Showing papers on "Grain boundary strengthening published in 2020"

The Hall–Petch and inverse Hall–Petch relations and the hardness of nanocrystalline metals

Abstract: We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

Hall-Petch and inverse Hall-Petch relations in high-entropy CoNiFeAlxCu1-x alloys

Abstract: Significant theoretical efforts have been made to understand the Hall-Petch and inverse Hall-Petch relations of nanocrystalline pure metals, metallic glasses and binary alloy systems. However, only a few studies have investigated the Hall-Petch or inverse Hall-Petch relations in high-entropy alloys. In this work, phase stability of single-crystalline CoNiFeAlxCu1-x and uniaxial compression of polycrystalline CoNiFeAlxCu1-x are investigated by molecular dynamics simulation. Calculations of cohesive energies indicate that FCC structured CoNiFeAlxCu1-x is more stable at low Al concentrations (x ≤ 0.4) and BCC structured CoNiFeAlxCu1-x is more stable for high Al concentrations (x > 0.4). Based on the phase stability, FCC structured polycrystalline CoNiFeAl0·3Cu0.7 and BCC structured polycrystalline CoNiFeAl0·7Cu0.3 are constructed to perform uniaxial compression. Hall-Petch and inverse Hall-Petch relations are observed in both FCC and BCC structured polycrystalline CoNiFeAlxCu1-x. The microstructural evolutions of polycrystalline CoNiFeAlxCu1-x reveal that the dominant deformation mechanisms in the Hall-Petch regime of FCC structures are dislocation slip and deformation twinning due to relatively low stacking fault energy and that of BCC structures is phase transformation plasticity. For the inverse Hall-Petch relation, the dominant deformation mechanisms for both FCC and BCC HEAs are the rotation of grains and migration of grain boundaries. It indicates that FCC and BCC HEAs exhibit similar Hall-Petch and inverse Hall-Petch relations with the conventional polycrystalline materials, but its grain size exponent and gradient are quite different from those of pure metals.

Synthesis and Mechanical Characterization of a CuMoTaWV High-Entropy Film by Magnetron Sputtering.

Abstract: Development of high-entropy alloy (HEA) films is a promising and cost-effective way to incorporate these materials of superior properties in harsh environments. In this work, a refractory high-entropy alloy (RHEA) film of equimolar CuMoTaWV was deposited on silicon and 304 stainless-steel substrates using DC-magnetron sputtering. A sputtering target was developed by partial sintering of an equimolar powder mixture of Cu, Mo, Ta, W, and V using spark plasma sintering. The target was used to sputter a nanocrystalline RHEA film with a thickness of ∼900 nm and an average grain size of 18 nm. X-ray diffraction of the film revealed a body-centered cubic solid solution with preferred orientation in the (110) directional plane. The nanocrystalline nature of the RHEA film resulted in a hardness of 19 ± 2.3 GPa and an elastic modulus of 259 ± 19.2 GPa. A high compressive strength of 10 ± 0.8 GPa was obtained in nanopillar compression due to solid solution hardening and grain boundary strengthening. The adhesion between the RHEA film and 304 stainless-steel substrates was increased on annealing. For the wear test against the E52100 alloy steel (Grade 25, 700-880 HV) at 1 N load, the RHEA film showed an average coefficient of friction (COF) and wear rate of 0.25 (RT) and 1.5 (300 °C), and 6.4 × 10-6 mm3/N m (RT) and 2.5 × 10-5 mm3/N m (300 °C), respectively. The COF was found to be 2 times lower at RT and wear rate 102 times lower at RT and 300 °C than those of 304 stainless steel. This study may lead to the processing of high-entropy alloy films for large-scale industrial applications.

High temperature strengthening via nanoscale precipitation in wrought CoCrNi-based medium-entropy alloys

Abstract: In order to make CoCrNi medium-entropy alloys (MEAs) applied for high temperature fields, we hereby demonstrate an approach of manipulating the mechanical properties of fcc structured MEA systems by the precipitation of nanoscale L12-(Ni, Co, Cr)3(Ti, Al, Ta) phase. A new kind of wrought CoCrNi MEAs with superior high temperature mechanical properties has been developed by composition design and hot processing. It is confirmed that the composite addition of Al, Ti and Ta element into CoCrNi MEA not only promotes the precipitation but also improves the thermal stability of nanoscale γ′ phase, resulting in significant enhancement at high temperature. Especially, the (CoCrNi)95Al2Ti2Ta1 MEA exhibits the yield and tensile strength at 700 °C up to ~620 MPa and ~939 MPa, respectively, which are higher than those of IN718 superalloys, and holds an acceptable engineering strain of 28.3%. The high temperatures strengthening mechanisms can be ascribed to the annealing twinning induced grain boundary strengthening, precipitation strengthening by the suppression of the local deformation around nanoscale γ′ grain boundaries, and Orowan bypass mechanism of dislocations. The excellent mechanical properties and processability makes the MEAs promising for high temperature components.

Simulation of the Hall-Petch effect in FCC polycrystals by means of strain gradient crystal plasticity and FFT homogenization

Abstract: The influence of grain size on the flow stress of various FCC polycrystals (Cu, Al, Ag and Ni) has been analyzed by means of computational homogenization of a representative volume element of the microstructure using a FFT approach in combination with a strain gradient crystal plasticity model. The density of geometrically necessary dislocations resulting from the incompatibility of plastic deformation among different crystals was obtained from the Nye tensor, which was efficiently obtained from the curl operation in the Fourier space. The simulation results were in good agreement with the experimental data for Cu, Al, Ag and Ni polycrystals for grain sizes > 20 µm and strains

Increasing strength and ductility of a Mg–9Al alloy by dynamic precipitation assisted grain refinement during multi-directional forging

Abstract: A fine-grained microstructure with enhanced strength and ductility was fabricated in a Mg–9Al alloy by tailoring multi-directional forging with dynamic precipitation. The dynamic precipitation concurred with dynamic recrystallization and inhibited grain growth, thus improved the strength by grain boundary strengthening. The improved ductility was explained by the drastic morphology alteration and decreased volume fraction of the precipitates.

Microstructure and mechanical properties of SPS sintered Al2O3–ZrO2 (3Y)–SiC ceramic composites

Abstract: Al2O3–ZrO2 (3Y)–SiC ceramic composites were prepared by spark plasma sintering in vacuum under 40 MPa of uniaxial compression between 1300 °C and 1400 °C. The effect of nano-size SiC and sintering temperature on the microstructure and mechanical properties of ceramic composites were investigated. The results showed that SiC particles was uniformly distributed in the composite, mainly within the matrix grains. Sintering temperature and SiC particles caused the shape of alumina grains to be columnar. The flexural strength of 22.3 vol% Al2O3-57.7 vol% ZrO2-20 vol% SiC sintered at 1350 °C composites was 1081 MPa, fracture toughness was 15.18 MPa m1/2 and hardness was 16.42 GPa. The addition of SiC particles leads to grain boundary strengthening, and the change of fracture mode and cracks deflection and bridging are considered to be the main reasons for the increase in material fracture toughness.

Strengthening and ductilization mechanisms of friction stir processed cast Mg–Al–Zn alloy

Abstract: AZ91 alloy, the most widely used Mg casting alloy, exhibits low strength/ductility and weak energy absorption, which is a function of its large grain size and the presence of a coarse and continuous network of β-Mg17Al12 intermetallic compounds. This work demonstrated that friction stir processing (FSP) enables enhancement of strength and energy absorption capability of AZ91 alloy. The influence of FSP treatment on various potential strengthening mechanisms, including grain boundary, solid solution, and sub-micron particle strengthening mechanisms, was studied. It is identified that the grain boundary strengthening plays a significant contribution to the strength of the FSP treated AZ91. FSP treatment altered the failure mechanism of the alloy from brittle cleavage-dominant mode to ductile dimple-dominant mode, which can increase the potential of cast Mg alloys to use in the safety-critical application. The significant improvement in energy absorption capability is a function of grain refining and the formation of ultra-fine sub-micron Mg17Al12 particles. The trends in mechanical properties of AZ91 treated by various severe plastic deformation approaches are disused.

The Effect of Lath Martensite Microstructures on the Strength of Medium-Carbon Low-Alloy Steel

Abstract: Different austenitizing temperatures were used to obtain medium-carbon low-alloy (MCLA) martensitic steels with different lath martensite microstructures. The hierarchical microstructures of lath martensite were investigated by optical microscopy (OM), electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM). The results show that with increasing the austenitizing temperature, the prior austenite grain size and block size increased, while the lath width decreased. Further, the yield strength and tensile strength increased due to the enhancement of the grain boundary strengthening. The fitting results reveal that only the relationship between lath width and strength followed the Hall–Petch formula of. Hence, we propose that lath width acts as the effective grain size (EGS) of strength in MCLA steel. In addition, the carbon content had a significant effect on the EGS of martensitic strength. In steels with lower carbon content, block size acted as the EGS, while, in steels with higher carbon content, the EGS changed to lath width. The effect of the Cottrell atmosphere around boundaries may be responsible for this change.

Regulating Surface and Grain-Boundary Structures of Ni-Rich Layered Cathodes for Ultrahigh Cycle Stability

Abstract: The wide applications of Ni-rich LiNi1- x-y Cox Mny O2 cathodes are severely limited by capacity fading and voltage fading during the cycling process resulting from the pulverization of particles, interfacial side reactions, and phase transformation. The canonical surface modification approach can improve the stability to a certain extent; however, it fails to resolve the key bottlenecks. The preparation of Li(Ni0.4 Co0.2 Mn0.4 )1- x Tix O2 on the surface of LiNi0.8 Co0.1 Mn0.1 O2 particles with a coprecipitation method is reported. After sintering, Ti diffuses into the interior and mainly distributes along surface and grain boundaries. A strong surface and grain boundary strengthening are simultaneously achieved. The pristine particles are fully pulverized into first particles due to mechanical instability and high strains, which results in serious capacity fading. In contrast, the strong surface and the grain boundary strengthening can maintain the structural integrity, and therefore significantly improve the cycle stability. A general and simple strategy for the design of high-performance Ni-rich LiNi1- x - y Cox Mny O2 cathode is provided and is applicable to surface modification and grain-boundary regulation of other advanced cathodes for batteries.

Effect of grain size on the strain rate sensitivity of CoCrFeNi high-entropy alloy

Abstract: Strain rate sensitivity (SRS) of a face-centered cubic (fcc) CoCrFeNi high-entropy alloy (HEA) with grain sizes ranging from 57 μm to 45 nm was investigated using nanoindentation, and was compared with those reported for conventional fcc metals. Experimental results show pronounced grain boundary strengthening in the HEA. Estimated values of the SRS parameter, m and activation volume, V*, indicate similar plastic deformation mechanisms in HEA and Ni in nanocrystalline regime that are grain boundary mediated. In coarse-grained regime, the high lattice friction stress in the HEA results in much higher m and smaller V* as compared to coarse grained Ni.

Developing a high-strength Al–11Si alloy with improved ductility by combining ECAP and cryorolling

Abstract: In this study, a high-strength Al–11Si alloy with an improved ductility was developed via a two-step processing route - equal-channel angular pressing (ECAP) and post-cryorolling. Specifically, the cast alloy was ECAP processed at 623 K for 16 passes, followed by multi-pass cryorolling with 50% thickness reduction. Herein, its mechanical properties was evaluated, and the underlying mechanisms for the improvement of both strength and ductility was investigated. Through systematic comparisons with other samples, including the cast, the cast-rolled, the ECAP-processed and the ECAP-annealed samples, we found the respective origin for the high strength and the improved ductility in the processed alloy. On the one hand, the high strength resulted from both grain boundary strengthening and dislocation strengthening caused by the thermo-mechanical processing. On the other hand, the improved ductility was primarily dictated by the uniform distribution of the very fine Si particles due to the attendant fragmentation of the Si phase during ECAP process. In addition, the results showed that the superior combination of strength and ductility could not be achieved by either processing step alone. This two-step processing route is also promising to benefit other Al–Si alloys for structural applications by optimizing their mechanical properties.