Hardness Of High-entropy Alloy Research Articles

Atomistic simulation of nanoindentation behavior of amorphous/crystalline dual-phase high entropy alloys

High-entropy alloys (HEAs) are a new type of multi-principal metal materials that exhibit excellent mechanical properties. However, the strength–ductility balance in the HEAs remains a challenge that needs to be addressed. The amorphous/crystalline (A/C) structure is a new design strategy to achieve high strength and excellent ductility of the HEAs. Here, the influences of amorphous layer spacing, indenter velocity, and indenter radius on the mechanical properties and microstructure evolution of the A/C dual-phase CoCrFeMnNi HEAs under nanoindentation were investigated by molecular dynamics (MD) simulation. The results indicate that the plastic deformation mechanism of the monocrystalline HEAs is mainly dominated by the nucleation and slip of dislocations, while the plastic deformation mechanism of the dual-phase HEAs is mainly dominated by the interaction between dislocations and amorphous phases. The results show that the average indentation force of the dual-phase HEAs increases with the increase of the amorphous layer spacing. The amorphous layer in the HEAs can hinder the expansion of dislocations, limiting them to the crystalline matrix between the two amorphous layers. The results also indicate that Young's modulus of the HEAs increases with the increase of the indentation velocity and indentation radius. However, the hardness of HEAs is positively correlated with the indenter velocity, and negatively correlated with the indenter radius. It should be noted that the critical indentation depth and critical indentation force for the plastic deformation of the dual-phase HEAs decrease with the increase of indenter velocity, which is opposite to that of the single-phase crystalline HEAs.

Tailoring microstructures and properties in Co-free FeNiCrCuAl high entropy alloys by Al addition

In this study, a series of Co-free FeNiCrCuAlx (x = 0.1, 0.3, 0.5, 0.8 and 1.0) high entropy alloys (HEAs) were developed by vacuum arc melting and casting method in high-purity argon atmosphere. The effects of Al addition on the microstructure and properties were investigated using X-ray diffraction, scanning electron microscopy, Vickers hardness tester, tensile test, wear test and electrochemical experiment. It was found that with the increase in the large atomic-sized Al content, the closely-packed FCC structure was destabilized for a transition to the loosely-packed BCC structure at x = 0.3. As x increased up to 1.0, the structure was mainly composed of BCC phase. The hardness of HEAs was enhanced by the influence of the contiguous atomic planes in BCC structures due to the relatively lower inter-planar spacing and higher lattice friction for dislocation motion, with the potential to increase the yield strength of the HEAs by 93.5 %. The friction coefficient decreased with increasing Al content, as indicated by stable profiles of Al0.8 and Al1.0 under progressive wear in comparison to those of Al0.1, Al0.3 and Al0.5 while the associated minimal wear depth and lower capacity for debris accumulation of the higher Al content HEAs confirmed their superior wear resistance. As FeNiCrCuAl0.5 presented the most superior corrosion resistance due to high resistance to ionic transport via the steady oxide film and the indication of a comparatively wider capacitive semicircle, undue Al additions would not be beneficial to improving plasticity or enhance corrosion resistance. Thus, FeNiCrCuAlx HEAs with Al contents in the range of 0.5–1.0 were found to possess optimized comprehensive properties, and can be suitably recommended as nuclear structural materials. This study will shed light on advancing the development of cutting-edge HEAs.

Influences of Si and C addition on microstructure and mechanical properties of Fe2.5CoNiCu high-entropy alloy

To evaluate the role of non-metallic element (Si and C) on the microstructural evolution and mechanical properties of Fe2.5CoNiCu high-entropy alloys (HEAs), two series of Fe2.5CoNiCuSix (x=0.1−0.3, molar fraction) and Fe2.5CoNiCuCx (x=0.15−0.4, molar fraction) HEAs were prepared by vacuum induction melting. The results show that the amounts of C and Si additions significantly stimulate the transformation of BCC/FCC dual phases into a single FCC structure, and the microstructural evolution appears to be more strongly influenced by the C addition. The Fe2.5CoNiCuSi0.2 HEA presents high hardness (HV 439.5) and tensile strength (868 MPa), which is attributed to the solution strengthening and grain refinement effect. Both the hardness and strength of Fe2.5CoNiCuCx HEAs increase with the addition of C, due to the formation of carbides and solution strengthening. This demonstrates that adding non-metallic element (C or Si) is a viable approach for enhancing the strength and hardness of HEAs.

Predicting phases and hardness of high entropy alloys based on machine learning

The mechanical properties of high entropy alloys are highly dependent on phase structure. Therefore, accurate prediction of the phase structure is a guide to composition selection and performance enhancement, and it saves both experimental and financial expenditure. In this study, seven classification models were selected to predict the phase structure of high-entropy alloys and six regression models were selected to predict the hardness, and it was found that the XGB model performed best in both classification and regression predictions, where in the prediction phase structure, the XGB model has an error rate of 4.5% in the 10% prediction set, 11.6% in the 20% prediction set and 6.25% in the 30% prediction set. In predicting the hardness, the RMSE and MAE evaluation indexes of the XGB model are 52.66, 26.49. The δ (atomic radius difference) feature parameter having the strongest contribution to the phase formation in the high-entropy alloys. In addition, valence electron concentration(VEC), y/n BCC (The presence or absence of BCC) and y/n lm (The presence or absence of other phases) were key features affecting the hardness. Predictive analysis of two high-entropy alloys, AlCoCrFeNi and AlCoCrFeNiTi0.5, and a characterization effect map based on the Shapley additive explanations (SHAP) framework were developed to provide decision support for hardness assessment.

Machine learning combined with solid solution strengthening model for predicting hardness of high entropy alloys

Traditional material calculation methods, such as first principles and thermodynamic simulations, have accelerated the discovery of new materials. However, these methods are difficult to construct models flexibly according to various target properties. And they will consume many computational resources and the accuracy of their predictions is not so high. In the last decade, data-driven machine learning techniques have gradually been applied to materials science, which has accumulated a large quantity of theoretical and experimental data. Machine learning is able to dig out the hidden information from these data and help to predict the properties of materials. The data in this work are obtained from the published references. And several performance-oriented algorithms are selected to build a prediction model for the hardness of high entropy alloys. A high entropy alloy hardness dataset containing 19 candidate features is trained, tested, and evaluated by using an ensemble learning algorithm: a genetic algorithm is selected to filter the 19 candidate features to obtain an optimized feature set of 8 features; a two-stage feature selection approach is then combined with a traditional solid solution strengthening theory to optimize the features, three most representative feature parameters are chosen and then used to build a random forest model for hardness prediction. The prediction accuracy achieves an <i>R</i><sup>2</sup> value of 0.9416 by using the 10-fold cross-validation method. To better understand the prediction mechanism, solid solution strengthening theory of the alloy is used to explain the hardness difference. Further, the atomic size, electronegativity and modulus mismatch features are found to have very important effects on the solid solution strengthening of high entropy alloys when genetic algorithms are used for implementing the feature selection. The machine learning algorithm and features are further used for predicting solid solution strengthening properties, resulting in an <i>R</i><sup>2</sup> of 0.8811 by using the 10-fold cross-validation method. These screened-out parameters have good transferability for various high entropy alloy systems. In view of the poor interpretability of the random forest algorithm, the SHAP interpretable machine learning method is used to dig out the internal reasoning logic of established machine learning model and clarify the mechanism of the influence of each feature on hardness. Especially, the valence electron concentration is found to have the most significant weakening effect on the hardness of high entropy alloys.

Prediction of phase and hardness of HEAs based on constituent elements using machine learning models

The phase and hardness of high entropy alloys (HEA) were predicted using machine learning models trained on large experimental datasets. The chemical composition was used as the set of input features. For phase prediction, a neural network was trained on 775 experimental samples and yielded a 93.4% prediction accuracy. In addition, the hardness of HEAs has been predicted using an ensemble model trained on an unprecedented large hardness dataset of 427 samples. The model showed an average regression value of 0.88, and most of the predicted values were within the 20% error region. Moreover, sensitivity analysis suggest that HEAs hardness is affected the most by Cu,Ti,Co, and Ni concentration (respectively). Due to the testing dataset size, and the aforementioned accuracy, the phase and hardness model may be reliably used for prediction and screening purposes.

Effect of annealing on irradiation resistance of (FeCoNi)86Al7Ti7 high entropy alloys

(FeCoNi)86Al7Ti7 High-entropy alloys show high hardness, fatigue resistance and excellent mechanical property, making it appealing for nuclear industry application. In this paper, (FeCoNi)86Al7Ti7 High-entropy alloys were fabricated using a selective laser melting technique (SLM). The microstructural evolution of original and annealed the SLM alloys were discussed in detail after irradiating. In addition, the influences of the strength and hardness of high entropy alloys were also investigated.

Relation Between Strength and Hardness of High-Entropy Alloys

Relation Between Strength and Hardness of High-Entropy Alloys

Fine-Grained FeCoNi(CuAl)x High Entropy Alloys: Phase Transformation, Microstructure Evolution and Mechanical Properties

A series of fine-grained FeCoNi(CuAl)x (x=0, 0.4, 0.6, 0.8, 1.0) medium-entropy alloy (MEA) and high-entropy alloys (HEAs) were fabricated by Mechanical Alloying (MA) and Spark Plasma Sintering (SPS). The effect of Al and Cu content (x) on phase composition, microstructure and mechanical properties of the alloys was investigated. Experimental results show that the crystal structure of FeCoNi(CuAl)x alloy transforms from single FCC phase for x=0 to FCC+BCC duplex phases for x=0.4~1.0, with the fraction of BCC phase gradually increasing with the increase of x. Adding a low content of Al and Cu elements to FeCoNi alloy can significantly hinder the grain growth during sintering process, the average grain size of FCC phase decreases from 0.95 to 0.30 µm at x=0.4. However, the grain sizes of FCC and BCC phases gradually grow up when x increases from 0.4 to 1.0. The variation in grain size indicates that the atomic diffusion rate of sintered alloy may be influenced by the sluggish diffusion effect in HEA as well as the content of Al and Cu with lower melting points. Mechanical properties of the HEAs are mainly affected by the volume fraction of BCC phase. The compressive yield strength and hardness of HEAs are improved at first and then slightly reduced, while the plasticity drops down continuously with the increase of x. The bulky HEA achieved excellent comprehensive mechanical properties with a compressive yield strength of 1467.7 MPa and plastic strain to failure of 24.9 % at x=0.6, due to the fine duplex microstructure.

Surface modification of CoCrFeNi high entropy alloy by low-temperature gaseous carburization

Low-temperature (470 °C) gaseous carburization is successfully employed in surface hardening of a CoCrFeNi high entropy alloy. After low-temperature gaseous carburization, a carburized case of expanded face-centered cubic (FCC) structure, i.e., a carbide-free supersaturated interstitial solid solution of carbon (maximum of ~3.0 wt%) in FCC, forms in the surface region of the high entropy alloy. Due to solid solution strengthening by interstitially dissolved carbon atoms and strain hardening caused by plastic accommodation of the lattice expansion, the surface hardness of high entropy alloy is significantly enhanced. The surface hardness reaches 1221 HV0.1N, which is about 5 times the hardness of the substrate.

Effect of carbon content on microstructure, hardness and wear resistance of CoCrFeMnNiCx high-entropy alloys

A set of CoCrFeMnNiCx (x = 0, 0.3, 0.6, 0.9, 1.2) high-entropy alloys (HEAs) were prepared using mechanical alloying and spark plasma sintering. The influence of carbon content on the microstructure, hardness, scratch resistance and wear resistance of the CoCrFeMnNiCx HEAs was systematically investigated. The results show that the CoCrFeMnNiCx HEAs have a dual-phase structure comprised of FCC and M7C3 phases. The M7C3 phase content and porosity are found to increase with increasing the carbon content. The hardness of the HEAs is significantly enhanced by carbon alloying from 327.8 HV for CoCrFeMnNi to 566.4 HV for CoCrFeMnNiC0.6. The M7C3 phase content and porosity are two competing factors that affect the hardness. The CoCrFeMnNiC0.6 exhibits the highest scratch and wear resistances of the prepared samples. The wear rate of CoCrFeMnNiC0.6 (0.47 × 10−5 mm3/N·m) is more than an order magnitude lower than that of the CoCrFeMnNi (6.5 × 10−5 mm3/N·m) and is of the same magnitude with the mating material Si3N4. This is due to the precipitation of hard M7C3 carbides and low porosity. The scratch and wear resistances are positively correlated with the hardness. Both delamination wear and abrasive wear contribute to the wear of CoCrFeMnNiCx HEAs.

Prediction of the Composition and Hardness of High-Entropy Alloys by Machine Learning

Machine learning with artificial neural network (ANN)-based methods is a powerful tool for the prediction and exploitation of the subtle relationships between the composition and properties of materials. This work utilizes an ANN to predict the composition of high-entropy alloys (HEAs) based on non-equimolar AlCoCrFeMnNi in order to achieve the highest hardness in the system. A simulated annealing algorithm is integrated with the ANN to optimize the composition. A bootstrap approach is adopted to quantify the uncertainty of the prediction. Without any guidance, the design of new compositions of AlCoCrFeMnNi-based HEAs would be difficult by empirical methods. This work successfully demonstrates that, by applying the machine learning method, new compositions of AlCoCrFeMnNi-based HEAs can be obtained, exhibiting hardness values higher than the best literature value for the same alloy system. The correlations between the predicted composition, hardness, and microstructure are also discussed.

The Valence Electron Structure of High-Entropy Alloys

Empirical Electron Theory in Solids and Molecules (EET) was used to analyze the valence electron structure of ZrTiHfVNb, ZrTiHfVTa and ZrTiHfNbMo high-entropy alloys. The parameters characterizing the valence electron structure of high-entropy alloys were calculated, which were used to discuss the hardness and melting temperature of high-entropy alloys. The results show that the hardness of high-entropy alloys is positively correlated to the shared electron pair number in valence electron structure. The theoretical melting temperatures of high-entropy alloys were predicted by the parameters characterizing the valence electron structure.

FABRICATION AND WEAR BEHAVIOR ANALYSIS ON AlCrFeNi HIGH ENTROPY ALLOY COATING UNDER DRY SLIDING AND OIL LUBRICATION TEST CONDITIONS

In this paper, AlCrFeNi high entropy alloy coating was fabricated on the surface of Q235 steel using hot pressing sintering process. The coating has the controlled thickness size and excellent mechanical properties. Scanning electron microscopy (SEM), XRD and hardness testing method were used to study the morphology, phase structure and hardness of high entropy alloys coating. The lattice distortion plays a significant role in increasing the hardness. Coating formation mechanism caused by the element diffusion under the hot pressing effect is also discussed in the paper. Simultaneously, the dry sliding and oil lubrication wear tests, wear morphology observation and wear mechanism discussion were completed. As the result shows, AlCrFeNi high entropy alloys coating exhibits superior wear resistance either at dry sliding or oil lubrication tests owing to its hard high entropy solid solution structure.

Additive Manufacturing of High-Entropy Alloys by Laser Processing

This contribution concentrates on the possibilities of additive manufacturing of high-entropy clad layers by laser processing. In particular, the effects of the laser surface processing parameters on the microstructure and hardness of high-entropy alloys (HEAs) were examined. AlCoCrFeNi alloys with different amounts of aluminum prepared by arc melting were investigated and compared with the laser beam remelted HEAs with the same composition. Attempts to form HEAs coatings with a direct laser deposition from the mixture of elemental powders were made for AlCoCrFeNi and AlCrFeNiTa composition. A strong influence of solidification rate on the amounts of face-centered cubic and body-centered cubic phase, their chemical composition, and spatial distribution was detected for two-phase AlCoCrFeNi HEAs. It is concluded that a high-power laser is a versatile tool to synthesize interesting HEAs with additive manufacturing processing. Critical issues are related to the rate of (re)solidification, the dilution with the substrate, powder efficiency during cladding, and differences in melting points of clad powders making additive manufacturing processing from a simple mixture of elemental powders a challenging approach.

Empirical design of single phase high-entropy alloys with high hardness

We collect the available basic properties of nearly 100 high-entropy alloys (HEAs) with a single face centered cubic (fcc) or body centered cubic (bcc) phase. HEAs crystallizing in the fcc structure are mainly composed of the late 3d elements (LTM-HEAs), whereas HEAs consisting of the early (refractory) transition elements and the LTM-HEAs containing an increased level of bcc stabilizer form the bcc structure. Guided by the solid solution theory, we investigate the structure and hardness of HEAs as a function of the valence electron concentration (VEC) and the atomic size difference (δ). The fcc structure is found for VEC between 7.80 and 9.50, whereas the structure is bcc for VEC between 4.33 and 7.55. High strength is obtained for an average valence electron number VEC ∼ 6.80 and for an average atomic size difference δ ≈ 6%. Based on these empirical correlations, one can design the high-entropy alloys with desired hardness.

Formation and Stability of Equiatomic and Nonequiatomic Nanocrystalline CuNiCoZnAlTi High-Entropy Alloys by Mechanical Alloying

Nanocrystalline equiatomic high-entropy alloys (HEAs) have been synthesized by mechanical alloying in the Cu-Ni-Co-Zn-Al-Ti system from the binary CuNi alloy to the hexanary CuNiCoZnAlTi alloy. An attempt also has been made to find the influence of nonequiatomic compositions on the HEA formation by varying the Cu content up to 50 at. pct (Cu x NiCoZnAlTi; x = 0, 8.33, 33.33, 49.98 at. pct). The phase formation and stability of mechanically alloyed powder at an elevated temperature (1073 K [800 °C] for 1 hour) were studied. The nanocrystalline equiatomic Cu-Ni-Co-Zn-Al-Ti alloys have a face-centered cubic (fcc) structure up to quinary compositions and have a body-centered cubic (bcc) structure in a hexanary alloy. In nonequiatomic alloys, bcc is the dominating phase in the alloys containing 0 and 8.33 at. pct of Cu, and the fcc phase was observed in alloys with 33.33 and 49.98 at. pct of Cu. The Vicker's bulk hardness and compressive strength of the equiatomic nanocrystalline hexanary CuNiCoZnAlTi HEA after hot isostatic pressing is 8.79 GPa, and the compressive strength is 2.76 GPa. The hardness of these HEAs is higher than most commercial hard facing alloys (e.g., Stellite, which is 4.94 GPa).