Coarsening of hafnium carbide particles in tungsten

The coarsening behaviour of finely dispersed HfC particles in a W-HfC alloy was investigated by monitoring the growth rate of the particles. An activation energy of 480 kJ mol^–1 was obtained for the process. Diffusion experiments of hafnium in tungsten were conducted at temperatures between 1773 and 2573 K using a secondary ion mass spectroscopy technique to determine the diffusion contribution to the coarsening process. The diffusion process at high temperature is controlled by lattice diffusion with an activation energy of 335 kJ mol^–1 whereas that at low temperature is governed by grain-boundary diffusion with an activation energy of 170 kJ mol^–1. It appears that the coarsening process is controlled by two energy barriers: one dictated by the diffusivity of hafnium and the other by the solubility limit as a function of temperature. The strain energy required to dissociate the carbide particles into individual species was also considered. The effects of the coarsening of HfC particles in a dispersion-strengthened W-0.4 mol% HfC alloy on recrystallization and creep deformation were illustrated using a concerted experimental modelling analysis. Results show that the strengthening effect of the HfC particles is significantly reduced at temperatures above 1800 K, due to particle coarsening.     

HfC前驱体的合成及其裂解行为研究

以HfCl₄、乙酰丙酮、无水甲醇、对苯二酚为原料, 采用一锅法合成了HfC陶瓷前驱体, 通过前驱体裂解制备得到了HfC陶瓷粉体。采用XRD、FTIR、SEM/EDS、TEM、SAED等分析手段对裂解产物的组成、形貌和微观结构进行了分析和表征, 利用TG-DSC和TG-MS对前驱体的裂解行为进行了研究。结果表明: HfC前驱体在600℃左右开始陶瓷化, 在1300℃左右开始形成HfC陶瓷相, 在1500℃及以上完全转化为HfC陶瓷及自由碳。HfC陶瓷相具有单晶结构, 颗粒粒径在50~100 nm之间。HfC陶瓷相的形成基于前驱体在低温段裂解形成的HfO₂的碳热还原反应, 裂解过程中形成的自由碳抑制了HfC晶体的生长。     

Phase Transitions and Oxidation Behavior During Oxyacetylene Torch Testing of TaC–HfC Solid Solutions

Tantalum carbide (TaC) and hafnium carbide (HfC) have some of the highest melting temperatures among the transition metal carbides, borides, and nitrides, making them promising materials for high-speed flight and high-temperature structural applications. Solid solutions of TaC and HfC are of particular interest due to their enhanced oxidation resistance compared to pure TaC or HfC. This study looks at the effect of Hf content on the oxidation resistance of TaC–HfC sintered specimens. Five compositions are fabricated into bulk samples using spark plasma sintering (2173 K, 50 MPa, 10 min hold). Oxidation behavior of a subset of the compositions (100 vol% TaC, 80 vol% TaC + 20 vol% HfC, and 50 vol% TaC + 50 vol% HfC) is analyzed using an oxyacetylene torch for 60 s. The TaC–HfC samples exhibit a reduction in the oxide scale thickness and the mass ablation rate with increasing HfC content. The improved oxidation resistance can be attributed to the formation of a Hf₆Ta₂O₁₇ phase. This phase enhances oxidation resistance by reducing oxygen diffusion and serving as a protective layer for the unoxidized material. The superior oxidation resistance of TaC–HfC samples makes these materials strong contenders for the development of high-speed flight coatings.     

Coarsening of particles connected by dislocations

This work theoretically reinvestigates the coarsening of particles controlled by diffusion through a dislocation network, when the dislocation spacing of the network is larger than the maximum particle size and the volume fraction of the particles vanishes. Both a three-dimensional network and a plane one, such as that which might be encountered at a low-angle grain boundary, are treated. It is considered here that the number of dislocation pipes for diffusion increases with distance away from the particle. Under certain reasonable assumptions, at ^1/4 steady-state coarsening kinetics is found. The origin of the differences between the present kinetic results (t^1/4) and those predictions previously reported in the specialized literature (t ^1/5), for similar dislocation geometries, are discussed. The effect of the dislocation annihilation phenomenon on the growth kinetics is also examined qualitatively.     

Carbon fiber reinforced hafnium carbide composite

Hafnium carbide is proposed as a structural material for aerospace applications at ultra high temperatures. The chemical vapor deposition technique was used as a method to produce monolithic hafnium carbide (HfC) and tantalum carbide (TaC). The microstructure of HfC and TaC were studied using analytical techniques. The addition of tantalum carbide (TaC) in the HfC matrix was studied to improve the microstructure. The microstructure of HfC, TaC and co-deposited hafnium carbide-tantalum carbide (HfC/TaC) were comparable and consisted of large columnar grains. Two major problems associated with HfC, TaC, and HfC/TaC as a monolithic are lack of damage tolerance (toughness) and insufficient strength at very high temperatures. A carbon fiber reinforced HfC matrix composite has been developed to promote graceful failure using a pyrolytic graphite interface between the reinforcement and the matrix. The advantages of using carbon fiber reinforcement with a pyrolytic graphite interface are reflected in superior strain capability reaching up to 2%. The tensile strength of the composite was 26 MPa and needs further improvement. Heat treatment of the composite showed that HfC did not undergo any phase transformations and that the phases comprising composite were are thermochemically compatible.     

Dissolution and coarsening of large niobium carbonitrides in a microalloyed steel

Observations and measurements of the kinetics of coarsening and dissolution of large cuboidal niobium carbonitrides during solution treatments of a high nitrogen niobium microalloyed steel are reported. At temperatures between 1473 and 1573 K a competitive coarsening and dissolution process was established where the larger niobium carbonitrides grew at the expense of the smaller, or employing niobium and nitrogen which remained in solid solution. In this temperature range growth or dissolution rates and critical sizes could be determined from the analysis of the evolution of particle size distribution. At higher temperatures (1623–1723 K), only a dissolution process existed, where the dissolution rates as a function of particle size was found to increase with increasing temperature.     

Advances in pressureless densification of boron carbide

The coarsening processes which occur concurrently with the sintering of B₄C were deduced using differential dilatometry, weight loss and grain size measurements. B₂O₃ coatings on B₄C particles stalled the onset of densification until these coatings decomposed away at 1840°C. Oxide gaseous species formed in this process were postulated to be one conduit of particle coarsening. At higher temperatures, the volatilization of B₄C itself were interpreted to be the second vapor phase coarsening mechanism, starting at temperatures above 2000°C. Carbon additions or soaking the compact in a hydrogen-containing atmosphere at 1350°C removed the oxide coatings at temperatures below those in which sintering or coarsening occurred. Accelerating densification above 2130°C was attributed to nonstoichiometric volatilization, leaving carbon behind to facilitate activated sintering.     

Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55Y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling

An ultrafine-grained(UFG) Mg-13Zn-1.55 Y alloy(ZW132) with a high volume fraction(7.4%) of icosahedral phase(I-phase, Mg_3Zn_6Y) particles was prepared by applying high-ratio differential speed rolling(HRDSR) on the cast microstructure following homogenization. The alloy exhibited excellent superplasticity at low temperatures(tensile elongations of 455% and 1021% 473 K-10~(-3)s~(-1) and 523 K-10~(-3)s~(-1),respectively). Compared with UFG Mg-9.25Zn-1.66 Y alloy(ZW92) with a lower volume fraction of I-phase particles(4.1%), which was prepared using the same processing routes, the UFG ZW132 alloy exhibited a higher thermal stability of grain size. Rapid grain coarsening, however, occurred at temperatures beyond523 K, leading to a loss of superplasticity. The high-temperature deformation behavior of the HRDSRprocessed ZW132 alloy could be well described assuming that the mechanisms of grain boundary sliding and dislocation climb creep competed with each other and considering that the grain-size was largely increased by accelerated grain growth at the temperatures beyond 523 K.     

Dispersion/Solution Strengthening of Molybdenum at High Temperatures

Tensile properties of pure molybdenum, a molybdenum - 0.5 at % hafnium carbide, and a molybdenum - 5 at % rhenium - 0.5 at % hafnium carbide were evaluated with a strain rate of 10'Vsec over a temperature region of 1200 to 2400 K in ultrahigh vacuum. The yield strengths, tensile strengths, solution and dispersion strength increments, strain-hardening exponents, and tensile elongations of these materials were determined. The effects of rhenium, hafnium carbide, and temperature on the tensile properties of molybdenum were examined. The fine size and superior thermostability of hafnium carbide: dispersoids resulted in a strong pinning effect on dislocations at high temperatures. Dispersion strengthening by hafnium carbide particles was significant in the entire temperature range employed. Solution strengthening by rhenium was effective up to approximately 1800 K. The microstructures of the post-test molybdenum-base alloys were characterized with a scanning electron microscope and a transmission electron microscope. The dominant deformation mechanism of these alloys was found to be dislocation sliding up to 1800 K. and was grain-boundary sliding at higher temperatures. The dominant strengthening mechanisms involved in these alloys included the long-range and the short-range interactions between the hafnium carbide dispersoids and the dislocations.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

High temperature tensile properties of oxide dispersion strengthened T91 and their correlation with microstructural evolution

Abstract

In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y₂O₃ particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M₂₃C₆ carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.     

Fracture-resistant ultralloys for space-power applications: Normal-spectral-emissivity and electron-emission studies of tungsten, rhenium, hafnium-carbide alloys at elevated temperatures

A series in this journal on high-temperature properties of “fracture-resistant ultralloys for space-power systems” preceded the present paper: the antecedent publications covered tungsten(W), rhenium(Re) alloys with and without thoria(ThO₂) (W, 23Re; W, 27Re; W, 30Re and W, 30Re, 1ThO₂). This paper reports radiative and thermionic effects of hafnium carbide(HfC) and Re variation in W alloys: normal spectral emissivity(ε_λ) is used in pyrometry to determine the true temperature of a surface. Effective work function (φ_e) is an important consideration in the selection of the electrode materials for high-temperature thermionic energy converters in space-power applications. The _0.535μ, ε_0.65μ and φ_e trends of W, Re, 0.35HfC with 5–20% Re were measured in the range of 1700–2500K. The results indicate that ε_λ decreases with increasing temperatures and Re contents. The presence of HfC produced higher ε_λ values than those of sintered materials with comparable W,Re alloy contents. The results also indicate that φ_e increases with rhenium contents. This can be explained as growth of the potential barrier at the metal, vacuum boundary associated with a volume effect—the decrease in the lattice constant of W.     

Thermodynamic modeling and preparation of hafnium carbide coatings in the hafnium–carbon–fluorine system

Thermodynamic modeling of the Hf–C–F system has been carried out in wide temperature and pressure ranges. Analysis of the calculated molecular composition of the vapor phase in equilibrium with Hf + HfC and the vapor phase in equilibrium with C + HfC has shown that the chemical vapor transport of hafnium under isothermal conditions is mediated by lower hafnium fluorides. The content of the lower hafnium fluorides increases with increasing temperature and decreasing total pressure in the system. Reactive hafnium carbide deposition on a carbon material, with CF₄ as a transport agent, has been studied experimentally. The coating obtained in this way is sufficiently dense and consists of hafnium carbide nanocrystals ranging in size from 100 to 200 nm.     

Thermal stability of MgO nanoparticles

Thermal stability of nanocrystalline MgO particles with average diameter of 11 nm was investigated by annealing of the cold isostatically pressed compacts between 600 °C and 900 °C for various durations. Sintering time versus grain radius at 800 °C demonstrated a linear line with the slope of ∼ 4 similar to that expected for surface diffusion. High resolution scanning electron microscope images from different specimens showed a porous microstructure of interconnected particles typical for initial sintering. Arrhenius plot of the grain size data revealed the activation energy of 161 ± 11 kJ mol^− 1 for the growth process in agreement with those reported for grain boundary grooving experiments. It was found that MgO particles undergo coarsening already at temperatures as low as 0.31 of the MgO melting point (3125 K). Increase in the particle diameter and decrease in the surface area were associated with surface diffusion mechanism that leads to initial sintering between the particles.     

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

The agglomeration process of MgO powder derived from Mg (OH)₂ was investigated at fixed temperatures of 600, 800, 900 and 1200° C; these temperatures were chosen as representative of four regions, i.e. below 600° C, 650 to 850° C, 850 to 1050°C and 1050 to 1200° C previously reported. At 600° C, coherent crystallites coalesced within the heating time of 60 min; on further heating till 300 min, the primary particles which consisted of crystallites grew rapidly. The original Mg (OH)₂ framework or pseudomorphs, composed of minute crystallites and primary particles, still remained in the powder. At 800° C, the pseudomorphs had disintegrated into fragments. The crystallite growth and primary particle growth were accelerated with increasing the heating times beyond 60 min. At 900° C, a further fragmentation of agglomerates occurred with increasing the heating times; the crystallite and primary particle growth in fragments brought about the pore coalescence. At 1200° C, the crystallite and primary particle growth proceeded with the coarsening of pores; on heating beyond 240 min, the crystallites and primary particles grew rapidly due to the entrapment of pores within them.     

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

Abstract

Three alloys based on nickel, with a high level of chromium (25 wt-%) and containing varied quantities of carbon, 0·25 and 0·50 wt-%, and hafnium, 3·7 and 5·6 wt-%, fabricated by casting, were subjected to a 46 h long exposure at 1200°C in dry industrial air. The aim of the work was to investigate the thermal stability of their carbide interdendritic network and to discover their general behaviour in high temperature oxidation. The volume fraction of the hafnium carbides decreased slightly during high temperature exposure but their fragmentation was rather limited. In contrast, chromium carbides appeared in the two alloys, which initially contained exclusively HfC, and this may result in a decrease in their high temperature properties. The evolution of the carbides appeared to be responsible for a moderate lowering of room temperature hardness. The behaviour of the three alloys during high temperature oxidation was very good, despite the unusually high content of hafnium. All were chromia-forming, although oxidation of Hf led to HfO₂ islands in the external scale and in the subsurface region. In summary, the behaviour of these three alloys showed that the HfC containing Ni–25Cr family is potentially interesting for applications at very high temperatures.     

Hydrogen combustion under conditions of a high-temperature supersonic flow

Some results of studying the combustion of hydrogen in a high-velocity air flow at stagnation temperatures below 3000 K are given. Flame shape and combustion zone length change at temperatures above 2000–2200 K due to deterioration in the fuel–oxidizer mixing process. It has been shown that dissociation processes have an effect on the process of hydrogen combustion at a high temperature. In experiments, they can be detected by a change in the intensity of radiation from the intermediate ОН* radical.     

Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels

The effects of Cr and/or Mo additions and tempering temperatures on mechanical properties in relation to the microstructural evolution during tempering were investigated in induction-tempered steels. The additions of Cr and/or Mo result in the finer distribution of cementite particles due to the decrease in the coarsening rates of cementite particles above tempering temperature of 400°C, while their influence is less effective at low tempering temperatures. Accordingly, the increments of tensile strength and yield strength by the addition of alloying elements become more pronounced at high temperatures above 400°C. The occurrence of maximum peak of yield strength at 400°C would be related to further precipitation of the cementite at low temperatures, and the subsequent spheroidization and coarsening process of the cementite at high temperatures. The addition of alloying elements does not change the minima in Charpy impact values, related to tempered martensite embrittlement, since alloying elements do not have an influence on the decomposition of retained austenite and the formation of the cementite at boundaries. The strain-hardening exponent, n, decreases up to 400°C and then continuously increases with tempering temperature. This abrupt increase of n at 300°C is related to the transformation of retained austenite during straining in induction-tempered steels.     

Effect of boron on the microstructure of spark plasma sintered Ultrafine WC

The microstructure of sintered carbide compacts generally contain facetted and abnormally grown grains. In the present work we show that the addition of a small quantity of boron to tungsten carbide powders can induce isotropic coarsening without any abnormal grain growth. The ability of boron to reduce faceting is brought about by the oxidation of boron at low temperatures which leads to isotropic wetting and roughening of particle boundaries during sinter-coarsening at elevated temperatures. Increase in boron content leads to enhanced grain growth and a limiting value to the boron concentration is suggested. Increase in the ambient pressure during sintering increases oxidation of boron and also the sintering temperature leading to a change in both grain shape and size. The isotropic coarsening behavior of WC in the presence of boron conforms to Jackson’s theory of crystal growth based on the energetics of a rough liquid–solid interface.     

Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al–Ni–Co–Zr–Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al₁₉(Ni,Co)₅Y₃ plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al₃Y and Al₃Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al₁₉(Ni,Co)₅Y₃ plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al₁₉(Ni,Co)₅Y₃ plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al₁₉(Ni,Co)₅Y₃ inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.