The Hf–Au system

The Hf–Au system has been reinvestigated by means of differential thermal analysis, X-ray diffraction and electron probe microanalysis. The existence of the seven intermetallic phases, previously reported, Hf₂Au, HfAu, Hf₇Au₁₀, HfAu₂, HfAu₃, HfAu₄ and HfAu₅ was confirmed. We specify here their type of formation. For the equiatomic compound, we have shown the presence of a martensitic transformation and of allotropic transitions.     

Martensite structure in Ti–Ni–Hf–Cu quaternary alloy ribbons containing (Ti,Hf)2Ni precipitates

The martensite structure in a Ti₃₆Ni₄₄Hf₁₅Cu₅ ribbon annealed at different temperatures is investigated. When the annealing temperature is <873 K, spherical (Ti,Hf)₂Ni particles 20–40 nm in diameter precipitate in the grain interior. Transmission electron microscopy analysis shows that (0 0 1) compound twins are dominant in the ribbon containing homogeneously distributed (Ti,Hf)₂Ni precipitates. When the annealing temperature is 773 K, the boundaries between the martensite domains with the (0 0 1) twins are blurry and vague. When the annealing temperature is 873 K, four types of boundaries among the martensite domains are found: {1 1 1}, (0 0 1)//{1 1 1}, {1 1 3} and (1 1 0)//{1 1 3} types. When the annealing temperature is 973 K, the (0 1 1) twins become dominant, and the martensite variants show mainly spear-like and mosaic-like morphologies. However, martensite domains with (0 0 1) twins also exist around the coarse (Ti,Hf)₂Ni precipitates. Fine (Ti,Hf)₂Ni precipitates should be responsible for the improvement in shape memory effect and the superelasticity of Ti–Ni–Hf–Cu ribbons.     

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

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

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Phase separation and microstructure evolution of rapidly quenched Gd–Hf–Co–Al alloys

Phase separated metallic glasses were prepared in the Gd–Hf–Co–Al system by rapid quenching of the melt. Due to the strong positive enthalpy of mixing between the main constituent elements Gd and Hf (ΔH_Gd–Hf = +11 kJ/mol) a heterogeneous microstructure is formed consisting of two amorphous phases, which are Gd-enriched and Hf-enriched, respectively. The size of the phase separated regions varies from 0.1 μm to 5 μm, depending on alloy composition and cooling rate. Different types of microstructure, such as an interconnected structure or a droplet structure were obtained as a function of cooling rate. The microstructure of the phase separated metallic glasses is determined not only by their composition, the critical temperature, and the shape of the miscibility gap, but also by the viscosity and diffusivity of the melt.     

High-strength Cu-based bulk glassy alloys in Cu–Zr–Ti and Cu–Hf–Ti ternary systems

New Cu-based bulk glassy alloys were formed in Cu–Zr–Ti and Cu–Hf–Ti systems by the copper mold casting method. The critical diameter is 4 mm for the Cu₆₀Zr₃₀Ti₁₀ and Cu₆₀Hf₂₅Ti₁₅ alloys which are larger than 1 mm for the Cu₆₀Zr₄₀ and Cu₆₀Hf₄₀ glassy alloys. The substitution of Zr or Hf for Ti causes an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (T_g), crystallization temperature (T_x), and the supercooled liquid region ΔT_x(=T_x−T_g) decrease for both Cu₆₀Zr_40−xTi_x and Cu₆₀Hf_40−xTi_x alloys. In contrast, the liquid temperature (T_l) has a minimum value of 1127 K for the Cu₆₀Zr₂₀Ti₂₀ alloy and 1175 K for the Cu₆₀Hf₂₀Ti₂₀ alloy, resulting in a maximum T_g/T_l of 0.63 and 0.62, respectively. The alloys with the highest T_g/T_l value showed the highest GFA for these Cu-based alloys. The bulk glassy alloys exhibit high tensile fracture strength of 2000–2160 MPa, compressive fracture strength of 2060–2150 MPa and compressive plastic elongations of 0.8–1.7%. The finding of the new Cu-based bulk glassy alloys with high GFA, high fracture strength above 2000 MPa and distinct plastic elongation is encouraging for the future development of a new type of bulk glassy alloy which can be used for structural materials.     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.     

Comparison of high temperature shape memory behaviour for ZrCu-based,Ti–Ni–Zr and Ti–Ni–Hf alloys

New ZrCu-based high temperature shape memory alloys with M_s close to 500 K are under development. The shape memory behaviour of this material is compared to those of Ti–Ni–Zr and Ti–Ni–Hf alloys. The optimal compositions show a shape recovery of not less than 3% at temperatures above 470 K.     

Microstructure evolution of eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the near-eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf(at%) alloy was directionally solidified at 1900 ℃ with withdrawal rates of 6, 18, 36, 50 mm·min~(-1) and then heat-treated at 1450 ℃ for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified(DS) alloy is composed of Nb_(ss)+Nb_5Si_3 eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes,the morphology and size of Nb_(ss)+Nb_5Si_3 eutectic cells.With the increase in withdrawal rates, the petaloid Nb_(ss)+Nb_5Si_3 eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nbssis formed gradually which isolates the Nb_5Si_3, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb_5Si_3 exist in the heat-treated(HT) samples identified by crystal form and element composition, which are supposed as α-Nb_5Si_3 and γ-Nb_5Si_3, respectively. This study exhibits significant merits in guiding the optimization of Nb–Si-based alloys' mechanical properties.     

Effect of NiAl Microcrystalline Coating on the High-Temperature Oxidation Behavior of NiAl–28Cr–5Mo–1Hf

The effect of an NiAl microcrystalline coating prepared by magnetron sputtering on the high-temperature oxidation behavior of NiAl–28Cr–5Mo–1Hf was investigated in static air at 1000–1150°C. The additions of Cr, Mo, and Hf changed the single -phase structure into a multiphase structure [-NiAl, -Cr(Mo), and Heusler phase]. The NiAl–28Cr–5Mo–1Hf alloy formed a nonprotective mixed scale of Al₂O₃+Cr₂O₃+HfO₂ and exhibited relatively large weight gains. The large weight gains were attributed to extensive internal oxidation. The sputtered NiAl microcrystalline coating remarkably improved the oxidation resistance of NiAl–28Cr–5Mo–1Hf due to the formation of a compact and adherent Al₂O₃ scale at all test temperatures. It was found that the --Al₂O₃ transformation caused the anomalous behavior of the oxidation–kinetics curves of the NiAl microcrystalline coating in the temperature range 1000–1150°C. A change in the morphology of scales occurred with the transformation.     

A quinary Ti–Zr–Hf–Be–Cu high entropy bulk metallic glass with a critical size of 12 mm

A quinary Ti₂₀Zr₂₀Hf₂₀Be₂₀Cu₂₀ high entropy bulk metallic glass (HE-BMG), with a critical diameter (D_c) of 12 mm—the largest size in the reported quinary high entropy alloys (HEAs), has been successfully prepared by copper mold casing. This novel HE-BMG possesses a supercooled liquid region ΔT (=T_x − T_g) of 78 K, indicating a better thermal stability than those of other HE-BMGs. In addition, the HE-BMG exhibits a relatively good compressive plasticity (2.2 ± 0.1%) among the HE-BMGs. The newly developed HE-BMG may offer insights for the indepth understanding of the fundamental issues associated with the glass formation and the unique structure-property relationship when combining the features of HEAs and BMGs together.     

Electrochemical reduction behavior of Hf(Ⅳ) in molten NaCl–KCl–K_2HfCl_6 system

The electrochemical reduction mechanism of hafnium ion(Ⅳ) was studied in NaCl–KCl–K_2HfCl_6 melts on a molybdenum electrode. The cyclic voltammetry study shows that Hf(Ⅳ) is reduced to hafnium metal in double two-electron process, that is: Hf(Ⅳ)+2e~-→Hf(Ⅱ) and Hf(Ⅱ)+2e~-→Hf, and the electrochemical reduction of Hf(Ⅳ) process was diffusion-controlled. The diffusion coefficients were calculated at several temperatures, and the results obey the Arrhenius law. According to the relationship of lnD versus 1/T,the corresponding activation energy was determined to be 158.8 kJ ·mol~(-1). The square wave voltammetry results further confirm the reduction mechanism of hafnium.     

TEM study of structural and microstructural characteristics of a precipitate phase in Ni-rich Ni–Ti–Hf and Ni–Ti–Zr shape memory alloys

The precipitates formed after suitable thermal treatments in seven Ni-rich Ni–Ti–Hf and Ni–Ti–Zr high-temperature shape memory alloys have been investigated by conventional and high-resolution transmission electron microscopy. In both ternary systems, the precipitate coarsening kinetics become faster as the Ni and ternary element contents (Hf or Zr) of the bulk alloy are increased, in agreement with the precipitate composition measured by energy-dispersive X-ray microanalysis. The precipitate structure has been found to be the same in both Hf- and Zr-containing ternary alloys, and determined to be a superstructure of the B2 austenite phase, which arises from a recombination of the Hf/Zr and Ti atoms in their sublattice. Two different structural models for the precipitate phase were optimized using density functional theory methods. These calculations indicate that the energetics of the structure are not very sensitive to the atomic configuration of the Ti–Hf/Zr planes, thus significant configurational disorder due to entropic effects can be envisaged at high temperatures. The precipitates are fully coherent with the austenite B2 matrix; however, upon martensitic transformation, they lose some coherency with the B19′ matrix as a result of the transformation shear process in the surrounding matrix. The strain accommodation around the particles is much easier in the Ni–Ti–Zr-containing alloys than in the Ni–Ti–Hf system, which correlates well with the lower transformation strain and stiffness predicted for the Ni–Ti–Zr alloys. The B19′ martensite twinning modes observed in the studied Ni-rich ternary alloys are not changed by the new precipitated phase, being equivalent to those previously reported in Ni-poor ternary alloys.     

Initial crystallization processes of Hf–Cu–M (M=Pd,Pt or Ag) amorphous alloys

Hf₇₀Cu₃₀, Hf₇₀Cu₂₀Pd₁₀, Hf₇₀Cu₂₀Pt₁₀, Hf₇₀Cu₂₀Ag₁₀, Hf₇₃Pd₂₇ and Hf₇₃Pt₂₇ amorphous alloys were prepared by a single roller melt-spinning method and the crystallization process was studied by differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopy. A single exothermic peak was observed in the DSC curves of Hf₇₀Cu₃₀, Hf₇₀Cu₂₀Ag₁₀ and Hf₇₃Pt₂₇ amorphous alloys, which corresponds to the precipitation of stable crystalline phases. Multiple exothermic peaks were observed in the DSC curves of Hf₇₀Cu₂₀Pd₁₀, Hf₇₀Cu₂₀Pt₁₀ and Hf₇₃Pd₂₇ alloys, of which the low-temperature one corresponds to the precipitation of nanoscale icosahedral quasicrystalline phase (I-phase). The I-phase is in a metastable state since further annealing leads its transformation to other stable crystals. The relation among the atomic configurations of the amorphous alloy, I-phase and stable crystalline phases is discussed.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Microstructure and room temperature mechanical properties of NiAl–Cr(Mo)–(Hf,Dy) hypoeutectic alloy prepared by injection casting

The NiAl–Cr(Mo)–(Hf,Dy) hypoeutectic alloys were prepared by conventional casting and injection casting techniques respectively, and their microstructure and room temperature mechanical properties were investigated. The results reveal that with the addition of Hf and Dy, the Ni₂AlHf Heusler phase and Ni₅Dy phase form along the NiAl/Cr(Mo) phase boundaries in intercellular region. By the injection casting method, some Ni₂AlHf Heusler phase and Ni₅Dy phase transform into Hf and Dy solid solutions, respectively. Moreover, the microstructure of the alloy gets good optimization, which can be characterized by the fine interlamellar spacing, high proportion of eutectic cell area and homogeneously distributed fine Ni₂AlHf, Ni₅Dy, Hf solid solution and Dy solid solutions. Compared with conventional-cast alloy, the room temperature mechanical properties of injection-cast alloy are improved obviously.     

Hf对Ni-Ti-Al合金高温变形行为影响

采用热模拟实验研究了Ni50Ti46-xAl4Hfx(x=0～6,at%)合金的微观组织特点和高温变形行为。合金由基体相NiTi和第二相Ti2Ni组成。Hf全部溶解在基体相和第二相中,并且促进了第二相的析出,显著提高了合金的高温强度。Ni-Ti-Al-Hf合金在700～800℃下变形行为的研究表明:合金具有良好的高温压缩性能,流变应力随着应变速率的减小或变形温度的增加而降低,其高温变形行为符合温度补偿幂规律描述。Hf的添加使合金的应力指数(n)和变形激活能(Q)上升,这主要是由于Hf的固溶强化、促进Ti2Ni相析出强化和高温稳定性造成的。     

含Hf铸造镍基高温合金中Ni_5Hf的转变和次生MC_(2)的形成

本文鉴定含Hf铸造镍基高温合金中的Ni_5Hf相.发现这类合金中存在着下列两种反应: Ni_5Hf+γ(C)→MC_(2)+γ或γ′(Hf)+γ(C)→MC_(2)+γ并讨论这些反应对合金组织稳定性的影响.