The tribological properties such as ‘specific wear rate’ and ‘coefficient of friction’ (COF) of a new high entropy alloy (HEA) of (Al)10(FeCoNiCu)90 were determined by sliding against the normal loads of 30 and 40 N and compared with that of the high-speed steel (HSS) in this work. The major wear mechanism in the HEA was ploughing due to abrasion. The specific wear rate determined for the HEA at 40 N load is slightly higher (∼7%) than that at 30 N load indicating small increase in wear rate on significant increase in the load. The specific wear rate of the HEA is about five times higher than that of HSS at 30 N load. At 30 N load, after 200 m of sliding, both the wear volume rate and COF decreased continuously for the HEA on increasing the sliding distance due to the presence of sufficient amount of lubricating wear debris.

The equiatomic powder of AlCoCuFeNi high entropy alloy (HEA) was produced by mechanical alloying technique via ball milling. The powder achieved excellent compositional homogeneity after milling. Heat treatment of the as-milled powder at 900 °C was carried out to investigate its phase stability. Both the as-milled and the milled + heat-treated powders crystallized only in FCC + BCC phases that confirmed the phase stability of the alloy powder at high temperature. The lattice parameters of the corresponding FCC and BCC phases of the milled + heat-treated powder were determined as 3.58 Å and 2.86 Å, respectively. Heat treatment increased the fraction of the BCC phase in the alloy powder by reducing that of the FCC phase in large amounts. Thermal analysis of the milled powder revealed that the transformation of the FCC phase to BCC commenced between 721 and 735 °C. The alloy powder can be beneficial for high-temperature structural and coating applications due to the yielding of large amount of strong BCC phase at high temperature.

A new powder combination of Al10(FeCoNiCu)90 is selected and verified for the potential to form solid solutions or else bulk metallic glasses (BMGs) in the alloy based on Guo et al.'s conditions and mismatch entropy criterion. The powder combination satisfies the above criteria to crystallize in solid-solution phases and inhibits the formation of BMGs in the alloy. The alloy is fabricated by mechanical alloying of the powder mixture followed by sintering. A homogeneous powder mixture is obtained after ball milling for 80 h. The 80 h-milled powder is crystallized in two face-centered cubic (FCC) phases. The lattice parameters of the FCC1 and FCC2 phases are 3.60 Å and 5.21 Å, respectively. The microstructure of the alloy ingot reveals spinodal decomposition. The alloy ingot crystallizes in a single-FCC structure. The lattice parameter of the corresponding FCC structure is determined as 3.66 Å. The compression test of the alloy ingot at room temperature reveals that the 0.2% offset yield stress of the high-entropy alloy (HEA) is as high as 308 MPa. The compressive stress at 40% strain of the HEA is 1406 MPa. The overall Vickers hardness and density of the alloy ingot are 160 ± 14 HV0.5 and 7.7 g cm−3, respectively.

Refractory high entropy alloys (RHEAs) exhibit high strength, high hardness and stable microstructure over a wide range of temperatures. However, these alloys are also characterized by poor ductility. Total strain to failure of 6.2% at room temperature was observed in equiatomic WMoVCrTa RHEA. Attempts are made to improve the mechanical properties by altering the composition of existing RHEAs. This report presents the results of mechanical properties viz., room and elevated temperature properties of W23Mo23V17Cr8Ta7Fe22 and WMoVCrTa RHEAs fabricated by mechanical alloying followed by vacuum arc melting. Both the arc melted alloys revealed presence of two Body-Centered Cubic (BCC) structured phases along with a small amount of intermetallic phase. The arc melted W23Mo23V17Cr8Ta7Fe22 alloy exhibited compressive yield strength of 1688 MPa and an ultimate strain of 7.3% at room temperature whereas at 1000 °C the corresponding values were 1514 MPa and 10.6%. At 1000 °C the arc melted equiatomic WMoVCrTa alloy exhibited compressive yield strength of 785 MPa and an ultimate strain of 5.7%, whereas at 1200 °C the corresponding values were 941 MPa and 8.3%. Yield stress anomaly was observed for the two alloys at elevated temperatures. Both alloys possess high hardness at room temperature. The two RHEAs appear to be viable materials for high-temperature applications like gas turbine blades, combustion chamber lining, high-temperature coating, nuclear reactor walls, etc., due to their combination of high hardness, high strength and yield strain at elevated temperatures.

Refractory high-entropy alloys (HEAs) possess outstanding mechanical strength at room and high temperature but lack the room temperature ductility. A novel refractory equiatomic powder combination of WMoVCrTa was selected and verified for the feasibility of formation of solid solutions or else bulk-metallic glasses (BMGs) in the alloy based on the Guo et al.'s criteria and mismatch entropy criterion. The powder combination satisfies the above two criteria to crystallize in solid solution phases and inhibit the BMGs. Mechanical alloying characteristics of the powder mixture were determined. The particle size of the powder mixture decreased continuously during initial milling and later increased after 32 h of mechanical alloying. A homogeneous mixture was obtained after milling for 64 h. Crystallite sizes of the constituent elements in the powder mixture decreased continuously on progressive milling. A nanocrystalline powder was obtained by mechanical alloying. The powder milled for 64 h revealed a major BCC1, a minor BCC2 and small unknown phases. The lattice parameters of those BCC1 and BCC2 phases are 3.16 Å and 2.88 Å respectively. The alloy ingots were fabricated from the milled powder by vacuum arc melting technique followed by heat treatment. The ingot crystallizes in three phases such as a major BCC1, a minor BCC2 and a minor laves phase. The lattice parameters of these BCC1 and BCC2 phases are 3.05 Å and 2.85 Å respectively. Thereby, the BCC1 lattice of the milled powder contracts slightly after ingot fabrication. A fine combination of compressive stress and strain of 995 MPa and 6.2% respectively was achieved by the alloy at room temperature. Vickers hardness of the alloy was as high as 773 ± 20HV0.5. The density of the alloy was 11.52 g/cm3. The combination of excellent room temperature stress-strain and high hardness properties can enable the refractory HEA as a potential candidate for structural applications.

Recent years have seen the emergence of high-entropy alloys (HEAs) consisting of five or more elements in equi-atomic or near equi-atomic ratios. These alloys in single phase solid solution exhibit exceptional mechanical properties viz., high strength at room and elevated temperatures, reasonable ductility and stable microstructure over a wide range of temperatures making it suitable for high temperature structural materials. In spite of the attractive properties, processing of these materials remains a challenge. Reports regarding fabrication and characterisation of a few refractory HEA systems are available. The processing of these alloys have been carried out by arc melting of small button sized materials. The present paper discusses the development of a novel refractory W-Mo-V-Cr-Ta HEA powder based on a new alloy design concept. The powder mixture was milled for time periods up to 64 hours. Single phase alloy powder having body centred cubic structure was processed by mechanical alloying. The milling characteristics and extent of alloying during the ball milling were characterized using X-ray diffractiometre (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). A single phase solid solution alloy powder having body-centred cubic (BCC) structure with a lattice parameter of 3.15486 Å was obtained after milling for 32 hours.