Addition Of HF Research Articles

Mechanical and tribological performance of CoCrNiHfx eutectic medium-entropy alloys

The excellent properties of the multi-principal element alloys (e.g., the CoCrNi medium-entropy alloy) make them a perfect candidate for structure materials. Their low strength and poor wear-resistance, however, limit considerably their applications. In this study, a lamellar eutectic microstructure was introduced by addition of Hf into CoCrNi alloy to produce a series of CoCrNiHfx (x = 0.1, 0.2, 0.3 and 0.4) eutectic medium-entropy alloys. A homogeneous eutectic microstructure with an alternate array of the soft FCC solid-solution phase and the hard Laves phase was identified for the as-cast CoCrNiHf0.3 alloy. After an investigation of the microstructure, mechanical and tribological properties, it was found that the hardness (plasticity) increases (decreases) with the increasing volume fraction of the Laves phase and the CoCrNiHf0.3 eutectic alloy exhibits both good plasticity and high strength. The wear behavior is strongly dependent on the applied normal load. For a low normal load, its tribological behavior follows the Archard's equation and a higher hardness due to Hf addition can resist plastic deformation and abrasive wear. When the normal load is high enough, the hypoeutectic or hypereutectic alloy, which possessing either high strength or good ductility but not at the same time, exhibit a poor wear resistance. In comparison, the full eutectic CoCrNiHf0.3 alloy with a superior combination of strength and toughness shows the best wear performance, as it can significantly reduce fracture during wear.

Energetic Behavior Study in Phase Transformations of High Temperature Cu–Al–X (X: Mn, Te, Sn, Hf) Shape Memory Alloys

Shape memory alloys (SMAs) are increasingly used in a wide range of thermomechanical and thermoelectric applications that are of a macro-, micro- and nano-scale. Though the use of commercial Ni–Ti (NiTiNOL) SMAs is preferred in most of these applications, there is an ever-increasing demand for cost-effective and easy-to-produce alternatives to NiTiNOL. In terms of functional properties, copper-based SMAs are comparable to Ni–Ti SMAs. Moreover, Cu-based SMAs are cheaper and easier to produce and process. In order to improve the functional properties of binary Cu-based SMAs (Cu–Al, Cu–Zn and Cu–Sn), ternary elements are added in small quantities. This is also one of the most common methods that is used to improve the functional properties of NiTi- and Fe-based SMAs. In this work, Cu–Al-based alloys containing ternary elements, Mn, Sn, Te and Hf, were produced by arc melting technique. Thermal (DSC, DTA) and microstructural (XRD, metallography) analyses were then carried out on the alloys prepared so as to characterize them for their thermal and microstructural characteristics. The results demonstrate improved shape memory properties for these alloys. The transformation temperatures of the binary Cu–Al alloys determined showed higher values (Af > 100 °C). Among the ternary alloys, the highest transformation temperatures were found for alloys with Hf addition, followed by those with Te addition. But on the other hand, the most intense peaks corresponding to the martensitic transformation were observed for Sn addition. The microstructural investigations revealed the presence of martensite in the alloys which was consistent with the results of thermal analysis.

Phase, microstructure and related mechanical properties of a series of (NbTaZr)C-Based high entropy ceramics

The microstructure and a few mechanical-related properties of a series of multi-equiatomic-cation (NbTaZr)C-based carbides, namely (NbTaZr)C, (NbTaZrW)C, (ZrNbTaHf)C and (NbTaZrHfW)C, were assessed to shed light into understanding the role of composition. Dense, pure and homogeneous rock-salt structured ceramics are achievable for all four compositions through spark plasma sinntering at 2200 °C for 10 min. Neither the hardness nor the fracture toughness is determined solely by the number of cations; among the four high entropy ceramics, (NbTaZrW)C exhibits the best combination of hardness and fracture toughness. Electronic structure and lattice distortion differences can explain part of the property differences, such as the lowered Young's modulus of (NbTaZrW)C and (NbTaZrHfW)C compared to (NbTaZr)C; but certainly, more in-depth theoretical investigations are needed to provide a full understanding of the effects of W and/or Hf addition on the hardness and fracture toughness of (NbTaZr)C.

The effect of Hf on solidification cracking inhibition of IN738LC processed by Selective Laser Melting

The effects of Hf addition on defects and mechanical properties of the “hard-to-weld” superalloy IN738LC manufactured by Selective Laser Melting (SLM) process were studied. The addition of Hf inhibits cracking in SLMed IN738LC. Cracking free and low porosity IN738LC parts were obtained in a wide range of process parameters. The study shows that the cracks were refilled by Hf-riched melt and the relative density was increased, thus the tensile properties at 700 and 850 °C were improved.

Tailoring the microstructure, martensitic transformation and strain recovery characteristics of Ti-Ta shape memory alloys by changing Hf content

In the present work, the microstructure features, martensitic transformation, mechanical properties and strain recovery characteristics of Ti-Ta based shape memory alloys were tailored by changing Hf contents. The single α´´ martensite phase was dominated in Ti-Ta alloy with 2 at.%H f. Upon Hf content exceeded 2 at.%, β phase started to appear. Moreover, the amount of β phase gradually increased with Hf content increasing. The martensitic transformation temperatures continuously decreased with the increased Hf content, which was attributed to the rising of valence electron concentration. Meanwhile, Hf addition improved the thermal cycling stability of Ti-Ta alloys due to the suppression of ω precipitation. The yield stress of Ti-Ta based alloys firstly decreased and then increased with Hf content increasing. In addition, the completely recoverable strain of 4 % can be obtained in Ti-Ta alloy with 6 at.% Hf as a consequence of the higher critical stress for dislocation slip. Besieds, the Ti-Ta based alloy containing 8 at.% Hf had the superior superelasticity behavior with the fully recoverable strain of 2 % at room temperature.

Preparation and strengthening mechanisms of in-situ HF catalyzed highly porous mullite with whisker frame structure

Using HF as catalyst and based on sol-gel chemical precursor powder molding and sintering process, porous mullite with whisker frame structure was in-situ synthesized in the blank body. Due to the high efficiency catalysis of HF, this structure has both of high porosity and high strength. The effects of HF addition and sintering temperature on the formation of mullite whisker frame, phase transition process, porosity and strength of porous materials were discussed, and the optimal process parameters were determined. The mechanism of strengthening the porous materials was preliminary analyzed, and under the catalysis of HF, the close connection between whisker frame lap joints was formed. When the porosity of mullite is up to 83.22%, the compressive strength of can reach 16.48 MPa.

Crystallization characteristic and structure of hafnium addition in germanium antimony thin films for phase change memory

The influences of Hf addition on the phase change character of Ge5Sb95 thin films were systematically studied. The heat induced crystallization was evaluated by in situ resistance measurement. By doping Hf element, both the resistance and the crystallization activation energy rise, resulting in better thermal stability and higher operating energy efficiency. Various analysis indicate that due to the existed amorphous germanium and antimony element, the hafnium addition can delay the growth of crystal grains and constrain the crystal size. A shift in the Raman mode corresponding to Sb after crystallization can be observed. By using the X-ray reflectance and atomic force microscope, it is observed that after doping Hf, the volume fluctuation and the surface roughness decrease. To evaluate the electrical characteristics, a phase change memory chip based on Hf doped Ge5Sb95 thin film was also fabricated successfully. The results show that proper incorporation of Hf with Ge5Sb95 is an helpful method for adjusting and optimizing the crystallization properties of the material in the applications of the phase change memory.

Porous structure obtained by anodizing niobium in NaOH

Porous structures can be produced as nanotubes, nanowires and membranes, and they can be used in gas sensors, magnetic and electronic devices, biotechnology, etc. These structures can be obtained by the anodizing process, in which the electrolyte must etch the oxide to occur the pore formation. In the case of niobium, porous oxides have been obtained successfully in electrolytes containing hydrofluoric acid (HF). However, studies have shown that niobium dissolves in electrolytes of NaOH with pH 14, and porous oxides of niobium have already been obtained by the anodizing process with the occurrence of sparking. Thus, the objective of this work is to obtain porous oxides on Nb without HF addition by studying the sparking phenomenon. Therefore, niobium was anodized at different current densities (10, 25, 50, 75 and 100 mA.cm-2). The samples were evaluated concerning morphology by SEM and chemical microanalysis by EDS. It was observed that sparking depends of the oxide barrier thickness and occurs at current densities higher than 50mA.cm-2. However, the increase of the current density, in spite of not increasing the thickness of the barrier layer, caused a greater incidence of sparking. According to the assignment proposed it was possible to obtain a porous structure in niobium with NaOH electrolyte, but this structure got an irregular aspect

Microstructure And Fracture Toughness of Nb-Si Based Alloys by with Ti, Zr and Hf Additions

Microstructure And Fracture Toughness of Nb-Si Based Alloys by with Ti, Zr and Hf Additions

The Effect of Hf Addition on the Boronizing and Siliciding Behavior of CoCrFeNi High Entropy Alloys

The Effect of Hf Addition on the Boronizing and Siliciding Behavior of CoCrFeNi High Entropy Alloys

Magnetic properties and microstructure of Sm-Co-Fe-Cu-Zr-Hf spark plasma sintered magnets

In this work, Sm24.6Co50Fe17·4Cu5·7Zr2.3-xHfx (x = 0–2.0) ribbons and magnets were prepared by melt-spinning method and spark plasma sintering (SPS) technique. The magnetic properties of the melt-spun ribbons with Hf addition are much higher than those of the Hf-free ribbons in the temperature range of 300–400 K. The optimal comprehensive magnetic properties of ribbons and isotropic magnets were obtained with 1.0 wt% Hf substitution. Microstructural results showed that Hf elements do not enter into the hard magnetic 1:7H and 2:17R phase in SPSed magnets and Zr–Hf rich phases are formed around grain boundaries as particulates. The enhancement of magnetic properties of the ribbons and SPSed magnets can be attributed to grain refinement and more uniform microstructure as a result of Hf addition. The micro-twin structure of 2:17R phase also have been discovered in the spark plasma sintered magnets. This work may provide a rapid preparation of Sm2Co17 magnets.

Effect of Hf addition on solidification and hot isostatically pressed microstructures of the Ti-48Al-2Cr-2Nb (at%) intermetallic alloy

The microstructural evolutions during solidification and hot isostatic pressing (HIP) of Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated using both experimental observations and thermodynamic calculations. The primary solidification phase of all the studied alloys was β, albeit it was predicted by Thermo-Calc to be α for the alloys containing 4% and 6%Hf. In agreement with the Thermo-Calc calculations, the solidification path of the base 4822 alloy encountered a peritectic reaction. Although alloying by 2%Hf did not change the solidification path, further increase in Hf content caused an additional eutectic reaction following the peritectic one. The eutectic microstructure was composed of two orthorhombic phases with TiAl2 and Al3Hf2 compositions and this finding was the first evidence of such phases in a quinary alloy with Cr, Nb, and Hf. It was found that Hf acted as a weak β stabilizer as it showed a low partitioning tendency in β and the β single-phase field was not formed in the high-Hf alloys. A finer dendritic microstructure was characterized in the high-Hf alloys due to the Hf solubility limit in the 4822 alloy. Interestingly, α2 laths dissolved in the Hf-containing alloys after HIP treatment and instead rode-like precipitates of Al3Hf2 emerged at newly formed γ/γ interfaces. The precipitation mechanism of Al3Hf2 particles was discussed.

Effect of hafnium transition metal additives on the microstructure of 01570 aluminum alloy

The study addresses the effect of hafnium on microstructure, mechanical properties and stability of Al3Sc type strengthening particles in Al – Mg system 01570 alloy with scandium and zirconium additions. As part of the study, 01570 alloy-based ingots with hafnium content of 0.1% and 0.2% (here and everywhere hereinafter % weight is indicated) were cast in steel chill molds with zirconium and without zirconium addition. The obtained ingots grains size was examined using optical microscope. It was established, that 0.2% Hf addition to the previously designed 01570 alloy can reduce the average nominal grain diameter by half. A number of anneals was performed at temperatures of 400 oC and 450 oC with 1, 10, 30 minutes, 1, 4, 8, 16, 32 and 56 hours soaking times, to study the rate of supersaturated solid solution decomposition and determine the moment of strengthening dispersed particles coalescence onset. Annealed samples were measured for microhardness, electrical resistance and examined using scanning electron microscopy. It was demonstrated, that samples with hafnium do not lose strengthening phase stability at traditional treatment temperature of 400 oC with considerable soaking time (up to 56 h), at 450 oC decrease in microhardness is observed only after 4 hours of soaking. The samples were pressure-formed, i.e., hot and cold rolled, to determine recrystallization onset, it was established, that even at 500 oC and short soaking period (up to 30 minutes) rolled samples with hafnium still have some not recrystallized microstructure, while the samples without hafnium have visible equiaxed recrystallized grains. Using transmission electron microscope, it was possible to establish that the dispersed particles obtained in the alloy with 0.2% Hf retain a size of about 9 nm after the treatment cycle, and the average distance between them is 62 nm.The study was supported by a grant from the Russian Science Foundation, project 18-79-10099.

Effect of Zr and Hf additions on microstructure and mechanical properties of Nb–Si based ultrahigh temperature alloys

To improve the fracture toughness, and reduce density and cost of Nb–Si alloy, the influence of the substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys is investigated in this study, and the microstructural evolution and mechanical properties, including fracture toughness and compressive properties are analyzed. The results show that the substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys can refine the microstructure, and promote the formation of (Nb, X)5Si3/Nbss eutectic. The fracture toughness KQ value increases with the increase of Zr content, since the high Zr content alloys exhibits the low c/a ratio and fine microstructure. Moreover, the addition of 1~4 at.% Hf in Nb–16Si–16Ti–4Zr alloy has less influence on the mechanical properties. The substitution of Zr for Hf in Nb–16Si–16Ti–xZr–yHf (x + y = 4) alloys can improve the fracture toughness of alloys, and the Zr element exhibits the low density and cost compared with Hf element, which indicates that Zr is a promising alloying element.

A chemical kinetic model for the decomposition of perfluorinated sulfonic acids

Perfluorinated sulfonic acids (such as perfluorooctanesulfonic, PFOS, and short-chain analogues) are notorious halogenated pollutants that exhibit severe toxicity, even at minute levels. Limited number of experimental studies addressed their thermal decomposition at elevated temperatures. Such scenarios are particularly relevant to open fires and incineration of materials laden with perfluoroalkyl compounds (PFCs). Herein, we construct a detail kinetic model that illustrates major chemical reactions underpinning initial degradation of 1-butanesulfonic acid (CF3(CF2)3SO2OH), as a model compound of PFOS, and perfluorinated sulfonic acids in general. Reaction rate parameters were estimated based on an accurate density functional theory (DFT) formalism. The kinetic model incorporates four sets of reactions, namely, unimolecular decomposition channels, hydrofluorination, hydrolysis, and fragmentation of the alkyl chain. Results are discussed considering recent experimental measurements. Temperature-dependent profiles for a large array of perfluoroalkyl acyl fluorides, short perfluorinated cuts, and perfluorinated cyclic compounds, are presented. SO2 emerges as the main sulfur carrier, with a minor contribution from SO3. HF addition to double carbon bonds in alkenes, and to carbonyl bonds in aldehydic structures signifies a major sink pathway for hydrogen fluoride. Addition of moisture was shown to expedite the destruction of relatively large perfluoroalkyl acyl fluorides into C1 species. Construction of this model could aid in a better understanding of the fate and chemical transformation of PFCs under a pyrolytic environment pertinent to waste incineration and fluorine mineralization.

Microstructures and mechanical properties of CoCrFeNiHfx high-entropy alloys

CoCrFeNiHfx (x = 0.1–0.6) high-entropy alloys with different Hf additions are investigated in this paper. As the Hf content increases, the alloy microstructure gradually transforms from the hypoeutectic type to the hypereutectic type, with primary phases of either FCC dendrites or Laves phases. The eutectic composition of the alloy is identified to be close to CoCrFeNiHf0.42. At room temperature, CoCrFeNiHfx alloys exhibit increased yield stresses accompanied by the degradation of ductility as raising the Hf content. CoCrFeNiHf0.1 and CoCrFeNiHf0.2 show strain hardening rates >2500 MPa at strain <0.3 and high strengths of ~3000 MPa at the strain of 0.5. At the high temperature of 800 °C, the alloy featuring a typical eutectic microstructure exhibits the maximum yield strength of 536 MPa compared to hypoeutectic and hypereutectic compositions. Nevertheless, as-cast eutectic and hypereutectic alloys show severe microstructural instability at 800 °C, as evidenced by the transformation of the eutectic microstructure from lamella to irregular morphology with significant coarsening.

Effects of Hf addition on the structure, mechanical and tribological properties of CrN film

Alloying elements in binary nitride is considered as a promising method to design hard and tough films. Hf atoms are proposed to be incorporated in CrN nitrid films to form ternary nitrides, which may possess the improved mechanical and tribological properties. In this study, Cr1-xHfxN [x = Hf/(Cr + Hf)] films were sputtering deposited by incorporating Hf into CrN film in an mixed atmosphere of Ar and N2. The influences of Hf addition on the microstructure, mechanical and tribological properties of Cr1-xHfxN films were investigated. The results show that the hardness and fracture toughness of CrN film is significantly improved due to solid solution strengthening by incorporating Hf atoms. Thanks to the improved hardness and toughness, the wear resistance of CrN film is enhanced. A high H/E and H3/E2 values, which are regarded as the reflection of toughness of hard films, are obtained from Cr0.69Hf0.31N film with higher hardness (30.49 GPa). The lowest wear rate of about 1.03 × 10−6 mm3/Nm is also presented for Cr0.69Hf0.31N film, which corresponds to higher H/E(~0.125) and H3/E2(~0.481 GPa).

In-field critical current performance of 4.0 μm thick film REBCO conductor with Hf addition at 4.2 K and fields up to 31.2 T

We present results on the in-field critical current (Ic) performance of 4.0 μm thick REBCO film with 15% Hf addition with fields up to 31.2 T and field orientations in the B║ab plane and B║c axis. Unlike the behavior at B║c, the critical current at B║ab is only very weakly dependent on field, decreasing from self-field to 31.2 T by only 22%, i.e. from the self-field value of ∼7700 A/4 mm width to ∼6300 and 5812 A/4 mm width at 14 and 30 T, respectively. These values are remarkably 3 and 5.7x higher than the corresponding critical currents at B║c. The in-field behavior of the present 15% Hf sample at field orientation B║c axis is nearly identical to the previously reported record values found in 4.3 and 4.6 μm thick 15% Zr samples in terms of critical current density. In contrast to the pinning force behavior in the B║c orientation, which saturates to a constant value of 1.7 TN m−3 above ∼5–6 T, the pinning force in the B║ab orientation increases near-linearly, reaching a remarkable value of over 11.5 TN m−3 at 31.2 T. These results demonstrate the potential of thick REBCO conductors at 4.2 K for high field and energy density applications, in particular where the magnetic field is contained near the ab-plane.

Microstructure and cyclic oxidation of a Hf-doped (Ni,Pt)Al coating for single-crystal superalloys

A diffusion-barrier-contained Hf-doped (Ni,Pt)Al coating was prepared by firstly composite electroplating and then aluminising. The TEM study of the microstructure showed that Hf serves as a solid solution in (Ni,Pt)Al lattice and forms HfO2 in the Hf-rich zone. This HfO2 is formed in situ during aluminisation under low oxygen pressure. It acts as a diffusion barrier to prevent the inter-diffusion during cyclic oxidation. In comparison with (Ni,Pt)Al coating, the oxide scale on Hf-dope (Ni,Pt)Al is more adherent. Moreover, the surface rumpling extent is much relieved due to slower Al depletion rate and higher creep resistance by Hf addition.

Microstructures and mechanical properties of oxide dispersion strengthened CoCrFeNi high-entropy alloy produced by mechanical alloying and spark plasma sintering

1.2 Hf and 1.5 Y2O3 (wt.%) were added into CoCrFeNi (CCFN) high-entropy alloy (HEA) to fabricate oxide dispersion strengthened CoCrFeNi (ODS-CCFN) composite by mechanical alloying and spark plasma sintering. Microstructures, including XRD, SEM, EBSD, and TEM of CCFN and ODS-CCFN HEAs were investigated to identify the effects of introducing nano-sized oxides into CCFN HEA. These HEAs compose of FCC matrix and a small amount of Cr7C3 and Cr2O3, and a few of MnCr2O4 is only detected in CCFN HEA. High-density Y2Hf2O7 with the average diameter of 6.9 ± 4.2 nm uniformly distribute in the matrix and Cr7C3 of ODS-CCFN. Lattice misfit calculations indicate that Y2Hf2O7 particles exhibit a coherent interface with the matrix, and a semi-coherent interface with Cr7C3. The average sizes of all phases, including grain, Cr7C3 and Cr2O3, are highly refined in ODS-CCFN. The effects of microstructure changes on properties of CCFN HEA were also detected. Compared with CCFN, the tensile yield strength of ODS-CCFN increases from 978 MPa to 1.25 GPa with a reasonable elongation decreases from 2.6% to 1.9%. The increase in yield strength for ODS-CCFN is mainly ascribed to the precipitation strengthening of Y2Hf2O7 and highly refined grains, Cr7C3 and Cr2O3. The electrical resistivity of ODS-CCFN is lower than that of CCFN, the reason may be that the existence of Y2Hf2O7 particles reduce the lattice distortion of the matrix, and make the electrical resistivity decrease. The corrosion resistance of ODS-CCFN HEA is highly improved in both sulfuric acid solution and sodium chloride solution because its passive current density is largely reduced by the addition of Hf and Y2O3.