The effects of composition on mechanical properties of W-4Re-Hf-C alloys

Arc melted W-4Re-Hf-C alloys containing up to about 0.8 mol pct HfC were fabricated into rod and sheet for evaluation of compositional effects on mechanical properties in the as-worked condition. The ductile-brittle transition temperatures (DBTT) of electropolished bend and tensile specimens were independent of HfC content in the range studied but dependent on the excess Hf or C above stoichiometric HfC. The lowest DBTT was found at Hf contents slightly in excess of stoichiometric. Tensile and creep strength variations at 0.2 to 0.5 mol pct HfC were also dependent on excess Hf and C. Maximum creep strengthening occurred also where the Hf content was slightly in excess of stoichiometric. Analyses of extracted second-phase particles indicated that they were hafnium carbide with a minor amount of tungsten carbide in solid solution. Creep strength was reduced by increasing tungsten carbide content in the particles.     

On the precipitation mechanism in the molybdenum based alloy MHC (Mo–Hf–C)

Abstract

The precipitation mechanism of small hafnium carbides in the sintered and thermo-mechanically processed molybdenum based alloy MHC (Mo–0·65Hf– 0·65C (at.-%)) is reported. Light and scanning electron microscopy revealed hafnium oxides, large hafnium carbides and molybdenum carbide layers at the grain boundaries in the as-sintered material. Additionally, atom probe tomography showed a residual dissolved content of 0·12 at.-%Hf, but no carbon in solid solution. After thermo-mechanical processing of the as-sintered material in a deformation dilatometer, transmission electron microscopy revealed small hafnium carbides with diameters of 10–100 nm. These carbides were preferentially located at dislocations and dislocation networks. Without deformation prior to aging, no formation of small hafnium carbides was observed. X-ray diffraction confirmed that decomposition of molybdenum carbides had occurred, which delivered the carbon for the formation of strain induced precipitates.     

The partitioning of refactory metal elements in hafnium-modified cast nickel- base superalloys

Data from the electron microprobe have been used to elucidate the role of hafnium in altering the microstructure of the nickel-base superalloys B-1900, Alloy 713 LC, Udimet 700, and Mar-M246. The addition of about 1.3 to 2.0 pct Hf to these alloys improves their strength and ductility at both room temperature and 760°C. The data indicate that hafnium partitions to the surface of the MC carbides, replacing titanium and some molybdenum and/or tungsten in the carbide. On the basis of the observed morphology and composition of the carbides, it is postulated that, primarily, hafnium modifies the solidification sequence, which a) results in the formation of discrete, uniformly distributed MC carbides, b) retards the formation of secondary carbides, and c) contributes indirectly to solid solution strengthening of the matrix. Subsidiary of American Metal Climax, Inc. AMAX Division.     

Regularities of the contact interaction of double carbides (Ti<Subscript>1 –<Emphasis Type="Italic">n</Emphasis></Subscript>Me<Stack><Subscript><Emphasis Type="Italic">n</Emphasis></Subscript><Superscript>IV,V</Superscript></Stack>)C with the Ni–Mo melt

Regularities of dissolution, phase formation, and structure formation during the interaction of double carbides (Ti_1–n Me _n ^{IV, V} )C with the Ni–25%Mo melt (t = 1450°C, τ = 1 h, vacuum 10^–1 Pa) are investigated for the first time by electron probe microanalysis and scanning electron microscopy. The role of each alloying metal in the composition and microstructure formation of studied compositions is revealed. It is established that Group IV alloying metals (Zr and Hf) almost do not enter the composition of the forming K-phase (carbide Ti_1–n Mo _n C _x ); therefore, its composition is independent of their concentration in double carbide. In contrast with zirconium and hafnium, Group V alloying metals (V and Nb) actively participate in the formation of the K-phase; however, the dependences of the composition of the K-phase and metallic matrix on the vanadium and niobium content are the opposite in this case. An interpretation of the causes of these distinctions is proposed.     

Diffusion Interaction in Fiber-Reinforced Composites of the Chromium Alloy – Silicon Carbide System

Diffusion interaction in the Cr – ZrC and Cr – HfC systems has been studied in the temperature range 1200-1400°C. The impurities contained in chromium or carbides were found to affect the interaction in the systems. Almost no interaction was observed when the systems were held at temperatures up to 1300°C for 100 h. At higher temperatures the carbides were reduced to ZrO₂, HfO₂, and Cr₂₃C₆ because of the presence of a slight amount of oxygen. Thermodynamic calculations indicated no interaction in these systems at temperatures up to 1600°C. A study of the interaction in the systems SiC – ZrC and SiC – HfC showed that a transition zone formed already in the stage of sample preparation by diffusion welding in vacuum (1300°C, vacuum of 10^_3 Pa, with a load applied for 20 min). During annealing (1300°C, 50 h) the transition zone stratified, forming a solid solution of silicon in Zr(Hf)C and SiC inclusions in the SiC – Zr(Hf)C solid solution. A transition zone formed on the zirconium carbide side when SiC interacted with Zr(Hf)C. The interaction in SiC – Zr(Hf)C casts doubt on the use of them as a barrier without antidiffusion layers.     

Structural Features of Al–Hf–Sc Master Alloys

Microstructural features of new master alloys of the Al–Hf–Sc system with metastable aluminides with a cubic lattice identical to the lattice of a matrix of aluminum alloys are investigated using optical microscopy, scanning electron microscopy, and electron probe microanalysis. Binary and ternary alloys are smelted in a coal resistance furnace in graphite crucibles in argon. Alloys Al–0.96 at % Hf (5.98 wt % Hf) and Al–0.59 at % Hf (3.77 wt % Hf) are prepared with overheating above the liquidus temperature of about 200 and 400 K, respectively. Alloys are poured into a bronze mold, the crystallization rate in which is ~10³ K/s. Metastable Al₃Hf aluminides with a cubic lattice are formed only in the alloy overheated above the liquidus temperature by 400 K. Overheating of ternary alloys, in which metastable aluminides Al _n (Hf_1–xSc _x ) formed, is 240, 270, and 370 K. Depending on the Hf-to-Sc ratio in the alloy, the fraction of hafnium in aluminides Al _n (Hf_1–xSc _x ) varies from 0.46 to 0.71. Master alloys (at %) Al–0.26Hf–0.29Sc and Al–0.11Hf–0.25Sc (wt %: Al–1.70Hf–0.47Sc and Al–0.75Hf–0.42Sc) have a fine grain structure and metastable aluminides of compositions Al _n (Hf_0.58Sc_0.42) and Al _n (Hf_0.46Sc_0.54), respectively. Sizes of aluminides do not exceed 12 and 7 μm. Their lattice mismatch with a matrix of aluminum alloys is smaller than that for Al₃Sc. This makes it possible to assume that experimental Al–Hf–Sc master alloys manifest a high modifying effect with their further use. In addition, the substitution of high-cost scandium with hafnium in master alloys can considerably reduce the consumption of the latter.     

SHS Metallurgy of Composite Materials Based on the Nb–Si System

Composite materials (CMs) based on niobium with functional and alloying additives (Si, Hf, Ti, Al, etc.) have prospects for industrial approval in aviation propulsion engineering. The authors previously showed that such CMs can be synthesized in an autowave mode (combustion mode) using highly exothermic mixtures of Nb₂O₅ with Al, Si, Hf, and Ti. It was found that hafnium actively participates in the reduction of Nb₂O₅, which complicates its introduction into the CM. This study is directed at investigating the possibility to synthesize Nb-based composite materials with a high Hf content using methods of centrifugal SHS metallurgy. It is shown in experimental investigations using a centrifugal installation under the effect of acceleration of 40 g that the replacement of active Hf by its less active compounds Hf–Al or Hf–Ti–Si–Al in the composition of the Nb₂O₅/Al mixtures makes it possible to transfer the combustion of the mixture from the explosion-like mode into the steady-state combustion mode. The content of Hf in the CM increases with an increase in the size of Hf–Al granules from 0–40 to 160–300 μm from 1.3 to 3.8 wt %. The introduction of Hf–Ti–Si–Al granules with a particle size from 1 to 3 mm into the initial charge makes it possible to form cast CMs based on niobium silicides with a Hf content up to 8.1 wt %. The integral composition and distribution of base and impurity elements in structure components of cast CMs, as well as their phase composition, were determined using electron microscopy and X-ray phase analysis. CMs with the maximal Hf content (8.1 wt %) contain three structural components: (1) the base, which includes Nb, Si, and Ti; (2) intergrain boundaries containing Nb, Ti, and Al; and (3) inclusions based on hafnium oxide. Three phases are revealed in the X-ray diffraction pattern of the CM, notably, solid solutions based on Nb and Nb5Si3, as well as a minor amount of Nb₃Si.     

The separation of zirconium and hafnium in a molten salt-molten zinc system

The commercial separation of zirconium and hafniumvia aqueous/organic extraction procedures is both difficult and expensive. The principal objective of the present work was to study an alternative procedure involving the oxidation-reduction equilibria of zir-conium and hafnium between a molten salt phase and molten zincvia the displacement reaction: Zr(IV)_{molten salt} + Hf_{molten zink} ⇌ Hf(IV)_{molten salt} + Zr_{molten zink} The reaction goes strongly to the right with an equilibrium constant: logK =0.432 ×10⁴/T - 1.565 for Na₂Zr(Hf)F₆ dissolved in NaKCl₂ solvent, thus showing promise as the basis for an anhydrous process for separating zirconium and hafnium. The rate of approach to chemi-cal equilibrium was studied in a baffled stirred reactor. The equilibration rate was found to be controlled by the hafnium mass transfer rate from the molten zinc to the metal-salt interface. The mass transfer rate could be estimated from the physical properties of the molten metal and the salt phases, using the Mayer correlation. Formerly with the Teledyne Wah Chang Cor-poration, Albany, OR.     

两种快速测定锆铪的光度法

研究了锆和铪的二甲酚橙络合物在高氯酸介质中、硫酸钠下存在的吸收光谱,结果表明:在高氯酸介质中,Zr(Ⅳ)-XO,λmax为555~558 nm;Hf(Ⅳ)-XO,λmax为535 nm。在常温下,Hf(Ⅳ)-XO在558 nm和500 nm处的吸光度比值稳定。对锆铪混样可用双波长K系数法测出锆的含量,用差减法求出铪的量,此法适用范围为锆(铪)0~25μg/25 mL;在低温(0~5℃)时,有硫酸钠存在的条件下,铪被有效隐蔽,测出锆的含量,再结合EDTA滴定法可求出铪的含量,该法适用范围为锆0~30μg/25 mL,铪0~40μg/25 mL。两种测定分析方法的相对误差均在10%以下。     

Interaction of tungsten with tungsten carbide in a copper melt

The chemical interaction between tungsten and tungsten carbide in a copper melt with the formation of W₂C at 1300°C is studied. It is shown that the mechanical activation of a composition consisting of copper melt + W and WC powders by low-temperature vibrations initiates not only the chemical interaction of its solid components but also their refinement.     

Hard Alloy WC–24% Ni Obtained in the Solid Phase from Ultrafine WC,NiO, and C Powders. Part 2. Mechanical Properties

Mechanical properties of WC–24 mass% Ni alloy prepared by a combination in single stage of metal phase synthesis and compaction of an ultrafine mixture of WC–Ni powders by high-energy compaction and sintering are studied. Tungsten carbide, nickel oxide, and carbon are selected as the starting powders. After milling the initial powders the average particle size is 200-300 nm. Previously compacted briquettes of WC + NiO + C are heated, sintered, and pressed in the range 950-1300°C at vacuum of 0.133 Pa. Briquettes are also sintered in the liquid phase at 1350°C for comparison. Ultimate strength in bending, fracture toughness, ultimate strength in compression, and Vickers hardness are determined for specimens prepared at different temperatures. The dependence of mechanical properties on specimen consolidation temperature is studied. It is shown that these dependences for pressed specimens have a maximum at 1200-1250°C. The high level of properties (ultimate strength in bending 2500 MPa, ultimate strength in compression 3100 MPa, fracture toughness 19 MPa·m^1/2, and hardness 10.0 GPa) are achieved for a WC + Ni + C powder mixture to which carbon is added in the form of a liquid carbon-containing compound. Introduction into the mixture of commercial carbon grade P803 leads to low specimen mechanical properties. The effect on mechanical properties of porosity and pore size, and also grain boundary quality between particles is studied.     

The effect of thermal exposure on the mechanical properties of aluminum-graphite composites

The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition, after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the as-received material was 80,000 psi (552 MN/m²), which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on the extent of wetting of the fibers by the aluminum. The strength of the composites in the transverse direction was generally very low, due to poor interfacial bonding. Aluminum carbide (A1₄C₃) formed at the surface of the fibers at temperatures greater than 500?C (773 K). Development of the carbide was shown to be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually unaffected by heat-treatment up to 500?C (773 K); beyond that temperature a drastic degradation of tensile strength occurred. The degradation could be correlated with the extent of carbide development at the interface. Thermal cycling of the composites below 500?C (773 K) resulted in an observable degradation of the composite strength. Scanning electron microscopy of fractured surfaces indicated that the relatively weak interface governs the mode of failure in tension.     

Comparison between ultrafine-grained WC–Co and WC–HEA-cemented carbides

The AlFeCoNiCrTi high-entropy alloy (HEA) powders were prepared by high-energy ball milling. The ultrafine-grained WC–HEA and WC–Co-cemented carbides were fabricated through planetary ball milling and heat-pressure sintering. The microstructures and properties of the sintered alloys were compared using scanning electron microscope, X-ray diffraction, mechanical property testing and electrochemical testing. It has been shown that the AlFeCoNiCrTi HEA can be used as a binder for the ultrafine-grained WC-based cemented carbide. The WC–HEA-cemented carbide has better performances than the WC–Co-cemented carbide. The suitable contents of HEA can inhibit the WC grain growth and improve the mechanical properties and corrosion resistance of cemented carbides.     

Mechanical behavior of zirconium and hafnium in tension and compression

The mechanical behavior and substructural evolution of highly textured hafnium (Hf) has been examined in tension and compression and compared to the mechanical response of zirconium (Zr). The quasi-static work-hardening rate as a function of strain for both metals exhibits a compression-tension asymmetry. Both Zr and Hf exhibit a downward work-hardening response in tension, while each displays a parabolic and then concave upward work-hardening behavior in compression. Additionally, Hf displays higher flow stresses than Zr both in tension and compression. The stress-strain and strain-hardening curves for Zr and Hf have been characterized in terms of their propensity for deformation twinning and evolution of substructure with strain. Differences in the work-hardening rates and flow stresses as a function of the sense of the applied load and material are discussed in terms of slip-twin interactions during deformation.     

DIBK-TBP萃取分离锆铪的热力学研究

对二异丁基甲酮(DIBK)和TBP从HSCN介质中协同萃取锆铪的性能及热力学进行研究,采用对数函数外推法求得DIBK-TBP体系萃取反应的热力学平衡常数分别为log(K12,Zr)=4.73和log(K12,Hf)=-5.09,锆铪与SCN-形成配合物Zr(SCN)3+和Hf(SCN)3+的稳定性常数分别为1×109.86和1×10-0.80,而铪的分配比在硫氰酸盐存在时要大于锆的分配比,说明过渡金属离子锆和铪在硫氰酸盐存在时与一般金属离子与配位体形成的配合物的稳定性常数愈大,金属离子的分配比愈大的规律相矛盾,并计算出萃取反应的焓变分别为ΔHZr=-11.43 kJ.mol-1和ΔHHf=-7.80 kJ.mol-1,说明对锆铪的萃取反应为放热反应,升高温度不利于萃取反应的进行,常温下自由能变分别为ΔGZr=-26.54 kJ.mol-1和ΔGHf=28.57 kJ.mol-1,熵变分别为ΔSZr=51.54 J.(K.mol)-1和ΔSHf=-124.07 J.(K.mol)-1,说明铪离子比锆离子更易与SCN-形成配位键,从而生成中性分子Hf(SCN)4与有机相发生溶剂化作用而进入有机相中。     

Synthesis of nanocrystalline titanium carbide alloy powders by mechanical solid state reaction

A high-energy ball mill operated at room temperature has been used for preparing titanium carbide (TiC) alloy powders, starting from elemental titanium (Ti) and carbon (C) powders. X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) have been used to follow the progress of the mechanical solid state reaction of Ti and C powders. A complete single phase of fcc-Ti₄₄C₅₆ alloy powders is obtained after a very short milling time (20 ks). The lattice parameter (a ₀ ) of the end product of Ti₄₄C₅₆ was calculated to be 0.4326 nm. The presence of excess starting reactant materials (Ti and/or C atoms) in the final product of the alloy powders could not be detected. The end product of Ti₄₄C₅₆ alloy powders possesses homogeneous, smooth spherical shapes with an average particle diameter of less than 0.5 μm. The internal structure of the particles is marked by fine cell-like features of about 3 nm. On the basis of the results of the present study, the mechanical alloying (MA) process appears to provide a powerful tool for the fabrication of Ti₄₄C{im56} alloy powders at room temperature. The mechanism of mechanical solid state reaction for formation of Ti₄₄C₅₆ alloy powders is discussed. Formerly Lecturer of Materials Science, Mining and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City 11884, Cairo-Egypt.     

Activated sintering of W-HfC composite materials

The features of consolidation of the particles during the activated sintering of tungsten powders with different values of dispersity (d _av = 2–3 and 0.8–1.0 μm) are investigated. Sintering was activated by introducing nickel additives (up to 0.5 wt %), tungsten nanoparticles (up to 30 wt %), and finely dispersed hafnium carbide (5–30 vol %) with subsequent milling in a vibrating mill. The uniaxial compaction of the samples has been performed under pressures from 50 to 1000 MPa, and sintering was performed in vacuum at 1850°C with holding for 1 h. It is shown that the additives of tungsten carbide increase the density of sintered billets and, in combination with dispersed hafnium carbide, tungsten-based composite materials with a grain size up to 2 μm can be obtained.     

溴邻苯三酚红—氯化十六烷基三甲铵光度法同时测定锆和铪

本文对Zr(Hf)—BPR—CTMAC体系的多元配合物进行了研究.发现在锆、铪共存时,可利用试剂加入顺序的不同,其锆、铪配合物之吸光度直有极大差异,则可分别测出锆、铪的含量,适用范围为锆(铪)0～37μg/25ml.     

Comparison of Microstructures and Mechanical Properties for Solid and Mesh Cobalt-Base Alloy Prototypes Fabricated by Electron Beam Melting

The microstructures and mechanical behavior of simple, as-fabricated, solid geometries (with a density of 8.4 g/cm³), as-fabricated and fabricated and annealed femoral (knee) prototypes, and reticulated mesh components (with a density of 1.5 g/cm³) all produced by additive manufacturing (AM) using electron beam melting (EBM) of Co-26Cr-6Mo-0.2C powder are examined and compared in this study. Microstructures and microstructural issues are examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD), while mechanical properties included selective specimen tensile testing and Vickers microindentation hardness (HV) and Rockwell C-scale hardness (HRC) measurements. Orthogonal (X-Y) melt scanning of the electron beam during AM produced unique, orthogonal and related Cr₂₃C₆ carbide (precipitate) arrays (a controlled microstructural architecture) with dimensions of ~2 μm in the build plane perpendicular to the build direction, while connected carbide columns were formed in the vertical plane, parallel to the build direction, with microindentation hardnesses ranging from 4.4 to 5.9 GPa, corresponding to a yield stress and ultimate tensile strength (UTS) of 0.51 and 1.45 GPa with elongations ranging from 1.9 to 5.3 pct. Annealing produced an equiaxed fcc grain structure with some grain boundary carbides, frequent annealing twins, and often a high density of intrinsic {111} stacking faults within the grains. The reticulated mesh strut microstructure consisted of dense carbide arrays producing an average microindentation hardness of 6.2 GPa or roughly 25 pct higher than the fully dense components.     

The age hardening characteristics of Nb-Base alloys containing carbon and/or silicon: Part I. (Nb-15 At. Pct Hf)

A study was made to investigate solutioning and aging reactions, and their effects on microstructure and microhardness, in fifteen experimental Nb-base alloys, involving Nb-C, Nb-Si-C, Nb-Si, Nb-15 Hf, Nb-15 Hf-C, Nb-15 Hf-C-Si, and Nb-15 Hf-Si* systems. Alloys were solution treated, quenched, then aged for 2 h at temperatures ranging from 800 to 1600°C, to examine the effect of temperature, and aged at 900 to 1100°C isothermally for times varying from 2 to 20 h. Hardness provided the means of evaluating structural effects. Microstructural changes were determined by optical and electron microscopy, and the phases were chemically extracted and identified by X-ray diffraction or electron diffraction. In general, all of the alloys containing hafnium exhibited age hardening characteristics. This age hardening appears to be due to precipitation of the complex monocarbide (Nb, Hf) (C, O, N) phase. Replacement of some of the carbon by silicon in the Nb-15 Hf-C alloys raises the overaging temperature from 800 to 1000°C and leads to strength retention for longer times as compared to the alloys which contain carbon only. Out of several alloys studied, the Nb-15 Hf-0.46 C-2.32 Si and Nb-15Hf-l.31C-0.96 Si showed the best aging characteristics.