Hf Alloys Research Articles

Impact of Hf alloying on the functional properties of Ni-Mn-Ga high temperature shape memory alloys

The paper reports on the functional properties and microstructure of polycrystalline Ni-Mn-Ga-Hf high temperature shape memory alloys, in which different amounts of Hf up to 4 at.% are added in substitution of Mn and Ga, while the nominal Ni content is kept constant. The increase in the amount of Hf promotes a decrease in the valence electron concentration (e/a), tetragonality (c/a) of the martensitic unit cell, and martensitic transformation temperatures, as well as a significant decrease of the transformation hysteresis. The non-modulated tetragonal martensite, typical of Ni-Mn-Ga high temperature shape memory alloys, is formed in all the alloys studied here, though the alloy with 4 at.% of Hf also contains a small fraction of modulated 14M martensite. Hf addition improves the thermomechanical response of the alloys under compressive stress up to 300 MPa and leads to nearly closed cycles with minimal irrecoverable strain and low hysteresis (8 K) for the alloys with 3 and 4 at.% of Hf. These alloys also demonstrate excellent stability under repetitive thermal cycling and little change upon aging at 870 K. A second phase rich in Hf and Ni starts to precipitate at 1 at.% of Hf addition and its volume fraction experiences an abrupt and progressive increase for higher Hf contents. The structure of the second phase looks like the usual f.c.c. γ phase reported in other Ni-Mn-Ga-based alloys, but it has a double lattice parameter. Two structural models based on the A6 face-centered tetragonal unit cell with space group I4/mmm (No. 139), equivalent to the double f.c.c. lattice, are proposed for this new phase.

First-principles study on electronic and mechanical properties of Ru2XAl (X = Mn, Zr, Ti, Hf) alloys

The magnetic properties of Heusler alloys have been a hot research topic. However, the current understanding of the properties of Ru-based Heusler alloys is still very limited. The magnetic, electronic, and mechanical properties of four Ru-based Heusler alloys, Ru2XAl (X = Mn, Zr, Ti, and Hf) were calculated by the first-principles. This article studied the basic mechanical parameters of four types of Heusler alloys, such as bulk elastic modulus, Cauchy pressure, Pugh’s ratio, shear modulus, Kleinman parameter, Young’s modulus, Poisson’s ratio, and elastic constant. The calculated results of the elastic constant prove that all four alloys are mechanically stable. Ru2MnAl alloy exhibits brittleness, while the other three alloys exhibit excellent ductility based on Poisson’s ratio and Pugh’s ratio. The Kleinman parameter of the four alloys are all greater than 0.5, indicating that they have stronger stability in the tensile state. In addition, the spin magnetic moment of Ru2MnAl alloy is 2.113 μB, which is consistent with the SP rule and other theoretical calculations.

Microstructure and oxidation behaviour of Mo–Si–B–Hf alloys at 800 °C and 1100 °C

Microstructure and oxidation behaviour of Mo–Si–B–Hf alloys at 800 °C and 1100 °C

Research on Microstructure, Mechanical Properties, and High-Temperature Stability of Hot-Rolled Tungsten Hafnium Alloy.

W-(0, 0.1, 0.3, 0.5) wt.% Hf (mass fraction, wt.%) materials were fabricated by the powder metallurgy method and hot rolling. The microstructure, mechanical properties, and high-temperature stability of alloys with varying compositions were systematically studied. The active element Hf can react with the impurity O segregated at the grain boundary to form fine dispersed HfO2 particles, refining the grains and purifies and strengthening the grain boundary. The average size of the sub-grains in the W-0.3 wt.% Hf alloy is 4.32 μm, and the number density of the in situ-formed second phase is 6.4 × 1017 m-3. The W-0.3 wt.% Hf alloy has excellent mechanical properties in all compositions of alloys. The ultimate tensile strength (UTS) is 1048 ± 17.02 MPa at 100 °C, the ductile fracture occurs at 150 °C, and the total elongation (TE) is 5.91 ± 0.41%. The UTS of the tensile test at 500 °C is 614 ± 7.55 MPa, and the elongation is as high as 43.77 ± 1.54%. However, more Hf addition will increase the size of the second-phase particles and reduce the number density of the second-phase particles, resulting in a decrease in the mechanical properties of the tungsten alloy. The isochronal annealing test shows that the recrystallization temperature of W-Hf alloy is 1400 °C, which is 200 °C higher than rolling pure tungsten.

Study on the gradient characteristics of reactive melt infiltration C/SiC-HfC composite

C/SiC-HfC composite was prepared by the process of unidirectional reactive melt infiltration (i.e., porous C/C composite was placed above the alloy infiltrant during the preparation process) with Si-19at.%Hf alloy powder, and the microstructure and ablation resistance of different parts of the composite were studied. It was found that the ceramic matrix composition of the as-prepared C/SiC-HfC composite presented unidirectional gradient distribution in the direction of infiltration, which led to the gradient characteristics of the ablation resistance. By adjusting the composition of alloy infiltrant and process method, composite with good ablation resistance and lightweight can be achieved by reactive melt infiltration process.

Basal and prismatic slip in hafnium

Metals with hexagonal crystal structure are found in many safety-critical applications, from titanium alloy fan blades in jet engines to magnesium alloys in the automotive industry. They also exist in nuclear reactors, for example hafnium alloy control rods whose structural integrity is critical to nuclear safety. However, mechanistic understanding of the micromechanical properties and deformation behaviour of Hf remains elusive. To aid in this understanding, we performed in situ scanning electron microscopy single crystal micropillar compression tests of a commercial Hf alloy, where the samples were aligned to activate 〈a〉 prismatic and 〈a〉 basal slip systems. We then employed transmission electron microscopy to examine the dislocation structure in the deformed pillars. These two slip systems exhibited distinctly different behaviour, where prismatic slip is planar and basal slip is wavy. Prismatic slip is the easiest deformation mode, while basal slip is at least twice as difficult. Generally, the characters of prismatic and basal slip in Hf show similarities to those in some other hexagonal metals such as Ti and Zr, except for the higher relative difficulty of basal slip. Our results provide fundamental knowledge and parameters that can be used for modelling degradation and predicting performance of Hf, and shed light on the general behaviour of basal and prismatic slip systems in low-c/a-ratio hexagonal metals.

Effects of intermediate aging on the microstructure, strength and texture evolution of the Cu–Cr–Zr-Hf alloys

Cu-0.4Cr-0.2Zr-0.2Hf (wt%) alloy plates were prepared by cryogenic rolling and intermediate aging process. The effects of intermediate aging and cryogenic rolling on the microstructural, mechanical and textural evolutions were studied by X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. The results show that the combination of cryogenic rolling and intermediate aging resulted in the formation of high-density dislocations, nanoscale deformation twins, deformation bands, and nano-precipitates in the Cu–Cr–Zr-Hf alloys. The nano-precipitates formed during the intermediate aging effectively increase the flow stress, and promote the formation of the nano-twins. Besides, the combination process can both reduce the fraction of the remnant grains and refine the width of remnant grains, promoting the homogeneous deformation of the Cu–Cr–Zr-Hf alloys. The reduction of the remnant grains decreases the Brass component, and increases the S component. The profuse nano-twins and Brass-type shear band contribute to the increase in the Y component.

Effect of Hf addition on structural transformation and aging stability of Cu–15Ni–8Sn alloy

The Cu–15Ni–8Sn-xHf (wt.%, x = 0–0.6) alloys were prepared by vacuum melting, and the effect of Hf on the micro-structure of as-cast, as-solution-treated and as-aged alloys was systematically investigated, as well as the alloy aging stability. The Hf addition provided a significant role on refining the solidification structure and reducing the micro-segregation degree, and it also made γ-D03 phases more likely to form in needlelike-shape rather than lamellar-shape. With the presence of more Hf atoms, their existence form was accordingly changed from solid solute atoms to nano-scale precipitation phases and then to micron-scale segregation phase. A small content of Hf (0.1 wt%) could effectively improve the aging stability by inhibiting the growth of γ-D03. Once its content reached or exceeded 0.3 wt%, the harmful discontinuous precipitation was completely inhibited, and the common lamellar-shape γ-D03 locating around the grain boundary was no longer formed. Even the alloy failed at higher temperatures, it could only be carried out through the continuous precipitation method, namely the generation of needlelike-shape γ-D03 uniformly in the matrix. The appropriate Hf addition was capable of extending the alloy aging window, and the hardness of Cu–15Ni–8Sn-0.3Hf alloy could still exceed 360 HV even after 48 h aging.

Structural stabilities and natural half-metallic properties of OsXCoSi (X=Ti, Zr, Hf) quaternary Heusler alloys series first-principles calculations

Based on first-principles calculation of density functional theory, this study investigates the structural stability, magnetic properties, and electronic properties of the three different phases (i.e. type 1, type 2, and type 3) of OsXCoSi (X=Ti, Zr, Hf) in a new quaternary Heusler alloy series. The corresponding equilibrium lattice constants of each type are optimized, and the change of formation enthalpy and elastic constant phonon spectrum show that the OsXCoSi (X=Ti, Zr, Hf) alloy is thermodynamically, dynamically and mechanically stable. Furthermore, the bonding features of each phase are discussed. It is found that all type 1 structures of OsXCoSi (X=Ti, Zr, Hf) exhibit natural half-metallicity (HM) in equilibrium lattice constant, and their equilibrium lattice constants in the ground state were determined to be 5.909 Å for OsTiCoSi, 6.155 Å for OsZrCoSi, and 6.100 Å for OsHfCoSi. Meanwhile, by testing the alloy under different pressures, the range of the integer magnetic moment non-equilibrium lattice constants for the three alloys OsTiCoSi, OsZrCoSi, and OsHfCoSi are 5.710 Å ∼ 6.329 Å, 5.696 Å ∼ 6.1557 Å and 5.716 Å ∼6.1009 Å, respectively, which is wide and is more close to the practical application for spin-polarized materials. In addition, its magnetic moment is consistent with the values given by the Slater–Pauling rule. Furthermore, the forming of the HM gap is examined by analysing the total and partial density of states, energy bands of alloy’s electronic property, with respect to the calculated results. What’s more, special attention is paid to the differences of the properties for series Heusler alloys. It is found that the electronics properties distinction is mainly based on valence electron changes. However, the lattice constants are susceptible to size of a nucleus.

Increasing strength and electrical conductivity of Cu-0.77%Cr-0.86%Hf alloy by rotary swaging and subsequent aging

Increasing strength and electrical conductivity of Cu-0.77%Cr-0.86%Hf alloy by rotary swaging and subsequent aging

Evaluation of the pumping performance of combinations of Ti–Zr–V–Hf alloy nonevaporable getter coatings with different morphologies, activation protocols controlled at cryogenic temperatures

The pumping performance of a range of Ti, V, Zr, and Hf alloy nonevaporable getter (NEG) coatings of order 5 μm thickness and with different modular structures has been investigated. For equal atomic percent Ti–Zr–V–Hf quaternary alloys, dual (columnar layer on the dense layer) coatings gave higher sticking probabilities than columnar and dense morphologies. The same alloy provided the highest sticking probability, followed by the Ti–Zr–V tertiary alloy, and then singular Zr. For a 24 h activation time, single element Zr coatings were shown to require an activation temperature of 300 vs 250 °C for the other alloys. H2 and N2 sticking probabilities increased by a factor of 100 and N2 terminal capacities by a factor of 1000, in the range 150–300 °C. For the dual coated Ti–Zr–V–Hf quaternary alloy at an activation temperature of 140 °C, H2 sticking probabilities increased by a factor of 2 when the activation time increased from 1 to 7 days and that of N2 increased by a factor of 6. The factor of 100 increase in measured sticking probability at LN2 temperature, compared to room temperature, to a value of 0.5 for H2, is transient in nature. Further investigations into this behavior are planned.

Advances in Diffusion Barrier Coatings for High-Temperature Applications

Diffusion barrier coating (DBC) systems on heat resistant alloys consist of a multi-layer structure: an outer Al-reservoir layer and an inner diffusion barrier layer (DBL). The outer Al-reservoir layer forms a protective Al2O3 scale and DBL acts as a barrier layer against alloy interdiffusion. Three kinds of DBL were developed: Re on Ni-based superalloys; W on stainless steels; and Cr on Ni–Cr alloys. It was found that DBC systems have excellent mechanical properties (creep and fatigue), improving alloy substrate performance, and enhance the anti-exfoliation properties of YSZ in thermal barrier coatings (TBC) in addition to providing excellent oxidation resistance. At temperatures higher than 1300 °C, however, the DBC design based on kinetics (diffusion) is insufficient to form and maintain a protective Al2O3 scale. In this case a self-maintaining coating (SMC) system designed on the basis of thermodynamics (phase stability) is required. The SMC system formed on Nb–Hf (C-103) alloy consists of a multi-layer structure: an outer Re (Al, Si)1.8 and inner NbSi2 layers, plus a transient Nb5Si3 layers formed during oxidation. At temperatures higher than 1300 °C the Al2O3 can be formed by changing the Al/Si ratio in the Re (Al, Si)1.8 in which Si was supplied from the decomposition reaction of NbSi2 to Si + Nb5Si3 during selective oxidation of Al. It is proposed that coating alloys should be designed for considering not only high temperature oxidation, but also alloy substrate mechanical properties and anti-exfoliation of oxide scales, based on both kinetic principles (DBC system) and thermodynamics (SMC system).

Effect of Hf alloying on magnetic, structural, and magnetostrictive properties in FeCo films for magnetoelectric heterostructure devices

Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron–cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1−xHfx (x = 0 – 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x ∼ 0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying.

Thermomechanical properties and the deformation mechanism of nickel monoaluminide-based alloys produced by L-PBF in combination with gasostatic treatment and aging

The synthesized NiAl-based material was manufactured by laser powder bed fusion (L-PBF) followed by gasostatic treatment and aging using spheroidized CompoNiAl-M6 (NiAl–8Cr–6Co–1Nb-0.9Hf) alloy powder (the 20–50 μm fraction). The structure was examined by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED); nanoparticles of the refractory phases were identified. The compressive mechanical properties of the alloy were measured at 20, 800, 900, and 1000 °C. Doping with niobium and hafnium was shown to strengthen the alloy with fine-grained Laves and Heusler phases, as well as the (HfxNby)C carbide phase, which reduce the size of NiAl grains and prevent grain boundary diffusion. Due to this fact, the material can be used at temperatures above 800 °C. Ultimate compressive strength (σb) at 900 °C was 575 MPa, while being 260 MPa at 1000 °C. The dislocation substructure of the deformed sample was examined, and the predominant deformation mechanisms (dislocation glide, dislocation climb, and shear of the crystal lattice of the matrix phase) were identified. In situ TEM nanotensile testing revealed that fine-grained refractory phases increase σb from 1360 MPa to 1870 MPa. Clusters and agglomerates of strengthening phases act as stress concentrators and cause premature alloy failure because of stress localization at the particle interface.

Hf and Ta co-doping MCrAlY alloy to improve the lifetime of coatings

Thermal barrier coating (TBC) is one of the key components of an aircraft turbine or stationary gas turbine. NiCoCrAlY is the most commonly used bond coat (BC) and its composition and impurities have an impact on the lifetime of the TBC. However, the performance of NiCoCrAlY is very unstable due to the influence of impurities. Reactive elements (RE) Hf and Ta are co-doped into the bond coat to improve the adhesion of thermally grown oxide (TGO) layers to extend the lifetime of TBC. Yttrium stabilized zirconia (YSZ) is sprayed on free-standing alloys of NiCoCrAlY and NiCoCrAlYHfTa, respectively. The samples are cyclically oxidized at 1050 °C until the peeling off of coatings. The lifetime of NiCoCrAlY coatings is only 240 h, while the lifetime of NiCoCrAlYHfTa coatings reaches 2664 h. The morphology and structure of the oxide scale are characterized to study the mechanisms of lifetime extension.

Effect of Hf on the lattice invariant shear and self-accommodation of martensite in Ti–Ni–Hf alloys

Effect of Hf on the lattice invariant shear and self-accommodation of martensite in Ti–Ni–Hf alloys

Heat-resistant wire made of Al – Zr – Hf alloy: composition and production

Heat-resistant wire made of Al – Zr – Hf alloy: composition and production

Thermodynamic modeling of the Ni–Ti–Hf system

Ni–Ti–Hf alloys have received extensive attention owing to their excellent shape-memory behavior. In the present work, the thermodynamic assessment of the Ni–Ti–Hf system was performed using the CALPHAD method after a critical review of the thermodynamic descriptions of boundary binaries and available experimental data in the ternary system. The solubility ranges of the third elements in binary intermetallic compounds are well described. Moreover, the wide homogeneity ranges of the B2 and NiTi2 phases are satisfactorily reproduced. The ternary τ phase was reasonably described using a two-sublattice model (Ni)0.44(Hf,Ti)0.56. Both the thermochemical properties and phase diagrams were well described using the present model parameters.

Comparative analysis of the aging kinetics in low-alloyed Cu – Cr – Hf and Cu – Cr – Zr alloys after high pressure torsion

The kinetics of phase precipitation during aging in the Cu – Cr – Hf and Cu – Cr – Zr alloys after high pressure torsion (HPT) has been studied and compared with quenching state. The effect of HPT on the kinetics of aging was revealed by the differential scanning calorimetry (DSC) and confirmed by changes in the microhardness and electrical resistivity. Analysis of the microstructure and phase precipitation after HPT and subsequent aging using a transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed three types of phases in each of the alloys: Cu matrix, chromium and zirconium- or hafnium-rich phases. By measuring the microhardness, a higher degree hardening after HPT and subsequent aging was revealed in the Cu – Cr – Hf alloy compared to the Cu – Cr – Zr alloy.

Synergistic effect of Zr and Hf alloying on cross fluvial microstructure of CoCrFeNi eutectic high-entropy alloys

In this study, selective vacuum arc furnace is used to fabricate CoCrFeNi-M0.4 (M = Zr, Hf, 50 %Zr + 50 %Hf) EHEAs, and the content of Zr and Hf element is controlled to solve the balance of strength and plasticity in HEAs. Through alloying with Zr or Hf, the cross fluvial eutectic structure of CoCrFeNiZr0.4 and CoCrFeNiHf0.4 are consists of FCC-CoCrFeNi matrix phase and intermetallic. CoCrFeNiZr0.2Hf0.2 EHEAs, which is synergistic alloying with Zr and Hf, have a brawny river eutectic structure of both soft FCC and hard Ni7Hf2 intermetallic. The synergistic precipitation strengthening and solid solution strengthening result in the fracture strength and ductility of CoCrFeNiZr0.2Hf0.2 reaching 3475.7 MPa and 14.1 %, respectively. Furthermore, CoCrFeNiZr0.2Hf0.2 exhibit a good combination of strength and plasticity through synergistic alloying effect compared with other CoCrFeNi HEAs.