Simulations of Coarsening Behavior for M23C6 Carbides in AISI H13 Steel

Based on the local equilibrium assumption, coarsening behavior of M23C6 carbide at 700℃ in H13 steel was simulated by DICTRA software. The results from the calculations were compared with transmission electron microscopy (TEM) observations. The results show the interfacial energy for M23C6 in H13 steel at 700℃ is thus probably 0.7 J·m-2, which fits the experiments well. The influence of composition and temperature on the coarsening rate was also investigated by simulations. Simulations show a decrease in the coarsening rate when V/Mo ratio is increased, while the coarsening rate increases with increasing temperature.     

Coarsening behavior for M23C6 carbide in 12 %Cr-reduced activation ferrite/martensite steel: experimental study combined with DICTRA simulation

Based on the multi-component aspects of thermodynamics and diffusion, coarsening behavior of M₂₃C₆ (M = Cr, Fe, W) carbide at 650 °C in 12 %Cr-reduced activation ferrite/martensite steel has been investigated experimentally using scanning transmission electron microscopy, combined with DICTRA simulation. Both the experimental measurements as well as the simulations indicate that the interfacial energy of M₂₃C₆ carbide in this steel at 650 °C is probably 0.5 J m^?2, and the coarsening rate of M₂₃C₆ carbide is very low. The influence of a change in Mn, V, and Ta content and temperature on the coarsening rate of M₂₃C₆ carbide is also investigated. The results show that the coarsening rate is increased by adding Mn and reduced by V and Ta addition, respectively, while an increase in the coarsening rate by an order of magnitude with increasing temperature per 50 °C between 600 and 750 °C. Precipitation of Laves (Fe₂W) phase during aging has a negligible effect on the coarsening of M₂₃C₆.     

6061 铝基碳化硅复合材料搅拌摩擦焊工艺研究

目的研究搅拌道次对6061铝基碳化硅复合材料的影响。方法对6061铝基碳化硅复合材料进行了搅拌摩擦焊实验。结果获得了型面良好,表面光滑的焊接截面,焊核区组织为细小的等轴晶,热机影响区为弯曲变形的晶粒,热影响区组织发生了明显的粗化。结论搅拌次数越多,Si C颗粒分布越均匀,热机影响区与热影响区附近硬度最低,热影响区硬度较低,焊核区硬度比母材稍高。     

Karbidpartikelbildung und Diffusion in Stählen

Carbide Particle Formation and Diffusion in Steels The adjustment of dispersed microstructures in Cr and Mn alloyed steels by precipitation of carbide particles M₃C was carried out from the martensitic initial condition by annealing in the temperature range T = 500°…700°C about the time t = 1…4290 min. In this connection it was already shown that the nucleation, the particle growth and coarsening run off parallel, where the coarsening is the dominating process. With the increase of the particle volume according to r³ α t (r – particle radius) final particle sizes were obtained asymptotically. The increase of the particle volume in the coarsening stage can be only maintained, if small particles will be dissolved again. This process can be written well with the Ostwald-ripening relationships. Permanent concentration changes at the nucleation sites resp. in the decompositing matrix are necessary. By means of the Ostwald-ripening law the effective chemical diffusion coefficients D_eff of the alloying elements C, Mn, Si Mo and Cr in the α-Fe-matrix could be evaluated. D_eff changes with the decomposition of the matrix by a few orders of magnitude. The diffusibility of the elementes depends on the alloying content in the matrix. C has as expected the greatest mobility, Cr shows the smallest one. The lattice bond seems to be of significance. Moreover the values of activation energy of carbide particle precipitation Q and the frequency factors D₀ in the Arrhenius-relationship were evaluated applied to the steels investigated. With this the necessary diffusion data relative to short till to longtime thermal treatment for the adjustment of heat-treated microstructures are available. The lower and the upper limiting values of the carbide particle size spectra can be taken from the plots referred to the Mn and Cr alloyed steels.     

Effects of growth rate on carbides and microporosity in DS200 + Hf superalloy

The effects of growth rate on the carbide morphology and microporosity were investigated using DS200 + Hf superalloy, between 16.7×10^–6 and 266.7×10^–6 m s^–1. The fact that the shape factor remained almost unchanged with the growth rate indicates that the shape of the carbide particles does not directly depend on the cooling rate in this alloy. The stability of carbide particles was considered in terms of the interfacial energy between the carbide and matrix interface and the fluctuation of carbide composition. It was observed that the carbide/--matrix interfacial area per unit volume as a function of growth rate remained almost unchanged (especially above 66.7×10^–6 m s^–1), indicating that the rate of coarsening of carbides during solidification is not affected by the carbide/matrix specific interface energy. One of the factors which determines the rate controlling step for the coarsening of carbide particles is suggested to be Ti in the interdendritic and grain-boundary regions, and Hf in the vicinity of the incipient melting region.     

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.     

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.     

The role of tungsten in the coarsening behaviour of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbide in 9Cr–W steels at 600 °C

Tungsten is sometimes used as a solute in the so called 9Cr creep-resistant steels for use in the power generation industry, in part because of its ability to reduce the coarsening rate of M₂₃C₆ carbides. The mechanism by which it does so is not fully understood and indeed has been shown to be inconsistent with multicomponent coarsening theory, which predicts an opposite effect. This makes the role of tungsten to the coarsening behaviour of the carbide remain unsolved over decades. The work presented here is an attempt to show that the influence of tungsten can be reconciled with experiments, if kinetic study ignores the presence of Laves phase (Fe₂W) in the system.     

Annealing kinetics of cold-rolled plain carbon steels

Softening of rimmed steel during batch annealing is due to recrystallization, grain growth and carbide coarsening. The post-recrystallization softening process is used commercially to control product quality and its kinetics can be characterized by an apparent activation energy. This parameter is shown to increase with decreasing carbon content owing to the influence of carbide particle dissolution on the grain growth rate. The latter effect is suppressed by low manganese and high sulphur contents because of the formation of fine manganese sulphide precipitates. The apparent activation energy of softening can be applied to improve the economics of industrial batch annealing.     

Applications of microstructural characterization and computational modeling in damage analysis of a turbine blade exposed to service conditions in a power plant

Microstructural characterization and computational modeling were used to analyze the damage produced by overheating of a turbine blade exposed to service conditions in a power plant. Various electron-optical techniques were used to characterize the microstructure. Localized overheating was reflected by the microstructural features of the blade material particularly the extent of interdiffusion between the coating and alloy substrate, coarsening of the γ′-phase, and re-precipitation of M₂₃C₆ carbide by a discontinuous mechanism at grain boundaries. Damage associated with these effects included creep cavities at grain boundaries and intergranular oxidation leading to ductile intergranular cracking at the leading edge of the blade. Most evidence pointed out that improper internal cooling of the blade resulted in excessive overheating at leading edge. Qualitatively, the temperature profile across the blade as indicated by microstructural variations was consistent with the results derived from computational modeling.     

Structure of centrifugally cast austenitic stainless steels: Part 2 Effects of Nb,Ti, and Zr

Abstract

The microstructures of several centrifugally cast stainless steels containing strong carbide formers (Nb, Ti, and Zr) have been examined as cast and after prolonged creep in the range 800–1000°C. These additions refine the eutectic carbide, changing the morphology and composition as illustrated by the behaviour of IN 519 (Fe–Ni–Cr–Nb) and IN 519 TZ (Fe–Ni–Cr–Nb–Ti–Zr). The carbides present have been identified by electron diffraction and by energy-dispersive X-ray analysis. During creep, the precipitation of both MC and M₂₃C₆ carbides was observed, the former being very much finer and very resistant to coarsening. The role of both the grain-boundary carbide networks and the matrix precipitation in determining the creep properties is discussed.

MST/136     

The limit of non-stoichiometry in silicon carbide

The extent of the silicon carbide single-phase stability field has been investigated. Samples were equilibrated at 2400 ° C by coarsening of fine-grain silicon carbide powder. The lattice parameter, density, and the silicon-to-carbon ratio were measured on silicon- and carbon-saturated samples. These two compositions were not distinguishable at a level of better than one part in one thousand by their molecular weights per mole of crystal sites; no native point defects measurably respond to the difference in silicon activity. The accuracy of the lattice parameter and density measurements require that the free energies of defect pair formation be larger than about 3eV. This applies to antisite pairs, Frenkel pairsand Schottky pairs. It is concluded that silicon carbide is largely stoichiometric. The crystal chemistry must be dominated by electrons, holes and impurities.     

Removal of surface carbides from spark-machined molybdenum and tungsten

When reactive metals such as molybdenum and tungsten are shaped by electrical discharge machining in the kerosene-based electrolytes generally employed for the process, they become covered with a thin, multiply-melted surface layer which contains carbide phases of variable composition. This surface layer is sometimes undesirable because its chemical and mechanical properties are different from the base metals. It has been demonstrated that the carbide can be removed by a high-temperature anneal in a reducing atmosphere. The anneal also removes surface cracks on molybdenum introduced by the machining, but the heat treatments employed here did not remove surface cracks on tungsten. The high-temperature anneal had the unfortunate side effect of inducing a significant degree of grain coarsening in the metals.     

Increase in the yield of silicon carbide whiskers from rice husk

Favourable conditions for the growth of good quality silicon carbide (SiC) whiskers from rice husk have been discussed in the light of available evidence on the probable growth mechanism and the theoretical understanding of the same. Preliminary results indicate an increase in whisker yield at lower temperatures and coarsening of whiskers with longer duration of conversion.     

Effect of a transverse thermal gradient on the microstructure of a directionally solidified 73C eutectic alloy

The thermal stability of a directionally solidified (d. s.) eutectic CoCrCAl-alloy in a thermal gradient is compared to isothermal conditions. A device has been designed and constructed for examination of d. s. eutectics in high temperature gradients at temperatures above 1300 K in different atmospheres and vacuum. Experiments in this gradient furnace show coarsening of the carbide fibres forming the aligned strengthening phase. A microstructural instability resulting from the transformation of carbide fibres has been observed. These effects are discussed in relation to isothermal conditions and their implications on the applicability of the alloy as high temperature gas turbine blade material.     

Modelling precipitation of niobium carbide in austenite: multicomponent diffusion,capillarity, and coarsening

Abstract

The growth of niobium carbide in austenite involves the diffusion of both niobium and carbon. These elements diffuse at very different rates. A model is presented for the overall transformation kinetics of niobium carbide precipitation in austenite that takes into account the multicomponent nature of the diffusion process while at the same time allows for the curvature of the transformation front. The inclusion of the curvature (capillarity) effect has, in a numerical scheme, permitted the precipitation and coarsening reactions to be treated in a single model. The model is compared with published experimental data.     

Structural characterization of some ion-nitrided steels

Four steels of various compositions were ion nitrided in a plasma consisting of 14vol.%N₂-86vol.%H₂ at 525 °C for 10 h. The nitrided depth is seen to be inversely proportional to the alloy content of the steels while hardenability characteristics exhibit the opposite behavior. The case-core interface is seen to be delineated by the different etching characteristics of the two domains and the slight grain coarsening introduced into the nitrided layer. Field ion microscopy shows the microstructure to consist of carbide particles within the matrix at the tempered and annealed conditions while nitriding is seen to introduce nitride particles in the vicinity of the undissolved carbide clusters.     

Phase decomposition and grain growth in (W,Ti) C-Co-alloys

The object of this study was to obtain information on the inhomogeneous structure of the WC-TiC solid-solution phase after sintering with cobalt or nickel acting as binder. For this purpose, the solubility of WC in TiC was determined at temperatures from 1500 to 2100° C. Furthermore, grain growth and phase decomposition behaviour were investigated in the temperature ranges 1450 to 1550° C and 1700 to 1850° C. Phase-decomposition of oversaturated mixed carbide solid solution which is not capable of further coarsening enabled the reaction constant at the solid-liquid interface to be determined.     

Kinetics of niobium carbide precipitation in a low carbon austenitic steel

An examination has been made of the kinetics of niobium carbide precipitation in a 18-10-1 austenitic stainless steel in the temperature range 650 to 750° C. Electrical resistance-time plots, thin film electron microscopy and hardness measurements have been employed to follow the ageing sequence. In these alloys the carbides precipitate on undissociated dislocations and in association with stacking faults; these processes are diffusion controlled and have an activation energy of ～318 kJ mol^?1. Prior to the reaction beginning a clear incubation period existed, e.g. ～30 h at 650° C and 20 min at 750° C. During the first 10% transformation the carbide nucleation rate increases and the associated faults nucleate and grow rapidly. The carbide nucleation rate appears to peak around this level and then falls away gradually to zero around 70% transformation. At this latter stage fault growth ceases, and transformation continues by a carbide growth process. The age-hardening peak occurs much beyond the end of the reaction by which time precipitate coarsening is in evidence. The precise effect mechanical deformation has upon stacking fault formation depends to a major extent on the niobium supersaturation in the quenched alloy.     

<Emphasis Type="Italic">In-situ</Emphasis> TEM observations of the coarsening of a nanolamellar structure in a cobalt based magnetic alloy

The thermal stability and coarsening of nanostructures is of both scientific interest and of engineering significance in order to produce thermally stable nanomaterials. Real time observations were carried out using ultra high vacuum (UHV) in situ TEM to investigate the coarsening process of a highly modulated nanolamellar structure obtained by crystallization of a Co based Co₆₅Si₁₅B₁₄Fe₄Ni₂ amorphous magnetic alloy. The coarsening process consisted of three steps: (a) precipitation of spherical fine precipitates; (b) continuous coarsening of the nanolamellar structure at the surface and precipitation at the grain boundaries; and (c) formation of a stable multiphase structure. Due to surface effects, continuous coarsening of nanolamellar structure was observed during in-situ annealing; this mechanism was different from that of the coarsening process found during conventional annealing. Discontinuous coarsening from grain boundaries, which dominates the coarsening process in the conventional annealing of bulk sample, also occurred in in-situ annealing of thin sample. The driving force for coarsening of the nanolamellar structure from interlamellar interfaces, grain boundaries and surfaces is discussed.