Hf Alloys Research Articles

Increase in the Strength and Electrical Conductivity of a Cu–0.8Hf Alloy after Rotary Swaging and Subsequent Aging

Increase in the Strength and Electrical Conductivity of a Cu–0.8Hf Alloy after Rotary Swaging and Subsequent Aging

First-Principles Study of Structural Stability and Mechanical Properties of Ta–W–Hf Alloys

In order to obtain the effect of W and Hf elements on the mechanical properties of Ta–W–Hf alloys, the structural, mechanical, and electronic properties of Ta–xW–6.25Hf (x = 6.25, 12.50, 18.75, 25.00, 31.25, 50.00) alloys were studied using first-principles calculation based on density functional theory, and the supercell method. The calculated formation enthalpy and elastic constants clarify that Ta–W–Hf alloys have structural and dynamical stability. The formation enthalpy and the cohesive energy decrease with the increase in W content, and the cohesive energy increases when Hf element is added to Ta–W alloy. In addition, bulk modulus (B), shear modulus (G), and Young’s modulus (E) for each of the Ta–xW alloys increase gradually with the increase in W concentration. The B, G, and E of Ta–xW–6.25Hf alloys is lower than that of Ta–xW alloy under the same W content conditions, suggesting that Hf alloying with higher Ta–W concentration becomes softer than the Ta–W alloy. Based on the mechanical characteristic, the B/G and Poisson’s ratio of Ta–W–Hf alloys are higher than those of Ta–W alloys with W content over 25%, the ductility of Ta–W–Hf alloys improves with the addition of Hf, and Hf can reduce the anisotropy of Ta–W–Hf alloys. Furthermore, the electronic density of states shows that alloying W and Hf improves the metallicity of Ta. The results in this work provide the underlying insights needed to guide the design of Ta–W–Hf alloys with excellent mechanical properties.

Twinning-induced plasticity (TWIP) effect in multi-phase metastable beta Zr-Nb alloys

• Twinning-induced plasticity (TWIP) strategy was first applied in Zr alloys for strain-hardening and ductility enhancements. • Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf alloys designed by electron-to-atom ratio exhibit multi-phase microstructures consisting of acicular body-centered tetragonal phase and monoclinic phase in beta grains. • TWIP-enhanced Zr-13.8Nb-0.7Hf alloy resulted in superior strain-hardening rate and ductility when comparing to Zr-15.5Nb-0.7Hf without mechanical twinning. • In-situ traction tests in TEM and SEM-EBSD were conducted to clarify the {332}<113> twinning operation and its interaction with body-centered tetragonal phase. The deformation mechanisms were studied in metastable Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf (wt.%) alloys exhibiting complex microstructures due to quenched-in precipitates (acicular body-centered tetragonal phase and globular monoclinic phase) in beta grains. Twinning-induced plasticity (TWIP) effect was observed in Zr-13.8Nb-0.7Hf via {332}<113> twinning operation, resulting in higher strain-hardening rate and uniform elongation than those of Zr-15.5Nb-0.7Hf deforming via only dislocation glide. In-situ characterizations, by applying tensile deformation during electron backscattered diffraction (EBSD) mapping and transmission electron microscopy (TEM) observation, were conducted to clarify the twinning process and the role of quenched-in precipitates when interacting with deformation twins.

APC Nb3Sn superconductors based on internal oxidation of Nb–Ta–Hf alloys

In the last few years, a new type of Nb3Sn superconducting composite, containing a high density of artificial pinning centers (APC) generated via an internal oxidation approach, has demonstrated a significantly superior performance relative to present, state-of-the-art commercial Nb3Sn conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr alloy. On the other hand, our recent studies have shown that internal oxidation of Nb–Ta–Hf alloys can also lead to dramatic improvements in Nb3Sn performance. In this work we follow up on this latter approach, fabricating a 61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy, and compare its critical current density (J c) and irreversibility field with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to improve the filamentary design of APC wires in order to improve their wire quality and electromagnetic stability. Our new modifications have led to significantly improved residual resistivity ratio and stability in the conductors, while still keeping non-Cu J c at or above the conductor J c specification required by the proposed Future Circular Collider. Further improvement via optimization of the wire recipe and design is ongoing. Finally, additional work needed to make APC conductors ready for applications in magnets is discussed.

Studies of the effect of Hf doping on the electrochemical performance of C15 Laves type metal hydride battery anode alloys

This paper focuses on the studies of the electrochemical performance of arc-melted Hf-modified Ti-Zr based AB2 Laves type metal hydride battery anode alloys with composition HfxZr24-xTi6.5V3.9Mn22.2Fe3.8Ni38La0.3Sn0.3 (x = 1–4). The phase composition and the morphology of the alloys were characterized by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Metal-gas interactions and electrochemical performances were studied by building PCT diagrams and by electrochemical tests performed in a 9 N KOH electrolyte solution at ambient temperature when using the half-cells in a three-electrode assembly.All studied alloys were two-phase and contained the Laves phase intermetallics of C15 (major constituent) and C14 (minor constituent) types. An increase in Hf content caused a growth of the abundance of the C14 phase and was accompanied by a decrease in the electrochemical discharge capacities, exchange current densities, and H diffusion rates as compared to the Hf-free alloy. Among the Hf-containing alloys, an introduction of 2 wt% Hf into the alloy's composition resulted in a higher H diffusion rate, while the highest H storage capacity has been reached for the alloy with the lowest, 1 wt%, content of Hf. The surface of the alloys contained catalysing hydrogen exchange metallic nickel clusters with the highest enrichment observed for the 2 wt% Hf alloy. An influence of Hf on the hydrogen storage and electrochemical performance can be understood in terms of the modification of the intrinsic properties of the alloys following a substitution (Zr/Ti) → Hf as C15 type alloys show the fastest hydrogen diffusion rates while a gradually increasing content of the hexagonal C14 type in the Hf-containing alloys slows down the H diffusion process.

Effect of Sc, Hf, and Yb Additions on Superplasticity of a Fine-Grained Al-0.4%Zr Alloy

This research was undertaken to study the way deformation behaves in ultrafine-grained (UFG)-conducting Al-Zr alloys doped with Sc, Hf, and Yb. All in all, eight alloys were studied with zirconium partially replaced by Sc, Hf, and/or Yb. Doping elements (X = Zr, Sc, Hf, Yb) in the alloys totaled 0.4 wt.%. The choice of doping elements was conditioned by the possible precipitation of Al3X particles with L12 structure in the course of annealing these alloys. Such particles provide higher thermal stability of a nonequilibrium UFG microstructure. Initial coarse-grained samples were obtained by induction casting. A UFG microstructure in the alloys was formed by equal-channel angular pressing (ECAP) at 225 °C. Superplasticity tests were carried out at temperatures ranging from 300 to 500 °C and strain rates varying between 3.3 × 10−4 and 3.3 × 10−1 s−1. The highest values of elongation to failure are observed in Sc-doped alloys. A UFG Al-0.2%Zr-0.1%Sc-0.1%Hf alloy has maximum ductility: at 450 °C and a strain rate of 3.3 × 10−3 s−1, relative elongation to failure reaches 765%. At the onset of superplasticity, stress (σ)–strain (ε) curves are characterized by a stage of homogeneous (uniform) strain and a long stage of localized plastic flow. The dependence of homogeneous (uniform) strain (εeq) on test temperature in UFG Sc-doped alloys is increasing uniformly, which is not the case for other UFG alloys, with εeq(T) dependence peaking at 350–400 °C. The strain rate sensitivity coefficient of flow stress m is small and does not exceed 0.26–0.3 at 400–500 °C. In UFG alloys containing no Sc, the m coefficient is observed to go down to 0.12–0.18 at 500 °C. It has been suggested that lower m values are driven by intensive grain growth and pore formation in large Al3X particles, which develop specifically at an ingot crystallization stage.

Creep behavior of 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy under constant load

In the last three decades, aerospace industry has required shape memory alloys with higher phase transformation temperatures. In response to this technical challenge, High Temperature Shape Memory Alloys that can be actuated at higher temperatures were developed. However, high temperature actuation applications also lead to observe much higher dislocation motion and thus plastic deformation due to creep and phase transformation. In this study, 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy was used for the investigation of creep deformation under tensile loads corresponding to stress magnitudes between 200 MPa and 500 MPa. Deformation behavior of the alloy in austenitic phase was investigated by means of creep deformation experiments at 450 °C, 550 °C, and 600 °C. 50at%Ni 25at%Ti 25at%Hf alloy has similar creep behavior as NiTi despite higher loading conditions. Almost no noticeable creep strain values were observed under 200 MPa and 300 MPa at 550 °C and 600 °C. Creep parameters such as the stress exponent and the activation energy values were calculated using power law creep model and viscous glide creep was found to be the main mechanism in 50at%Ni 25at%Ti 25at%Hf high temperature shape memory alloy.

Effect of Copper on the Formation of L12 Intermetallic Phases in Al–Cu–X (X = Ti, Zr, Hf) Alloys

Transition metal trialuminides of the Al3X type of groups 4 and 5 of the periodic system have reduced density, high melting points, and corrosion resistance. Aluminides with a cubic lattice of the Al3Sc type can be used as a nucleating phase for aluminum alloys. However, low plasticity and a tetragonal lattice limit their application. In this work, we stabilized the metastable cubic lattice of Al3X-type aluminides by replacing aluminum with copper. The conditions for the formation of L12 metastable aluminides in the Al–Cu–TM (TM: Ti, Zr, Hf) alloys were studied using a wide range of copper concentrations. A high concentration of copper (hypereutectic alloys) is the one of the necessary conditions for the formation of (Al1−xCux)3Ti, (Al1−xCux)3Zr, (Al1−xCux)3Hf aluminides. With an increase in the copper concentration, the number of metastable aluminides sharply increased. The process of their formation strongly depended on the sequence of dissolution of the corresponding components in the melts. The low volume fraction of precipitated titanium aluminides was the result of insufficient supersaturation of α-Al with titanium (at the peritectic temperature) compared to that for alloys with zirconium and hafnium. Under identical synthesis conditions in the crystal lattice of metastable aluminides formed in experimental Al–Cu–Ti, Al–Cu–Zr, Al–Cu–Hf alloys, copper was found to substitute up to 8, 10, and 13 at.% of aluminum, respectively. The crystallographic and dimensional similarities of the lattices in metastable transition metal aluminides and in α-Al suggest their usefulness as modifying additions in aluminum-based alloys.

Microstructure evolution of Ni-rich NiTi(-Hf) alloy during thermal oxidation at different temperatures

The oxidation behavior of Ni-rich NiTi(-Hf) alloy at the temperature range from 400 to 950 °C is investigated in this paper. For Ni-rich NiTi alloy, the results show that the oxidation kinetics follow the logarithmic law at 400 °C and obey the parabolic law from 500 to 950 °C. After high-temperature oxidation, the observed multi-layered scale structure consists of TiO2+Ti2O3+NiTiO3, Ni(Ti)+Ti2O3+rich Ti, and Ni3Ti from outside to the inside. The addition of the Hf element improves the high-temperature oxidation resistance. A schematic oxidation mechanism of Ni-rich NiTi(-Hf) alloy is proposed to explain the observed results.

Local lattice distortions, phase stability, and mechanical properties of NbMoTaWHfx alloys: A combined theoretical and experimental study

Refractory high-entropy alloys (RHEAs) are of growing interest due to their potentially superior mechanical performances at elevated temperatures. Inherent lattice distortions are believed to be a major contributor to strength in solid solution phases of RHEAs. Here, we investigate the NbMoTaWHfx alloy series (x = 0, 0.27, 0.57, 0.92, 1.33, 1.82) using first-principles simulations, thermodynamic modeling, and experimental techniques. The first-principles results suggest that Hf alloying is an effective means to enhance atomic-scale lattice distortions in BCC NbMoTaWHfx solid solutions. X-ray diffraction on prepared as-cast samples shows that the alloys with Hf content x≤0.92 are single phase BCC alloys, whereas a dual BCC phase microstructure is observed for x = 1.33 and 1.82. Elemental mappings from scanning electron microscopy for the dual-phase alloys are checked with predictions from thermodynamic modeling for equilibrium and non-equilibrium solidifications. Room-temperature compressive mechanical tests reveal that yield and ultimate strengths increase strongly with the addition of Hf and saturate for x>0.92, whereas the compressive plasticity is slightly improved by Hf but remains limited. We predict the compositional effects on poly-crystal elastic moduli for the constituent BCC phases and the dual-phase composites and find a linear behavior between modulus-normalized yield strength and lattice distortion on average.

Effect of hafnium addition on microstructure and room temperature mechanical properties of the Ti-48Al-2Cr-2Nb intermetallic alloy

Microstructures, phase transformations and mechanical properties of the Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated. The alloys were fabricated by vacuum arc remelting followed by hot isostatic pressing and homogenization treatment. The results showed that Hf alloying leads to a significant microstructure refinement in terms of both colony size and inter-lamellar spacing. Homogenization at 1400 °C resulted in a fully lamellar (FL) microstructure in 4822 and 2Hf alloys, while nearly lamellar (NL) in 4Hf and 6Hf alloys. Differential thermal analysis (DTA) demonstrated that Hf addition up to 2 at% has a slight contribution to phase transition sequences, but a significant implication to the phase equilibrium of the alloys with further Hf content. Based on the DTA data and annealing at 1450 °C, the solvus temperature of the eutectic phases was estimated to be over the range of 1430–1440 °C. Although the eutectic phases of Al3Hf2 and TiAl2 formed during solidification of the high-Hf alloys did not undergo any phase transition, both size and volume fraction of the eutectic cells increased due to the solvus of these metastable eutectic phases. The orientation relationship {111}Tetragonal//{001}Orthorhombic detected between the eutectic phases and their surrounding matrix confirmed the occurrence of γc (TiAl) → eutectics (Al3Hf2, TiAl2) phase transformation. Small punch tests results showed that the 2Hf and 4Hf alloys exhibit a higher maximum load (Fm) than the base 4822 alloy due to the solid solution effect of Hf and finer inter-lamellar spacing. Nevertheless, the brittle behavior of the eutectic phases dramatically deteriorated mechanical performance such that the 6Hf alloy possessed the lowest Fm and displacement. The major fracture mode changed from trans-lamellar to inter-lamellar as the Hf content increased from 4 to 6 at%.

Effect of Hf on the microstructure and martensitic transformation behavior in Ti–Pd–Hf alloy

We investigated the microstructure and the crystallography of martensite variants in Ti–Pd–Hf alloy using electron microscopy. The martensitic transformation temperature decreased with increasing amount of Hf substituted into the Ti50−xPd50Hfx alloy. No indication of a martensitic transformation was observed in the as-quenched Ti25Pd50Hf25 and Ti10Pd50Hf40 alloys, even when the samples were cooled to 150 K. The Ti40Pd50Hf10, Ti35Pd50Hf15, and Ti30Pd50Hf20 alloys were composed of a B19 orthorhombic phase having plate-like habit-plane variants consisting of both major and minor correspondence variants several hundred nanometers in width and several tens of micrometers in length at room temperature. Lattice-invariant shear of these alloys was a {111}B19 Type I twin. A solution-treated sample of the Ti25Pd50Hf25 alloy consisted of a B2 cubic matrix with local atomic displacement and lattice distortions, and rounded H-phase particles several tens of nanometers in diameter were precipitated inside the B2 matrix upon aging of the specimen at 823 K for 10.8 ks. The as-quenched Ti10Pd50Hf40 alloy contained rounded H-phase particles several tens of nanometers in diameter embedded in a B2 matrix at room temperature. The martensitic transformation temperature in the Ti50−xPd50Hfx alloy was drastically decreased by both the fully coherent nanosized H-phase particles and the short-range order originating from clustering of solute atoms.

The role of temperature in the microstructural evolution of HPT-processed NiTiHf high-temperature shape memory alloy

Ni50.3Ti29.7Hf20 alloy was subjected to high-pressure torsion (HPT) process at room temperature (RT) and 300 °C. The mix of amorphous phase and nanocrystalline debris was found by TEM observation in sample processed at RT. Two types of structure were clearly distinguished after HPT at 300 °C: the first one is martensite-like plates oriented in single direction; the second is nanocrystalline structure with narrow amorphous bands. The fraction of amorphous phase is significantly larger after RT deformation.

Surface tension measurement of Ti–Zr and Ti–Hf alloys at high-temperature by the pendant drop method

High-temperature alloy melts introduced into carbon/carbon (C/C) composites by reactive melt infiltration (RMI) to form ultra-high-temperature ceramics (UHTCs) is currently an important method for improving their ablation resistance. As a fundamental thermo-property, surface tension has an important impact on the infiltration of alloy melts into the C/C composites. However, the surface tension of binary alloy systems, such as Ti–Zr and Ti–Hf, used for RMI has been little studied. In this work, experimental measurements through the pendant drop method and theoretical calculations of the surface tension of Ti–Zr and Ti–Hf alloys with different compositions have been carried out. Meanwhile, inevitably, impurities are introduced during the determination of surface tension at high temperatures, mainly carbon and oxygen. The effect of impurities on the phase composition, distribution, and surface tension of the alloys is investigated. The results demonstrated that the experimental measurements are in good agreement with the theoretical values of Butler's equation. The surface tension of Ti–Zr and Ti–Hf alloys increases with the content of Zr or Hf at the same temperature. Small amounts of oxygen are mainly dissolved in the lattice and have no significant effect on the surface tension. Carbon slightly decreases the surface tension of the alloy.

The influence of Hf content on oxide scale microstructure and HfO2 formation mechanisms in two-phase (γ' + β) Ni-Al alloys

Hafnium is often doped into alumina-forming alloys to enhance their oxidation resistance, though the effect varies with the doping amount. In this paper, two-phase (γ' + β) Ni-30Al alloys with different Hf contents were prepared and the cyclic oxidation behaviour at 1100 °C was investigated. The oxidation rate increased remarkably with the increase of Hf content, because excessive Hf leads to severe internal oxidation. And it is interesting to note that oxide scales that formed on the alloys exhibited very different microstructures. On the Ni-30Al-0.1Hf alloy the scale mainly consisted of single α-Al2O3. But on the Hf over-doped alloys (Hf > 0.1 at.%), the scales microstructure varied with locations. The scale in-situ formed on β phase consisted of an outer spinel layer and an inner α-Al2O3 layer; while, the one formed on γ' phase consisted of an outmost NiO layer, a middle spinel layer dispersed with fine HfO2 particles and an inner α-Al2O3 layer. Moreover, Hf-rich oxide pegs were prone to be formed at scale/alloy interface on original γ' phase. However, when Hf content reached to 2 at.%, the pegs were also found at scale/alloy interface on original β phase. The different oxidation behaviour and peg formation mechanisms of the alloys are discussed.

Исследование распада пересыщенного твердого раствора в высокомагниевых алюминиевых сплавах со скандием, легированных гафнием

Magnesium-rich aluminum alloys with small scandium additives are widely used in many branches of modern industry due to the high level of their mechanical properties. However, the issue of low thermal stability of Al3Sc particles, which does not allow performing deformation processing of this group of alloys at a temperature above 400 °С, continues to be relevant. Hafnium addition can become one of the ways to solve this problem as hafnium forms a shell around the Al3Sc particles and, due to the low diffusion coefficient in the aluminum matrix, reduces their coagulation rate. The paper studies the influence of addition of 0.2 % and 0.5 % Hf on the electrical conductivity and the process of supersaturated solid solution decomposition, as well as on the size and quantity of nanoparticles in the 1570 magnesium-rich aluminum alloy at its thermal treatment. The authors studied the kinetics of supersaturated solid solution decomposition in the 1570, 1570–0.2Hf, and 1570–0.5Hf alloys by the electrical conductivity measuring and constructed C-curves describing the supersaturated solid solution decomposition in the studied alloys in the temperature range of 260–440 °С. Besides, using transmission electron microscopy, the strengthening nanoparticles of the 1570 and 1570–0.5Hf alloys were studied during heating to 370 °C and 4-hour soaking. The study showed that hafnium addition significantly slows down the supersaturated solid solution decomposition in the 1570 alloy. The authors identified that in the alloys with hafnium additives, the supersaturated solid solution decomposition is the most intense at a temperature of 350 °С, and in the alloys without hafnium – at a temperature of 430 °С. The transmission microscopy data confirm that the 1570 alloy without hafnium contains 3–4.5 times more nanoparticles than the 1570–0.5Hf alloy.

Effect of Hf on the Microstructure and Martensitic Transformation Behavior in Ti–Pd–Hf Alloy

Effect of Hf on the Microstructure and Martensitic Transformation Behavior in Ti–Pd–Hf Alloy

Mixing enthalpy of the Co–Ti–Hf liquid alloys and regularities of the function change in the row of the ternary (Co, Ni, Cu)–Ti–Hf glass-forming melts

The partial molar mixing enthalpies of titanium and hafnium in liquid Co–Ti–Hf alloys were studied at 1900 K by high-temperature isoperibolic calorimetry along xCo/xHf = 3 and xCo/xHf = 3 sections. The partial mixing enthalpies of undercooled liquid titanium and hafnium in liquid alloys and the integral mixing enthalpy are negative over the studied composition range. The thermodynamic mixing functions of liquid (Co, Ni, Cu)–Ti–Hf alloys were modeled in the framework of the associate solution model. Regularities in the change of thermodynamic mixing functions in liquid glass-forming (Co, Ni, Cu)–Ti–Hf alloys are discussed considering donor–acceptor interaction of the components in the melts.

Investigation of Thermal Stability of Microstructure and Mechanical Properties of Bimetallic Fine-Grained Wires from Al-0.25%Zr-(Sc,Hf) Alloys.

Thermal stability of composite bimetallic wires from five novel microalloyed aluminum alloys with different contents of alloying elements (Zr, Sc, and Hf) is investigated. The alloy workpieces were obtained by induction-casting in a vacuum, preliminary severe plastic deformation, and annealing providing the formation of a uniform microstructure and the nucleation of stabilizing intermetallide Al3(Zr,Sc,Hf) nanoparticles. The wires of 0.26 mm in diameter were obtained by simultaneous deformation of the Al alloy with Cu shell. The bimetallic wires demonstrated high strength and improved thermal stability. After annealing at 450–500 °C, a uniform fine-grained microstructure formed in the wire (the mean grain sizes in the annealed Al wires are 3–5 μm). An increased hardness and strength due to nucleation of the Al3(Sc,Hf) particles was observed. A diffusion of Cu from the shell into the surface layers of the Al wire was observed when heating up to 400–450 °C. The Cu diffusion depth into the annealed Al wire surfaces reached 30–40 μm. The maximum elongation to failure of the wires (20–30%) was achieved after annealing at 350 °C. The maximum values of microhardness (Hv = 500–520 MPa) and of ultimate strength (σb = 195–235 MPa) after annealing at 500 °C were observed for the wires made from the Al alloys alloyed with 0.05–0.1% Sc.

Examining oxidation in \u03b2-NiAl and \u03b2-NiAl+Hf alloys by stochastic cellular automata simulations

We present results from a stochastic cellular automata (CA) model developed and employed for examining the oxidation kinetics of NiAl and NiAl+Hf alloys. The rules of the CA model are grounded in diffusion probabilities and basic principles of alloy oxidation. Using this approach, we can model the oxide scale thickness and morphology, specific mass change and oxidation kinetics as well as an approximate estimate of the stress and strains in the oxide scale. Furthermore, we also incorporate Hf in the grain boundaries and observe the “reactive element effect”, where doping with Hf results in a drastic reduction in the oxidation kinetics concomitant with the formation of thin, planar oxide scales. Interestingly, although we find that grain boundaries result in rapid oxidation of the undoped NiAl, they result in a slower-growing oxide and a planar oxide/metal interface when doped with Hf.