Evolution of microstructure of nickel base superalloy at high temperatures

Abstract

The evolution of primary and secondary γ′ precipitates in the high γ′ volume fraction Rene 80 Ni based superalloy has been examined during aging at elevated temperatures for periods up to 1750 h. While the increase in average dimension of particles followed the cube rate Lifshitz, Slypzof and Wagner (LSW) law, r³_t – r³₀=Kt, there were significant discrepancies between the experimental and theoretical particle size distributions (PSDs) and inconsistency with the kinetic constants associated with the two populations of particles. These differences are attributed to the influence of elastic coherency strains which have not been considered in conventional capillarity driven coarsening models. During thermal exposure at 871°C, coalescence of primary cuboidal γ′ was predominant in early stages of aging, while the microstructure was relatively stable at longer aging times. The stability of the microstructure at longer aging times is attributed to the formation of the network of closely spaced dislocations at the γ/γ′ interface which would cause the loss of internal misfit stresses associated with the growth. Secondary spheroidal γ′ particles were initially coarsened and their volume fraction gradually decreased until they completely dissolved after 500 h at 871°C or 1 h at 982°C.     

Microstructure and physical properties of Al/diamond composite fabricated by pressureless infiltration

Abstract

The Al/diamond composite was fabricated using a pressureless infiltration method. The microstructure and physical properties of the composite were investigated. The composite has a very low coefficient of thermal expansion (CTE) of 3·9 × 10^?6 K^?1. The thermal conductivity (TC) of the composite is 12% higher than that of the Al alloy matrix. The lower TC of the composite than the expected value was attributed to the existence of interfacial low conducting phases and the porocity of the composite.     

Influence of dynamic strain aging on plastic instability behaviour of near alpha titanium alloy Timetal 834

Abstract

The deformation behaviour of near alpha titanium alloy Timetal 834 was investigated by analysing Considére criterion in temperature range between 300 and 500°C at an interval of 25°C. In the dynamic strain aging (DSA) regime (400–475°C) studied in this material, Considére criterion was satisfied partially at peak DSA temperature (450°C). This was attributed to maximum DSA effect at 450°C which eventually resulted in specimen fracture at low ductility levels. Strain hardening exponent (n) values was determined using Ludwik plot, Considére criterion and obtained experimentally from uniform strain. The n values obtained from Ludwik plot and Considére criterion were greater than the uniform strain in DSA regime studied.     

Diffusion annealing and laser surface alloying with aluminium to enhance oxidation resistance of carbon steels

Abstract

The oxidation behaviour of a carbon steel was investigated after modifying the surface in three different ways, namely: (a) physical vapour deposition of aluminium by a resistance heating evaporation technique; (b) diffusion annealing of predeposited aluminium; and (c) laser surface alloying with predeposited aluminium. While diffusion annealing at a temperature of 1023 K was found to result in the formation of Al₁₃Fe₄ and Al₅Fe₂ phases, laser surface alloying resulted in a case containing Al₁₃Fe₄ and Al₂Fe phases. The oxidation behaviour of all the above specimens was studied at 873 K for a total duration of up to 200 h. The mass change and morphological modifications to the surface were monitored at regular intervals of 25 h. It was observed that aluminising results in a substantial enhancement of the high temperature oxidation resistance in comparison with the untreated base material, by virtue of the superior oxidation resistance of the intermetallic phases formed on the surface. Furthermore, laser surface alloying was found to result in better oxidation resistance, in comparison with diffusion annealed and as deposited specimens.     

Characterisation of thermally treated and untreated polyethylene–polypropylene blends using DSC,TGA and IR techniques

Abstract

The thermal characteristics of thermally treated and untreated very low density polyethylene, isotactic polypropylene and their blends were investigated. Injection moulded blends containing five different weight percentages of VLDPE/iPP were prepared and thermally treated at 100°C for 2, 4, 7 and 14 days. Differential scanning calorimetry, thermogravimetry and infrared spectral analysis techniques were used to study the effect of thermal treatment and blending ratio on the thermal and chemical stability. The addition of PE had caused the T _m, heat of fusion and percentage crystallinity of PP main melting peak to decrease, indicating that both polymers are partially miscible. T _m has been found to increase with aging time, however, the heat of fusion is not significantly affected. The initial and final decomposition temperatures, maximum decomposition rate temperature, order of decomposition reaction, activation energy and activation enthalpy were calculated, in a dynamic nitrogen atmosphere, and discussed in terms of blending ratios and aging times. The IR spectra of all blends at different aging times do not show any degradation products.     

Phase transformation and microstructure of MgO–Al2O3–SiO2 system glass–ceramics under different heat treatment conditions

Abstract

Effects of heat treatment conditions on phase transformation, microstructure and thermal expansion coefficient (TEC) in MgO–Al₂O₃–SiO₂ system glass–ceramics were investigated by means of differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnesium aluminium titanate (MAT) precipitated firstly at 850°C and β-quartz solutions (β-QSS) formed at 950°C. Further increasing temperature to 1000°C, MAT disappeared and β-QSS became master phase, following little amount of α-cordierite, MgTi₂O₅, rutile and sapphirine. When glass was treated at 1050°C, β-QSS content decreased and α-cordierite became master phase. As temperature reached higher than 1100°C, β-QSS and sapphirine disappeared, and α-cordierite became master phase accompany with rutile and MgTi₂O₅ as secondary phase. The microstructure transformed gradually from particle shape crystallites to slat shape network with the increase in heat treatment temperature. By controlling heat treatment condition, an ideal glass–ceramics with proper TEC for matching sealing to 4J29 alloy has been obtained.     

Plans for the future for the Packaging,Transport, Storage and Security of Radioactive Material Journal

Abstract

The regulatory driven design of radioactive material transportation packages leads package vendors to perform analyses that demonstrate the ability of packages to meet the regulatory requirements. For risk assessment and communication, the analysis of package response to thermal environments that are more severe than those described in the regulations is required. In general, experimental and analytical assessments of casks exposed to thermal insults other than the regulatory environment are performed in the USA by the Department of Energy national laboratories. This paper provides a brief summary of some recent thermal analyses of spent fuel transportation packages exposed to thermal environments different from regulatory standards. The analyses were performed by Sandia National Laboratories under several different projects for multiple customers. These analyses examined the response of spent fuel packages exposed to severe thermal environments different from the regulatory hypothetical accident condition. One assessment determined the response of four generic casks to very long duration engulfing fires. The results from these analyses included fire durations necessary to reach critical temperatures of the fuel and seals. In another assessment, two certified spent fuel casks were analysed for exposure to 1 h pool fires. The height of the cask above the pool was varied to study the effect of the vapour dome on the heating of the casks. Another assessment investigated the effect of offset long duration fires on rail cask performance, which showed that casks can withstand offset fires of much longer duration than the regulatory fire. Other assessments examined the response of packages to thermal environments resulting from propane fires and realistic liquid hydrocarbon fires that included various positions of the transportation rail car in the simulation.     

用逐步升压法和Weibull参数估计油纸绝缘寿命

为评估油纸绝缘老化寿命 ,采用逐步升压法加速电老化油纸绝缘模型 ,利用Weibull概率分布对同一电压等级下多次试验得到的击穿寿命统计分析 ,利用反幂函数和指数函数拟合试验数据 ,求得两个模型中的电压耐受系数 ,用以评估油纸绝缘的电老化寿命。同时用恒定电压法试验作对比分析 ,结果表明 ,两种加压方法计算结果吻合 ,而逐步升压法简单 ,数据分散性小     

Rapid sintering of iron powders under action of electric field

Abstract

A new rapid sintering technique for iron powders compacted under the action of an electric field with high current density has been advanced. The results show that the sintering densification of iron powder could be finished in less than 6 min at a temperature of 800^u C reached at a heating rate of 600 K s^?1, and the relative density of the sintered compact was over 95%. Moreover, the sintering densification was almost finished in the heating stage of the compact.     

Heat transfer analysis of blast furnace cast steel cooling stave

Abstract

Cast steel blast furnace (BF) cooling staves are widely used in the Chinese steel industry. A heat transfer mathematical model of a BF cast steel cooling stave has been developed and verified by thermal state experiments. Calculation of a cooling stave working under steady state has been carried out based on the model. Effects of two factors, thickness of scale on the cooling water pipes and gas clearance between the pipes and main body, which are difficult for experimental measurement but determined mathematically, on the temperature field of the stave body are discussed. The results indicate that much importance should be attached to the two factors during manufacturing of cooling staves as they highly influence cooling capability of cooling stave and hence BF operation.