Spherical tip nanoindentation experiments on two typical face-centered cubic high-entropy alloys (HEAs), CoCrFeNi and CoCrFeMnNi in as-solutionized and aged conditions were performed using tips of ...two different radii. Large datasets of the strength at the first pop-in were obtained and their statistical nature were analyzed to gain insights into the micromechanisms responsible for the onset of incipient plasticity in HEAs that are notionally monophasic. In all the cases examined, the probability density distributions were bimodal in nature. The deconvoluted distributions were utilized to estimate the activation volumes of the underlying deformation mechanisms. They show that when the probed material's volume is relatively small, heterogeneous dislocation nucleation aided by monovacancies occurs at lower indentation stresses; this followed by homogeneous dislocation nucleation at high loads, resulting in strengths corresponding to the theoretical strengths. When a substantially larger volume is sampled, by using a larger radius tip, either the preexisting dislocation mediated ones at low stresses or vacancy cluster /grain boundary aided heterogeneous dislocations nucleation at higher stresses become dominant. Increasing the chemical short-range order in the alloy via high temperature aging leads to overall strengthening of the alloy by enhancing stress required for the homogeneous dislocation nucleation. Implications of such plurality of mechanisms with overlapping strength distributions at HEA's disposal in imparting high strength-ductility combinations are discussed.
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The influence of hydrogen on the mechanical behavior of the CoCrFeMnNi high-entropy alloy (HEA) was examined through tensile and nanoindentation experiments on specimens hydrogenated via gaseous and ...electrochemical methods. Results show that the HEA's resistance to gaseous hydrogen embrittlement is better than that of two representative austenitic stainless steels, in spite of the fact that it absorbs a larger amount of hydrogen than the two steels. Reasons for this were discussed in terms of hydrogen-enhanced localized plasticity mechanism and the critical amount of hydrogen required for it. These were further substantiated by additional experiments on electrochemically charged specimens.
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Time-dependent plastic deformation behavior of nanocrystalline (nc) and coarse-grained (cg) CoCrFeMnNi high-entropy alloys (HEAs) was systematically explored through a series of spherical ...nanoindentation creep experiments. High-pressure torsion (HPT) processing was performed for achieving nc microstructure in the HEA, leading to a reduction in grain size from ∼46 μm for the as-cast state to ∼ 33 nm at the edge of the HPT disk after 2 turns. Indentation creep tests revealed that creep deformation indeed occurs in both cg and nc HEAs even at room temperature and it is more pronounced with an increase in strain. The creep stress exponent, n, was estimated as ∼3 for cg HEA and ∼1 for nc HEA and the predominant creep mechanisms were investigated in terms of the values of n and the activation volumes. Through theoretical calculations and comparison of the creep strain rates for nc HEA and a conventional face-centered-cubic nc metal (Ni), the influence of sluggish diffusion on the creep resistance of nc HEA was analyzed. In addition, sharp indentation creep tests were performed for comparison purposes and the results confirmed that the use of a spherical indenter is clearly more appropriate for investigating the creep behavior of this HEA.
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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.
The influence of annealing on the constitutive stress-strain response of nanocrystalline (nc) CoCrFeMnNi high-entropy alloy (HEA) was investigated through a series of nanoindentation experiments ...using five different three-sided pyramidal indenters. The nc HEA, produced by high-pressure torsion (HPT), was subjected to annealing at 450 °C for 1 and 10 h. Microstructural analysis using transmission electron microscopy (TEM) showed that three different nano-scale precipitates (NiMn-, FeCo-, and Co-rich phases) form in the primary single-phase matrix of nc HEA after annealing. The strain-dependent plastic flow response of nc HEA pre- and post-annealing was estimated using the indentation strain and constraint factor, revealing a significant strain softening in nc HEA, which becomes pronounced after annealing. TEM analysis of the deformed material underneath the indenter suggests that the plastic deformation aids in the dissolution of the annealing-induced intermetallic precipitates, which could be the mechanism for the pronounced softening. The dissolution mechanism was rationalized by the destabilization of precipitates during plastic deformation due to the increase in interface energy.
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The influence of charging method on hydrogen (H) distribution and the resultant nanomechanical behavior of CoCrFeMnNi high-entropy alloy was examined and compared with another face-centered cubic ...structured alloy, an austenitic stainless steel. Through thermal desorption spectroscopy measurement and theoretical analysis, it was revealed that electrochemical (E-) charging induces steep gradient of H concentration near the surface while H was homogenously distributed after gaseous (G-) charging. Nanoindentation results show significant hardening in E-charged alloys while the hardness of G-charged alloys remains invariant. These differences were rationalized in terms of the nature of H distributions induced by different charging methods.
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A four-component equimolar high-entropy alloy (HEA) with the composition of HfNbTiZr and body-centered cubic (bcc) structure was processed by HPT at RT. The evolution of the dislocation density, the ...grain size and the hardness was monitored along the HPT-processed disk radius for different numbers of turns between ¼ and 20. It was found that most of the increase of the dislocation density and the refinement of the grain structure occurred up to the shear strain of ∼40. Between the strains of ∼40 and ∼700, only a slight grain size reduction was observed. The saturated dislocation density and grain size were ∼2.1 × 1016 m−2 and ∼30 nm, respectively. The saturation in hardness was obtained at ∼4450 MPa. These values were similar to the parameters determined in the literature for five-component HEAs processed by HPT. The analysis confirmed that the main component in the strength was given by the friction stress in the HPT-processed bcc HfNbTiZr HEA. It was also revealed that the contribution of the high dislocation density to the strength was significantly higher than the effect of the small grain size.
•The microstructure and the hardness of a HPT processed HfNbTiZr HEA were studied.•The dislocation density increased to 210 × 1014 m−2 at the shear strain of ∼40.•The minimum grain size of 28 nm was achieved at the shear strain of ∼400.•The saturation hardness was 4450 MPa.•Most of the hardening (60–90%) was caused by the friction stress.
The existence of an indentation size effect (ISE) in the onset of yield in a Zr-based bulk metallic glass (BMG) is investigated by employing spherical-tip nanoindentation experiments. Statistically ...significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different structural states (as-cast and structurally relaxed) of the BMG. Hertzian contact mechanics were employed to convert the pop-in loads to the maximum shear stress underneath the indenter. Results establish the existence of an ISE in the BMG of both structural states, with shear yield stress increasing with decreasing tip radius. Structural relaxation was found to increase the yield stress and decrease the variability in the data, indicating “structural homogenization” with annealing. Statistical analysis of the data was employed to estimate the shear transformation zone (STZ) size. Results of this analysis indicate an STZ size of ∼25 atoms, which increases to ∼34 atoms upon annealing. These observations are discussed in terms of internal structure changes that occur during structural relaxation and their interaction with the stressed volumes in spherical indentation of a metallic glass.
Rural settlement is the basic spatial unit for compact communities in rural area. Scientific exploration of spatial-temporal differentiation and its influencing factors is the premise of spatial ...layout rationalization. Based on land use data of Liangshan Yi Autonomous Prefecture (hereinafter referred to as Liangshan Prefecture) in Sichuan Province, China from 1980 to 2020, compactness index, fractal dimension, imbalance index, location entropy and the optimal parameters-based geographical detector (OPGD) model are used to analyze the spatial-temporal evolution of the morphological characteristics of rural settlements, and to explore the influence of natural geographical factors, socioeconomic factors, and policy factors on the spatial differentiation of rural settlements. The results show that: (1) From 1980 to 2020, the rural settlements area in Liangshan Prefecture increased by 15.96 km
2
. In space, the rural settlements are generally distributed in a local aggregation, dense in the middle and sparse around the periphery. In 2015, the spatial density and expansion index of rural settlements reached the peak. (2) From 1980 to 2020, the compactness index decreased from 0.7636 to 0.7496, the fractal dimension increased from 1.0283 to 1.0314, and the fragmentation index decreased from 0.1183 to 0.1047. The spatial morphological structure of rural settlements tended to be loose, the shape contour tended to be complex, the degree of fragmentation decreased, and the spatial distribution was significantly imbalanced. (3) The results of OPGD detection in 2015 show that the influence of each factor is slope (0.2371) > traffic accessibility (0.2098) > population (0.1403) > regional GDP (0.1325) > elevation (0.0987) > poverty alleviation (0). The results of OPGD detection in 2020 show that the influence of each factor is slope (0.2339) > traffic accessibility (0.2198) > population (0.1432) > regional GDP (0.1219) > poverty alleviation (0.0992) > elevation (0.093). Natural geographical factors (slope and elevation) are the basic factors affecting the spatial distribution of rural settlements, and rural settlements are widely distributed in the river valley plain and the second half mountain area. Socioeconomic factors (traffic accessibility, population, and regional GDP) have a greater impact on the spatial distribution of rural settlements, which is an important factor affecting the spatial distribution of rural settlements. Policy factors such as poverty alleviation relocation have an indispensable impact on the spatial distribution of rural settlements. The research results can provide decision-making basis for the spatial arrangement of rural settlements in Liangshan Prefecture, and optimize the implementation of rural revitalization policies.
The influence of hydrogen on the onset of plastic deformation in a CoCrFeMnNi high-entropy alloy (HEA) was examined through the analysis of the load at which first pop-in during spherical ...nanoindentation experiments occurs on hydrogen-charged and subsequently aged specimens. Results reveal that the dissolved hydrogen lowers the plastic flow resistance, indicated by the shear yield strength, τy, by modifying defect formation energies. Aging, subsequent to charging, leads to recovery of τy, but only partially. The results are discussed in terms of the vacancy-mediated dislocation nucleation, which is supported by the estimated activation volume for deformation.
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