Professor of Materials Science Phone: +46(0)739431247 Address: Materials & Manufacturing Technology
Chalmers University of Technology
SE-412 96 Göteborg
SWEDEN
A B S T R A C T Refractory high-entropy alloys (RHEAs) are promising candidates for new-generatio... more A B S T R A C T Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials , but they generally suffer from room temperature brittleness and unsatisfactory high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance.
CoCrFeMnNi is a prototype fcc-structured high-entropy alloy. Numerous efforts have been paid to s... more CoCrFeMnNi is a prototype fcc-structured high-entropy alloy. Numerous efforts have been paid to strengthen CoCrFeMnNi, by replacing Mn with other elements for an enhancement of the solid solution strengthening. 4d transition metals, including Zr, Nb, and Mo, are of interest for this purpose , since they have much larger atomic radii than that of Mn. However, Nb and Mo are known to have a low solid solubility in fcc-structured CoCrFeNi. Compared to Nb and Mo, Zr has an even larger atomic radius. The solid solubility of Zr in fcc-structured CoCrFeNi was investigated in this work, combining both experimental studies and thermodynamic calculations. In addition, based on previous results and new results obtained here, methods to predict the solid solubility in CoCrFeNiM x (M ¼ Zr, Nb, and Mo) alloys were developed. Particularly, the average d-orbital energy level, Md, was re-evaluated in the present work, for an improved predictability of the solid solubility in fcc-structured high entropy alloys containing 4d transition metals. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4983762]
The development of texture during cold-rolling and recrystallization was investigated in a AlCoCr... more The development of texture during cold-rolling and recrystallization was investigated in a AlCoCrFeNi 2.1 eutectic high entropy alloy (EHEA). For this purpose, the as-cast alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 C to 1200 C. The microstructure of the as-cast EHEA showed a nano-lamellar mixture of L1 2 and B2 phases. The B2 phase was significantly harder than the L1 2 phase. Development of an ultrafine microstructure was observed after 90% cold-rolling. During cold-rolling, progressive disordering of the L1 2 phase was observed while the B2 phase maintained the ordered structure. The progressive disordering and development of a predominantly brass type texture in the L1 2 phase correlated well with profuse shear band formation during cold-rolling. The B2 phase showed the presence of the {111}<110> component which was typical for cold-rolled B2 alloys. An ultrafine duplex structure of equiaxed L1 2 and B2 phases developed after complete recrystallization that showed significant resistance to grain growth up to very high annealing temperatures (~1300 C). The remarkable resistance to grain growth compared to conventional or even other single or dual phase HEAs was due to the formation of a homogeneous duplex structure where growth of one phase was effectively retarded by the other phase. The strong presence of the a-fiber components, but weak BR ({236}<385>) and D ({113}<332>) components in the recrystallization texture of the L1 2 /FCC phase was due to the absence of strong preferential nucleation or growth. Presence of ND-fiber (ND//<111>) with strong {111}<011> component in the recrystallization texture of the B2 phase indicated ease of nucleation from similarly oriented regions in the deformed microstructure.
High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable compo... more High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable composi-tional inhomogeneity, mainly because they contain multiple elements with high concentrations. As a result, large-scale production of HEAs by casting is limited. To address the issue, the concept of eutectic high entropy alloys (EHEAs) was proposed, which has led to some promise in achieving good quality industrial scale HEAs ingots, and more importantly also good mechanical properties. In the practical large-scale casting, the actual composition of designed EHEAs could potentially deviate from the eutectic composition. The influence of such deviation on mechanical properties of EHEAs is important for industrial production, which constitutes the topic of the current work. Here we prepared industrial-scale HEAs ingots near the eutectic composition: hypoeutectic alloy, eutectic alloy and hypereutectic alloy. Our results showed that the deviation from eutectic composition does not significantly affect the mechanical properties, castability and the good mechanical properties of EHEAs can be achieved in a wide compositional range, and at both room and cryogenic temperatures. Our results suggested that EHEAs with simultaneous high strength and high ductility, and good liquidity and castability can be readily adapted to large-scale industrial production. The deformation behavior and microstructure evolution of the eutectic and near-eutectic HEAs were thoroughly studied using a combination of techniques, including strain measurement by digital image correlation, in-situ synchrotron X-ray diffraction, and transmission electron microscopy. The wavy strain distribution and the therefore resulted delay of necking in EHEAs were reported for the first time.
Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (... more Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials.However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.
The effect of thermo-mechanical processing on the evolution of microstructure and mechanical prop... more The effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi 2.1 high entropy alloy. For this purpose, the alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 °C to 1200 °C. The as-cast alloy revealed eutectic lamellar mixture of (Ni, Al) rich but Cr depleted B2 phase and Al-depleted L1 2 phases, having volume fractions of $ 35% and 65%, respectively. Nanosized precipitates enriched in Cr and having disordered BCC structure were found dispersed inside the B2 phase. Cold-rolling resulted in progressive disordering of the L1 2 phase but the B2 phase maintained the ordered structure. The disordering of the L1 2 phase was accompanied by the evolution of ultrafine lamellar structure and profuse shear band formation. Annealing of the 90% cold-rolled material at 800 °C resulted in the formation of a duplex microstructure composed of two different phases with equiaxed morphologies, having significant resistance to grain growth up to 1200 °C. The annealed materials showed disordered FCC and precipitate-free B2 phases. This indicated that quenching of the annealed specimens to room temperature was sufficient to prevent the ordering of the L1 2 phase and the formation of the Cr-rich nano-precipitates which were dissolved in the B2 phase during annealing. Significant improvement in tensile properties compared to the as-cast alloy could be achieved by thermo-mechanical processing. All the specimens annealed at 800 °C to 1200 °C were having good tensile ductility over 10% as well as high tensile strength greater than 1000 MPa. These indicated that the properties of the EHEA could be successfully tailored using thermo-mechanical processing for a wide range of engineering applications.
The development of microstructure and mechanical properties was investigated in a heavily cold-ro... more The development of microstructure and mechanical properties was investigated in a heavily cold-rolled and annealed AlCoCrFeNi 2.1 high-entropy alloy. The as-cast alloy having a eutectic morphology consisting of alternate bands of ordered L1 2 and B2 phases was 90% cold-rolled. The deformed microstructure showed profuse shear banding and disordering of the L1 2 , but no transformation of the B2 phase. A duplex microstructure consisting of ultrafine equiaxed grains (∼ 0.60 μm) of disordered face centered cubic and B2 was observed after annealing at 800°C. The annealed material showed remarkable strength–ductility combination having ultimate tensile strength ∼ 1.2 GPa and elongation to failure ∼ 12%.
The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 hi... more The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 high entropy alloy (HEA) with a composite structure reinforced by TiC nanoparticles. The initial alloy was prepared by mechanical alloying (MA) in a planetary ball mill, compacted by spark plasma sintering (SPS) and heat treated at different temperatures. Mechano-chemical reactions during the MA process as well as the microstructure and hardness of the SPS-ed compacts prior to and after the heat treatment were investigated. During MA, Cr-based supersaturated solid solution with the BCC structure was formed. After SPS at 1100 °C, the BCC solid solution decomposed into nano-grained microstructure consisting of FCC and ordered BCC solid solutions, s phase, and in-situ formed TiC nanoparticles. The high hardness of the alloy (762 HV) was retained after the subsequent heat treatment at 1100 °C (603 HV). It was shown that the fabrication of TiC reinforced nanocomposites from elemental powders without the use of expensive nanograined powders can be achieved.
The formation of core–shell structure is an interesting phenomenon occurring during the solidific... more The formation of core–shell structure is an interesting phenomenon occurring during the solidification process, due to the liquid phase separation. The formation of core–shell structure in high-entropy alloys, a new class of advanced metallic materials, has not been reported previously, and thus constitutes an intriguing scientific question. Here, we firstly report the formation of core–shell structure in one laser cladded high-entropy alloy, where we show the nanosized-Y2O3 powder addition, serves as the catalyst for the liquid phase separation.
High-entropy alloys (HEAs) are currently at the research frontier of metallic materials. Understa... more High-entropy alloys (HEAs) are currently at the research frontier of metallic materials. Understanding the solidsolubility limit in HEAs, such a highly concentrated multicomponent alloy system, is scientifically intriguing. It is also technically important to achieve desirable mechanical properties by controlling the formation of topologically or geometrically closed packed phases. Previous approaches to describe the solidsolubilities in HEAs could not accurately locate the solubility limit and have to utilize at least two parameters. Here, we propose to use a single parameter, the average energy of d-orbital levels, Md, to predict the solidsolubility limit in HEAs. It is found that Md can satisfactorily describe the solidsolubilities in fcc structured HEAs containing 3 d transition metals, and also in bcc structured HEAs. This finding will greatly simplify the alloys design and lends more flexibility to control the mechanical properties of HEAs. When 4 d transition metals are alloyed, Md alone cannot describe the solidsolubility limit in fcc structured HEAs, due to the large increase of the bond strength that can be gauged by the bond order, Bo. The potential opportunities and challenges with applying the molecular orbital approach to HEAs are discussed.
Duplex structure Cu-Cr alloys are widely used as contact materials. They are generally designed b... more Duplex structure Cu-Cr alloys are widely used as contact materials. They are generally designed by increasing the Cr content for the hardness improvement, which, however, leads to the unfavorable rapid increase of the electrical resistivity. The solidification behavior of Cu100−xCrx (x = 4.2, 25 and 50 in wt.%) alloys prepared by laser rapid solidification is studied here, and their hardness and electrical conductivity after aging are measured. The results show that the Cu-4.2%Cr alloy has the most desirable combination of hardness and conductive properties after aging in comparison with Cu-25%Cr and Cu-50%Cr alloys. Very importantly, a 50% improvement in hardness is achieved with a simultaneous 70% reduction in electrical resistivity. The reason is mainly attributed to the liquid phase separation occurring in the Cu-4.2%Cr alloy, which introduces a large amount of well-dispersed sub-micron-scale Cr-rich particulates in the Cu-rich matrix.
Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an
... more Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an important complementary energy technology in the future. While many efficient TE materials exist in the lower temperature region, few are efficient at high temperatures. Here, we present the high temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature TE materials. We show that their TE properties can be controlled significantly by changing the valence electron concentration (VEC) of the system with appropriate substitutional elements. Both the electrical and thermal transport properties in this system were found to decrease with a lower VEC number. Overall, the large microstructural complexity and lower average VEC in these types of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These findings highlight the possibility to exploit HEAs as a new class of future high temperature TE materials.
This paper summarises existing phase selection rules for cast high entropy alloys. Essentially,
... more This paper summarises existing phase selection rules for cast high entropy alloys. Essentially,
they are almost all based on the parametric approach, utilising various descriptors comprising
mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch,
electronegativity and valence electron concentration. The overview starts from phase selection
rules for solid solutions, intermetallic compounds and the amorphous phase in high entropy
alloys. Further discussions are relevant to selection rules for solid solution phases in high entropy
alloys, more specifically, for face centred cubic and body centred cubic type solid solutions.
Finally, some challenges and future prospects of phase selection rules for high entropy alloys are
addressed.
Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), whic... more Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), which was characterized using scanning transmission electron microscopy (STEM) combined with atom probe tomography (APT). Despite the fact that X-ray diffraction exhibits a single face-centered-cubic (fcc) phase feature of the as-cast alloy prepared by melt spinning, selected area electron diffraction reveals weak L12 ordering in the as-spun alloy. High-resolution STEM shows the presence of two coherent nanophases with distinct L12 and fcc structures, coupling with compositional segregations. The ordering of the L12 domains is enhanced after annealing at 500 °C. Electron energy loss spectroscopy and APT analyses reveal that the L12 nano-phase is enriched with Fe, Co, Cr and Ni, while the fcc domains are a Cu-rich phase. The nano-scale phase separation can effectively minimize the lattice distortions caused by the atomic size difference in the constituent elements, which may offer structural insights into the unusual mechanical behavior and phase stability of fcc HEA.
High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous ac... more High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous achievement of both still constitutes a tough challenge. The inferior castability and compositional segregation of HEAs are also obstacles for their technological applications. To tackle these problems, here we proposed a novel strategy to design HEAs using the eutectic alloy concept, i.e. to achieve a microstructure composed of alternating soft fcc and hard bcc phases. As a manifestation of this concept, an AlCoCrFeNi2.1 (atomic portion) eutectic high-entropy alloy (EHEA) was designed. The as-cast EHEA possessed a fine lamellar fcc/B2 microstructure, and showed an unprecedented combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700°C. This new alloy design strategy can be readily adapted to large-scale industrial production of HEAs with simultaneous high fracture strength and high ductility.
Recently, high-entropy alloys (HEAs) have attracted much interest in the materials community, as ... more Recently, high-entropy alloys (HEAs) have attracted much interest in the materials community, as they offer massive opportunities to observe new phenomena, explore new structure, and develop new materials. Particularly, it is attractive to prepare high-performance HEA coatings by laser-induced rapid solidification, which can be formed on the surface of components and parts in a variety of sizes and shapes with a lower cost in comparison with those bulk material fabrication methods. From the technical point of view, laser-induced rapid solidification could hamper the compositional segregation, improve the solubility in solid-solution phases, and lead to the strengthening effect by the grain refinement. This article reviews the recent work on the typical microstructural features and the mechanical and chemical properties in laser-induced rapidly solidified HEAs, and these data are compared with conventional Co- and Ni-based alloy coatings. The article concludes with suggestions for future research and development in HEAs, from considerations of their characteristic properties.
A crossover behavior in the initial creep stage during nanoindentation with a constant load for a... more A crossover behavior in the initial creep stage during nanoindentation with a constant load for a high-entropy-based alloy CoFeNi is found. The holding time and stress in the indentation tests are evaluated to reveal the intrinsic mechanism of the crossover point. It indicated that the initial creep behavior is dominated by the strain-hardening at the beginning and then transit into the dislocation migration induced viscous stage. The transition presents the crossover, where the strain hardening effect predominates in a very short initial period while the viscous stage accounts for most of the initial creep stage with a time-dependent strain rate.
High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase sele... more High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase selection in HEAs has attracted much attention recently, especially on forming of the solid solutions. Up to now, phase diagrams of most HEAs are still not well developed, and the empirical phase selection rules play an important role in HEAs area. In this brief review, the physical factors controlling the phase stability in HEAs are discussed, and the phase selection rules are identified. Different from previous results, the rules on equilibrium phase selection within a certain temperature range are carefully reviewed and presented in this article.
The application of high-entropy alloys (HEAs) as coating materials has become an active research ... more The application of high-entropy alloys (HEAs) as coating materials has become an active research topic recently. Here an fcc structured CoCrCuFeNi HEA coating with a thickness of ∼1.2 mm was laser cladded onto a Q235 steel. The alloy coating possessed an excellent thermal stability in that no phase transformations occurred up to 1000 °C (0.86Tm), and the dendritic morphology of the as-solidified microstructure could be kept to higher than 750 °C (0.7Tm). After annealing the as-solidified coating at 750 °C for 5 h, the lattice distortion in the rapidly solidified alloy was reduced, resulting in a moderate decrease of both the hardness and electric resistivity. Interestingly, profuse stacking faults ribbons were observed in the dendritic region of the alloy after annealing, driven by the thermal stress. This phenomenon provided a direct experimental evidence of the low stacking fault energy in HEAs. The thermodynamic origin of the thermal stability for HEAs was proposed.
A B S T R A C T Refractory high-entropy alloys (RHEAs) are promising candidates for new-generatio... more A B S T R A C T Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials , but they generally suffer from room temperature brittleness and unsatisfactory high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance.
CoCrFeMnNi is a prototype fcc-structured high-entropy alloy. Numerous efforts have been paid to s... more CoCrFeMnNi is a prototype fcc-structured high-entropy alloy. Numerous efforts have been paid to strengthen CoCrFeMnNi, by replacing Mn with other elements for an enhancement of the solid solution strengthening. 4d transition metals, including Zr, Nb, and Mo, are of interest for this purpose , since they have much larger atomic radii than that of Mn. However, Nb and Mo are known to have a low solid solubility in fcc-structured CoCrFeNi. Compared to Nb and Mo, Zr has an even larger atomic radius. The solid solubility of Zr in fcc-structured CoCrFeNi was investigated in this work, combining both experimental studies and thermodynamic calculations. In addition, based on previous results and new results obtained here, methods to predict the solid solubility in CoCrFeNiM x (M ¼ Zr, Nb, and Mo) alloys were developed. Particularly, the average d-orbital energy level, Md, was re-evaluated in the present work, for an improved predictability of the solid solubility in fcc-structured high entropy alloys containing 4d transition metals. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4983762]
The development of texture during cold-rolling and recrystallization was investigated in a AlCoCr... more The development of texture during cold-rolling and recrystallization was investigated in a AlCoCrFeNi 2.1 eutectic high entropy alloy (EHEA). For this purpose, the as-cast alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 C to 1200 C. The microstructure of the as-cast EHEA showed a nano-lamellar mixture of L1 2 and B2 phases. The B2 phase was significantly harder than the L1 2 phase. Development of an ultrafine microstructure was observed after 90% cold-rolling. During cold-rolling, progressive disordering of the L1 2 phase was observed while the B2 phase maintained the ordered structure. The progressive disordering and development of a predominantly brass type texture in the L1 2 phase correlated well with profuse shear band formation during cold-rolling. The B2 phase showed the presence of the {111}<110> component which was typical for cold-rolled B2 alloys. An ultrafine duplex structure of equiaxed L1 2 and B2 phases developed after complete recrystallization that showed significant resistance to grain growth up to very high annealing temperatures (~1300 C). The remarkable resistance to grain growth compared to conventional or even other single or dual phase HEAs was due to the formation of a homogeneous duplex structure where growth of one phase was effectively retarded by the other phase. The strong presence of the a-fiber components, but weak BR ({236}<385>) and D ({113}<332>) components in the recrystallization texture of the L1 2 /FCC phase was due to the absence of strong preferential nucleation or growth. Presence of ND-fiber (ND//<111>) with strong {111}<011> component in the recrystallization texture of the B2 phase indicated ease of nucleation from similarly oriented regions in the deformed microstructure.
High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable compo... more High entropy alloys (HEAs) usually possess weak liquidity and castability, and considerable composi-tional inhomogeneity, mainly because they contain multiple elements with high concentrations. As a result, large-scale production of HEAs by casting is limited. To address the issue, the concept of eutectic high entropy alloys (EHEAs) was proposed, which has led to some promise in achieving good quality industrial scale HEAs ingots, and more importantly also good mechanical properties. In the practical large-scale casting, the actual composition of designed EHEAs could potentially deviate from the eutectic composition. The influence of such deviation on mechanical properties of EHEAs is important for industrial production, which constitutes the topic of the current work. Here we prepared industrial-scale HEAs ingots near the eutectic composition: hypoeutectic alloy, eutectic alloy and hypereutectic alloy. Our results showed that the deviation from eutectic composition does not significantly affect the mechanical properties, castability and the good mechanical properties of EHEAs can be achieved in a wide compositional range, and at both room and cryogenic temperatures. Our results suggested that EHEAs with simultaneous high strength and high ductility, and good liquidity and castability can be readily adapted to large-scale industrial production. The deformation behavior and microstructure evolution of the eutectic and near-eutectic HEAs were thoroughly studied using a combination of techniques, including strain measurement by digital image correlation, in-situ synchrotron X-ray diffraction, and transmission electron microscopy. The wavy strain distribution and the therefore resulted delay of necking in EHEAs were reported for the first time.
Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (... more Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials.However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.
The effect of thermo-mechanical processing on the evolution of microstructure and mechanical prop... more The effect of thermo-mechanical processing on the evolution of microstructure and mechanical properties was investigated in an AlCoCrFeNi 2.1 high entropy alloy. For this purpose, the alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 °C to 1200 °C. The as-cast alloy revealed eutectic lamellar mixture of (Ni, Al) rich but Cr depleted B2 phase and Al-depleted L1 2 phases, having volume fractions of $ 35% and 65%, respectively. Nanosized precipitates enriched in Cr and having disordered BCC structure were found dispersed inside the B2 phase. Cold-rolling resulted in progressive disordering of the L1 2 phase but the B2 phase maintained the ordered structure. The disordering of the L1 2 phase was accompanied by the evolution of ultrafine lamellar structure and profuse shear band formation. Annealing of the 90% cold-rolled material at 800 °C resulted in the formation of a duplex microstructure composed of two different phases with equiaxed morphologies, having significant resistance to grain growth up to 1200 °C. The annealed materials showed disordered FCC and precipitate-free B2 phases. This indicated that quenching of the annealed specimens to room temperature was sufficient to prevent the ordering of the L1 2 phase and the formation of the Cr-rich nano-precipitates which were dissolved in the B2 phase during annealing. Significant improvement in tensile properties compared to the as-cast alloy could be achieved by thermo-mechanical processing. All the specimens annealed at 800 °C to 1200 °C were having good tensile ductility over 10% as well as high tensile strength greater than 1000 MPa. These indicated that the properties of the EHEA could be successfully tailored using thermo-mechanical processing for a wide range of engineering applications.
The development of microstructure and mechanical properties was investigated in a heavily cold-ro... more The development of microstructure and mechanical properties was investigated in a heavily cold-rolled and annealed AlCoCrFeNi 2.1 high-entropy alloy. The as-cast alloy having a eutectic morphology consisting of alternate bands of ordered L1 2 and B2 phases was 90% cold-rolled. The deformed microstructure showed profuse shear banding and disordering of the L1 2 , but no transformation of the B2 phase. A duplex microstructure consisting of ultrafine equiaxed grains (∼ 0.60 μm) of disordered face centered cubic and B2 was observed after annealing at 800°C. The annealed material showed remarkable strength–ductility combination having ultimate tensile strength ∼ 1.2 GPa and elongation to failure ∼ 12%.
The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 hi... more The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 high entropy alloy (HEA) with a composite structure reinforced by TiC nanoparticles. The initial alloy was prepared by mechanical alloying (MA) in a planetary ball mill, compacted by spark plasma sintering (SPS) and heat treated at different temperatures. Mechano-chemical reactions during the MA process as well as the microstructure and hardness of the SPS-ed compacts prior to and after the heat treatment were investigated. During MA, Cr-based supersaturated solid solution with the BCC structure was formed. After SPS at 1100 °C, the BCC solid solution decomposed into nano-grained microstructure consisting of FCC and ordered BCC solid solutions, s phase, and in-situ formed TiC nanoparticles. The high hardness of the alloy (762 HV) was retained after the subsequent heat treatment at 1100 °C (603 HV). It was shown that the fabrication of TiC reinforced nanocomposites from elemental powders without the use of expensive nanograined powders can be achieved.
The formation of core–shell structure is an interesting phenomenon occurring during the solidific... more The formation of core–shell structure is an interesting phenomenon occurring during the solidification process, due to the liquid phase separation. The formation of core–shell structure in high-entropy alloys, a new class of advanced metallic materials, has not been reported previously, and thus constitutes an intriguing scientific question. Here, we firstly report the formation of core–shell structure in one laser cladded high-entropy alloy, where we show the nanosized-Y2O3 powder addition, serves as the catalyst for the liquid phase separation.
High-entropy alloys (HEAs) are currently at the research frontier of metallic materials. Understa... more High-entropy alloys (HEAs) are currently at the research frontier of metallic materials. Understanding the solidsolubility limit in HEAs, such a highly concentrated multicomponent alloy system, is scientifically intriguing. It is also technically important to achieve desirable mechanical properties by controlling the formation of topologically or geometrically closed packed phases. Previous approaches to describe the solidsolubilities in HEAs could not accurately locate the solubility limit and have to utilize at least two parameters. Here, we propose to use a single parameter, the average energy of d-orbital levels, Md, to predict the solidsolubility limit in HEAs. It is found that Md can satisfactorily describe the solidsolubilities in fcc structured HEAs containing 3 d transition metals, and also in bcc structured HEAs. This finding will greatly simplify the alloys design and lends more flexibility to control the mechanical properties of HEAs. When 4 d transition metals are alloyed, Md alone cannot describe the solidsolubility limit in fcc structured HEAs, due to the large increase of the bond strength that can be gauged by the bond order, Bo. The potential opportunities and challenges with applying the molecular orbital approach to HEAs are discussed.
Duplex structure Cu-Cr alloys are widely used as contact materials. They are generally designed b... more Duplex structure Cu-Cr alloys are widely used as contact materials. They are generally designed by increasing the Cr content for the hardness improvement, which, however, leads to the unfavorable rapid increase of the electrical resistivity. The solidification behavior of Cu100−xCrx (x = 4.2, 25 and 50 in wt.%) alloys prepared by laser rapid solidification is studied here, and their hardness and electrical conductivity after aging are measured. The results show that the Cu-4.2%Cr alloy has the most desirable combination of hardness and conductive properties after aging in comparison with Cu-25%Cr and Cu-50%Cr alloys. Very importantly, a 50% improvement in hardness is achieved with a simultaneous 70% reduction in electrical resistivity. The reason is mainly attributed to the liquid phase separation occurring in the Cu-4.2%Cr alloy, which introduces a large amount of well-dispersed sub-micron-scale Cr-rich particulates in the Cu-rich matrix.
Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an
... more Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an important complementary energy technology in the future. While many efficient TE materials exist in the lower temperature region, few are efficient at high temperatures. Here, we present the high temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature TE materials. We show that their TE properties can be controlled significantly by changing the valence electron concentration (VEC) of the system with appropriate substitutional elements. Both the electrical and thermal transport properties in this system were found to decrease with a lower VEC number. Overall, the large microstructural complexity and lower average VEC in these types of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These findings highlight the possibility to exploit HEAs as a new class of future high temperature TE materials.
This paper summarises existing phase selection rules for cast high entropy alloys. Essentially,
... more This paper summarises existing phase selection rules for cast high entropy alloys. Essentially,
they are almost all based on the parametric approach, utilising various descriptors comprising
mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch,
electronegativity and valence electron concentration. The overview starts from phase selection
rules for solid solutions, intermetallic compounds and the amorphous phase in high entropy
alloys. Further discussions are relevant to selection rules for solid solution phases in high entropy
alloys, more specifically, for face centred cubic and body centred cubic type solid solutions.
Finally, some challenges and future prospects of phase selection rules for high entropy alloys are
addressed.
Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), whic... more Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), which was characterized using scanning transmission electron microscopy (STEM) combined with atom probe tomography (APT). Despite the fact that X-ray diffraction exhibits a single face-centered-cubic (fcc) phase feature of the as-cast alloy prepared by melt spinning, selected area electron diffraction reveals weak L12 ordering in the as-spun alloy. High-resolution STEM shows the presence of two coherent nanophases with distinct L12 and fcc structures, coupling with compositional segregations. The ordering of the L12 domains is enhanced after annealing at 500 °C. Electron energy loss spectroscopy and APT analyses reveal that the L12 nano-phase is enriched with Fe, Co, Cr and Ni, while the fcc domains are a Cu-rich phase. The nano-scale phase separation can effectively minimize the lattice distortions caused by the atomic size difference in the constituent elements, which may offer structural insights into the unusual mechanical behavior and phase stability of fcc HEA.
High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous ac... more High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous achievement of both still constitutes a tough challenge. The inferior castability and compositional segregation of HEAs are also obstacles for their technological applications. To tackle these problems, here we proposed a novel strategy to design HEAs using the eutectic alloy concept, i.e. to achieve a microstructure composed of alternating soft fcc and hard bcc phases. As a manifestation of this concept, an AlCoCrFeNi2.1 (atomic portion) eutectic high-entropy alloy (EHEA) was designed. The as-cast EHEA possessed a fine lamellar fcc/B2 microstructure, and showed an unprecedented combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700°C. This new alloy design strategy can be readily adapted to large-scale industrial production of HEAs with simultaneous high fracture strength and high ductility.
Recently, high-entropy alloys (HEAs) have attracted much interest in the materials community, as ... more Recently, high-entropy alloys (HEAs) have attracted much interest in the materials community, as they offer massive opportunities to observe new phenomena, explore new structure, and develop new materials. Particularly, it is attractive to prepare high-performance HEA coatings by laser-induced rapid solidification, which can be formed on the surface of components and parts in a variety of sizes and shapes with a lower cost in comparison with those bulk material fabrication methods. From the technical point of view, laser-induced rapid solidification could hamper the compositional segregation, improve the solubility in solid-solution phases, and lead to the strengthening effect by the grain refinement. This article reviews the recent work on the typical microstructural features and the mechanical and chemical properties in laser-induced rapidly solidified HEAs, and these data are compared with conventional Co- and Ni-based alloy coatings. The article concludes with suggestions for future research and development in HEAs, from considerations of their characteristic properties.
A crossover behavior in the initial creep stage during nanoindentation with a constant load for a... more A crossover behavior in the initial creep stage during nanoindentation with a constant load for a high-entropy-based alloy CoFeNi is found. The holding time and stress in the indentation tests are evaluated to reveal the intrinsic mechanism of the crossover point. It indicated that the initial creep behavior is dominated by the strain-hardening at the beginning and then transit into the dislocation migration induced viscous stage. The transition presents the crossover, where the strain hardening effect predominates in a very short initial period while the viscous stage accounts for most of the initial creep stage with a time-dependent strain rate.
High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase sele... more High-entropy alloys (HEAs) have been investigated considerably in the last decade. The phase selection in HEAs has attracted much attention recently, especially on forming of the solid solutions. Up to now, phase diagrams of most HEAs are still not well developed, and the empirical phase selection rules play an important role in HEAs area. In this brief review, the physical factors controlling the phase stability in HEAs are discussed, and the phase selection rules are identified. Different from previous results, the rules on equilibrium phase selection within a certain temperature range are carefully reviewed and presented in this article.
The application of high-entropy alloys (HEAs) as coating materials has become an active research ... more The application of high-entropy alloys (HEAs) as coating materials has become an active research topic recently. Here an fcc structured CoCrCuFeNi HEA coating with a thickness of ∼1.2 mm was laser cladded onto a Q235 steel. The alloy coating possessed an excellent thermal stability in that no phase transformations occurred up to 1000 °C (0.86Tm), and the dendritic morphology of the as-solidified microstructure could be kept to higher than 750 °C (0.7Tm). After annealing the as-solidified coating at 750 °C for 5 h, the lattice distortion in the rapidly solidified alloy was reduced, resulting in a moderate decrease of both the hardness and electric resistivity. Interestingly, profuse stacking faults ribbons were observed in the dendritic region of the alloy after annealing, driven by the thermal stress. This phenomenon provided a direct experimental evidence of the low stacking fault energy in HEAs. The thermodynamic origin of the thermal stability for HEAs was proposed.
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metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through
controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.
For questions, please contact:
Assistant Professor Dr. Sheng Guo
Materials and Manufacturing Technology
E-mail: sheng.guo@chalmers.se
important complementary energy technology in the future. While many efficient TE materials exist
in the lower temperature region, few are efficient at high temperatures. Here, we present the high
temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature
TE materials. We show that their TE properties can be controlled significantly by changing the valence
electron concentration (VEC) of the system with appropriate substitutional elements. Both
the electrical and thermal transport properties in this system were found to decrease with a lower
VEC number. Overall, the large microstructural complexity and lower average VEC in these types
of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These
findings highlight the possibility to exploit HEAs as a new class of future high temperature TE
materials.
they are almost all based on the parametric approach, utilising various descriptors comprising
mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch,
electronegativity and valence electron concentration. The overview starts from phase selection
rules for solid solutions, intermetallic compounds and the amorphous phase in high entropy
alloys. Further discussions are relevant to selection rules for solid solution phases in high entropy
alloys, more specifically, for face centred cubic and body centred cubic type solid solutions.
Finally, some challenges and future prospects of phase selection rules for high entropy alloys are
addressed.
metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through
controlled alloying. A new ductile RHEA, Hf0.5Nb0.5Ta0.5Ti1.5Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries.
For questions, please contact:
Assistant Professor Dr. Sheng Guo
Materials and Manufacturing Technology
E-mail: sheng.guo@chalmers.se
important complementary energy technology in the future. While many efficient TE materials exist
in the lower temperature region, few are efficient at high temperatures. Here, we present the high
temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature
TE materials. We show that their TE properties can be controlled significantly by changing the valence
electron concentration (VEC) of the system with appropriate substitutional elements. Both
the electrical and thermal transport properties in this system were found to decrease with a lower
VEC number. Overall, the large microstructural complexity and lower average VEC in these types
of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These
findings highlight the possibility to exploit HEAs as a new class of future high temperature TE
materials.
they are almost all based on the parametric approach, utilising various descriptors comprising
mixing enthalpy, configuration entropy, mismatch entropy, melting points, atomic size mismatch,
electronegativity and valence electron concentration. The overview starts from phase selection
rules for solid solutions, intermetallic compounds and the amorphous phase in high entropy
alloys. Further discussions are relevant to selection rules for solid solution phases in high entropy
alloys, more specifically, for face centred cubic and body centred cubic type solid solutions.
Finally, some challenges and future prospects of phase selection rules for high entropy alloys are
addressed.