Oxidation of hafnium carbide and hafnium carbide with additions of tantalum and praseodymium

The oxidation behavior of HfC, HfC-25 wt. % TaC, and HfC-7 wt.% PrC₂ has been studied between 1200–2200° C. Parabolic growth of the oxide layer has been observed for both HfC and HfC-TaC over the entire temperature range. A break in the temperature dependence of the oxidation kinetics occurs around 1600°C. At lower temperatures, the kinetics are limited by gaseous diffusion via pores in the oxide. Above 1800°C, gaseous diffusion through pores becomes less important as scale-growth kinetics are dominated by bulk (ambipolar) diffusion of oxygen and electrons through the oxide.     

Properties of tungsten-rhenium and tungsten-rhenium with hafnium carbide

Historically, tungsten-25wt.% rhenium alloy has been manufactured into wire for the thermocouple market, but recent demands for high-temperature structural components have forced the development of novel processing techniques for tungsten-rhenium and tungsten-rhenium with hafnium carbide. With a melting temperature of 3,050°C, and a recrystallization temperature near 1,900°C, tungsten-rhenium alloys are being used in aerospace, temperature measuring, and friction stir welding applications. The mechanical properties and microstructures of tungsten-25wt.% rhenium and tungsten-25wt.% rhenium with hafnium carbide are reported at ambient temperature, 1,371°C, and 1,926°C, after processing by three methods: hot isostatic pressing, swaging, and extrusion.     

Nickel titanium and nickel titanium hafnium shape memory alloy thin films

Shape memory alloy (SMA) coatings of NiTi and NiTiHf have been deposited onto Si substrates using pulse DC sputtering. Coatings of NiTi with compositions containing 45 to 65 at.% Ti have been fabricated by co-sputtering NiTi with Ti. NiTiHf coatings with Hf compositions ranging from 2 to 30 at.% Hf have been fabricated by co-sputtering NiTi with Hf. XRD results reveal the as-deposited coatings as amorphous. A high temperature, 1100 °C anneal followed by a low temperature, 550 °C anneal was employed to crystallise the coatings. The XRD then shows the coatings to be martensitic at room temperature.Two sets of samples were produced for characterisation; one set was used for indentation studies and the other set used to prepare freestanding films required for differential scanning calorimetry (DSC) studies.Using the DSC, a NiTi coating containing 52 at.% Ti shows an endothermic austenite peak phase transformation, (A_p) at around 105 °C and an exothermic peak martensite phase transformation, (M_p) at 65 °C, resulting in a hysteresis of 40 °C. For a NiTi coating containing 65 at.% Ti the hysteresis remained unchanged at 40 °C, but there was a decrease in the phase transformation enthalpies when compared with the coatings containing 52 at.% Ti. Calculated phase transformation enthalpies in the NiTi coatings ranged from 6 to 13 J/g for the austenite phase and − 8 to − 11 J/g for the martensite phase.The NiTiHf coating shows SMA behaviour for a film containing 30 at.% Hf. DSC reveals an ‘R’ phase transition in this film. It is understood that this phase is present in films that have high internal stresses and is understood to nucleate near Ti₃Ni₄ precipitates. Phase transformation temperatures occur at 98 °C and 149 °C during heating and occur at 99 °C during cooling. Phase transformation enthalpies range between 2 and 3 J/g for the austenite phase and − 7 J/g for the martensite phase.A scratch tester equipped with a 5 mm spherical tip has been utilised with loads ranging from 1 to 5 N to determine the recovery properties of the films. The results in this study conclude that NiTi films containing 65 at.% Ti deform 3 times more than films containing 52 at.% Ti. For NiTiHf thin films, increasing the Hf composition from 2 at.% to 30 at.%, doubled the deformation measured in the coatings.     

Synthesis mechanism of titanium carbide nano-fiber during self-propagating high temperature synthesis

Titanium carbide nano-fiber was synthesized by self-propagating high temperature synthesis (SHS) method. The final products after the SHS reaction were titanium carbide containing excess carbon and metallic titanium, which were removed by additional leaching process. TEM observation revealed that the average diameter is about 20 nm. Neutron diffraction analysis was carried out to study non-stoichiometric number of the titanium carbide. The non-stoichiometric numbers of the titanium carbide were 0.89–0.94. The Rietveld refinement of each patterns converged to good agreement (÷2=0.49–1.34). The formation mechanism of the carbide is related to a liquid-solid reaction including the preferential diffusion process of carbon atom into liquid titanium. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Ablation resistance and thermal conductivity of carbon/carbon composites containing hafnium carbide

Ablation properties and thermal conductivity of carbon/carbon (C/C) composites containing hafnium carbide (HfC) were investigated. The C/C composites containing 6.5 wt.% HfC exhibit the best thermal conductivity and ablation resistance. The improvement of the thermal conductivity is attributed to the increased phonon–defect interaction produced by the thermal motion of CO released from the reaction of carbon and ZrO₂. High thermal conductivity of the composites can slow down the ablation of carbon. When the HfC mass fraction is greater than 6.5 wt.%, cracks generated act as diffusion channels for an oxidizing atmosphere and thus accelerate the ablation of the composites.     

Grain growth of titanium carbide in nickel

The infiltration technique was employed to study the grain growth of titanium carbide in liquid nickel. Three successive stages of growth were observed; formation of idiomorphic grains, spheroidization accompanied by rapid growth, and a period of coalescence and closer packing of the carbide grains. The primary mode of grain growth appears to be by dissolution of high energy surfaces, transport of carbide through the liquid phase and isothermal redeposition.     

High-temperature oxidation of titanium carbide in oxygen

The kinetics of titanium carbide oxidation in oxygen over the temperature range of 600–1200°C and oxygen pressure from 0.1 to 740 Torr have been studied with a vacuum microbalance. Layer-by-layer x-ray analysis, petrography, metallography, and gas chromatography have been used to analyze the oxidation products. A paralinear nature of the oxidation of material was established, and the rate constants of the process were calculated for the corresponding parabolic and linear portions of the kinetic curves. It was shown that a gaseous product, CO₂, formed, as well as a solid product, TiO₂ (rutile), both stoichiometric and nonstoichiometric. The lower oxides, Ti₃O₅, Ti₂O₃, TiO, were noted in the scale at temperatures from 700 to 800° and low oxygen pressures, their relative quantity rising with decreasing pressure. Based on x-ray analysis and microhardness measurements, it was concluded that titanium oxicarbides formed in the TiC, directly adjacent to the scale. A possible oxidation mechanism of titanium carbide is proposed.     

Synthesis of titanium carbide nano-powders by thermal plasma

The paper presents a thermodynamic analysis for predicting the conditions for the plasma synthesis of TiC powders. The paper also investigates the effects of feeding rate and molar ratio. The experimental results show that TiC powders are synthesized by thermal plasma and the average size of the TiC powders is less than 100 nm.     

Synthesis of nano-structured titanium carbide by Mg-thermal reduction

Nano-structured titanium carbides were synthesized by liquid-magnesium reduction of vaporized TiCl₄ + CCl₄ solution. Fine TiC particles were produced by the reaction of released Ti and C atoms, and vacuum was used to remove the residual phases of MgCl₂ and excess Mg. Characterization of products was performed with various reaction parameters.     

Vacancies,interstitials and their complexes in titanium carbide

Titanium carbide is a prototype transition-metal compound and is used in various applications due to its exceptional hardness and stability. Here we use first-principles calculations to elucidate the properties and interactions of TiC point defects and their role on the thermal stability of TiC films. We find that the stability of defects and defect complexes depends strongly on structural details. We also show via calculations of diffusion barriers that, while carbon interstitials are relatively mobile species, the migration of carbon vacancies is suppressed unless the sample is heated to extremely high temperatures. We close with a discussion on defect-related effects and a comparison with similar physical traits in transition-metal nitrides.     

Synthesis of titanium carbide by induction plasma reactive spray

A novel method capable of sufficient mixing of titanium powder and methane of carbon source was developed in the synthesis of titanium carbide by induction plasma reactive spray. X-ray diffraction analysis, optical microscopy, scanning electron microscopy, and microhardness test were used to characterize the spray-formed deposit.The experimental results show that both primary carburization of the titanium particles inside the plasma flame and secondary carburization of the growing deposit on high temperature substrate contribute to the forming of titanium carbide. The transitional phase of TiC1-x has the same crystal structure as TiC, but has a slightly low lattice constant. The deposit consists of fine grain structure and large grain structure. The fine grain structure, harder than large grain structure, shows grain boundary fracture.     

Effects of dispersants on dispersibility of titanium carbide aqueous suspension

In this study, the dispersing phenomenon of titanium carbide suspensions has been investigated using various dispersants. The effect of pH and dispersant concentration on the dispersibility of the powder has been studied via sedimentation and zeta potential test. To optimize the pH range for the best dispersibility, the sedimentation test has been carried out in various dispersant concentrations in wide pH range. The zeta potential of TiC suspensions, both with and without polyelectrolyte addition, is examined as a function of pH. Zeta potential studies show that the isoelectric point of TiC powder is at pH 3.1. The use of an anionic polyelectrolyte, tetramethylammonium hydroxide in the optimum concentration, significantly increased the stability of suspension. The maximum value of the zeta potential − 60 mV is obtained in 0.4 wt.% at pH 8. The addition of a cationic dispersant, polyethylenimine, significantly alters the isoelectric point and shifts to the basic pH. The maximum stability of suspension was achieved in 2 wt.% at pH 8. The result showed that nonionic dispersant polyethylene glycol 400 is not a good dispersant for TiC suspension. The surface charge and potential do not change in the presence of this dispersant.     

Laser alloying of metal surfaces by injecting titanium carbide powders

The process of injecting wwar resisting particles into a laser melted surface has proven to be quite versatile, being applicable to a wide range of material combinations. The present study examines the microalloying of the metal surfaces using a pulsed Nd YAG laser. Alloying was achieved by injecting fine particles of titanium carbide into the shallow laser melted metal surface. A vacuum cell was used to prevent the effect of oxygen during the alloying process. Metals which were alloyed include En58 steel, titanium, nickel, aluminium and tantalum. A carbide volume fraction of 8–16% was achieved for the metals employed in the experiment.     

TC4钛合金和YG8硬质合金的钎焊工艺

针对钛合金和YG8型硬质合金异种材料的真空钎焊工艺和接头可靠性问题展开研究,采用润湿性实验、金相显微镜、显微硬度计、万能拉伸试验机、扫描电子显微镜等实验及测试手段,对Ag94AlMn钎焊试样接头的微观组织结构、维氏硬度、接头剪切强度等进行试验分析。结果表明,银基钎料与钛合金、硬质合金界面冶金结合良好,焊缝表面组织均匀,无微裂纹。钎缝组织为Ag基固溶体,硬质合金母材Co、W元素和钛合金母材Ti、V元素向钎缝内扩散甚少,几乎不发生母材溶蚀;TC4与YG8真空钎焊异种金属真空钎焊,选择银基钎料以及钎焊温度920℃、保温时间10 min的工艺参数,接头剪切强度最高。     

Formation of composite coatings based on titanium carbide via electrospark alloying

This work concerns studying the coatings prepared via electrospark alloying. To deposit the coatings, we used STIM-2/30 electrodes derived from a combination of self-propagating high-temperature synthesis (SHS) and extrusion. It was found that the composite coating is formed from titanium carbides and a solid solution of nickel in iron and contains both large (4–5 μm) and small carbides (less than 1 μm); in addition, large grains of titanium carbide are formed in the central portion of the coating, and the grain size decreases to 100 nm while approaching the transition zone. Large grains of titanium carbide in the coating consist of dispersed carbides with sizes less than 1 μm. It is determined that the composition of the substrate has an effect on the size of the transition and diffusion zones. It ranges from 17 μm for steel 9KhSA to 26 μm for steel 20; that is, the higher the degree of alloying, the less the depth of the modified layer.     

The axial compressive failure of titanium reinforced with silicon carbide monofilaments

Monofilament-reinforced titanium has been subjected to compressive loading, with a range of angles between the fibre axis and the loading direction. Under axial loading, the failure stress is about 4 GPa, which is well below levels predicted for kink band formation. It is proposed that compressive failure occurs under these circumstances by the crushing of individual fibres. A model is proposed for prediction of the composite strength as controlled by this mechanism. Observed strengths are consistent with monofilament crushing stresses of about 8–10 GPa. Composites were also studied after a post-consolidation heat treatment and with weak fibre–matrix interfacial bonding. In both cases, slightly higher compressive strengths were recorded than for the standard material. These increases are attributed to an enhanced matrix yield stress and to a higher monofilament compressive strength, respectively. Under off-axis loading, strengths fell from about 4 GPa at low misalignment angles to just above 1 GPa at an angle of 16°. A transition occurs between fibre crushing at low angles and kink band formation at higher angles. The transition range is around 3–4°, which is consistent with model predictions. Microstructural studies confirmed that the expected failure modes were operative in these two regimes.     

High temperature plasticity of titanium carbide in the low area of homogeneity

This paper deals with the study of structural and mechanical properties of a ceramic material which is composed on the basis of nonstoichiometric titanium carbide in the low area of homogeneity. Material for the investigation was produced by means of self-propagating high temperature synthesis (SHS) with subsequent high effort compaction. It was established that under tensile and compression straining the material had unusually high plasticity at temperatures above the toughbrittle transition (TBT) temperature. The strain rate and stress pattern strongly affect the TBT temperature. Optical and scanning electron microscopy as well as x-ray structural and energy dispersion analysis helped to establish the regularities of titanium carbide structure evolution under various conditions of sample deformation. The paper presents data on plasticity and mechanical properties of the nonstoichiometric titanium carbide. At high temperatures and low strain rates, the nonstoichiometric titanium carbide displays features of superplastic flow. At relatively high strain rates, dynamic recrystallization occurs in the titanium carbide, which results in considerable refining of microstructure, which, under certain temperature rate conditions, also results in the transition to superplastic state.