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Equilibrium phase diagrams for ten MeX(NbTiZr)100-X alloy systems, where Me is Al, Cr, Fe, Hf, Mo, Re, Si, Ta, V or W and X ranges from 0 to 25 at% were calculated using the PanNb2018a database recently developed by CompuTherm, LLC. By the type of phases and sequence of formation, these alloy systems can be divided into two groups. The first group of quaternary alloys, containing Al, Hf, Mo, Ta, or V has a single-phase BCC region below the solidus line over the entire X range and can be considered as potential candidates for the development of single-phase high entropy alloys. The second group consists of the quaternary alloy systems in which the fourth element (Cr, Fe, Re, Si or W) has limited solubility in BCC NbTiZr, which leads to the formation of an additional phase below the solidus and above the solubility limit. These quaternary alloy systems can be used for the development of precipitation or dispersoid strengthened complex concentrated alloys. To verify CALPHAD calculations three representative alloys, Cr10Nb30Ti30Zr30, Ta10Nb30Ti30Zr30 and Re10Nb30Ti30Zr30, were prepared by arc melting. The alloy densities were 6.56, 7.81 and 7.85 g/cm3, respectively. The phase compositions of the produced alloys agreed satisfactorily with CALPHAD calculations. Mechanical properties were also studied and compared with those of NbTiZr. At room temperature (RT) all the alloys showed high hardness exceeding 350 Hv and high compression yield stress exceeding 1000 MPa. RT compression ductility of Re and Ta containing alloys was above 50%, but Cr-containing alloy showed low ductility of 5%. With an increase in temperature ≥800 °C, compression strength decreased more rapidly for the Cr-containing alloy, which at 1200 °C became softer than NbTiZr. The least temperature dependence of the strength was observed for the Ta-containing alloy, which became the strongest at 1200 °C.

Refractory complex concentrated alloys (RCCAs) represent an emerging class of materials that have displayed significant potential for increased mechanical performance at extreme temperatures over conventional refractory alloys and Ni-base superalloys. While limited work has been conducted in this area, several RCCAs have shown remarkable levels of oxidation resistance in comparison to the majority of uncoated commercial refractory alloys. However, the oxidation mechanisms in these types of alloys have not been well understood. This study aims to develop the foundation for understanding RCCA oxidation mechanisms through the systematic evaluation of NbTiZrV and NbTiZrCr, which display vastly different oxidation behaviors. In both cases, the predominant mechanism appeared to be internal oxidation, where a mixture of simple and complex oxides was observed after prolonged oxidation times. This resulted in extended linear oxidation kinetics for the NbTiZrV alloy and linear to near-parabolic oxidation kinetics for the NbTiZrCr alloy. Under this mindset, novel design strategies for promoting increased oxidation resistance in both RCCAs and dilute refractory alloys based on sluggish complex oxides are proposed.

This study reports the microstructure, phase composition, mechanical properties and oxidation behavior of NbTaTi and NbTaZr ternary equiatomic alloys. These ternary components, together with earlier reported NbTiZr and MoNbTi, are commonly observed in refractory high entropy alloys (RHEAs) with a single-phase BCC or multi-phase crystal structures. Therefore, the reported information about these ternary alloys is thought to be useful for the development of advanced RHEAs. While NbTiZr and MoNbTi are single-phase BCC structures, NbTaTi and NbTaZr are multi-phase alloys. NbTaTi consists of a BCC matrix and discontinuous, Ti-rich precipitates with BCC and FCC crystal structures. NbTaZr consists of a co-continuous spinodal mixture of two BCC phases, one of which is rich in Ta and Nb and another is rich in Zr, and it also contains a Zr-rich HCP phase in the form of coarse grain-boundary particles. NbTaTi has compression yield stress of 724 MPa at 25 °C, 268 MPa at 800 °C and 129 MPa at 1200 °C. NbTaZr is stronger, with the compression yield stress of 1027 MPa at 25 °C, 530 MPa at 800 °C and 183 MPa at 1200 °C. Both alloys show rapid oxidation with formation of complex oxides during holding at 1200 °C.

The recently developed refractory metal high entropy superalloys (RSA) have been proposed as novel materials for high temperature service. A key requirement of any material used in such applications is to retain its properties throughout the life of a component. Consequently, it is critical that the microstructures that give rise to these properties are stable across the temperature ranges experienced in service, yet, at present, very little is known about the microstructural stability of RSA. To address this issue, here we report on the microstructural evolution of two alloys from the AlMoNbTaTiZr RSA system following long duration thermal exposures of 1000 h at 1200, 1000 and 800 °C. At these temperatures, the initial microstructures were found to be unstable, forming new intragranular precipitates. In AlNbTa0.5TiZr0.5, both Al–Zr hexagonal and Al–Nb–rich orthorhombic phases were observed following exposure at 1200 and 1000 °C, whilst additional fine scale phases were found to have formed following exposure at 800 °C. In AlMo0.5NbTa0.5TiZr0.5, the Al–Zr phase was observed to precipitate within the grains following exposure at both 1200 and 1000 °C. A refractory metal–rich secondary bcc phase was observed following exposure at 1000 °C and 800 °C and a Zr–rich phase precipitated at 800 °C. The formation of these additional phases would undoubtedly affect the properties of these alloys, altering their in-service performance. Consequently, these results highlight the need to consider microstructural stability in the future development of RSA and to gain an enhanced understanding of the phase equilibria of these systems.

A simple thermodynamic criterion is proposed to predict the presence or absence of equilibrium intermetallic phases in a high entropy alloy at a given temperature T. The criterion was verified using 45 currently available HEAs, for which equilibrium phases and respective annealing temperature are reported. The present model shows good correlation with experiment and gives an improved ability to predict formation of solid solution and intermetallic phases compared to earlier models.

A refractory high-entropy alloy HfNbTaTiZr was successfully rolled at room temperature up to 86.4% reduction in thickness (true thickness strain is −2.3). This represents the first successful attempt to cold roll a HEA with a BCC crystal structure. The microstructure and properties of the rolled sheets were determined in the as-rolled condition and after annealing at 800 °C, 1000 °C, and 1200 °C. Cold rolling resulted in extensive grain elongation, formation of deformation bands within the grains, and development of crystallographic textures that depended on the rolling reduction. The 86.4% cold-rolled sheet had true tensile stress of 1295 MPa and tensile ductility of 4.7%. After annealing at 1000 °C and 1200 °C, complete recrystallization of the cold-rolled sheet occurred. After annealing at 1000 °C, the true tensile stress and ductility of the sheet were 1262 MPa and 9.7%, respectively.

Crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiVx (x = 0, 0.25, 0.5, 0.75, 1) high entropy alloys were examined. The alloys were produced by vacuum arc melting and studied in as-solidified and homogenized (annealing at 1000 °C for 24 h) conditions. The CoCrFeMnNi alloy was a single-phase fcc solid solution in both conditions. The CoCrFeMnNiV0.25 alloy had a single-phase fcc structure in as-solidified condition, but ∼2 vol.% fine particles of a sigma phase precipitated after annealing. The alloys with x = 0.5, 0.75 and 1.0 contained the sigma phase already in as-solidified condition. The sigma-phase volume fraction increased with an increase in the V content, and in CoCrFeMnNiV the sigma phase became the matrix phase. After homogenization treatment, the volume fraction of the sigma phase increased in all three alloys by ∼8% due to additional precipitation of fine particles inside the fcc phase. Phase composition and microstructure of the alloys was analyzed employing criteria for solid solution/intermetallic phase formation. The effect of alloys’ chemical composition on the volume fraction of constitutive phases was discussed. A modified valence electron concentration (VEC) criterion, which takes into account localized lattice distortions around V atoms, was suggested to correctly predict sigma phase formation in the CoCrFeNiMnVx alloys. It was demonstrated that the volume fraction of sigma phase was proportional to the cumulative Cr and V concentration. Mechanical properties of the alloys were greatly affected by the sigma phase. The CoCrFeMnNi and CoCrFeMnNiV0.25 alloys were soft and ductile, but an increase in the sigma-phase volume fraction resulted in continuous strengthening and loss of ductility.

Microstructure and mechanical properties of equimolar composition alloys FeCrCoNi, FeCrCoNiV, FeCrCoNiMn and FeCrCoNiMnV were studied in as-solidified and annealed conditions. The FeCrCoNi and FeCrCoNiMn alloys were single-phase FCC solid-solutions in both conditions. However, the FeCrCoNiV and FeCrCoNiMnV alloys consisted of the intermetallic σ-phase matrix with a tetragonal crystal lattice and precipitates of a disordered FCC phase. The crystal structures of these alloys were found to be not affected by annealing. A number of criteria were considered to explain phase composition of the studied alloys. It was shown that poor compatibility of V with other alloying elements caused significant distortions of FCC solid solution and thus promoted formation of the σ phase. Tensile and compressive properties of these alloys together with their microhardness were determined. Significant strengthening accompanied by the loss of ductility due to formation of the σ phase was demonstrated in the V containing alloys. The characteristics of the microstructure formation in the studied alloys were discussed.

Several Nb-based alloys containing Ti, Mo and W were produced and their microstructure, phase composition and mechanical properties in the temperature range of 23–1200 °C were reported and compared with a high-strength commercial Nb alloy C-3009, which contains Hf and W alloying elements. Replacement of Hf with Ti and W with Mo led to alloys with considerably reduced density and cost, as well as with reduced micro-segregation of the alloying elements. The strengthening effect from W was higher than from Mo at temperatures above ∼600 °C and more amount of Mo was required to compensate the strength decrease. The strengthening effects from Hf and Ti were comparable at temperatures up to 1000 °C; however, at 1200 °C the effect from Hf was superior. It was concluded that the Nb–Mo–Ti system can be used as a baseline for the development of high strength, high-temperature alloys using alloying with other elements.

A refractory alloy, Ta20Nb20Hf20Zr20Ti20, was produced by vacuum arc-melting. The as-solidified alloy had a dendritic structure, which was not affected by hot isostatic pressing (HI Ping) conducted at T = 1200 deg C and p = 207 MPa for 3 hours. The alloy had a single-phase body-centered cubic (BCC) structure with the lattice parameter a= 340.44 pm. The alloy density and Vickers microhardness after HIPing were p = 9.94 g/cm3 and Hv = 3826 MPa. Room temperature compression testing of HIPd samples revealed excellent ductility. The samples deformed homogeneously without any evidence of cracking at least up to 50% compression strain. Continuous strengthening followed yielding at 807 MPa. A simple model of solid-solution strengthening is proposed to explain the behavior.

A number of ternary Ca–Mg–Cu amorphous alloys with compositions ranging from 40 to 70 at.% for Ca, 5–30 at.% for Mg and 10–36 at.% for Cu were produced by a copper mold casting method as wedge-shaped samples with the thickness varying from 0.5 to 10 mm. The maximum thickness at which an alloy remains fully amorphous, glass transition temperature, crystallization temperature, temperature interval of the super-cooled region, solidus and liquidus temperatures, as well as heats of crystallization and melting, are reported for these alloys. The effect of the alloy composition on glass forming ability is discussed. In the studied composition range, the Ca 50 Mg 22.5 Cu 27.5 alloy has the best glass forming ability, being fully amorphous after casting to the thicknesses of at least 10 mm.

Al 85 Ni 10 Y 2.5 La 2.5 alloy powder produced by gas atomization was compacted using equal channel angular extrusion (ECAE). The powder particle size was below 40 μm (−325 mesh grade), the powder was partially (∼60%) amorphous and it contained intermetallic phases. Differential scanning calorimetry showed that crystallization of the amorphous phase starts at about 220 °C by precipitation of Al-based f.c.c. particles and continues at temperatures above 300 °C by formation of intermetallic phases. Four-pass B C route compaction of the powder was conducted in a copper can at 280 °C, to avoid additional formation of intermetallic phases and retain some level of amorphicity of the compacted material. A non-homogeneous deformation of the compacted material during ECAE process was detected. The powder was completely consolidated and no voids were observed in the regions where powder particles were severely deformed. In other regions, non-deformed round particles, cracks, and high porosity were observed. Microstructural analysis showed that the severely deformed powder particles contained amorphous and/or f.c.c. phases only, while the non-deformed and/or fractured powder particles were fully crystalline and consisted mostly of intermetallic phases.

Amorphous Al 85 Ni 10 Y 2.5 La 2.5 alloy powder with the particle size of −500 mesh was produced by gas atomization. Crystallization kinetics of the amorphous phase were studied using differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy. Crystallization occurred in the temperature range 230–450°C in three exothermic reaction steps. The onset and peak temperatures for these reactions shifted toward higher temperatures when the heating rate was increased; this allowed the activation energies of the mechanisms controlling each step of the crystallization to be determined as 148, 336, and 274 kJ/mol, respectively. XRD and SEM analysis of the powder heated to different temperatures showed that precipitation of Al-based particles occurred during the first exothermic reaction, crystallization of the Al 4 La phase and an increase in the volume fraction of the Al-based phase took place during the second exothermic reaction, and development of the Al 3 Ni phase occurred during the third exothermic reaction. After annealing at 550°C, the powder consisted of Al-based matrix reinforced with very fine intermetallic particles of different morphologies. The mechanism of crystallization is discussed.

A low-density titanium alloy was synthesized from blended elemental powders of TiH 2 , Mg, and Si by mechanical alloying and/or heat treatment. The titanium hydride was used in place of titanium. Phase transformations occurring in the system during heating at a constant rate were studied with the use of DTA and XRD. During heating of the blended elemental powders decomposition of titanium hydride occurred in the temperature range 550–750°C and some silicon went into solid solution in titanium while the majority of the silicon reacted exothermically with magnesium at about 500°C producing an intermetallic phase Mg 2 Si. This phase was stable on heating up to 950°C, where a eutectic component of this phase began to melt leading to formation of a liquid solution of magnesium in silicon, followed by a reaction of the liquid silicon with titanium and formation of a Ti 5 Si 3 phase. A third reaction in the system was detected at about 1100°C due to formation of MgO, so that after annealing at 1150°C three stable phases, Ti(Si), Ti 5 Si 3 , and MgO, were present in the alloy. No decomposition of the Ti 5 Si 3 phase or formation of Mg 2 Si were detected either during subsequent cooling or a second heating of the alloy. Completely different kinetics of the phase reactions occurred in the mechanically alloyed powders. Magnesium and silicon dissolved in the titanium hydride during mechanical alloying. Decomposition of the titanium hydride occurred at 320–600°C, the Mg 2 Si phase was formed after heating to 450°C, and the Ti 5 Si 3 phase was detected after heating to 570°C. The Mg 2 Si decomposed completely at a temperature of 650°C with the formation of MgO and Ti 5 Si 3 . After heating to 1150°C, three stable phases, TiN 0.3 , Ti 5 Si 3 , and MgO, were present in the alloy. A discussion of the results is given.