Laves phase fields in Cr–Zr–Nb and Cr–Zr–Hf alloy systems

The phase fields of the Laves phase in the Cr–Zr–Nb and Cr–Zr–Hf alloy systems were investigated at 1573 K. The Laves phase formed in the Cr–Zr–Nb alloy system had a broad off-stoichiometric range in the center of ZrCr₂–NbCr₂ pseudo-binary line, while the Laves phase formed in the Cr–Zr–Hf alloy system had a uniform and limited off-stoichiometric range along ZrCr₂–HfCr₂ pseudo-binary line. The results are discussed, in terms of geometric concept of atomic sizes of the constituent atoms.     

Site preference of early transition metal elements in C15 NbCr2

The site preference of early 3d (Ti, V), 4d (Zr, Mo) and 5d (Hf, Ta, W) transition metal elements in C15 NbCr₂ Laves phase was studied using first-principles calculations. According to the present calculations, at T = 0 K, Zr, Hf and Ta consistently have a preference for the Nb sites in Nb-rich, Cr-rich and stoichiometric NbCr₂, while the site preference of Ti, V, Mo and W varies strongly with alloy composition. Using a statistical–mechanical Wagner–Schottky model based on the canonical ensemble, the finite temperature site occupancy behavior of those transition metal elements in NbCr₂ was further predicted. It was found that the site preference of Ti, V, Mo and W also depends strongly on temperature. The calculated results compare favorably with the experimental measurements using ALCHEMI and synchrotron X-ray diffraction techniques.     

High temperature oxidation characteristics of Nb–10W–XCr alloys

The effect of Cr content on the microstructure and cyclic oxidation behavior of Nb–10W–XCr alloys with four different compositions has been investigated. Experiments were conducted in air at 900 °C and 1300 °C; the oxidation kinetics have been evaluated in terms of weight change per unit area with respect to exposure time. Alloy's microstructure consists of Nb solid solution phase regions surrounded by a network of NbCr₂ Laves phase. A trend of improvement in oxidation resistance with increase of the intermetallic phase is observed at 1300 °C and oxidation kinetics follow a parabolic behavior. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates than alloys with lower Cr content. The oxidation products are a mixture of CrNbO₄, and Nb₂O₅ and the amount of each oxide present in the mixture is related to the intermetallic phase content. Results delineate the influence of microstructure and composition on the oxidation mechanisms of these alloys that represent a promising base for high-temperature intermetallic alloy development.     

Physical metallurgy and mechanical properties of transition-metal Laves phase alloys

This paper provides a comprehensive review of the recent research on the phase stability, point defects, and fracture toughness of AB₂ Laves phases, and on the alloy design of dual-phase alloys based on a soft Cr solid solution reinforced with hard XCr₂ second phases (where X=Nb, Ta and Zr). Anti-site defects were detected on both sides of the stoichiometric composition of NbCr₂, NbCo₂, and NbFe₂, while they were observed only on the Co-rich side of ZrCo₂. Only thermal vacancies were detected in the Laves phase alloys quenched from high temperatures. The room-temperature fracture toughness cannot be effectively improved by increasing thermal vacancy or reducing stacking fault energy through control of phase stability. Microstructures, mechanical properties, and oxidation resistance of dual-phase alloys based on Cr–NbCr₂, Cr–TaCr₂, and Cr–ZrCr₂ were studied as functions of heat treatment and test temperature at temperatures to 1200°C. Among the three alloy systems, Cr–TaCr₂ alloys possess the best combination of mechanical and metallurgical properties for structural use at elevated temperatures.     

Compositional pathways and capillary effects during isothermal precipitation in a nondilute Ni–Al–Cr alloy

For a Ni–5.2Al–14.2Cr at.% alloy with moderate solute supersaturations, the compositional pathways, as measured with atom-probe tomography, during early to later stage γ′(L1₂)-precipitation (R = 0.45–10 nm), aged at 873 K, are discussed in light of a multi-component coarsening model. Employing nondilute thermodynamics, detailed model analyses during quasi-stationary coarsening of the experimental data establish that the γ/γ′ interfacial free-energy is 22–23 ± 7 mJ m⁻². Additionally, solute diffusivities are significantly slower than model estimates. Strong quantitative evidence indicates that an observed γ′-supersaturation of Al results from the Gibbs–Thomson effect, providing the first experimental verification of this phenomenon. The Gibbs–Thomson relationship for a ternary system, as well as differences in measured phase equilibria with CALPHAD assessments, are considered in great detail.     

The mechanical properties and the deformation microstructures of the C15 Laves phase Cr2Nb at high temperatures

Compression tests between 1250 and 1550 °C and 10⁻⁵ and 5 × 10⁻³ s⁻¹ and transmission electron microscopy have been employed to investigate the high temperature mechanical properties and the deformation mechanisms of the C15 Cr₂Nb Laves phase. The stress-peaks in the compression curves during yielding were explained using a mechanism similar to strain aging combined with a low initial density of mobile dislocations. The primary deformation mechanism is slip by extended dislocations with Burgers vector 1/2〈1 1 0〉, whereas twinning is more frequent at 10⁻⁴ s⁻¹. Schmid factor analysis indicated that twinning is more probable in grains oriented so as to have two co-planar twinning systems with high and comparable resolved shear stresses. Twinning produced very anisotropic microstructures. This may be due to synchroshear: a self-pinning mechanism which requires co-operative motion of zonal dislocations.     

Niobium and titanium diffusion in the high niobium-containing Ti–54Al–10Nb alloy

Titanium and niobium diffusion in the ternary Ti–54Al–10Nb alloy (the composition are in at.%) is investigated in an extended temperature interval by the radiotracer technique. The mechanical sectioning was performed by grinding or sputtering in order to measure the tracer diffusivities at higher and lower temperatures, respectively. Almost linear Arrhenius dependencies are established for Nb (Ti) diffusion with the pre-exponential factor and the activation enthalpy of 1.9×10⁻⁵ (4.0×10⁻⁴) m² s⁻¹ and 280 (304) kJ mol⁻¹, respectively. Niobium is a slower diffuser in comparison to Ti in the ternary as well as in the binary (α₂-Ti₃Al and γ-TiAl) titanium aluminides. The heavy alloying of γ-TiAl with 10 at.% niobium, however, enhances significantly both the Ti and the Nb diffusivities. This effect is attributed to an elastic distortion of the L1₀ structure of TiAl induced by the oversized Nb atoms, which may decrease the activation barriers for atomic diffusion.     

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

Cr effects on magnetic and corrosion properties of Fe–Co–Si–B–Nb–Cr bulk glassy alloys with high glass-forming ability

Effects of the Cr addition on glass formation, magnetic and corrosion properties of {[(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05]_0.96Nb_0.04}_100−xCr_x (x = 1, 2, 3, 4 at.%) alloys have been investigated. It was found that the addition of Cr element slightly decreases the glass-forming ability (GFA), but is very effective in increasing corrosion resistance and improving soft magnetic properties for this Fe–Co–B–Si–Nb bulk glassy alloy within the composition range examined. The Fe–Co–B–Si–Nb–Cr alloys exhibit high GFA. Full glassy rods with diameters up to 4 mm can be synthesized by copper mold casting. The Fe-based bulk glassy alloys (BGAs) exhibit a high saturation magnetization of 0.81–0.98 T as well as excellent soft magnetic properties, i.e., extremely low coercive force of 0.6–1.6 A/m and super-high initial permeability of 26,400–34,100. Furthermore, corrosion measurements show that corrosion rate and corrosion current density of these Fe-based BGAs in 0.5 M NaCl solution decrease from 7.0 × 10⁻¹ to 1.6 × 10⁻³ mm/year and 3.9 × 10⁻⁶ to 8.7 × 10⁻⁷ A/cm², respectively, with increasing Cr content from 0 to 4 at.%. The success of synthesizing the new Fe-based BGAs exhibiting simultaneously high GFA as well as excellent good soft magnetic properties combined with high saturation magnetization and enhanced corrosion resistance allows us to expect future progress as a new type of soft magnetic materials.     

High hydrogen permeability in the Nb-rich Nb–Ti–Ni alloy

The microstructure and the hydrogen permeability of the Nb-rich Nb–Ti–Ni alloy, i.e., the Nb₅₆Ti₂₃Ni₂₁ alloy were investigated and compared with those of the Nb₄₀Ti₃₀Ni₃₀ alloy. The Nb₅₆Ti₂₃Ni₂₁ alloy consisted of a combination of the primary phase bcc- (Nb, Ti) solid solution with the eutectic phase {bcc- (Nb, Ti) + B2-TiNi}. The volume fraction of the former and the latter phases were 62 and 38 vol.%, respectively. The Nb₅₆Ti₂₃Ni₂₁ alloy showed the higher Φ value of 3.47 × 10⁻⁸ (mol H₂ m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 1.8 times higher than that of the Nb₄₀Ti₃₀Ni₃₀ alloy, which has been reported to be highest in the Nb–Ti–Ni system. The present work demonstrated that the Nb-rich Nb–Ti–Ni alloys consisting of only the primary phase bcc- (Nb, Ti) and the eutectic phase {bcc- (Nb, Ti) + B2-TiNi} are promising for the hydrogen permeation membrane.     

Study of three-phase equilibrium in the Nb-rich corner of Nb–Si–Cr system

The full understanding of the Nb–Si–Cr ternary system is important for the development of Nb silicide based composites, which show great potential for high temperature applications. There was, however, disparity in experimental observations of phase equilibrium in the vicinity of the Nb-corner. Two kinds of three-phase equilibrium, Nb_ss+C14+αNb₅Si₃ and Nb_ss+CrNbSi+αNb₅Si₃, in the Nb corner of the Nb–Si–Cr system have been reported in the literature. This work aims to clarify the three-phase equilibrium near the Nb-corner, by studying phase equilibrium in the Nb–18Si–15Cr ingot. Such a composition was chosen with the assistance of CALPHAD calculations to avoid unnecessary load of work. The alloy ingot was prepared by clean melting followed by heat treatment at 1000 and 1500 °C. The C14 Laves phase formed in all the samples and was stable at both temperatures. The results confirmed that Si has the effect of stabilising the C14 Laves phase down to at least 1000 °C. The three-phase equilibrium Nb_ss+C14+αNb₅Si₃, instead of Nb_ss+CrNbSi+αNb₅Si₃, was observed in this work. The current work demonstrates that ingot metallurgy is necessary to check the reliability of the information obtained about the ternary and higher-order phase diagrams, especially for the regions where the exact phase boundaries are in question. The investigation of the selected alloy was greatly helpful to clarify the confusion of the three-phase field near the Nb corner in the Nb–Si–Cr ternary system. The work confirmed the CALPHAD prediction of phase equilibrium near the Nb corner, showing the power to combine phase diagram predictions with experimental work for cost effective alloy development.     

Hydrogen permeation and structural features of melt-spun Ni–Nb–Zr amorphous alloys

Amorphous (Ni_0.6Nb_0.4)_100−xZr_x (x = 0, 20, 30, 40 and 50 at.%) alloys were prepared by the melt-spinning technique, and the hydrogen permeation through those alloy membranes was examined. The local atomic structure in these alloys was also investigated by radial distribution function (RDF) analysis. Moreover, hydrogen solubility and diffusivity were also measured in order to discuss the mechanism for hydrogen permeation. The permeability of the Ni–Nb–Zr amorphous alloys increases with Zr content and temperature. The maximum hydrogen permeability is 1.59 × 10⁻⁸ mol m⁻¹ s⁻¹ Pa^−1/2 at 673 K for the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy. The (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed larger hydrogen solubility and diffusivity than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy. As the result, the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed higher hydrogen permeability than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy at 673 K. The RDF analysis shows that the atomic distance between the Zr atoms increases by hydrogenation. The chemical ordering such that the number of Zr coordinates is much higher than that of Ni and Nb coordinates was found in the (Ni_0.6Nb_0.4)₇₀Zr₃₀ and (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloys. The relation between the amorphous local structure and the permeation was discussed in detail.     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

High-field magnetization and specific heat of UCu2T3Al7 alloys where T = Cr,Mn and Fe(II)

The UCu₂T₃Al₇ alloys, where T = Cr, Mn and Fe, crystallize in the ThMn₁₂-type tetragonal structure. Earlier investigation of magnetic and electrical properties revealed their complex magnetic behavior except of the Cr compound, which is paramagnetic, whereas their electrical resistivity is weakly temperature dependent. At present we report on the magnetization measurements at 4.2 and 77 K in the steady magnetic field up to 14 T and in the pulsed field up to 34 T with a pulse duration of 10 ms at T = 4.2 K. These experiments confirmed paramagnetism of the Cr alloy and showed lack of saturation for the compounds of the Mn and Fe with the “saturation” magnetic moment amounting to 3.1 and 5.0 μ_B/f.u., respectively. In turn, the specific heat was measured in the temperature range 1.2–70 K in magnetic field μ₀H = 0 and 7 K using a home-made and fully automatic calorimeter. The investigation of the specific heat at temperature 2–300 K has been done using Quantum Design PPMS machine. The magnetic field does not in principle influence the obtained results, whereas the coefficient of the electronic specific heat, γ is strongly enhanced and amounts to 410, 330 and 150 mJ mol^?1 K^?2 for the Cr, Mn and Fe compounds, respectively.     

Applications of an extended Miedema's model for ternary alloys

The extension of Miedema's semi-empirical model to ternary systems by means of an energy minimization scheme was implemented to demonstrate a number of physical phenomena associated with select ternary alloys. In order to gain a thermodynamic understanding of glass forming ability of Zr based alloys, a combination of extended Miedema's model and lattice strain energies has been invoked. The extended Miedema approach has also been used to study the phase selection during crystallization for amorphous Zr–Cu–Ni alloys. Also extended Miedema's model was used to illustrate its applicability to study the phase stability of Mo–Nb–Si alloys around M₃Si composition (M = Mo, Nb), by predicting the amount of Nb (30 at.%) required to destabilize the structure.     

Structural stability of the Laves phase Cr2Ta in a two-phase Cr–Cr2Ta alloy

Detailed microstructural analysis of a two-phase alloy of composition Cr–9.8 at.% Ta and lying in the Cr–Cr₂Ta region of the Cr–Ta binary system confirmed that the existing phase diagram is inaccurate; in the cast and annealed condition (1273 K/24 h), blocky primary Cr₂Ta precipitates were observed although the phase diagram indicates the eutectic composition to be ∼13 at.% Ta. The eutectic structure is composed of Cr solid solution and the Laves phase Cr₂Ta; the morphology is primarily lamellar although the rod morphology was occasionally observed. The Laves phase eutectic microconstituent exhibits the C14 (2H) hexagonal structure with a low stacking fault (basal faults) density and an average composition corresponding to 28.5 at.% Ta. After a prolonged high-temperature anneal (1573 K/168 h), the morphology breaks down to form discrete particles of Cr₂Ta; the C14, C36 and C15 structures were all recognized in this annealed condition, often more than one form being present in a single precipitate. The C15 structure was not twinned but contained some stacking faults on the {111} planes. Composition measurements confirmed that these structural transformations were accompanied by composition changes, the precipitates becoming more Ta-rich as they transitioned from the C14 via the C36 to the C15 phase. These observations are coupled with the results from earlier studies to present a discussion on factors that influence the stability and C14/C36/C15 transformation kinetics.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Precipitation in Fe–15Cr–1Nb alloys after oxygenation

The effect of O on the phase relations at 950 °C in Fe–15Cr–1Nb alloys is experimentally investigated. Fe–15Cr–1Nb alloys are oxygenated by subjecting high-purity Fe–15Cr–1Nb to an O atmosphere at 600 °C. Both the high-purity and the oxygenated Fe–15Cr–1Nb alloys are heat treated for up to 500 h at 950 °C, quenched and investigated by scanning electron microscopy, transmission electron microscopy and electron probe microanalysis. The results show that Fe₂Nb is in equilibrium with α (Fe, Cr) with 0.29 at.% Nb in solid solution in the pure Fe–15Cr–1Nb alloy. The presence of a small amount of O induces the precipitation of a Fe₆Nb₆O_x phase with a cubic crystal structure and lattice parameter 1.13 nm, thereby decreasing the Nb in solid solution in α (Fe, Cr) with increasing O content.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.