The ability to predict and understand phases in high-entropy alloys (HEAs) is still being debated, and primarily true predictive capabilities derive from the known thermodynamics of materials. The ...present work demonstrates that prior work using high-throughput first-principles calculations may be further utilized to provide direct insight into the temperature- and composition-dependent phase evolution in HEAs, particularly Al-containing HEAs with a strengthening multiphase microstructure. Using a simple model with parameters derived from first-principles calculations, we reproduce the major features associated with Al-containing phases, demonstrating a generalizable approach for exploring potential phase evolution where little experimental data exists. Neutron scattering, in situ microscopy, and calorimetry measurements suggest that our high-throughput Monte Carlo technique captures both qualitative and quantitative features for both intermetallic phase formation and microstructure evolution at lower temperatures. This study provides a simple approach to guide HEA development, including ordered multi-phase HEAs, which may prove valuable for structural applications.
Abstract
The mechanical behavior of a single phase (fcc) Al
0.3
CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening ...mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to shear localization was studied by dynamically-loading hat-shaped specimens to induce forced shear localization. However, no adiabatic shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to shear localization, which makes this material an excellent candidate for penetration protection applications such as armors.
The factors of the atomic size difference Delta and the enthalpy of mixing ΔHmix of the multi‐component alloys were summarized from the literatures. The formation zones of solid‐solution phases, ...intermediate phases, and bulk metallic glasses were determined and the validity was verified by experimental results. For forming the solid solution, the alloys should have high entropy of mixing, lower Delta, and not too negative and positive enthalpy of mixing.
With a suite of multi-modal and multi-scale characterization techniques, the present study unambiguously proves that a substantially-improved combination of ultrahigh strength and good ductility can ...be achieved by tailoring the volume fraction, morphology, and carbon content of the retained austenite (RA) in a transformation-induced-plasticity (TRIP) steel with the nominal chemical composition of 0.19C–0.30Si–1.76Mn–1.52Al (weight percent, wt%). After intercritical annealing and bainitic holding, a combination of ultimate tensile strength (UTS) of 1100MPa and true strain of 50% has been obtained, as a result of the ultrafine RA lamellae, which are alternately arranged in the bainitic ferrite around junction regions of ferrite grains. For reference, specimens with a blocky RA, prepared without the bainitic holding, yield a low ductility (35%) and a low UTS (800MPa). The volume fraction, morphology, and carbon content of RA have been characterized using various techniques, including the magnetic probing, scanning electron microscopy (SEM), electron-backscatter-diffraction (EBSD), and transmission electron microscopy (TEM). Interrupted tensile tests, mapped using EBSD in conjunction with the kernel average misorientation (KAM) analysis, reveal that the lamellar RA is the governing microstructure component responsible for the higher mechanical stability, compared to the blocky one. By coupling these various techniques, we quantitatively demonstrate that in addition to the RA volume fraction, its morphology and carbon content are equally important in optimizing the strength and ductility of TRIP-assisted steels.
This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available ...to be used to design the alloys. We have proposed that the parameters of
δ
and Ω can be used to predict the phase formation of HEAs, namely Ω ≥ 1.1 and
δ
≤ 6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoV
x
and CoCrFeNiAlNb
x
were prepared. Their microstructures mainly consist of simple body-centered cubic solid solutions at low Nb contents. TiZrNbMoV
x
alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to-brittle transition even when the temperatures are lowered from 298 K to 77 K.
High entropy alloy is a new class of structural metallic materials. No work, so far, has been carried-out to understand high strain-rate plastic deformation behavior and resulting microstructure. ...This work focuses on understanding the deformation behavior of an Al0.1CrFeCoNi HEA at high strain-rate (HSR). HSR plastic deformation in compression mode was carried out using split-Hopkinson pressure bar. The pre- and post-deformation microstructures were studied using electron microscopes. A high strain-rate sensitivity of yield strength, significant work hardening, and profuse twinning are main characteristics observed during deformation of the alloy at HSR. Overall, the deformation behavior of the alloy was consistent with low stacking fault energy materials.
Display omitted
•High strain-rate sensitivity was observed at very high strain-rates.•The alloy showed extensive work hardening ability.•Out of three regions, region-II of work hardening rate plot showed a positive slope.•At high strain-rates nano-sized secondary twins were observed.
Additive manufacturing (AM) is an emerging technology that has been widely used for fabricating components with highly complex shapes. In this work, C-containing FeCoCrNi high entropy alloy with full ...density was successfully fabricated using selective laser melting (SLM). The effects of processing parameters on the microstructures and mechanical properties were investigated, and the underlying mechanism was discussed. The density of HEA is dependent on the SLM processing parameter. The increase of the SLM scanning power or the decrease of the scanning speed leads to the increase of the density of HEA. The grain size reduced when the high scanning power or the low scanning speed was used. The C-containing HEAs with full density has a yield strength of 650 MPa, and an elongation of 13.5%.
•Carbon can be fully dissolved in the SLMed FeCoCrNiC0.05.•The C-containing HEAs with full density has a yield strength of 650 MPa.•The elongation shows notable changes with varying the processing condition.
Two refractory high-entropy alloys with near-equiatomic concentrations, W–Nb–Mo–Ta and W–Nb–Mo–Ta–V, were produced by vacuum arc melting. Despite containing many constituents both alloys have a ...single-phase body-centered cubic (BCC) structure. The lattice parameters
a
=
3.2134(3)
Å for the quaternary alloy and
a
=
3.1832(2)
Å for the quinternary alloy were determined with high-energy X-ray diffraction using a scattering vector length range from 0.7 to 20
Å
−1. The alloy density and Vickers microhardness were
ρ
=
13.75
g/cm
3 and
H
v
=
4455
MPa for the W–Nb–Mo–Ta alloy and
ρ
=
12.36
g/cm
3 and
H
v
=
5250
MPa for the W–Nb–Mo–Ta–V alloy. The exceptional microhardness in these alloys is greater than any individual constituent, suggesting the operation of a solid-solution-like strengthening mechanism.
A serrated flow, which occurs in a material undergoing mechanical deformation, is a complex process of great engineering significance. Here statistical, dynamical, and multifractal modeling and ...analyses were performed on the stress-time series to characterize and model the stress-drop behavior of an Al0.5CoCrCuFeNi high-entropy alloy (HEA). Results indicate that the spatiotemporal dynamics of the serrated flow is affected by changes in the strain rate and test temperature. The sample entropy, in general, was found to be the highest in the samples tested at 500 °C. The higher complexity in the serrated flow at this temperature appeared to be associated with the stress-drop behavior that had intermediate values in terms of the maximum stress drop, the multifractality of the data set, and the histogram distributions. Moreover, the sample entropy was the lowest for the samples tested at 600 °C. The lower complexity values were associated with a wider multifractal spectrum and a less uniform and sparser distribution of the stress-drop magnitudes. In terms of the serration types, Type-C serrations were related to the lowest complexity values, widest multifractal spectra, and higher probability of exhibiting larger stress drops. Conversely, Type-A and B serrations were associated with the higher complexity, narrower spectra, and lower probability of higher stress drops. Furthermore, the body-centered-cubic (BCC) structure and the fully-ordered L12 nano-particles were found to emerge in the samples at 600 °C and are thought to be linked to the decreased spatiotemporal correlations in the stress-drop behavior.
•Link between complexity, multifractal, and stress-drop magnitude statistics.•Spatiotemporal dynamics of serrations is affected by strain rate and temperature.•Serrations exhibited the most complex behavior at a strain rate of 2 × 10-3 s-1.•Higher spatiotemporal correlations in the serrated flow at 500oC.
High-entropy alloys (HEAs) with a single-phase face-centered cubic (FCC) structure generally have excellent plasticity. However, its limited room temperature strength and hardness result in lower ...wear resistance, which hinders tribological and engineering applications. In this paper, the pack-boronizing method was used to improve the tribological properties of Al0·1CoCrFeNi HEAs with a single-phase FCC structure. The boronizing treatment was carried out at 900 °C for 2, 4, 6, and 8 h, respectively. The structure and phase formation of boronized alloys were studied. Moreover, the reciprocating sliding wear behavior of boronized alloys against Si3N4 balls in dry and deionized water was investigated. A boronized layer with a double-layer structure was formed on the HEA surface, with a total thickness of about 17.3–58.5 μm. The wear mechanism of boronized alloy was transformed from the abrasive and delamination wear to the polishing effect with increasing the boronizing time in air. In contrast, the wear mechanism of boronized alloys in the deionized water gradually changed from the two-body to three-body wear with increasing the boronizing time. Meanwhile, the wear mechanism of the Si3N4 counterface was also transformed to three-body wear gradually.
•The maximum hardness of boronized HEAs is higher 7 times than that of annealed HEAs.•The wear rate of boronized HEAs was much lower than that of as-cast HEAs.•The wear mechanism of boronized HEAs in air was abrasive wear and polishing effect.•Wear mechanism of HEAs in deionized water was two-body and three-body abrasive wear.•The lubrication of deionized water led to lower friction coefficient and wear rate.
PDF
