Volume 31, Issue 1

The influence of temperature at constant strain rate has been evaluated with respect to superplastic behavior of dispersion strengthened Al-Al 4 C 3 composite with 4 vol.% of Al 4 C 3 phase. The dispersion strengthened Al-Al 4 C 3 was prepared by powder metallurgy. Material can be characterized by grain size around 1 μm and dispersed particle Al 4 C 3 size around 50 nm. This material showed superplastic behavior for strain rate 10 -3 s -1 by test temperature 573 K. The mechanism of superplastic deformation was also investigated. For the presented material with low content of disperse particle slip on grain boundaries is typical. The final fracture is transcrystalline with dimples with their mean size 1.25 μm. The dimples are initialized by Al 4 C 3 particles in size from 40 to 50 nm.

A series of Poly(amide-imide)/montmorillonite nanocomposites containing N -pyromellitimido- L -phenyl alanine moiety in the main chain were synthesized by a convenient solution intercalation technique. Poly(amide-imide) (PAI) 5 as a source of polymer matrix was synthesized by the direct polycondensation reaction of N -pyromellitimido- L -phenyl alanine 3 with 4,4'-diamino diphenyl ether 4 in the presence of triphenyl phosphite (TPP), CaCl 2 , pyridine and N -methyl-2-pyrrolidone (NMP). The resulting nanocomposite films were characterized by Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The results showed that organo-modified clay was dispersed homogeneously in PAI matrix. TGA indicated an enhancement of thermal stability of new nanocomposites compared with the pure polymer.

The present paper deals with the tribological behaviour of the boride and carbide hardmetals evaluated by performing comparative dry sliding pin-on-disc experiments using normal contact loads. Analyses of the wear performance results, microstructural evaluation and processing conditions effect indicate that microstructure inhomogenities play an important role in abrasive wear behaviour of cermets. In term of grain size and chemical composition, the addition of VC also play an important role in increasing the wear resistance.

Given that Sn-based lead-free solders are considered as possible substitution for standard lead-tin solders, properties of the alloys from ternary Bi-Sb-Sn system were characterized by microstructural SEM-EDS analysis, Brinell hardness tests, and electrical conductivity measurements. Isothermal cross section at 298 K and liquidus surface projection were calculated by application of CALPHAD method and software package PANDAT 8.1. The experimentally determined phase compositions in analyzed microstructures were found to be in a good agreement with phases on calculated cross sections.

Tensile tests were performed on specimens obtained from 20 mm plate in normalised and tempered condition and quenched and tempered thick section tubeplate forging to assess the influence of processing route and section size on room and elevated temperatures (300–873 K) tensile properties of 9Cr-1Mo ferritic steel. Yield and ultimate tensile strength values of plate and tubeplate forging exhibited a gradual decrease up to intermediate temperatures followed by a rapid decrease at high temperatures. Ductility values displayed a gradual decrease to a minimum at intermediate temperatures followed by an increase at high temperatures. The fracture mode remained transgranular. At intermediate temperatures, 9Cr-1Mo steel in both the product forms exhibited similar serrated flow, an important manifestation of dynamic strain ageing. At all temperatures, thick section tubeplate forging exhibited lower strength and higher ductility values compared to normalised and tempered steel. The inferior strength values were attributed to coarse microstructure of the forging.

In order to predict correctly stress-strain curve for tensile tests at elevated and cooling temperatures, a modification of a Johnson–Cook (J-C) model and a new method to determine (J-C) material parameters are proposed. A MATLAB tool is used to determine material parameters by fitting a curve to follow Ludwick and Voce's hardening law at various elevated temperatures. Those hardening law parameters are then utilized to determine modified (J-C) model material parameters. The modified (J-C) model shows the better prediction compared to the conventional one. An FEM tensile test simulation based on the isotropic hardening model for metal sheet at elevated temperatures was carried out via a user-material subroutine, using an explicit finite element code. The simulation results at elevated temperatures were firstly presented and then compared with the measurements. The temperature decrease of all elements due to the air cooling process was then calculated when considering the modified (J-C) model and coded to VUMAT subroutine for tensile test simulation. The modified (J-C) model showed the good comparability between the simulation results and the corresponding experiments.

La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-Δ (LSCF) nano powders were synthesized via liquid citrate, spray drying, ethanol based sol-gel, auto combustion and EDTA/citrate complexing methods. All primary powders were calcined at 1000 ° C. XRD results showed that auto combustion method results in the purest powder. The results of SEM micrographs showed that this sample is nano scale and has narrow particle size distribution Effects of pressing pressure, sintering temperature and dwell time on membranes microstructure were investigated. The result showed that the most and the least effective parameters on densification and grain growth are temperature and pressing pressure. The optimum pressing pressure, sintering temperature, and dwell time were 2000 bar, 1200 ° C, and 8 h, which showed the highest relative density, favorable crystal growth, and nonporous crack-free structure. Effects of sintering conditions on oxygen permeation flux were studied and the results showed that oxygen permeation flux increases with average grain size.

By means of hot isostatic pressing (HIP) treatment, microstructure observation and creep properties measurement, the effects of the HIP temperatures on the microstructure and creep properties of FGH95 nickel-base superalloy are investigated. The results show that, when the HIP temperature is lower than solubility of γ' phase, the coarser γ' phase is precipitated in the previous particle boundary (PPB) regions, and the quantity and size of the coarser γ' phase which is distributed in the regions decrease as the HIP temperature increases. No feature of the grain growing up is detected after the alloy is solution treated at 1140 ° C. Moreover, there are a few of carbide particles distributing along the grain boundary and in the grain. After HIP treated at 1180 ° C and fully heat treated, coarser γ' phase is dissolved in the alloy, and the depleted zone of the fine γ' phase has disappeared. In addition, the grains grow up obviously in the alloy, and the γ' phase and fine carbide particles are dispersedly precipitated in the grains and along boundaries, which can enhance the creep resistance of the alloy. The deformation mechanisms of the alloy are that the dislocations slip in the matrix or shear into γ' phase during creep.

Nickel-Iron based superalloy (43Ni-14Cr, wt%) is used as high temperature material in space based transportation systems. It retains its high strength ( >100 MPa) upto 800 ° C alongwith good cryogenic properties, hot corrosion resistance and phase stability at elevated temperatures. The alloy was melted in vacuum induction melting furnace and was refined through vacuum arc refining process. Hot working and heat treatment of the alloy was carried out and the material was characterized for its mechanical properties at different temperatures. Microstructural analysis was conducted at different stages of processing in order to optimize the processing parameters. This paper brings out the work carried out on the process development for this alloy.

Vacuum induction refining is a process that can be applied to remove phosphorus from molten silicon for the production of solar grade silicon. Pure silicon was doped by phosphorus to make molten silicon containing around 17 ppmw phosphorus. The kinetics of phosphorus removal from this silicon was studied at 0.5 Pa through the application of vacuum induction refining. It was observed that vacuum removal of phosphorus occurs through a first-order reaction. The rate constants of phosphorus evaporation were determined as 2.28 × 10 -6 m / s and 4.93 × 10 -6 m / s at 1500 ° C and 1600 ° C, respectively. Moreover, an apparent activation energy 213.1 kJ / mol for phosphorus evaporation from molten silicon was calculated. It was found that mass transfer of phosphorus in the melt is not rate limiting in the inductively stirred silicon melt. The vacuum removal of phosphorus is mix-controlled by chemical reaction and gas phase mass transfer. Under medium vacuum conditions, the mass transfer in the gas phase is more rate-limiting than the chemical reaction at higher refining temperatures.

Fe-40Ni-15Cr alloys with different grain size were prepared by powder metallurgy (PM) and mechanical alloying (MA) through hot pressing technique. Their oxidation behavior was examined at 1073–1173 K in 1.01 × 10 2 kPa pure oxygen. Results show that the oxidation kinetics of the coarse grained PMFe-40Ni-15Cr and nanocrystalline MAFe-40Ni-15Cr alloys deviates from the parabolic law and their instantaneous parabolic rate constants change with time irregularly. At 1073 K, mass gain of MAFe-40Ni-15Cr alloy is lower than that of PMFe-40Ni-15Cr alloy. At 1173 K, mass gain of MAFe-40Ni-15Cr alloy is higher than that of PMFe-40Ni-15Cr alloy for initial 4 h and afterwards becomes lower than that of PMFe-40Ni-15Cr alloy up to 24 h. PMFe-40Ni-15Cr alloy formed iron oxide scales, while MAFe-40Ni-15Cr alloy formed the continuous and protective scales of chromia on the alloy surface at 1073–1173 K. The reason may be that grain refinement caused the increment of the grain boundary which leads to the faster diffusion of the reactive component Cr from the alloy to alloy/scale interface. Finally, MAFe-40Ni-15Cr alloy formed the continuous and protective scales of chromia.

The oxidation behavior of Fe-Ni alloys containing 10, 20, 43, 50, 57 and 80 (mass%) nickel was studied over the range 700 ° C–950 ° C in laboratory air atmosphere using thermogravimetric, XRD and SEM/EDAX techniques. The results showed that some oxidation kinetic curves of Fe-Ni alloys follow the parabolic law, some follow a cubic law, while the oxidation kinetic curves of 90Fe-10Ni is linear. XRD and SEM/EDAX results showed that the oxide scale consists of an outer layer of Fe 2 O 3 and an inner layer of Ni-Fe-O and NiO compound. Ni content in the alloy has an important role for the formation of NiFe 2 O 4 oxide. 20Fe-80Ni and 43Fe-57Ni can form NiFe 2 O 4 at 700 ° C–950 ° C, while 50Fe-50Ni can form NiFe 2 O 4 only oxidized at higher temperature of 950 ° C.