Volume 33, Issue 5

Hexagonal 1D AlN whiskers were synthesized by direct nitridation of aluminum and alumina. The effect of the atmosphere was investigated based on thermodynamic calculation. A green body compacting alumina with different particle size in between 44–420 μm in graphite crucible was found to be favorable for the large-scale synthesis of AlN at 1650 °C for 3 h in flowing nitrogen atmosphere. XRD result indicated that hexagonal AlN phase with high purity has been successfully synthesized. SEM and TEM analysis indicated that the whiskers possessed uniform morphology with 100–200 nm in diameter and length up to microns. The formation mechanism was further proposed based on the experimental results.

Four different layers of various silicon, carbon and nitrogen contents on the Ti6Al4V alloy and (001)Si wafers have been deposited by means of Plasma Assisted Chemical Vapor Deposition (PACVD) method. The layers were obtained from reactive gas mixture containing SiH 4 , CH 4 , NH 3 and Ar. After deposition the structure and chemical composition of modified surfaces have been analyzed with use of SEM/EDS technique. Based on these results and thermodynamic calculations, the diffusion coefficients, D, for nitrogen and carbon in alloy were discussed. Scratch test shown that silicon carbonitride layers have good adhesion to metal surface. In order to determine atomic structure of obtained layers, FTIR spectra for layer-(001)Si and layer-Ti6Al4V were registered.

The hot ductility of eco-friendly Bi-S based free cutting steels micro-alloyed with and without yttrium was studied using a Gleeble-1500 thermal–mechanical simulator over the temperature range 850–1200 °C. The results showed that the addition of rare earth yttrium had a substantial improvement in the hot ductility of Bi-S based free-cutting steel, especially at 1000 °C. The beneficial effect of yttrium on the hot ductility of Bi-S based free-cutting steel at the temperature no less than 1000 °C was mainly associated to the refinement of austenite grain size, which could effectively reduce the segregation density of bismuth at the grain boundary, and the lowering of the DRX onset temperature by yttrium addition. At 850–950 °C, the improvement of the hot ductility in these steels by yttrium addition might be attributed to the reduction of the low melting point sulphides at grain boundary and the refinement of the austenite grain size. However, the hot ductility of these steels micro-alloyed with yttrium was still poor at 850–950 °C, which was mainly owning to the presence of pro-eutectoid ferrite films and the absence of dynamic recrystallization as well as the segregation of liquid bismuth films at austenite grain boundaries.

New technological challenges in oil production require materials that can resist high temperature oxidation. In-Situ Combustion (ISC) oil production technique is a new method that uses injection of air and ignition techniques to reduce the viscosity of bitumen in a reservoir and as a result crude bitumen can be produced and extracted from the reservoir. During the in-situ combustion process, production pipes and other mechanical components can be exposed to air-like gaseous environments at extreme temperatures as high as 700 °C. To protect or reduce the surface degradation of pipes and mechanical components used in in-situ combustion, the use of nickel-based ceramic-metallic (cermet) coating produced by co-electrodeposition of nanosized Al 2 O 3 and TiO 2 have been suggested and earlier research on these coatings have shown promising oxidation resistance against atmospheric oxygen and combustion gases at elevated temperatures. Co-electrodeposition of nickel-based cermet coatings is a low-cost method that has the benefit of allowing both internal and external surfaces of pipes and components to be coated during a single electroplating process. Research has shown that the volume fraction of dispersed nanosized Al 2 O 3 and TiO 2 particles in the nickel matrix which affects the oxidation resistance of the coating can be controlled by the concentration of these particles in the electrolyte solution, as well as the applied current density during electrodeposition. This paper investigates the high temperature oxidation behaviour of novel nanostructured cermet coatings composed of two types of dispersed nanosized ceramic particles (Al 2 O 3 and TiO 2 ) in a nickel matrix and produced by coelectrodeposition technique as a function of the concentration of these particles in the electrolyte solution and applied current density. For this purpose, high temperature oxidation tests were conducted in dry air for 96 hours at 700 °C to obtain mass changes (per unit of area) at specific time intervals. Statistical techniques as described in ASTM G16 were used to formulate the oxidation mass change as a function of time. The cross-section and surface of the oxidized coatings were examined for both visual and chemical analyses using wavelength dispersive x-ray spectroscopy (WDS) element mapping, X-ray Diffraction (XRD) and Energy-dispersive X-ray spectroscopy (EDS). The results showed that the volume fraction for each type of particle in the nickel matrix corresponded to its partial molar concentration in the electrolyte solutions. Increase in volume fraction of particles in the nickel matrix was correlated to lower oxidation rates. It was concluded that formation of Ni 3 TiO 5 and NiTiO 3 compounds can reduce the oxidation rate of cermet coatings by capturing some inward diffusing oxygen ions resulting in a lower number of nickel cations diffusing upward into the oxide layer.

MoSi 2 functionally graded coating on Mo substrate is prepared by liquid-phase siliconizing technology. The SEM, GDS and XRD analysis shows that the silicon content in gradient layer appears in three changing regularities. Along the Mo substrate to the surface of the coating, the phase composition of gradient coating changes as follows: Mo → transition layer Mo (main phase) + Mo 3 Si + Mo 5 Si 3  → intermediate layer MoSi 2  → surface layer MoSi 2 (main phase) + Si. According to the Si-Mo diffusion couple and Arrhenius equation, the activation energy ( Q ) was obtained as 430 kJ·mol −1 , and the Arrhenius dependence of D on temperature can be described as D  = 4.9 × 10 −3 exp(−430000/RT) m 2 /s. The diffusion temperature has an important influence on silicon diffusion coefficient. As the diffusion temperature increases, silicon diffusion coefficient is also gradually increasing.

Eupatorium adenophorum was utilized as raw materials for the preparation of activated carbon via microwave assisted steam activation. Influences of the three vital process parameters – activation temperature, activation duration and steam flow rate – have been assessed on the adsorption capacity and yield of Eupatorium adenophorum activated carbon (EAAC). The process parameters were optimized utilizing the Design Expert software and were identified to be an activation duration of 45 min, an activation temperature of 950 °C and a steam flow rate of 0.7 ml/min, with the resultant iodine number and yield being 1,010 mg/g and 20.13% respectively. The validity of process model to optimize the process parameters was verified using the analysis of variance (ANOVA). The key parameters that characterize quality of the porous carbon such as the BET surface area, total pore volume and average pore diameter were estimated to be 1,142 m 2 /g, 0.84 ml/g and 3.3 nm respectively, for the sample corresponding to the optimized process conditions. Additionally the pore structure is characterized using Scanning Electron Microscope (SEM). The present work strongly supports utilization of Eupatorium adenophorum as a potential precursor through microwave heating.

Iron-based alloys are the most hopeful interconnects of intermediate temperature solid oxide fuel cell (IT-SOFC). Fe-16Cr alloy is one of the common iron-based alloys. Oxidation of the alloy is the key issue in the running process, which could be described in terms of diffusion path. Chemical potential, which is one of the primary factors to the diffusion process, is determined by the thermodynamics. The equilibrium phase in the oxidation of Fe-16Cr alloy was analyzed for better understanding of the mechanism of alloy oxidation in varied atmosphere. A novel form of thermodynamic diagram associated to the oxidation process was given. Phases in the equilibrium are changing with the quantitative increase of reacted fuel gas during the process. The diagram of Fe-16Cr alloy in atmospheres at cathode and anode sides were calculated. The diagram gives the intuitionistic phase transformation process corresponding to the practical oxidation process in the view of thermodynamics.

By means of direct aging, microstructure observation and creep property measurement, the microstructure and creep behaviors of GH4169G superalloy are investigated. Results show that, after direct aging, the grain size is inhomogeneous in the alloy, and some δ precipitates discontinuously distribute in the grain and along the boundaries, which may improve the bonding strength of the boundaries. Under the experimental conditions, the creep activation energy of the alloy during steady-state creep are calculated to be Q = 594.7 kJ/mol. During creep, the deformation features of the alloy are twinning deformation and dislocations slipping in the matrix. As creep goes on, deformed dislocations pile up near the boundary regions to induce stress concentration for promoting the initiation and propagation of cracks along boundaries.

This work investigates the effect of rejuvenation heat treatment conditions for refurbishment of the long-term serviced gas turbine blades, which were made of as-cast nickel base superalloy grade, Inconel 738. The reheat treatment conditions consist of solutionizing treatments at temperatures of 1,438, 1,458 and 1,478 K for 14.4 ks and aging treatments at temperatures of 1,133, 1,148 and 1,163 K for 43.2, 86.4, 129.6 and 172.8 ks. The results show that increase in aging times results in continuous increase of size and area fraction of gamma prime ( γ ′) particles. The higher solutionizing temperature leads to the lower area fraction and smaller size of gamma prime particles. Regarding the microstructure characteristics, the most proper reheat treatment condition should be solutionizing at temperature of 1,438 K for 14.4 ks and aging at temperature of 1,133 K for 172.8 ks, which provides the highest area fraction of gamma prime particles in proper size.

The characteristics of instability have been investigated on two low nickel austenite stainless steel at the temperatures of 950–1200 °C through the uniaxial tensile test and tensile unload test. The results show geometrical instability is not concurrent with load instability in high temperature tensile. The deformation of specimens are uniform before the load instability, then appear non-uniform in some local area of specimen but they do not directly lead to the geometric instability. With the increase of deformation temperature, load instability strain and strain hardening exponent n are both no obvious variation, and load instability true strain is close to n and depends on n . The geometric instability strain and strain rate sensitivity coefficient m both increase with the increase of temperature, and the strain between load instability and geometric instability depends on m and is the main part of the geometric instability strain.

The oxidation mechanism of copper selenide was investigated at deselenization temperatures of copper refining anode slimes. The isothermal roasting of synthetic, massive copper selenide in flowing oxygen and oxygen – 20% sulfur dioxide mixtures at 450–550 °C indicate that in both atmospheres the mass of Cu 2 Se increases as a function of time, due to formation of copper selenite as an intermediate product. Copper selenide oxidises to copper oxides without formation of thick copper selenite scales, and a significant fraction of selenium is vaporized as SeO 2 (g). The oxidation product scales on Cu 2 Se are porous which allows transport of atmospheric oxygen to the reaction zone and selenium dioxide vapor to the surrounding gas. Predominance area diagrams of the copper-selenium system, constructed for selenium roasting conditions, indicate that the stable phase of copper in a selenium roaster gas with SO 2 is the sulfate CuSO 4 . The cuprous oxide formed in decomposition of Cu 2 Se is further sulfated to CuSO 4 .

In this paper, the design of an electromagnetic levitation system and a technique for non-conductive silicon heating and conductive silicon levitation is described. The aim of the work is to describe the various parameters including coil design, applied power and specimen weight that govern the temperature of levitated silicon and silicon-iron alloy droplets.

Microstructural evolution of secondary γ′ precipitates in superalloy FGH96 under various aging treatments after 1200 °C solution treatment was investigated. The results indicate that particle size of secondary γ′ precipitates increases from about 73 μm to about 587 μm with the increasing of aging temperature in the range of 850–1120 °C, and the shape of secondary γ′ precipitates changes from spherical to cuboidal, and to butterfly-like with the increase of aging temperature. In case of the aging temperature of 1120 °C, the γ′ precipitates change into butterfly-like and the size is the largest. When the aging temperature reached 1140 °C, the γ′ precipitates are refined.