi alloy coatings deposited by atmospheric plasma spray technology were heated at 440 °C, 460 °C, and 480 °C for a short-time respectively to achieve a better understanding of the relationship between their microstructures and properties. Optical microscopy, scanning electron microscopy, X-ray diffraction, transmission electron microscopy and tribological tests were conducted to evaluate the microstructures and wear performance of the coatings. Both the hardness and wear resistance of the coatings increase at the initial stage of short-time heat treatment followed by decreasing. The heat-treated coatings show 15.4% higher maximum hardness and 60% lower minimum wear volume than the as-sprayed one. The as-sprayed NiCrBSi coating possesses a mixed microstructure mainly composed of the crystalline phases and the amorphous phase (about 10 vol%). After 3-min heat treatment at 480 °C, the volume fraction of the amorphous phase decreases to 1.6 vol%, while the fractions of compounds (such as Ni3B and Cr3B4) and γ-Ni increase correspondingly. The precipitates of Ni3B and Cr3B4 are responsible for the hardening of the coatings. As heat treatment proceeds, the γ-Ni grains coarsen, leading to a deceasing tendency in the hardness and wear resistance of the coatings. As the abrasive modes of the coatings are not varied after short-time heat treatment, the hardness of the coatings plays a dominant role in the wear resistance of the coatings.

Journal ArticleDOI
Z.H. Han, S. Liang1, Jie Yang, Ran Wei2, Chunguo Zhang1 
TL;DR: In this paper, a CoCrFeMnNi high-entropy alloy with a smaller average grain size and gradient microstructure was produced by asymmetric rolling and subsequent annealing treatment, which gave rise to a superior combination of strengthductility.

Journal ArticleDOI
TL;DR: In this paper, a hybrid manufacturing process combining layer by layer laser deposition of graphene/metal nanocomposites and laser shock peening has been investigated through modeling and experiments, and it was shown that the shock interaction with the graphene and metal interface generates dislocations pile-up in front of graphene and large stress intensity around the interface.