Single Bcc Phase Research Articles

Study on Microstructure and High Temperature Stability of WTaVTiZrx Refractory High Entropy Alloy Prepared by Laser Cladding.

The extremely harsh environment of the high temperature plasma imposes strict requirements on the construction materials of the first wall in a fusion reactor. In this work, a refractory alloy system, WTaVTiZrx, with low activation and high entropy, was theoretically designed based on semi-empirical formula and produced using a laser cladding method. The effects of Zr proportions on the metallographic microstructure, phase composition, and alloy chemistry of a high-entropy alloy cladding layer were investigated using a metallographic microscope, XRD (X-ray diffraction), SEM (scanning electron microscope), and EDS (energy dispersive spectrometer), respectively. The high-entropy alloys have a single-phase BCC structure, and the cladding layers exhibit a typical dendritic microstructure feature. The evolution of microstructure and mechanical properties of the high-entropy alloys, with respect to annealing temperature, was studied to reveal the performance stability of the alloy at a high temperature. The microstructure of the annealed samples at 900 °C for 5-10 h did not show significant changes compared to the as-cast samples, and the microhardness increased to 988.52 HV, which was higher than that of the as-cast samples (725.08 HV). When annealed at 1100 °C for 5 h, the microstructure remained unchanged, and the microhardness increased. However, after annealing for 10 h, black substances appeared in the microstructure, and the microhardness decreased, but it was still higher than the matrix. When annealed at 1200 °C for 5-10 h, the microhardness did not increase significantly compared to the as-cast samples, and after annealing for 10 h, the microhardness was even lower than that of the as-cast samples. The phase of the high entropy alloy did not change significantly after high-temperature annealing, indicating good phase stability at high temperatures. After annealing for 10 h, the microhardness was lower than that of the as-cast samples. The phase of the high entropy alloy remained unchanged after high-temperature annealing, demonstrating good phase stability at high temperatures.

Strength and oxidation resistance of Laves phase-containing refractory Nb-Ti-Zr-Cr alloys: Effect of chemical complexity

For a refractory Nb-Ti-Zr-Cr alloy system, we show that the effect of increasing chemical complexity, which is accepted as the main source for unique properties of high-entropy alloys, on the strength and oxidation resistance is sensitive to the particular elements added. A transition from pure Nb to NbTi and then to NbTiZr alloys that retained a single-phase body-centred cubic (bcc) structure improved the ambient-to-high-temperature strength and oxidation resistance at 1000 °C. However, the further addition of Cr that resulted in the Laves phase formation broke the trend. In the case of dual-phase bcc + Laves phase alloys, binary Nb100-xCrx alloys were much stronger at T > 600 °C than microstructurally similar multicomponent refractory complex concentrated alloys (RCCAs). Meanwhile, ternary (NbTi)100-xCrx alloys had the best oxidation resistance due to an exclusive formation of a protective Cr2O3 layer. This study emphasises opposite strategies for designing high-performance single-phase bcc and Laves phase-containing Nb-Ti-Zr-Cr RCCAs.

Simulation and study of the milling parameters on CuFeTaTiW multicomponent alloy

The CuFeTaTiW multicomponent alloy has been devised as an interlayer thermal barrier in nuclear fusion reactors. In order to predict the phase constitution of this alloy, two different lines of work were performed: (a) simulation using Molecular dynamics and Monte Carlo and (b) study of the influence of mechanical alloying parameters on the structures formed. The simulation results show that the most stable structure is achieved starting from a bcc type-structure and using Monte Carlo simulation. In fact, in these conditions the separation into two bcc phases Fe-Ta-W and Cu-Ti is predicted at room temperature. However, the experimental preparation of the materials with mechanical alloying revealed that from 2 h of milling a single bcc phase is formed. The structure of the milled powder was not much influenced by the amount of the process control agent and the by the size of the W starting particles, but generally there was formation of Ta2H from the reaction between the powders and the process control agent.

Microstructural, Phase, and Thermophysical Stability of CrMoNbV Refractory Multi-Principal Element Alloys

Structural survivability of refractory multi-principal element alloys (RMPEAs) at high temperatures hinges on a combination of microstructure, phase, and thermophysical stabilities. In this study, we investigate the high-temperature stability of three compositions from the CrMoNbV system: Cr7Mo18Nb35V40, Cr25Mo25Nb25V25, and Cr40Mo45Nb10V5. The microstructure, high-temperature phase stability, coefficient of thermal expansion (CTE), and yield strength were evaluated up to 1300oC. Results show that all three as-cast compositions have dendritic microstructures with an average grain size of ~100µm and no preferred texture. High-temperature analysis of phase stability shows that Cr7Mo18Nb35V40 maintains a single-phase BCC structure from room temperature up to 1300oC, while Cr25Mo25Nb25V25 forms an undesirable C15 Laves phase and Cr40Mo45Nb10V5 exhibits BCC phase separation. The CTE measured was 9.58, 9.52 and 8.80 ppm/oC, and the yield strength at 1250oC was 503, 629 and 628MPa for Cr7Mo18Nb35V40, Cr25Mo25Nb25V25 and Cr40Mo45Nb10V5, respectively. The low density, CTE and high-temperature yield strength of the three alloys sets the CrMoNbV system above current superalloys. More importantly, the microstructural, phase, and thermophysical stabilities of the Cr7Mo18Nb35V40 alloy over the equiatomic and Cr-rich alloys highlight the importance of high-temperature survivability over yield strength as an RMPEA design principle.

Atomic mobilities, diffusion coefficients, and kinetic coefficients in Ti-rich Ti–Mo–Nb system

Diffusion coefficient is an important physical property to control the microstructure, thus the establishment of atomic mobility databank for Ti-based alloy systems is vital to the design of novel Ti alloys. In the present work, ternary Ti–Mo–Nb diffusion couples within the single bcc phase were prepared and measured after annealing at 1273 K for 25 h, and interdiffusion coefficients of bcc Ti–Mo–Nb alloys at 1273 K were determined from the experimental composition profiles by using Matano–Kirkaldy method. Subsequently, the atomic mobility parameters of bcc Ti–Mo–Nb system under the CALculation of PHAse Diagrams (CALPHAD) framework were assessed by using recently proposed diffusion models, which were verified by the comparisons between the experimental interdiffusion coefficients/composition profiles and the model-predicted results. Moreover, diffusion models were carefully discussed for the ternary assessment. Finally, relationships among atomic mobility, kinetic coefficient, and diffusion coefficients were discussed and demonstrated via ternary contour maps. The present work contributes to the development of diffusion databank for computational design of biomedical Ti alloys.

Phase evolution and mechanical properties of Ti1.5NbVAlx(x=0.25, 0.5, 0.75, 1) lightweight refractory medium-entropy alloys

A series of Ti1.5NbVAlx(x = 0.25, 0.5, 0.75 and 1) lightweight refractory medium-entropy alloys was synthesized through vacuum arc-melting, and the effects of Al addition on the phase types, microstructure and properties were investigated. Al0.25 alloy showed a single-phase BCC structure, and the increase in Al content induced a transition from disordered-BCC to ordered-B2, which caused short-range ordered strengthening. The relationship between Al content and ordered B2 was revealed by TEM. The mechanical properties of this alloy system were measured by compression and tensile tests and results indicated that Al0.25 and Al0.5 show excellent tensile strength (800, 857 MPa) even without any microstructural homogenization or optimization, meanwhile, the tensile ductility is 20 and 19 %. TEM observation demonstrates that multiple slip systems are activated in Al0.5 alloy and highly concentrated dislocation bands cross each other to form substructures, which has an important effect on the strength and elongation. Furthermore, the four alloys showed excellent compression plasticity, and the deformation was more than 50 % without fracture. When the Al content increased to 0.75, the maximum compressive yield strength σ0.2 = 1.29 GPa. This work is of great significance for the design of high specific yield strength and good ductility RH/MEAs.

Design, fabrication, and characterization of high-entropy alloys (HEAs) of the type TiVCrX (X = Al, Mn, Mo)

According to Zhang et al. (Prog Mater Sci 61:1, 2014, https://doi.org/10.1016/j.pmatsci.2013.10.001), High-Entropy Alloys (HEAs) are an emerging class of advanced materials that contain multiple elements in equiatomic or near equiatomic concentrations. In recent years, extensive efforts have been made to design HEAs with suitable microstructures and superior properties. This work reports the preliminary results of fabrication and characterization of three HEAs (TiVCrAl, TiVCrMn, TiVCrMo) with equiatomic compositions. These HEAs were designed with the purpose of producing a single-phase BCC, according to calculations with the HEAPS (Martin et al. in Comput Phys Commun 278:108398, 2022, https://doi.org/10.1016/j.cpc.2022.108398) and CALPHAD programs. Microstructures consisting of BCC + C14 Laves phases (Mn), a mixture of two cubic phases (Mo) and a single BCC phase, were found, basically consistent with CALPHAD predictions, although the phase volume fractions and compositions were not accurately predicted.Graphical abstract

Design crystallographic ordering in NbTa0.5TiAlx refractory high entropy alloys with strength-plasticity synergy

To make up for the poor strength of high plasticity NbTa0·5Ti refractory medium entropy alloys (MEAs), light metal Al was introduced as alloying element. In this work, the NbTa0·5Ti-Alx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) series non-isoatomic refractory high entropy alloys (RHEAs) were prepared by arc melting, and phase equilibrium was predicted by CALPHAD. The effects of Al content and annealing temperature on microstructure and phase evolution, and its mechanical properties were studied. The NbTa0·5Ti-Alx alloys changed from single phase BCC to two-phase A2+B2 crystal structure after adding Al. The hardness and strength of the as-cast alloys are increased by the solution strengthening and precipitation strengthening effect, but the brittleness is increased. The precipitation of Laves of plate-like NbAlTi2 and finer-scale A15 of (needle, particle)-like AlTi3 precipitates at and near grain boundaries after annealing. Higher annealing temperature is beneficial to eliminate dendrites formed by element segregation in the arc melting cooling process and promote grain growth (up to ∼200 μm). This work designed a new alloys with excellent compression plasticity and enriched the field of composition design and aging treatment of the Al-containing second generation RHEAs， so that their microstructure can be better controlled to achieve a balance of strength and plasticity.

Wear and corrosion resistance of Al1.2CoCrFeNiScx high entropy alloys with scandium addition

The present study focuses on investigating the microstructure, hardness, wear, and corrosion resistance of Al1.2CoCrFeNiScx high entropy alloys, where the Sc content (x) varies between 0, 0.1, 0.2, and 0.3. The results indicate that the addition of Sc promotes the formation of the FCC phase and improves the hardness and wear resistance, but reduces the corrosion resistance of the alloy. The alloy's evolution can be observed as a transition from a single BCC phase to a combination of Fe, Cr-rich BCC phase, Al, Co, Ni-rich BCC phase, and Ni, Sc-rich FCC phase. As the Sc content increases from 0 to 0.3, the hardness increases from 438 HV to 581 HV, representing a 32.6 % increment. This increase can be mainly attributed to grain size strengthening and solid solution strengthening. Additionally, the average friction coefficient experiences a notable reduction from 0.999 to 0.574, indicating a 42.5 % decrease. With respect to wear resistance, the Al1.2CoCrFeNiSc0.3 alloy exhibits superior performance, demonstrated by its lower friction coefficient, reduced wear rate, and smaller volume, width and depth of wear marks. Furthermore, the addition of Sc to the Al1.2CoCrFeNiScx alloys leads to an overall decrease in corrosion potential, accompanied by an increase in corrosion current density, suggesting that Sc has a detrimental effect on the corrosion resistance of the alloy. Moreover, the Al1.2CoCrFeNi alloy displays pitting corrosion, while the Al1.2CoCrFeNiScx alloys demonstrate intercrystalline corrosion, which preferentially occurs on the Ni, Sc-rich phase due to the high electrochemical activity and low equivalent chemical potential.

Influence of eutectoid structures on the mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta alloys

Hf–Ta alloys have recently received much attention due to their favorable oxidation resistance and mechanical properties at temperatures beyond 1200 °C. However, at lower temperatures the oxidation resistance becomes poor and the alloys experience brittle to ductile transition. Both effects are likely due to their two-phase structure formed during eutectoid transformation. In the present work, we report mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta (compositions are in atomic percent), which experience eutectoid transformation at ∼1083 °C and contain two phases, Hf-rich HCP and Ta-rich BCC, below this temperature. Alloys of the compositions corresponding to these two eutectoid phases, Hf-90%Ta (BCC) and Hf-3%Ta (HCP), were also produced and studied for comparison. The results show that in the temperature range of 25–1000 °C (i.e., below the eutectoid transformation) the compression strengths of Hf-90%Ta and Hf-3%Ta are considerably smaller than the strength of Hf-27%Ta or Hf-42%Ta, while above the eutectoid transformation, where the Hf-27%Ta and Hf-42%Ta alloys have single-phase BCC structures, the Hf-90%Ta and Hf-3%Ta become stronger than either Hf-27%Ta or Hf-42%Ta. It was concluded that the interfaces in the eutectoid structure of Hf-27%Ta produce considerable strengthening, which can exceed the strengths of the phases which comprise the microstructure. At 800 °C and 1000 °C, Hf-90%Ta oxidized rapidly in comparison to Hf-3%Ta due to the extensive formation of Ta2O5. It is suggested that the observed poor oxidation behavior of Hf-27%Ta below the temperature of the eutectoid transformation is likely controlled by the oxidation kinetics of the Hf-90%Ta BCC phase in the eutectoid structure of this alloy.

Isothermal oxidation behavior of W-based alloy coatings doped with Ta, Cr, Ti, V

Tungsten-based multi-alloy coatings with the addition of Ta, Cr, Ti, and V were prepared by double glow plasma surface alloying (DGPSA) technology. The alloyed specimens showed bulges and pits on the surface with an island growth mechanism. The coating exhibits a single BCC phase. The hardness shows an increasing trend with the increase of alloyed elements. The maximum hardness value of WTaCrTiV is 1748 HV100g. The oxidation behavior of coatings at 800 °C was investigated, with emphasis on the effect of elemental doping on the antioxidant properties. With the addition of Ta, Cr, Ti, and V elements, the antioxidant properties of multi-alloy coatings were effectively improved compared the tungsten matrix. The WTaCrTi coating shows the optimum antioxidant properties with a dense Cr2O3 layer. As the number of elements in multi-alloy increases, the mixing entropy increases, leading to a decrease in the Gibbs free energy, which results in a more stable alloy.

A new insight into the phase stability in high entropy alloys

A comparative model has been used to evaluate the phase stability of the high entropy alloys (HEAs) by using chromium equivalent (Creq) and nickel equivalent (Nieq). 296 numbers of HEAs have been chosen for the present study and the experimentally observed phases are classified as single-phase FCC, single-phase BCC, FCC+BCC, FCC+Intermetallics (IM), and BCC+IM. The alloying elements were divided into BCC stabilizers and FCC stabilizers in order to calculate Creq and Nieq, respectively. An effort has been made to evaluate the solid solution stability range of 296 numbers of HEAs in terms of Creqand Nieq to explore a larger domain in the arena of HEAs. Valence electron concentration (VEC) of the selected HEAs has been correlated with the Creq and Nieq. Moreover, Creq and Nieqhave been inter-linked with the calculated melting temperature of the HEAs.

HARDENING OF LIGHTWEIGHT MULTI-PRINCIPAL ELEMENT TITANIUM-BASED ALLOY UNDER AR ION IRRADIATION

Among new prospective materials multi-principal element alloys (MPEA) have attracted considerable attention in recent years due to their excellent corrosion and irradiation resistance as well as their good mechanical properties over a wide temperature range. The new lightweight multi-principal element titanium-based alloy 61Ti-10Cr-7Al11V-11Nb (at. %) with high ductility at room and elevated temperatures is studied. This single-phase bcc alloy was irradiated with 1.4 MeV Ar ions at room temperature and mid-range doses from 1 to 10 displacements per atom. The effect of irradiation is studied by examining the hardening. A comparison was performed with irradiation-induced hardening behaviour of MPEA, 316 austenitic stainless steel irradiated under an identical condition. It was shown that hardness increases with irradiation dose for all the materials studied, but this increase is lower in multi-principal element alloys both face-centered cubic (FCC) and body-centered cubic (BCC) structures than in stainless conventional steel.

A lightweight NbMoZrTi refractory high entropy alloy with high specific strength

Refractory high entropy alloys are an obstacle to their use as engineering applications due to their low room temperature ductility and high density. In the present work, a new low-density Nb-Mo-Zr-Ti RHEA was prepared by vacuum arc melting method. The as-cast alloy exhibits a single BCC phase with lower density and higher hardness of 7.4331 g/cm3 and 522 HV. The compressive yield strength of 1462 MPa at room temperature, peak strength of 2158 MPa, and strain at break of 24.6 %. The specific yield strength of this alloy is the highest among all other reported RHEAs, and the ultra-high specific yield strength and excellent strong plastic synergistic effect provide important experience in designing and preparing lightweight structural materials for practical applications.

Synthesis of Li-Mg-Al-Ti based lightweight high entropy alloys by mechanical alloying and investigation of conditions for solid solution formation

Li-Mg-Al-Ti-M based lightweight high entropy alloys (lightweight HEAs) were synthesized through mechanical alloying and the possibility of solid solution formation was systematically investigated. The quaternary Li-Mg-Al-Ti system exhibited a HCP structure. Solid solutions were formed in the alloys with V or Nb among the 15 types of LiMgAlTiM alloys. Furthermore, distinct BCC single phase solid solution were successfully synthesized by adjusting the composition of the Li-Mg-Al-Ti-V system. The typical indicators of solid solution formation which have been used in HEAs were applied to this study. As a result, it was found that the effective indicators for mechanical alloying differ from those used in conventional melting methods, meaning their dependency on the alloy fabrication process.

Hydrogen storage properties of MgTiVZrNb high-entropy alloy and its catalytic effect upon hydrogen storage in Mg

Herein, a new approach relying on six parameters has been proposed to design the high-entropy alloy (HEA) with a single-phase BCC solid solution based on the Mg–Ti–V–Zr–Nb system. The Mg10Ti30V25Zr10Nb25 alloy was selected among 5152 calculated compositions. Subsequently, the HEA is prepared by mechanical alloying and introduced into the Mg to study the effect of HEA on the hydrogen storage performance of Mg/MgH2 system. Results show that the HEA exhibits excellent hydrogen absorption kinetics at room temperature, while the hydrogen absorption capacity is about 1.196 wt%. Moreover, the addition of HEA can significantly improve the hydrogen absorption and desorption kinetics of Mg at low temperatures. The dehydrogenation activation energy of the composites reduces from 121.94 ± 4.08 to 91.66 ± 1.31 kJ mol−1, and HEA exhibits good catalytic activity. The initial hydrogenation and dehydrogenation temperatures of Mg‒15 wt% HEA composite are about 418 K and 526 K, which are 78 K and 47 K lower than pure Mg, respectively. The Mg‒15 wt% HEA composite has good cycling stability.

An excellent synergy in yield strength and plasticity of NbTiZrTa0.25Cr0.4 refractory high entropy alloy through the regulation of cooling rates

The solidification process often determines the microstructure of alloys from the atomic scale to the macroscopic scale, which distinguishes the performance of one alloy from another. Among all modification measures, the cooling rate plays a vital role in the solidification process. Hereinto, to thoroughly investigate the cooling rate effects on phase components, microstructures and mechanical properties, a series of NbTiZrTa0.25Cr0.4 refractory high entropy alloys were prepared under a wide range of cooling rates. The results show that the microstructures with combined dendrites and interdendrites gradually coarsened as the cooling rate decreased, coupled with the transition from a single BCC phase to BCC + Laves dual-phase. During the microstructural evolution, the prominent strengthening mechanism (i.e., solution strengthening) of the alloy prepared under high cooling rates became weak, while the secondary strengthening mechanism (i.e., second phase strengthening associated with Laves phase) was gradually reinforced. The reinforced second phase strengthening compensated for the weakened solution strengthening; as a result, the yield strength first decreased and then increased as the cooling rate decreased without sacrificing the plasticity of the alloys. It was found that the alloy prepared under an appropriate cooling rate of 62.5 K/s exhibited the maximum yield strength of 1452 MPa and maintained a competitive plasticity of approximately 23.2%, which exceeded the NbTiZrTa0.25Crx system and other reported refractory high entropy alloys. This study provides insight into the development of high-temperature alloys and a promising avenue to enhance the performance of an alloy by regulating the cooling rates.

Accelerating First Principles Calculation of Multi-Component Alloy Steady-State Structure and Elastic Properties in Full Component Space.

The FeNiCrAlCoCuTi alloy system has great advantages in mechanical properties such as high hardness and toughness. It has high performance potential and research value and the key in research is designing alloy compositions with target properties. The traditional method, experimental analysis, is highly inefficient to properly exploit the intrinsic relationship between material characteristics and properties for multi-component alloys, especially in investigating the whole composition space. In this work, we present a research way that uses first principles calculation to obtain the properties of multi-component alloys and uses machine learning to accelerate the research. The FeNiCrAlCoCuTi alloy system with its elastic properties is used as an example to demonstrate this process. We specifically design models for each output, all of which have RMSE values of less than 1.1, and confirm their effectiveness through experimental data in the literature, showing that the relative error is below 5%. Additionally, we perform an interpretable analysis on the models, exposing the underlying relationship between input features and output. By means of spatial transformation, we achieve the prediction of the full-component spatial performance from binary to multiple components. Taking the FeNiCrAlM (M = Co, Cu, Ti) quinary alloy system as an example, we design a single-phase BCC structure composed of an Fe0.23Cr0.23Al0.23Ni0.03Cu0.28 alloy with a Young's modulus of 273.10 GPa, as well as a single-phase BCC structure composed of an Fe0.01Cr0.01Al0.01Ni0.44Co0.53 alloy with a shear modulus of 103.6 GPa. Through this research way, we use machine learning to accelerate the calculation, which greatly shortens research time and costs. This work overcomes the drawbacks of traditional experiments and directly obtains element compositions and composition intervals with excellent performance.

Exploring the mechanical and tribological properties of AlCrFeNiTi high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

The present study explores the mechanical and tribological properties of AlCrFeNiTi high entropy alloy (HEA) processed by mechanical alloying (MA) and spark plasma sintering (SPS). The results show the formation of a single-phase BCC structure after MA at 30 h, and after SPS a single BCC phase decomposed into dual BCC1; AlNi2Ti, and BCC2; CrFe phases. The maximum hardness of ∼960 ± 10 HV and ultimate compressive strength of ∼1650 ± 50 MPa were achieved. The nano hardness of ∼951 ± 21 HVIT and the elastic modulus of ∼256 ± 22 GPa were observed. Further, the ball-on-disc dry sliding tests were performed to study the friction and wear behavior of HEA at different loads and sliding velocities. The average coefficient of friction is about 0.3 for all the applied loads, suggesting no significant difference with load. In contrast, the average coefficient of friction slightly increased from 0.23 to 0.27 with increasing velocity. The specific wear rate increased from 2.66 to 5.06 × 10−6 mm3/Nm and 2.2 to 3.61 × 10−6 mm3/Nm with increasing load from 5 to 15 N and velocity from 0.08 to 0.12 m/s, respectively. Moreover, a transition in wear mechanism was observed from abrasive to oxidative and delamination wear with increasing load and sliding velocity.

Characterization and formation mechanism of non-metallic inclusions in single bcc-phase high entropy alloy

This work aims to provide a fundamental study of inclusion characteristics in the single bcc-phase high entropy alloy (HEA), Mn-rich and Al-contained multicomponent system (Al-Cr-Mn-Fe-Co-Ni) was selected as the prototype alloy in this study. According to the differential thermal analysis (DTA) measurements, the solidus (TS) and liquidus (TL) temperatures of this alloy were in the range of 1225–1228 °C (1226 ± 2 °C) and 1268–1271 °C (1270 ± 2 °C), respectively. Non-metallic inclusions were investigated in a two-dimensional (2D) cross-section method as well as extracted by a three-dimensional (3D) electrolytic extraction method. It was found that AlN was the dominant inclusion phase, also a small amount of Al2O3 inclusions were observed. They formed in the liquid alloy and mostly presented as Al2O3-AlN agglomerates, where the size range of the AlN inclusions was larger than that of Al2O3. The theoretical calculation showed that AlN inclusion has a higher coagulation coefficient and collision rate than those of Al2O3 inclusions, which agrees well with the experimental observations. The inclusion characteristics of Al2O3 and AlN were closely related to the relative contents of O and N in the presence of high Al content in the alloy. The impurity elements of N and O were the key issues in controling the stable inclusion phase in high entropic alloy.