Single-phase Alloys Research Articles

Far-from-equilibrium electrosynthesis ramifies high-entropy alloy for alkaline hydrogen evolution

High-entropy alloys (HEAs) provide an ideal platform for developing highly active electrocatalysts and investigating the synergy of mixed elements. Far-from-equilibrium synthesis holds great potential for fabricating HEAs at the nanoscale by rapidly shifting the thermodynamic conditions and manipulating the growth kinetics. While far-from-equilibrium synthesis of nanomaterials has been successful under thermochemical conditions, it is markedly challenging under electrochemical environments, as the use of an electrolyte limits the accessible temperature window and the temporal tunability of temperature. Herein, we demonstrate that applying a large electrochemical overpotential would create a far-from-equilibrium condition as changing the temperature of the system by considering the equation ΔG=ΔH−TΔS+nFΔψ. An electrochemical far-from-equilibrium approach is thus setup for constructing hierarchical and self-supporting high-entropy alloy nanostructures. The large overpotential drives the simultaneous reduction of multiple cations and the subsequent formation of a single-phase alloy. As a proof-of-concept, hierarchical Fe0.22Co0.18Ni0.18Cr0.14Cu0.28 was fabricated and used as an electrocatalyst for the hydrogen evolution reaction in alkaline media. The noble-metal-free HEA exhibits an overpotential of 84 mV at a current density of 10 mA cm–2, which is among the lowest even compared to noble metal-based electrocatalysts. This work opens a new avenue for building a variety of HEAs for energy and catalysis applications.

Development of eutectic high entropy alloy by addition of W to CoCrFeNi HEA

High entropy alloys have shown a remarkable combination of physical and mechanical properties. The introduction of eutectic microstructure, consisting of a tough fcc phase, and a hard-intermetallic phase, can help in obtaining even better synergy of strength and ductility. The presence of multiple principal alloying elements in HEAs and absence of corresponding multicomponent phase diagrams makes designing of eutectic high entropy alloys a tedious task. In the present study, systematic investigation of CoCrFeNi-Wx system has been carried out for the development of eutectic microstructure. Experimental results validated the presence of eutectic reaction in the calculated phase diagram. CoCrFeNi-Wx HEAs remained single fcc phase alloys at smaller amount of W (x = 0.1) but changed to hypoeutectic (x = 0.25, 0.5, 0.75) and hypereutectic alloys (x = 1.0) with increase in the amount of tungsten. It has been shown that calculated pseudo binary phase diagrams can provide a very good starting point for the development of eutectic HEAs. Mechanical characterization of the developed HEAs revealed that development of eutectic mixture of a soft (fcc) and a hard phase (intermetallic/bcc) can help in obtaining outstanding combination of mechanical properties.

Structure and thermoelectric property evolution of TiFe1.1−xNixSb with the filling of Ni

TiFeSb is a multiphase material with the half-Heusler structure as the main phase. In this work, we discover that TiFeSb can be stabilized by filling Ni to the vacant 4d site of the half-Heusler structure, and a single-phase TiFeNi0.1Sb alloy is obtained. To improve the thermoelectric properties, TiFe1.1−xNixSb (x = 0.1–1.0) samples with more valence electrons are designed and synthesized. Structural analysis reveals that the samples with x = 0–0.3 are single-phase alloys with the half-Heusler-like structure, the samples with x = 0.4–0.7 are two-phase materials with the double half-Heusler as the main phase, and the samples with x = 0.8–1.0 are single-phase alloys with the full-Heusler-like structure. Meanwhile, both promising p-type (x = 0–0.3) and n-type (x = 0.8–1.0) electrical properties are realized in TiFe1.1−xNixSb, which can be well explained by the Slater-Pauling rule. In combination with greatly reduced lattice thermal conductivity owing to the disordered crystal structure, the p-type TiFe0.8Ni0.3Sb and n-type TiFe0.3Ni0.8Sb achieve dimensionless thermoelectric figure of merits zT about 0.6 and 0.4 at 973 K, respectively. We also demonstrate that the thermoelectric properties of these samples can be further optimized by conventional doping or alloying methods. By replacing Ni (or Fe) with Cu, the carrier concentration of p-type TiFe0.8Ni0.2Cu0.1Sb is decreased, and the zT value reaches 0.8 at 973 K, which is the record of vacancy-filled Heusler alloys.

Study on the microstructure evolution mechanism of copper single phase alloys under deep undercooling conditions

This article mainly used the deep undercooling technology to treat Cu60Ni40, Cu60Ni38Co2, and Cu60Ni35Co5 alloys to obtain different undercoolings. By using high speed cameras, the rapid solidification process was recorded to analyze the relationship between its evolution law and the magnitude of undercooling. When observing its microstructure, it was also found that there was a critical undercooling degree throughout its entire solidification process. The influence of Co element addition on solidification rate, recalescence effect, and critical undercooling of copper based single-phase alloys under undercooling conditions was explored. EBSD analysis was conducted on the refined microstructure of the alloy that achieved the maximum undercooling, and the significance of grain orientation and orientation difference distribution was found. TEM tests were conducted on a Cu60Ni38Co2 alloy with a maximum undercooling of 259 K, and it was found that there are high-density dislocation networks in some areas within them. Finally, the Vickers hardness of the above alloys was measured and recorded using a microhardness tester (HVS-1000Z), and the evolution between microhardness and undercooling, as well as the changes in microhardness under critical undercooling, were analyzed. Based on the analysis of EBSD, TEM, and microhardness data above, it is found that grain refinement at high undercooling is dominated by recrystallization.

Influence of Heat-Treatment on Microstructures and Mechanical Properties of CuCrFeMnNi High-Entropy Alloy

High entropy alloys (HEAs) are defined as a multi-element alloy including more than 4 elements with near equi-atomic percentage. In general, the configurational entropy of the HEAs is known to be sufficient to stabilize a single solid solution, such as body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal-closed pack (HCP). Compared to BCC single-phase alloys, FCC single-phase alloys draw extensive attention because they are advantageous in manufacturing and processing. FCC-based HEAs show excellent ductility but limited strength, so many research on improving strength has been conducted. Outstanding mechanical properties with a balance of strength and ductility are rarely achieved in single-phase FCC-based HEAs. This is why most alloys for structural applications exhibit a multi-phase microstructure. In this study, we aimed to develop multi-phase FCC-based HEA with superior mechanical properties than single-phase CoCrFeMnNi HEA, via Co substitution in CoCrFeMnNi HEA by Cu, which has a high mixing enthalpy. It was found that the CuCrFeMnNi HEA is composed of two FCC phases and one BCC phase. The CuCrFeMnNi HEA was cold-rolled, and subsequently aged at 500, 700, 900<sup>o</sup>C for 1 hour. As the annealing temperature increased, the volume fraction of the FCC phase (FCC1 + FCC2) increased and the residual stress was gradually relieved by recrystallization. Furthermore, small amount of sigma phase was formed at 900<sup>o</sup>C. The effect of the microstructural evolution on the mechanical properties, such as hardness and tensile properties at room temperature, will be discussed.

Solution Combustion Synthesis and Characterization of Magnesium Copper Vanadates.

Magnesium vanadate (MgV2O6) and its alloys with copper vanadate were synthesized via the solution combustion technique. Phase purity and solid solution formation were confirmed by a variety of experimental techniques, supported by electronic structure simulations based on density functional theory (DFT). Powder X-ray diffraction combined with Rietveld refinement, laser Raman spectroscopy, diffuse reflectance spectroscopy, and high-resolution transmission electron microscopy showed single-phase alloy formation despite the MgV2O6 and CuV2O6 end members exhibiting monoclinic and triclinic crystal systems, respectively. DFT-calculated optical band gaps showed close agreement in the computed optical bandgaps with experimentally derived values. Surface photovoltage spectroscopy, ambient-pressure photoemission spectroscopy, and Kelvin probe contact potential difference (work function) measurements confirmed a systematic variation in the optical bandgap modification and band alignment as a function of stoichiometry in the alloy composition. These data indicated n-type semiconductor behavior for all the samples which was confirmed by photoelectrochemical measurements.

Production of multi-principal-component alloys by pendent-drop melt extraction

In order to examine the influence of the production technique on the performance of hydride-forming multi-principal-component alloys, a series of alloys in the Ti–Zr–V–Nb–Ta-Hf system has been obtained by arc-melting and electron beam melting combined with pendant-drop melt extraction (EBM-PDME). Structure analysis shows that EBM-PDME method ensures the synthesis of single-phase alloys with BCC structure for all studied compositions. During hydrogenation, the complete and reversible BCC to FCC phase transformation with maximum hydrogen capacity up to 2.0 wt% occurs. Mechanical and thermophysical investigations demonstrate high values of tensile strength, plasticity and thermal conductivity, which allow the alloys to be used as materials for membrane hydrogen separation.

Mechanical properties of an Al10Nb20Ta15Ti30V5Zr20 A2/B2 refractory superalloy and its constituent phases

Refractory superalloys (RSAs) are a new class of high-temperature structural materials consisting of a nanometer-sized mixture of two coherent A2 (disordered BCC) and B2 (ordered) phases. While these RSAs typically exhibit superior mechanical properties at elevated temperatures, they suffer from limited plasticity at lower temperatures, often attributed to the continuous ordered B2 matrix in these two-phase alloys. To knowledgably control the microstructure and mechanical properties of these alloys, the properties of the constitutive phases in RSAs should be known. Currently this information is absent. In the present work, the microstructure and mechanical properties (at 20–1200 °C) of a candidate Al0.5NbTa0.8Ti1.5V0.2Zr RSA and two single-phase alloys, which compositions correspond to the compositions of the A2 and B2 phases in the selected RSA, are reported. Interestingly, and quite unexpectedly, the intrinsic mechanical behavior of the constituent A2 and B2 phases, deciphered for the first time in the present study, revealed that they are softer and substantially more ductile than the parent two-phase RSA. Furthermore, the ordered B2 phase exhibits a deformation twinning mechanism leading to twinning induced plasticity (TWIP) coupled with a low APB energy and activity of 1/2<111> dislocations. These mechanisms are thought to be responsible for the high plasticity of the ordered B2 phase. The results presented here question the current assumption that the ordered B2 phase in these RSAs has limited plasticity and strongly indicate that the nature of the B2/A2 interface boundaries and coherency strengthening play an important role in controlling the mechanical properties of the two-phase RSA.

Microstructure, hydrogen permeability and ductile-to-brittle transition-hydrogen concentration of (V, Nb)-Ti-Co quaternary alloys

The ongoing development of hydrogen permeation alloys promotes the wide application of hydrogen energy, especially in the industry of hydrogen production from the water-gas shift reaction (WGS). However, before being widely used, it is still necessary to improve their comprehensive properties, such as hydrogen permeation flux and durability. Pd and its alloy membranes have been deeply studied and have appeared in the market in recent years. Vanadium-based alloys, as a new substitute for these palladium membranes, not only have lower cost but also higher hydrogen permeability. However, huge challenges remain-especially the poor mechanical stability and high susceptibility to hydrogen embrittlement (HE) which is closely related to high absorbed hydrogen concentration of Vanadium. To solve these problems, this paper investigated the structure, hydrogen diffusion, hydrogen dissolution, and hydrogen embrittlement properties of vanadium-based quaternary (V-Nb-Ti-Co) alloys. On this basis, the boundary of ductile brittle transition hydrogen concentration (DBTC) was described. It was found that new phases (such as Co2(V, Nb)3, Co7Nb6 etc.) will be formed in the structure when V is gradually replaced by Nb. Accordingly, the above changes the gradual substitution of V by Nb increased the hydrogen diffusivity and decreased hydrogen absorption. The overall effect is that the permeability decreases first and then increases, but larger hydrogen absorption will weaken the mechanical strength. As a result, the DBTC of the V-Nb-Ti-Co alloys shifts to the right somewhat (about 0.36 (H/M)), expanding the gap of the V-based single-phase alloys. This work demonstrated that the lower limit of DBTC can be adjusted by adding an appropriate amount of Nb in V-based alloys, thus simultaneously increasing the hydrogen permeability and hydrogen embrittlement resistance. This provides help and guidance for the design of novel hydrogen purification alloys.

Phase formation in CrFeCoNi nitride thin films

As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, \ensuremath{\Delta}$G$ of ${(\mathrm{CrFeCoNi})}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ solid solutions has a maximum at $x=0.20\ensuremath{-}0.25$, and \ensuremath{\Delta}$G$ becomes lower when $x&lt;0.20$ and $x&gt;0.25$. Thin films of ${(\mathrm{CrFeCoNi})}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ ($0.14\ensuremath{\ge}x\ensuremath{\le}0.41$) grown by magnetron sputtering show stabilization of the metallic fcc when $x\ensuremath{\le}0.22$ and the stabilization of the NaCl B1 structure when $x&gt;0.33$, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen $(x=0.22)$ grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics. The results from the study further aid in making correlations between the observed mechanical properties and the crystal structure of the films.

Magneto transport and critical behavior in metastable Fe15Cu85

FexCu[Formula: see text] binary alloys attracted much attention owing to their rich magnetic phase diagram and the challenges of achieving single phases in these systems. Despite early efforts put toward studying the physical properties of low-Cu doping in Fe–Cu alloys, a detailed understanding of the magnetic, structural and electrical properties of high Cu doping in Fe–Cu alloys remains not clear. In this report, we use X-ray diffraction (XRD), scanning electron microscopy, electrical transport and magnetic measurements to gain more insight into Fe[Formula: see text]Cu[Formula: see text] single-phase alloy system. Experimental findings show that the samples exhibit a second-order phase transition, which is consistent with the classical mean-field model. Moreover, negative magnetoresistance is observed at all temperatures and explained through classical models.

A strategy to regulate the yield ratio of a metastable high Zr-containing β titanium alloy: Synergistic effects of the β domain, β stability and β/α interfaces by varying the α phase content

To meet the demands of both processing and serving, to the best of our knowledge, this is the first report exhibiting large range control of the yield ratio from 0.31 to 0.96 with decent elongations over 10% in the same alloy with low-cost thermal treatments. The yield ratio of the metastable Ti-30Zr-5Mo alloy was regulated via adjusting trigger stress of the stress-induced phase transformation and work-hardening ability through changing the α phase content. Materials with acicular α phase of different contents were successfully prepared via low-cost thermal treatment. The effects of the α phase content on the stress-induced α' martensite phase transformation and work hardening behavior were then investigated. In the Ti-30Zr-5Mo alloy with dual phases, due to the crystal difference and element partitioning, the hardness of the α phase is higher than that of the β matrix, and the hardness difference between the phases increases with increasing α phase. In addition to Mo, Zr plays an important role in stabilizing the β phase in high-Zr-containing alloys. Deformation initiates in the β phase of both single-phase and duplex-phase alloys. The deformation mechanism of the β phase is dependent on both the β domain and β stability. Due to the low trigger stress and excellent work hardening ability, stress-induced α' martensite phase transformation is helpful to lower the yield ratio. As the α phase content increases, the trigger stress increases, and when the α phase content increases to 40%, dislocation slip dominates rather than stress-induced α' martensite phase transformation, and a high yield strength of 944 MPa is obtained. The α/β phase interfaces act as effective obstacles to hinder dislocation movement and provide working hardening, and the obstruction effect is more significant with an increase in the hardness difference between the α and β phases. The stress-induced α' phase transformation and/or the deformation coordination between the α and β phases guarantee decent elongations of no less than 10% in the large control of the yield ratio from 0.31 to 0.96 with a yield strength from 254 to 1013 MPa. It paves the way to develop “Unititaniam” alloys for wide possible applications.

Room-Temperature Thermoelectric Performance of n-Type Multiphase Pseudobinary Bi2Te3-Bi2S3 Compounds: Synergic Effects of Phonon Scattering and Energy Filtering.

Bismuth telluride-based alloys possess the highest efficiencies for the low-temperature-range (<500 K) applications among thermoelectric materials. Despite significant advances in the efficiency of p-type Bi2Te3-based materials through engineering the electronic band structure by convergence of multiple bands, the n-type pair still suffers from poor efficiency due to a lower number of electron pockets near the conduction band edge than the valence band. To overcome the persistent low efficiency of n-type Bi2Te3-based materials, we have fabricated multiphase pseudobinary Bi2Te3-Bi2S3 compounds to take advantages of phonon scattering and energy filtering at interfaces, enhancing the efficiency of these materials. The energy barrier generated at the interface of the secondary phase of Bi14Te13S8 in the Bi2Te3 matrix resulted in a higher Seebeck coefficient and consequently a higher power factor in multiphase compounds than the single-phase alloys. This effect was combined with low thermal conductivity achieved through phonon scattering at the interfaces of finely structured multiphase compounds and resulted in a relatively high thermoelectric figure of merit of ∼0.7 over the 300-550 K temperature range for the multiphase sample of n-type Bi2Te2.75S0.25, double the efficiency of single-phase Bi2Te3. Our results inform an alternative alloy design to enhance the performance of thermoelectric materials.

Study on the Influence of Cu Content on Phase Transition Characteristics of Fe75Ni(25-x)Cu x Elastocaloric Refrigeration Alloys

In this study, the mechanical alloying (AM) and powder metallurgy (PM) techniques were adopted to synthesize the Fe75 Ni(25-x) Cu x elastocaloric refrigeration alloys to research the effect of Cu contents on phase transition characteristics and microstructure of Fe75Ni(25-x) Cux elastocaloric refrigeration alloys. The conclusion was drawn that, with increasing Cu content, the phase transition characteristics changed and presented different changing trends. It was found that the phase enthalpy change value was 43.34J/g when the Cu content was 7at%, which was about 1.26 times of Fe15Ni (wt%) alloy. In addition, the FeNiCu alloy was a single-phase alloy with a crystal space group structure of Im-3m when the Cu content was 7at%, and the FeNiCu alloy was a two-phase alloy with crystal space group structures of Im-3m and Fm-3m when the Cu content was more than 7at%. This study provided an experimental basis for the theoretical investigation of the phase transition mechanism of FeNiCu elastocaloric refrigeration alloys.

Preparation and Microstructure of Multi-Component High Entropy Alloy Powders Fabricated by Gas Atomization Method

As an attractive high-entropy alloy, AlCrCoNiCu high-entropy alloy has excellent corrosion resistance, wear resistance, and anti-bacterial capabilities, and is considered to be a potential substitute material for marine and nuclear industry materials with great potential. One key to further optimizing the performance of high entropy alloy was to prepare high entropy alloy powder materials with uniform composition, good flow-ability, and stable performance. In this work, the AlCrCoNiCu high entropy alloy powder was prepared by the gas atomization method. The results indicated that the powder was spherical in shape, homogeneous in composition, and composed of a face-center cubic (FCC) phase. After adding Fe and Mn elements, FCC and body-center cubic (BCC) phases appeared and the particle size of the powder was mainly located at 10–50 μm. Furthermore, the larger the particle size was, the more obvious the surface roughness was. With the decreasing powder size, its shape became relatively regular, and the surface roughness decreased. This work provided an experimental and theoretical reference for preparing high-performance single-phase and multi-phase high entropy alloy spherical powders.

Influence of Gd-rich precipitates on the martensitic transformation, magnetocaloric effect, and mechanical properties of Ni–Mn–In Heusler alloys—A comparative study

A multi-stimuli cooling cycle can be used to increase the cyclic caloric performance of multicaloric materials like Ni–Mn–In Heusler alloys. However, the use of uniaxial compressive stress as an additional external stimulus to a magnetic field requires good mechanical stability. Improvement in mechanical stability and strength by doping has been shown in several studies. However, doping is always accompanied by grain refinement and a change in transition temperature. This raises the question of the extent to which mechanical strength is related to grain refinement, transition temperature, or precipitates. This study shows a direct comparison between a single-phase Ni–Mn–In and a two-phase Gd-doped Ni–Mn–In alloy with the same transition temperature and grain size. It is shown that the excellent magnetocaloric properties of the Ni–Mn–In matrix are maintained with doping. The isothermal entropy change and adiabatic temperature change are reduced by only 15% in the two-phase Ni–Mn–In Heusler alloy compared to the single-phase alloy, which results from a slight increase in thermal hysteresis and the width of the transition. Due to the same grain size and transition temperature, this effect can be directly related to the precipitates. The introduction of Gd precipitates leads to a 100% improvement in mechanical strength, which is significantly lower than the improvement observed for Ni–Mn–In alloys with grain refinement and Gd precipitates. This reveals that a significant contribution to the improved mechanical stability in Gd-doped Heusler alloys is related to grain refinement.

Stabilizing far-from-equilibrium (Mo,Ti)S2 thin films by metal sulfurization at reduced temperature

We report the synthesis of large-area, high-Ti-content, Mo1−xTixS2 alloy thin films in the 2H phase at temperature as low as 500 °C using a scalable two-step method of metal film deposition, followed by sulfurization in H2S. Film processing at higher temperature accelerates Ti segregation, film coarsening, and the formation of TiS2 in the 1T phase. Crystal growth at higher temperature results in the formation of multiple binary sulfide phases, in agreement with the equilibrium phase diagram. Making highly metastable, smooth, and uniform single-phase alloy films, therefore, hinges on developing low-temperature processing. Our results are relevant to the development of technologies based on designer transition metal dichalcogenide alloys, including in photonic integrated circuits and gas sensing.

Single-phase lightweight high-entropy alloys with enhanced mechanical properties

To develop single-phase lightweight high-entropy alloys (LWHEAs), we perform massive screening over a vast material space containing more than 560,000 possible compounds by employing thermodynamic principles and examine the mechanical properties of the resulting alloys based on density functional theory (DFT) calculations. Considering LWHEAs composed of five principal elements including light atoms such as Li, Mg and Al, we identify 40 optimal atomic compositions for single-phase solid solutions in body-centered-cubic structure. Combined with DFT calculations, it is demonstrated that the valence electron concentration plays an important role in determining elastic moduli and that the identified LWHEAs are ductile as is evidenced by the satisfied Pugh criteria and positive Cauchy pressure. In cases of shear modulus and yield strength, pd-hybridization between Al and transition metal atoms is shown to make a significant contribution through forming strong directional bonds. Together with an observation that the inclusion of Li and Mg atoms is instrumental in reducing the overall density, the present results provide a valuable guidance to develop novel, low-density high-entropy alloys.

Tensile behavior of hexagonal rare-earth-based low, medium, and high entropy alloys: Strengthening effect of configurational entropy

In the equiatomic hexagonal close packed high entropy alloy Ho-Dy-Y-Gd-Tb, there is a discrepancy, called the Gibbs paradox, between the extraordinary similarity of the constituent rare earth elements and its calculated configurational entropy. Equiatomic alloys made from one to five rare earth elements, with varying configurational entropy, are investigated for this work to evaluate this paradox in the context of tensile behavior. Assuming comparable microstructures and minimized solid solution strengthening due to the similarity of the constituent elements, a possible strengthening effect of the high configurational entropy is assessed. The alloys are successfully prepared via arc melting with comparable grain sizes and near equiatomic compositions. X-Ray diffraction, and elemental mappings with energy-dispersive X-Ray spectroscopy indicate single-phase microstructures in all alloys except for Ho–Y and Ho-Y-Gd. Tensile tests at room temperature show that the tensile strength of the single-phase alloys lies in the range of the pure elements, which confirms the hypothesis of minimized solid solution strengthening. Those alloys containing additional phases as well as the high entropy alloy show pronounced brittle behavior. No direct correlation between tensile behavior at room temperature and configurational entropy is observed. We state that this alloy system is a suitable candidate for evaluating the Gibbs paradox and disproving an entropy effect on the tensile behavior of this alloy system. This work contributes to understanding mechanical properties and the influence of configurational entropy on the hexagonal close packed high entropy alloy Ho-Dy-Y-Gd-Tb.

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.