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The mechanical properties of the bcc refractory high-entropy alloy (RHEA) NbMoTaW with columnar and equiaxed microstructures and a nanoscale metal oxide layer on the grain boundaries are investigated at ambient temperatures (RT) to 1200 °C. Under compression, the alloy shows a yield strength, σ y , of ⁓1390 ± 20 MPa at RT and retains a high yield strength, σ y , of ⁓301.5 MPa at 1200 °C. However, in tension, the fracture strengths, σ f , of both columnar and equiaxed microstructures are far lower - below 70 MPa - and the material fails in the elastic regime without showing any measurable ductility at all temperatures. The fracture mechanisms in tension transition from transgranular cleavage at RT to a mixture of transgranular and intergranular fracture at 800 °C to a complete intergranular fracture at 1200 °C due to selective weakening of the metal oxide layer on the grain boundaries driven by a structural change. The transition in the fracture mechanism in the equiaxed microstructure promotes extrinsic toughening at higher temperatures, resulting in a ⁓6.4 times higher fracture toughness (K Ic ⁓10.3 MPa√m) at 1200 °C compared to that at RT. NbMoTaW is a well-known RHEA, but the poor tensile strength and fracture toughness measured in this study highlight the critical importance of investigating the mechanical properties of these high-temperature RHEAs in tension. [Display omitted] [ABSTRACT FROM AUTHOR]

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

Abstract Equilibrium phase diagrams for ten Me X (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, Cr 10 Nb 30 Ti 30 Zr 30 , Ta 10 Nb 30 Ti 30 Zr 30 and Re 10 Nb 30 Ti 30 Zr 30 , 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. Highlights • Mo, Ta, V, Hf and Al have unlimited solubility range in BCC NbTiZr. • Cr, Fe, Re, Si and W have limited solubility in BCC NbTiZr resulting in secondary phases. • Alloying of NbTiZr with Ta and Re improve high-temperature mechanical properties. • Alloying of NbTiZr with Cr increases RT strength but softens the alloy at T ≥ 1000 °C. [ABSTRACT FROM AUTHOR]

Phase composition, microstructure and mechanical properties of six complex concentrated alloys, NbTiZr, NbTiZrV, NbTiZrVMo, NbTiZrVTa, NbTiZrVCr, and NbTiZrVAl 0.24 , are reported. The alloy density was in the range from 6.34 g/cm 3 to 8.43 g/cm 3 . After homogenization annealing at 1400 °C or 1200 °C, only the ternary NbTiZr alloy had a single-phase BCC crystal structure. NbTiZrV and NbTiZrVTa produced predominantly single-phase BCC structure with clusters of fine V-rich precipitates inside NbTiZrV grains and fine, Zr-rich grain boundary precipitates in NbTiZrVTa. The other three alloys contained BCC and Laves phases, and NbTiZrVMo also contained a second BCC phase. After compression deformation at 1000 °C, noticeable changes in the phase composition were found in NbTiZrV, NbTiZrVTa and NbTiZrVAl 0.24 , while the phase composition of other alloys retained unchanged. NbTiZr, NbTiZrV and NbTiZrVTa showed good ductility and noticeable work hardening rate at room temperature, while the ductility of other alloys rapidly decreased with increasing the amount of Laves phase. The RT yield stress varied from 975 MPa for NbTiZ to 1510 MPa for NbTiZrVMo. At 1000 °C, all the alloys showed excellent compression ductility; however only NbTiZrVCr and NbTiZrVMo were stronger than NbTiZr. The results indicate that increasing the composition complexity of refractory near-equiatomic alloys by increasing the number of the alloying elements does not necessary result in a simpler phase structure and/or better mechanical properties. The type of the alloying elements seems to be more important than their number. Additions of V, Al or Ta decrease while additions of Cr or Mo increase the 1000 °C strength of NbTiZr. [ABSTRACT FROM AUTHOR]

Methods of optical and electron microscopies were used to study the structure of particles of niobium carbide in a cast refractory Fe-Cr-Ni-C alloy modified by Nb and Ti. Particles of niobium carbide in the structure of the cast alloy are predominantly multiphase polycrystalline clusters that are inhomogeneous in the chemical composition and crystal structure. The misorientation angle between individual crystals that compose the carbide particles is 30°-60°. The polycrystalline character of carbides is probably associated with significant thermal stresses that arise at the interphase boundaries in the structure of the alloy upon the primary cooling of the ingot. To explain the polymorphism of the cluster of niobium carbide, a further analysis of the structural and geometrical crystallography is required. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties at 25–1200 °C of new refractory alloys, Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA), all compositions are in atomic percent, are reported. The alloys are developed as an alternative to a denser, more expensive and difficult-to-process refractory alloy WC-3009. After production by arc melting and hot isostatic pressing at 1400 °C, 207 MPa for 3 h, the alloys have a BCC crystal structure with the average grain size of ∼150–200 μm and contain low volume fraction of (Ti,N)-rich precipitates near grain boundaries. NTW shows excellent malleability at all studied temperatures. Its yield stress at 25 °C, 1000 °C and 1200 °C is 874 MPa, 353 MPa and 202 MPa, respectively. The NTW-R and NTW-RA alloys are stronger at all the studied temperatures; however, they lack in compression ductility at 25 °C. Extensive deformation twinning and formation of intergranular voids precede the fracture of these two alloys at 25 °C. • Nb-18Ti-12W (NTW), Nb-18Ti-12W-5Re (NTW-R) and Nb-18Ti-12W-5Re-1Al (NTW-RA) have a single-phase BCC crystal structure. • NTW shows excellent malleability and medium strength at 25–1200 °C. • NTW-R and NTW-RA are stronger than NTW at all studied temperatures but they lack in ductility at 25 °C. • Extensive deformation twinning and formation of intergranular voids precede the fracture of NTW-R and NTW-RA at 25 °C. • NTW-R and NTW-RA have excellent combination of strength and ductility at 600-1200°C. [ABSTRACT FROM AUTHOR]

Hf–Ta based alloys have recently been investigated as potential candidates for high-temperature structural applications. While most attentions have been given to the properties above ~1200 °C where the alloys are mainly single-phase BCC structures and have excellent oxidation performance due to formation of super-oxides, structural properties at lower temperatures are equally important for applications in which an alloy may experience a range of temperatures. In the present work, microstructure, phase composition, mechanical properties and oxidation behavior of six Hf-25Ta-5Me alloys (Me is Mo (HTM alloy), W (HTW), 0.5Mo + 0.5 W (HTMW), Cr (HTC) or Zr (HTZ), the compositions are in at.%) are reported at temperatures below the eutectoid transformation. The alloys were prepared by arc melting followed by hot isostatic pressing for 3 h at 1400 °C and 207 MPa. All the alloys display coarse grains of partially or fully transformed (by a eutectoid reaction) high-temperature BCC phase. The eutectoid regions consist of fine lamellae of Hf-rich HCP and Ta-rich BCC phases. The ternary alloys containing W or Cr also contained small amounts of a cubic Laves phase. At 25 °C, the HTW alloy was the strongest (yield stress σ y = 1966 MPa) but brittle and HTZ was the weakest (σ y = 1120 MPa) but ductile among the studied alloys. Other alloys showed intermediate behaviors. In general, the room temperature ductility of the alloys increased with decreasing σ y. All alloys maintained high strength up to 800 °C, but displayed a noticeable strength decrease at 1000 °C. At 1000 °C, HTMW was the strongest alloy (σ y = 468 MPa) and HTC was the weakest alloy (σ y = 368 MPa). All alloys had excellent deformability at 1000 °C. Oxidation behavior of the alloys was studied at 800 °C and 1000 °C and compared with that of Hf–27Ta binary alloy. Although the ternary additions improved oxidation resistance, overall oxidation performance of the studied alloys at 800 °C and 1000 °C was poor. • Hf-25Ta-5Me (Me = Mo, W, Mo+W, Nb, Cr or Zr) alloys have a complex microstructure caused by eutectoid transformation. • The alloys have high strength, exceeding 1000 MPa, at temperatures up to 800 °C. • The strength of the alloys decreases rapidly between 800 °C and 1000 °C. • Interfaces in the eutectoid structures provide a noticeable contribution to strengthening at 25 °C - 800 °C, not at 1000 °C. • Ternary additions to Hf-27Ta improve oxidation resistance at 800 °C and 1000 °C in air, although it is still poor. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure, phase composition and mechanical properties of the AlMo0.5NbTa0.5TiZr and Al0.4Hf0.6NbTaTiZr high-entropy alloys are reported. The AlMo0.5NbTa0.5TiZr alloy consists of two body-centered cubic (bcc) phases with very close lattice parameters, a 1 =326.8pm and a 2 =332.4pm. One phase was enriched with Mo, Nb and Ta and another phase was enriched with Al and Zr. The phases formed nano-lamellae modulated structure inside equiaxed grains. The alloy had a density of ρ =7.40gcm−3 and Vickers hardness H v =5.8GPa. Its yield strength was 2000MPa at 298K and 745MPa at 1273K. The Al0.4Hf0.6NbTaTiZr had a single-phase bcc structure, with the lattice parameter a =336.7pm. This alloy had a density ρ =9.05gcm−3, Vickers microhardness H v =4.9GPa, and its yield strength at 298K and 1273K was 1841MPa and 298MPa, respectively. The properties of these Al-containing alloys were compared with the properties of the parent CrMo0.5NbTa0.5TiZr and HfNbTaTiZr alloys and the beneficial effects from the Al additions on the microstructure and properties were outlined. A thermodynamic calculation of the solidification and equilibrium phase diagrams was conducted for these alloys and the calculated results were compared with the experimental data. [Copyright &y& Elsevier]

Uniaxial tension tests are carried out for the Mo–10wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25°C to 725°C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500°C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500°C. Under uniaxial tension at temperatures higher than 650°C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications. [ABSTRACT FROM AUTHOR]

The structure, the phase composition, the pattern of distribution of alloying elements in the structure, and the long-term strength of refractory alloy 45Kh26N33S2B2 in cast condition are studied. The degree of segregation of chemical elements in the γ-solid solution and in the carbide eutectic and the compositions of the segregated carbides and nitrides are determined. The instability of the cast structure of the studied alloy is explained. Recommendations are developed on application of cast alloy 45Kh26N33S2B2. [ABSTRACT FROM AUTHOR]

Abstract: Room temperature and elevated temperature mechanical properties of four multi-principal element alloys, NbTiVZr, NbTiV2Zr, CrNbTiZr and CrNbTiVZr, are reported. The alloys were prepared by vacuum arc melting followed by hot isostatic pressing and homogenization. Disordered BCC solid solution phases are the major phases in these alloys. The Cr-containing alloys additionally contain an ordered FCC Laves phase. The NbTiVZr and NbTiV2Zr alloys showed good compressive ductility at all studied temperatures while the Cr-containing alloys showed brittle-to-ductile transition occurring somewhere between 298 and 873K. Strong work hardening was observed in the NbTiVZr and NbTiV2Zr alloys during deformation at room temperature. The alloys had yield strengths of 1105MPa and 918MPa, respectively, and their strength continuously increased, exceeding 2000MPa after ∼40% compression strain. The CrNbTiZr and CrNbTiVZr alloys showed high yield strength (1260MPa and 1298MPa, respectively) but low ductility (6% and 3% compression strain) at room temperature. Strain softening and steady state flow were typical during compression deformation of these alloys at temperatures above 873K. In these conditions, the alloys survived 50% compression strain without fracture and their yield strength continuously decreased with an increase in temperature. During deformation at 1273K, the NbTiVZr, NbTiV2Zr, CrNbTIZr, and CrNbTiVZr alloys showed yield strengths of 58MPa, 72MPa, 115MPa and 259MPa, respectively. [Copyright &y& Elsevier]

Abstract: The crystal structure, microstructure, density and Vickers hardness of four multi-principal element alloys, NbTiVZr, NbTiV2Zr, CrNbTiZr, and CrNbTiVZr, are reported. The characteristics of these potential new high-temperature structural alloys are explored. The alloys were prepared by vacuum arc melting followed by hot isostatic pressing (at 1200°C, 207MPa for 1h) and homogenization annealing (at 1200°C for 24h). The alloys have densities of 6.52, 6.34, 6.67 and 6.57gcm−3, and Vickers microhardness values of 3.29, 2.99, 4.10 and 4.72GPa, respectively. The NbTiVZr alloy is essentially a single-phase alloy consisting of a coarse-grained disordered body-centered cubic (bcc) phase with fine, submicron-size precipitates inside the grains. The NbTiV2Zr alloy contains three disordered bcc phases. The CrNbTiZr and CrNbTiVZr alloys contain a disordered bcc phase and an ordered Laves phase. The lattice parameters and compositions of the identified phases are reported. The experimental data are compared with the results of the thermodynamic modeling of non-equilibrium and equilibrium phases in these alloys. [Copyright &y& Elsevier]

Abstract: The present study reports on microstructural evolution upon static annealing treatment and elevated-temperature low-cycle fatigue (LCF) of an ultrafine-grained (UFG) body-centered cubic (bcc) niobium–zirconium (NbZr) alloy, processed by equal channel angular processing (ECAP) at room temperature. UFG NbZr showed recovery and recrystallization at homologous temperatures, which are in the same range as those of another UFG bcc material, i.e. interstitial free (IF) steel. Unlike the UFG IF steel, the UFG NbZr featured a distinct plateau of decreased hardness due to recovery at temperatures below the recrystallization limit. This was attributed to the absence of dynamic recovery during ECAP due to the low homologous temperature of T h =0.11 (T h =0.16 for IF steel) at room temperature processing. Strain-controlled elevated-temperature LCF tests performed in vacuum revealed stable cyclic deformation response up to 600°C (T h =0.32). At higher temperatures, but still below the static recrystallization limit (≈900°C, T h =0.43), cyclic softening, rapid decrease of mean stress and premature failure were observed. As compared to the UFG IF steel, cyclic stability is preserved up to higher T h due to the stabilizing effect of solid solution alloying elements, i.e. mainly Zr. In the case of the UFG IF steel, localized grain coarsening at the crack tip caused premature failure upon elevated-temperature LCF below the static recrystallization temperature. The more stable microstructure in the UFG NbZr did not show any localized alterations in the vicinity of the crack tip, but instead slightly coarsened throughout the whole gauge length. In combination with the results obtained on the UFG IF steel in previous studies, a comprehensive summary of the microstructural evolution of UFG bcc materials at elevated temperatures is presented. [Copyright &y& Elsevier]

Microstructure and compression properties of (Hf 0.73 Ta 0.27) 100-X Mo X alloys, where X = 0, 5, 16, 21 and 30 at. %, are reported. The alloys were prepared by vacuum arc melting and hot isostatically pressed at 207 MPa for 3 h at 1400 °C. The alloys with 0, 5, 16 and 21 at. % Mo contained two phases, BCC and HCP, while the alloy with 30 at. % Mo contained three phases, BCC, HCP and cubic Laves (C15). The Hf 73 Ta 27 , Hf 69 Ta 26 Mo 5 and Hf 61 Ta 23 Mo 16 alloys experienced eutectoid transformation with the formation of a mixture of Hf-rich HCP and Ta-rich BCC phases. The volume fraction of the eutectoid regions decreased from 55% to 0% with increasing Mo from 0 to 21 at.%. The alloy with 30% Mo (Hf 51 Ta 19 Mo 30) experienced eutectoid transformation with the formation of a mixture of the Mo-rich cubic Laves (C15) phase and Hf-rich HCP phase. The volume fraction of the eutectoidally transformed regions in Hf 51 Ta 19 Mo 30 was ~20%. Among the studied alloys, Hf 73 Ta 27 had the highest room temperature yield stress of 1738 MPa. The yield stress continuously decreased with increasing Mo concentration to 1468 MPa at 16% Mo and then increased to 1672 MPa at 30% Mo. The room temperature ductility strongly depended on the amount of Mo. The maximum true fracture strain of 0.137 was achieved in Hf 69 Ta 26 Mo 5 , but it rapidly decreased to 0.027 in Hf 51 Ta 19 Mo 30. All the studied alloys were ductile at 1000–1400 °C and no fracture was observed after 0.7 true strain. The yield stress increased almost linearly with increasing the amount of Mo at these high temperatures. • Hf73Ta27, Hf69Ta26Mo5 and Hf61Ta23Mo16 are two-phase alloys consisting of Ta and Mo rich BCC and Hf-rich HCP phases. • Additions of Mo stabilizes and increases the volume fraction of the BCC phase in these three alloys. • Hf58Ta21Mo21 and Hf51Ta19Mo30 contain three phases: Ta-rich BCC, Hf-rich HCP and Mo-rich cubic Laves phases. • The HCP and Laves phases are present inside eutectoidally transformed areas in Hf51Ta19Mo30. • Hf73Ta27 is the strongest at 25 °C but the softest at T ≥ 1000 °C among the studied alloys. • At 25 °C, Hf61Ta23Mo16 has minimum true yield and peak stresses. • At 1000 °C, 1200 °C and 1400 °C, yield stress increases linearly with increasing the concentration of Mo. [ABSTRACT FROM AUTHOR]

[Display omitted] Due to the vast compositional space of multi-principal element alloys (MPEAs), the rational design of MPEAs for optimized microstructures is difficult. Therefore, a high-throughput first-principles study of Mo-V-Nb-Ti-Zr, a refractory MPEA, was conducted to gain insights into the underlying microstructures. Using Monte-Carlo simulations powered by cluster expansion, we uncover the principles governing the MPEA's microstructures across a large compositional space that includes non-equiatomic compositions and encompasses the constituent binaries, ternaries, and quaternaries. In the spirit of Hume-Rothery rules for complete solid solubility, we present a quantitative expression for predicting solid solution formation from the composition. Within a consistent framework, our results reproduce the microstructural observations (solid solution vs. segregation) from numerous experiments and provide microstructural predictions for unexplored regions in the compositional space. Our work illuminates the MPEA's microstructures in terms of the separation and clustering tendencies of the elements, presenting a set of simple but powerful design principles for future experiments to rationally design MPEAs with the desired microstructures for superior mechanical properties. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laser-processed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

• The LDEDed technique successfully fabricates 90W with fine W particles and excellent mechanical properties. • The microstructure and mechanical properties exist heterogeneities along the building direction. • The crack initiations of LDEDed 90W mainly originate at the W-M interface and some micro-voids. Additive manufacturing provides a novel route to manufacture tungsten-based alloy components with complex geometries. In this study, a laser directed energy deposition (LDED) additive manufacturing technique is introduced to fabricate 90W-7Ni-3Fe alloy, and the microstructure and mechanical properties are systematically investigated. To facilitate comparison with conventional processing methods, the 90W-7Ni-3Fe alloy is also processed by liquid phase sintering (LPS). The results show that the microstructure of LDEDed samples consisting of matrix (M) phase and W particles phase exhibits significant heterogeneity along the building direction, which is related to the complicated thermal history experienced. As a result, the tensile properties also show a variation at different build heights. The tensile strengths in the bottom and middle regions are better to the top and comparable to the LPSed samples, but overall the plasticity is poor. The tensile fracture shows a mixed fracture characteristic (i.e. W particle cleavage fracture, W-M interfacial debonding, matrix phase rupture, and W-W inter particle fracture), and crack initiations mostly originate from the W-M interface and micro-voids. Besides, benefitting from the higher W solubility in the matrix and smaller W grain size, the hardness of the matrix phase and W particles in the LDEDed samples are higher than those in the LPSed samples. The fundamental understanding will provide a theoretical basis for future fabrication and post treatment to enhance the mechanical properties of LDEDed tungsten-based alloys. [ABSTRACT FROM AUTHOR]

• The oxidation behaviour of pesting-resistant Mo-20Si-52.8Ti was investigated. • The scale forms slowly at 800 °C: cubic kinetics following linear kinetics. • It is composed of a Ti(+IV)-oxide and SiO2 islands. • The scale comprises strongly off-stoichiometric Ti- and Si-oxide at 1200 °C. • Here, diffusion-controlled parabolic kinetics prevail. The oxidation behaviour of the pesting-resistant eutectic alloy Mo-20Si-52.8Ti was investigated by addressing the microstructural and chemical constitution by grazing incidence X-ray diffraction GIXRD and atom probe tomography APT of oxide scales formed at 800 and 1200 °C in order to understand their protective character. The duplex Si-Ti-oxide scale formed at 800 °C in air, i.e. within the so-called " pesting regime ", is considerably slow growing with a scale thickness of (11 ± 3) μm after 1000 h of exposure. It is composed of a Ti(+IV)-oxide with the chemical composition of approximately TiO 2.1 confirmed by APT and SiO 2 islands, which are most likely amorphous. The oxide scale formed at 1200 °C, a potential application temperature, is characterised by a top single-phase scale and an underlying duplex scale with a considerably increased thickness of (68 ± 11) μm in total. It consists of rutile TiO 2 and some cristobalite SiO 2 as determined by GIXRD. APT analysis of the top Ti-oxide scale confirms the stoichiometry TiO 2.2. The underlying duplex scale comprising Ti- and Si-oxide is characterised by strongly off-stoichiometric compounds, namely TiO 1.4 and SiO 1.6. Initial linear oxide scale growth kinetics are found at 800 °C, which change to cubic kinetics for exposure times longer than 100 h. At higher temperatures of 1100 and 1200 °C, diffusion-controlled, almost parabolic kinetics prevail. [ABSTRACT FROM AUTHOR]

Phase equilibria in the Nb–Ni–Si ternary system were experimentally determined at 1323 K and 1473 K using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) techniques. The 1323 K isothermal section of the phase diagram was proposed in combination with previous knowledge of phase stabilities in the system at 1073 K. The results at 1473 K, on the other hand, positively allowed the identification of three new intermetallics phases, not present at lower temperatures, which still need to be crystallographically characterized. The new phases are found in equilibrium either with the Nb 5 Si 3 compound or with the Nb-rich bcc solid solution, which are characterized by very high melting temperatures. • Phase equilibria in Nb–Ni – Si were determined at 1323 K and 1473 K. • Compound NbNi3 dissolves Si and G phase is not in equilibrium with Ni. • Coupound known as H phase was attested both at 1323 K and 1473 K. • Compound known as T phase is a line compound. • Three new Nb-rich compounds stable at 1473 K were identified. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of five Nb alloys containing 8 to 17 at.% Mo, 8 or 35 at.% Zr, up to 7 at.% Ti, up to 2 at.% Al and up to 2 at.% Cr are reported. These alloys have been developed to replace heavy, expensive and difficult to process commercial Nb alloys, such as C-3009, for use at temperatures up to 1600 °C. The density of the alloys is in the range from 7.6 to 8.6 g/cm3. The alloys have a BCC matrix phase, and they also contain small amounts of secondary phases, which are rich in Zr and have BCC, FCC, hexagonal or monoclinic crystal structures depending on the concentration of other alloying elements, including oxygen and nitrogen. In the temperature range from 25 °C to 1600 °C, the alloys with a smaller amount of Zr are ductile and have higher specific strength than C-3009. The alloy containing 35 at.% Zr is stronger, but less ductile, than other alloys at temperatures up to 600 °C; however, it loses the strength rapidly at higher temperatures and becomes softer than other alloys at T > 1000 °C. The possible strengthening mechanisms responsible for the observed temperature dependence of the yield stress of the alloys are also discussed. • Microstructure and mechanical properties of five Nb alloys containing Mo, Zr, Al and/or Cr are reported. • The alloys have low density, in the range of 7.6 to 8.6 g/cm3. • The alloys are essentially single-phase BCC structures, with small amount of Zr-rich secondary phases. • Four reported alloys have excellent malleability and specific strength superior to C-3009 at temperatures from 25 °C to 1600 °C [ABSTRACT FROM AUTHOR]

Two Nb-Mo-Ti alloys were modified by additions of 2 at.% Fe. These small additions of Fe did not form secondary phases but noticeably increased the strength of the Nb-Mo-Ti alloys in the temperature range of 25 to 1200 °C due to solid solution strengthening effect. The oxidation resistance at 1200 °C in air was also improved considering both oxidation kinetics and the fraction of retained, un-oxidized metal alloy. • Nb-Mo-Ti-Fe alloys with single-phase BCC structures and densities of ~7.8 g/cm3 are reported. • Additions of 2 at.% Fe considerably increase strength of the alloys at temperatures from 25°C to 1200°C. • Nb-Mo-Ti-Fe alloys have good combination of strength and ductility. • The properties also depend on the amount of interstitial elements. • The alloys have slightly improved oxidation behavior relative to conventional refractory alloys. [ABSTRACT FROM AUTHOR]

The examined samples were W-Re and Mo-Re thin alloy layers with high content of Rhenium, applied in industry as protective coatings. The 550 nm of W-50.4%Re and 580 nm of Mo-39.5%Re alloys were deposited on crystalline silicon and amorphous silica substrates by magnetron sputtering method. For thermal characterization of investigated layers the scanning thermal microscopy (SThM) and high frequency photothermal radiometry (HF-PTR) measurements were carried out. The thermal methods were supported by the microstructural studies performed by the atomic force microscopy (AFM) and X-ray diffraction (XRD). The SThM method allowed determining the thermal conductivity (κ) of the alloy layers. From the HF-PTR the effective thermal conductivity (κ eff) was determined directly. Estimation of the thermal boundary resistance (R th) between particular alloy layer and its substrate enabled determination of the real κ of the layer with its thermal diffusivity (α) and effusivity (ε) from the HF-PTR fitting. The results showed, that W-Re and Mo-Re layers deposited on Si are characterized by higher κ , α , and ε values, and lower R th in comparison to those deposited on SiO 2. The κ values for W-Re and Mo-Re layers deposited on Si and SiO 2 substrates were estimated in the range from 7 W·m−1 K−1 to 12 W·m−1 K−1, and 1.6 W·m−1 K−1 to 4 W·m−1 K−1, respectively. The correlation between the alloy layer structure, it's thermophysical properties and substrate structure was observed. The AFM and XRD measurements confirmed the short range order formation in Mo-Re layers deposited on Si. • Thermophysical characterization of W-Re and Mo-Re layers deposited on Si and SiO 2. • Thermal conductivity determination by SThM and PTR methods. • Thermal diffusivity and effusivity determination of W-Re and Mo-Re films. • Estimation of interfacial thermal boundary resistance. • Influence of different substrates on thermophysical properties of W-Re and Mo-Re. [ABSTRACT FROM AUTHOR]

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. • Nb–Mo–Ti–W alloys with single-phase BCC structures and densities from 7.7 to 8.9 g/cm3 are reported. • Increasing the amount of Mo and W increases high-temperature strength of the alloys. • Nb–Mo–Ti alloys have good combination of strength and ductility. • W-containing alloys are stronger at high temperatures, but they have limited ductility. • Some of the reported alloys are candidates to replace currently used dense and more expensive Nb-based alloys. [ABSTRACT FROM AUTHOR]