Addition Of HF Research Articles

First-principles study on the effects of Er/Hf ratio on the properties of L12-Al3(Er,Hf)

L12-structured tri-aluminides in additively manufactured (AM) Al alloys refine α-Al grains while playing a significant role in precipitation strengthening. To precipitate the tri-aluminide dispersoids with an improved nucleation efficiency and coarsening resistance in AM Al alloys, the joint addition of Er and Hf into Al is proposed to form ternary L12-Al3(Er,Hf) precipitates. The work applies the first-principles method of projected augmented wave (PAW) under the framework of density functional theory (DFT), to calculate the lattice constant, formation energy, binding energy, bulk modulus, and density of states of L12-Al3(Erx,Hf1-x) (x = 0, 0.25, 0.5, 0.75, 1). The effects of Er/Hf ratios on certain key properties and features of the L12-Al3(Er,Hf) compounds and an Al alloy that contains the compounds are discussed.

Effect of Hf addition on initial microstructure of (Ni, Pt)Al coating and inhibiting the detrimental sulfur during high temperature exposure at 1150 °C

Single-phase (Ni, Pt)Al coatings with and without Hf addition were prepared via electroplating a Pt/Pt-Hf layer and subsequent gaseous phase aluminisation. The initial microstructure and oxidation behaviour of two coatings were investigated comparatively. The results show that finer grains were obtained in Hf-doped Pt-plating layer and as-received (Ni, Pt)Al coating, which contributes to better high temperature performance and oxide scale peeling resistance. Additionally, Hf can effectively retard the θ-Al2O3 transformed to α-Al2O3 during the early oxidation and enhanced oxidation resistance during the stable oxidation. The effect of Hf on inhibiting detrimental sulfur in acid-(Ni, Pt)Al during high-temperature exposure were intensively discussed.

Microstructure and First Hydrogenation Properties of Ti30V60Mn(10−x)Crx (x = 0, 3.3, 6.6, 10) + 4 wt.% Zr

In this paper, we studied the effect of the Cr/Mn ratio on the microstructure, crystal structure and hydrogen absorption properties of the quaternary alloys of compositions Ti30V60Mn(10−x)Crx (x = 0, 3.3, 6.6 and 10) + 4 wt.% Zr. The addition of Hf instead of Zr was also investigated. We found that all alloys are single-phase BCC (Body Centred Cubic) but with regions of high concentration of Zr (or Hf). The first hydrogenation at room temperature under 2 MPa of hydrogen happens quickly without any incubation time. The Ti30V60Mn3.3Cr6.6 + 4 wt.% Zr alloy showed the fastest kinetics and highest hydrogen absorption (3.8 wt.%). For this composition, replacing Zr with Hf made the first hydrogenation slower and reduced the capacity to 3.4 wt.%. No activation was observed for the same alloy without additives. As the alloy without additives did not absorb hydrogen at all, it means that the presence of these high concentrations of Zr (or Hf) is essential for quick first hydrogenation.

Effect of Hf addition on the microstructure and mechanical properties of electron beam welded Mo-La2O3 alloy

Electron beam welding of La2O3 dispersion strengthened molybdenum alloy is conducted in this research. Hf interlayers with different thicknesses are introduced to joining Mo alloy via in-situ alloying to avoid grain boundary oxidation and improve the mechanical properties of joints. The porosity ratio within the weld reduces from 7.53% (without Hf interlayer) to 1% ∼ 2% (with Hf interlayers of 0.1 mm–0.35 mm). The microstructure of the weld is mainly composed of Mo base solid solution for the joints with a Hf interlayer of 0.05 mm. As the thickness of the Hf interlayers increases from 0.1 mm to 0.35 mm, HfMo2 is generated within the weld and the content of HfMo2 phase increases significantly. The introduction of Hf promotes the grain refinement of the weld owing to the generation of HfO2 precipitates, which provide more heterogeneous nucleation sites for the liquid metal to nucleate. In addition, the formation of HfO2 consumes the O elements in the weld and prevents the generation of MoO2 along grain boundaries, which is beneficial for the improvement of the mechanical properties of the joints. The results show that the introduction of Hf significantly improves the strength of the joint. The ultimate strength of the joint with a Hf interlayer of 0.1 mm especially for the joint with a Hf interlayer of 0.1 mm reaches 333 MPa, which is 6 times more than that of the joint without a Hf interlayer of 54 MPa.

Predicting short-range order evolution in WTaCrVHf refractory high-entropy alloys

Short-range order (SRO) in multicomponent concentrated alloys affects their mechanical response. Hence, is paramount to understand how composition modifies the chemical ordering in the system to design materials with optimal properties. We present here a methodology to predict the SRO and thermodynamic properties in chemically complex systems and apply it to the WTaCrVHf quinary alloy. We observe that the addition of Hf significantly modifies the SRO, mainly at intermediate to low temperatures, matching experimental observations.

Influence of Strengthening Elements and Heat Treatment on Microstructure and Fracture Toughness of NiAl-Cr(Mo)-Based Eutectic Alloy.

Due to their potential improvement of high-temperature properties, the refractory metal hafnium (Hf) and the rare earth holmium (Ho) have attracted much attention. In the present research, NiAl-Cr(Mo) eutectic alloys with different Ho and Hf additions were fabricated by conventional smelting method and heat-treated to study the synergetic influence of strengthening elements and heat treatment. The samples were characterized using XRD, SEM, and TEM, and the three-point bending test was performed to obtain fracture toughness. The results demonstrate that Hf addition leads to the formation of Ni2AlHf Heusler phase and that Ho promoted the formation of Ni2Al3Ho phase. The microstructure of the alloy is affected by thermal treatment, with the coarsening of eutectic lamellae after heat treatment. The mechanical properties are improved by Hf and Ho additions, with increased fracture toughness. Overall, this study provides insights into the microstructure and properties of NiAl-Cr(Mo) eutectic alloys and highlights the potential of Hf and Ho addition to improve room-temperature properties. Specifically, the as-cast NiAl-Cr(Mo)-Hf eutectic alloy contains a relatively fine NiAl/Cr(Mo) eutectic lamella but coarse eutectic cell with Ni2AlHf phase embellished along the cell boundary. Minor Ho addition induces the formation of Ni2Al3Ho phase, which leads to the coarsening of the intercellular region but contributes to the refining of eutectic cell. In addition, the synergetic effect of Ho and Hf promotes the precipitation of Ni2Al3Ho and Ni2AlHf phases in the intercellular zone and increases the interface dislocations. Heat treatment benefits the solid solution of Ni2Al3Ho and Ni2AlHf phases, which improves their size and distribution by secondary precipitation. The Ni2AlHf phase in the NiAl-Cr(Mo)-Hf eutectic alloy becomes fine and uniformly distributed, but the NiAl/Cr(Mo) eutectic lamella in the eutectic cell becomes coarse. In comparison, heat treatment mainly optimizes the size and distribution of the Ni2Al3Ho and Ni2AlHf phases in the NiAl-Cr(Mo)-Hf-Ho eutectic alloy. Furthermore, heat treatment helps to eliminate the interface dislocations in the large NiAl precipitates and the NiAl/Cr(Mo) phase interfaces, which also contributes to fracture toughness by decreasing stress concentration. Minor Ho addition decreases the fracture toughness of as-cast NiAl-Cr(Mo)-Hf eutectic alloy from 6.7 to 6.1 MPa·m1/2, which should be ascribed to the coarsened intercellular region including aggregated Ni2Al3Ho and Ni2AlHf phases. However, minor Ho-doped NiAl-Cr(Mo)-Hf eutectic alloy obtained the highest fracture toughness of 8.2 MPa·m1/2 after heat treatment. This improved fracture toughness should be mainly attributed to the refined and well-distributed Ni2Al3Ho and Ni2AlHf phases in the heat-treated NiAl-Cr(Mo)-Hf-Ho eutectic alloy.

Effect of Homogenization on the Transformation Temperatures and Mechanical Properties of Cu15Ni35Hf12.5Ti25Zr12.5 and Cu15Ni35Hf15Ti20Zr15 High-Entropy Shape Memory Alloys.

The major challenge of high-temperature shape memory alloys (SMAs) is the collocation of phase transition temperatures (TTs: Ms, Mf, As, Af) with the mechanical properties required for application. Previous research has shown that the addition of Hf and Zr into NiTi shape memory alloys (SMAs) increases TTs. Modulating the ratio of Hf and Zr can control the phase transformation temperature, and applying thermal treatments can also achieve the same goal. However, the influence of thermal treatments and precipitates on mechanical properties has not been widely discussed in previous studies. In this study, we prepared two different kinds of shape memory alloys and analyzed their phase transformation temperatures after homogenization. Homogenization successfully eliminated dendrites and inter-dendrites in the as-cast states, resulting in a reduction in the phase transformation temperatures. XRD patterns indicated the presence of B2 peaks in the as-homogenized states, demonstrating a decrease in phase transformation temperatures. Mechanical properties, such as elongation and hardness, were improved due to the uniform microstructures achieved after homogenization. Moreover, we discovered that different additions of Hf and Zr resulted in distinct properties. Alloys with lower Hf and Zr had lower phase transformation temperatures, followed by higher fracture stress and elongation.

Effects of hafnium content on microstructures and properties of newly developed (Ti,Hf)(C,N) ceramics

In this study, novel (Ti,Hf)(C,N) ceramics with varying hafnium contents were fabricated via carbothermal reduction–nitridation and subsequent spark plasma sintering. The influence of Hf addition on the mechanical properties, wear properties, and corrosion resistance of the (Ti,Hf)(C,N) ceramics was systematically studied. The introduction of Hf promoted the sintering densification of the ceramics in the sintering process. The prepared (Ti,Hf)(C,N) ceramics exhibited excellent mechanical and wear properties owing to refinement and solution-strengthening mechanisms. The (Ti0.9,Hf0.1)(C0.5,N0.5) ceramic demonstrated higher Vickers hardness and fracture toughness, measuring 1997 HV5 and 4.28 MPa m1/2, respectively, compared to the pure Ti(C0.5,N0.5) ceramic which exhibited values of 1635 HV5 and 3.94 MPa MPa m1/2. The wear scar depth of the (Ti0.9,Hf0.1)(C0.5,N0.5) ceramic sample was 57.36% to that of the Ti(C0.5,N0.5) ceramic. Additionally, the addition of Hf improved the corrosion resistance of (Ti,Hf)(C,N) ceramics in a 0.5 M NaOH solution. The potential applications of (Ti,Hf)(C,N) ceramics include machining tools and wear-resistant parts.

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.

Perspectives on novel refractory amorphous high-entropy alloys in extreme environments

Two new refractory amorphous high-entropy alloys (RAHEAs) within the W–Ta–Cr–V and W–Ta–Cr–V–Hf systems were herein synthesized using magnetron-sputtering and tested under high-temperature annealing and displacing irradiation using in situ Transmission Electron Microscopy. While the 14W-41Ta-26Cr-19V in at.% RAHEA (defined as WTaCrV RAHEA) was found to be unstable under such tests, additions of Hf in this system composing a new quinary 24W-40Ta-18Cr-5V-13Hf in at.% RAHEA (defined as WTaCrVHf RAHEA) was found to be a route to achieve stability both under annealing and irradiation. A new effect of nanoprecipitate reassembling observed to take place within the WTaCrVHf RAHEA under irradiation indicates that a duplex microstructure composed of an amorphous matrix with crystalline nanometer-sized precipitates enhances the radiation response of the system. It is demonstrated that tunable chemical complexity arises as a new alloy design strategy to foster the use of novel RAHEAs within extreme environments. New perspectives for the alloy design and application of chemically-complex amorphous metallic alloys in extreme environments are presented with focus on their thermodynamic phase stability when subjected to high-temperature annealing and displacing irradiation.

Microstructure and High-Temperature Ablation Behaviour of Hafnium-Doped Tungsten-Yttrium Alloys

W is a widely used refractory metal with ultra-high melting point up to 3410 °C. However, its applications are limited by poor ablation resistance under high-temperature flame and air flow, which is crucial for aerospace vehicles. To improve the ablation resistance of W under extreme conditions, W-Y alloys doped with different Hf mass fractions (0, 10, 20, and 30) were prepared using the fast hot pressing sintering method. Microstructure and ablation behaviours at 2000 °C were investigated. Results showed that adding an appropriate amount of Hf improved the properties of the W-Y alloy evidently. In particular, the hardness of the alloy increased with the increased content of Hf. The formation of the HfO2 layer on the surface during ablation decreased the mass and linear ablation rates, indicating enhanced ablation resistance. However, excessive Hf addition will result in crack behaviour during ablation. With a Hf content of 20 wt.%, the alloy exhibited high stability and an excellent ablation resistance.

Void swelling in additively manufactured 316L stainless steel with hafnium composition gradient under self-ion irradiation

Compositionally-graded austenitic 316L stainless steel (SS) samples with five different Hafnium (Hf) concentrations (up to 1 wt.%) were additively manufactured by directed energy deposition and then were irradiated using 5 MeV Fe2+ ions to 50 displacements per atom (dpa) at 500, 550, and 600 °C, respectively. A rastering beam was used to ensure homogenous irradiations for the large areas of ∼1.40 cm2. Composition-dependent void evolution was evaluated. Void size, void number density, and void swelling all tend to decrease with increasing Hf concentration at all three temperatures. When the nominal Hf concentration increases to 1 wt.% in the doped additively manufactured (AM) 316L SS, the void swelling is over an order of magnitude lower than that in pure AM 316L. Atom probe tomography results showed that about 0.14 wt.% Hf dissolved in the matrix at the 1 wt.%. Hf nominal concentration. The suppression of void swelling by Hf addition shows qualitative agreement with the numerical calculation based on the vacancy trapping mechanism, indicating that the oversized Hf reduces the steady state vacancy concentration, and hence the incubation period of swelling is extended, resulting in a dramatic difference in void swelling. Other factors including grain boundaries, dislocation and dislocation cells, Hf-rich particles, and delta ferrite are secondary to this mechanism. This work shows that additive manufacturing enabled microalloying is promising for developing void swelling resistant materials.

Effects of Hf and Zr Additions on Microstructure, Phase Components and Martensitic Transformation Temperatures of Ni3Ta High-Temperature Shape Memory Alloys

Effects of Hf and Zr Additions on Microstructure, Phase Components and Martensitic Transformation Temperatures of Ni3Ta High-Temperature Shape Memory Alloys

Effect of Hf addition on evolution of microstructure and strengthening behavior in Ni-based ODS superalloy

In this study, two kinds of Zr-containing Ni-based ODS superalloys, ODS-Zr (Ni-20Cr-0.5Ti-0.3Al-0.6Zr-0.6Y2O3, wt%) and ODS-Zr+Hf (Ni-20Cr-0.5Ti-0.3Al-0.6Zr-0.8Hf-0.6Y2O3, wt%), were prepared by high-energy milling and hot isostatic pressing (HIP) process. Evolution of microstructure and tensile property caused by Hf addition was analyzed and discussed. The result showed that the addition of Hf changes significantly oxide type and refines oxide size. Y4Zr3O12, Y2TiO5 and three-type Y-Al oxides including monoclinic-structure Y4Al5O12 (YAM), orthogonal and hexagonal YAlO3(YAM and YAH) are found in ODS-Zr. In contrast, only Y2Hf2O7, Y4Zr3O12 and Y2TiO5 but no Y-Al oxides are viewed in ODS-Zr+Hf. The average diameter of oxides in ODS-Zr and ODS-Zr+Hf is 16.6 nm and 13.0 nm, respectively. The size difference between two alloys is ascribed to different oxide constitution in two alloys. Moreover, Hf addition enhances tensile strength of alloys at 25 °C and 700 °C meanwhile improves tensile ductility at 700 °C.

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.

Enhanced plastic deformation capacity in hexagonal-close-packed medium entropy alloys via facilitating cross slip

• A novel Hf x (ZrTi) 100-x (x=0, 20, 33 and 40 at.%) MEAs with single phase HCP was developed. • Addition of Hf enhanced the strain hardening rate and tensile ductility. • Effects of Hf on stacking fault energy and dislocation slip behavior were revealed. • Facilitated cross slip and pyramidal dislocations is responsible for the outstanding mechanical properties. Alloys with a hexagonal close-packed (HCP) lattice often suffer from intrinsic brittleness due to their insufficient number of slip systems, which limits their practical uses. In this paper, nevertheless, we show that remarkably tensile ductility in HCP Hf-Zr-Ti medium entropy alloys (MEAs) was achieved, particularly in the MEAs with a higher content of Hf. Both first-principles calculation and experimental analyses reveal that addition of Hf increases basal I2 stacking fault energy and decreases prismatic stacking fault energy in these HCP MEAs, which promotes the source of pyramidal dislocations due to the facilitated cross slips of basal dislocations and eventually give rise to the observed large tensile ductility. Our current findings not only shed new insights into understanding deformation of HCP alloys, but also provide a basis for controlling alloying effects for developing novel HCP complex alloys with optimized properties.

From cyclic (alkyl)(amino)carbene (CAAC) precursors to fluorinating reagents. Experimental and theoretical study.

Addition of anhydrous HF to the hydrochloride [MeCAACH][Cl(HCl)0.5] resulted in the formation of salts with high HF content. By stepwise removal of HF in vacuo, we selectively prepared [MeCAACH][F(HF)2] (3) and [MeCAACH][F(HF)3] (4). We also characterised a salt with [F(HF)4]- anions within the structure of [MeCAACH][F(HF)3.5] (5). Compounds with a lower content of HF were not accessible under vacuum conditions. MeCAAC(H)F (1) was selectively prepared by abstraction of HF from 3 with CsF or KF, while [MeCAACH][F(HF)] (2) was prepared by mixing 3 and 1 in a 1 : 1 ratio. Compound 2 proved to be quite unstable as it tends to disproportionate into 1 and 3. This observation triggered our computational study, in which the structural relationships between CAAC-based fluoropyrrolidines and dihydropyrrolium fluorides were investigated using different DFT methods. The study showed that the results were very sensitive to the computational method used. For a correct description, the quality of the triple-ζ basis set was crucial. Surprisingly, the isodesmic reaction of [MeCAACH][F] + [MeCAACH][F(HF)2] → [MeCAACH][F(HF)] + [MeCAACH][F(HF)] did not confirm the low thermodynamic stability of 2. Furthermore, the use of 3 as a nucleophilic fluorinating reagent was tested on a range of organic substrates, as it is the most stable compound in this series. It was found to have the potential to fluorinate benzyl bromides, 1- and 2-alkyl bromides, silanes and sulfonyls with good to excellent yields of the target fluorides.

Comparing the role of Zr and Hf atoms on microstructure and mechanical properties optimization of Ti2AlN reinforced Ti48Al 0.5W composites

In order to further optimize the mechanical properties of TiAl composites, Ti2AlN/Ti48Al0.5W composites with 1 at% Zr/Hf addition were prepared by arc melting, which were termed as Base alloy, Zr-containing alloy and Hf-containing alloy. The grain size, element distribution, phase content and compressive properties were investigated and compared in this work. With 1 at% Zr/Hf addition, the lamellae colony size decreased from 33 µm to 25/23 µm, and Zr atoms almost dissolved into the single γ phase in the form of solid solution atoms, while Hf atoms distributed in both α2/γ lamellae and single γ phase, and part Hf atoms also existed in the form of B2 phase, which illustrated the higher solid solution of Zr atoms in TiAl composites comparing with Hf atoms. The γ phase content increased and the α2 phase content decreased, especially with 1 at% Zr addition, which acted as γ-phase stabilizing element. With 1 at% Hf addition, the compressive strength and strain increased from 2010 to 2221 MPa and from 23.9 % to 26.2 %, respectively, which was attribute to the solid solution strengthening of Hf atoms by substituting Ti atoms, refinement of α2/γ lamellae and increased γ phase content. Compared with 1 at% Hf addition, 1 at% Zr addition led to higher compressive strength and strain, which were 2344 MPa and 26.8 %, respectively, and this result was due to the higher concentration of Zr atoms in γ phase and higher content of γ phase. On account of density, cost and performance, Zr atoms are more suitable as an alloying element to further optimize the microstructure and mechanical properties of TiAl composites.

Effect of HF, H2SiF6, and Al3+ ions on the crystal growth process and morphology of α-type hemihydrate calcium sulfate in phosphoric acid solution

The reaction of calcium dihydrogen phosphate solution with sulfuric acid in the presence or absence of HF, H2SiF6, and Al3+ ions was used to simulate the crystal growth process of α-type hemihydrate calcium sulfate (α-HH) with 30% P2O5 at a reaction temperature of 95 °C. The changes in Ca2+ ion concentration in the solution were measured at different reaction times. Then, the growth rates and reaction steps of α-HH crystals were fitted using the crystal growth kinetic equation. The growth morphologies of α-HH crystals were characterized using scanning electron microscopy, X-ray diffractometry, X-ray photoelectron spectrometry, and differential scanning calorimetry, and the influence mechanism of HF, H2SiF6, and Al3+ ions on the crystal growth process of α-HH were deeply investigated. Results show that the addition of HF, H2SiF6, and Al3+ ions decreased the growth rate constants of α-HH. Without HF, H2SiF6 and Al3+ ions, the growth rate constants of α-HH was 0.7452 × 10−3 (mol·min−1·m−2). When 0.060 mol·L−1 HF was added, the growth rate constants of α-HH decreased to 0.4954 × 10−3 (mol·min−1·m−2). When 0.015 mol·L−1 Al3+ ions and 0.060 mol·L−1 HF were added, the growth rate constants of α-HH decreased to 0.4548 × 10−3 (mol·min−1·m−2). The inhibition of α-HH growth by HF, H2SiF6, and Al3+ ions can be ranked as follows: HF, H2SiF6, and Al3+ ions > HF and Al3+ ions > H2SiF6 > HF. The reaction steps of α-HH in phosphoric acid solution in the presence or absence of HF, H2SiF6, and Al3+ ions were close to 2, and the growth of α-HH crystals was mainly affected by surface diffusion, adsorption, and helical growth mechanism. HF, H2SiF6, and Al3+ ions inhibited the growth of α-HH crystals along the C-axis, resulting in a decrease in aspect ratio, and the morphology of α-HH crystals is short columnar. These impurity ions decreased the intensity of diffraction peaks and crystallinity of the (200), (310), and (400) crystal planes of the α-HH crystals.

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.