نشریه مهندسی متالورژی و مواد
سال بیست و چهارم شماره 2 (پاییز و زمستان 1392)

In the present study, the mechanical properties and fracture mechanisms of two powder metallurgy low alloy steels, i.e. Distaloy DH and Distaloy HP, were investigated. All samples were sintered at 1120 ºC for 30 min in N2/H2 atmosphere. For evaluation of the effect of cooling rate on final properties, samples were cooled from sintering temperature with two different cooling rates. In addition, density, hardness, and tensile and fatigue strengths of the samples were measured and microstructure and fractured surfaces were examined using standard methods. The results showed that pores characterizations, i.e. pores shape and distribution, have strong effects on the mechanical properties of sintered steels. For example, an increase in the density by 4.5% resulted in an increase in the tensile strength by 44% and fatigue strength by 55% for Distaloy DH steel containing 0.2% Carbon. These values for Distaloy HP steel were measured to be 26% and 47%, respectively. The results of experiments as well as those obtained from microstructural examinations for two investigated materials indicated that the type of microstructure and distribution of metallurgical phases affect the mechanical and fatigue behaviour of samples. Distalloy HP steel sample with 0.5%C and 7.2 g/cm3 density after being cooled at 1.2 ºC/sec showed the best mechanical properties among all the samples investigated. Study of fractured surfaces indicated that the major mechanisms in crack growth region of the samples are ductile fracture, brittle fracture and formation of fatigue striations.

In the present investigation, a metal matrix composite consisting of A356 aluminum alloy matrix has been produced by the addition of boron nitride powder with three different conditions, i.e. raw powder, coated powder using Ni-P electroless bath and coated aluminum particles with Ni-P/BN surface composite. Moreover, parameters such as impeller shape and rotation speed have been examined in order to obtain their optimized conditions. Tensile and hardness measurement tests were carried out to choose the composite sample with superior mechanical properties. Surface of the powders were analyzed by EDAX. In addition, surface of the coated powder and the fractured surface of composite samples were examined using scanning electron microscope (SEM). The results showed the optimum value of added powder to be 6% after being coated on aluminum particles using the Ni-P electroless bath. Furthermore, the agitator of four-blade type with a radial flow was found to be a good choice for the manufacture of aluminum/boron nitride composite. It was shown that unlike single phase alloys, tensile strength of composite materials is not proportional to their hardness, e.g. a decrease in the tensile strength may cause an increase in the hardness of the composite.

This study is focused on the formation of colloidal silica by Amberlite cation exchange resin via alkaline water glass solution (Na2SiO3). Concentration of the colloidal silica and pH value of the solution was controlled by using different amounts of alkaline sodium silicate solution and silicic acid concentrations in constant temperature and titration rate of silisic acid. The colloidal silica formed at these conditions was characterized by Dynamic Light Scattering (DLS), Particle size Analysis, Zeta Potential Analyzer, Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM), X-Ray Powder Diffraction (XRD) and Accelerated Surface Area and Porosimetry Analyzing (ASAP). The results showed that the average particle size of colloids and their stability were also influenced by above-mentioned parameters including temperature, titration rate of silicic acid, pH value, silicic acid concentration and alkaline sodium silicate solution concentration. Results also showed an average particle size of 26 nm and a fairly large BET surface area of about 160 m2/g for these colloidal dispersions.

In this study, the preparation of transparent glass-ceramics with lithium-mica nanocrystals by melting-quenching method was investigated. The effect of heat treatment time and temperature on the crystalline phases, microstructure and transparency of the samples were studied. In order to obtain a fine microstructure, an optimum condition for heat treatment has been found to be very important in controlling the crystallization process. The optimum condition of crystallization process was obtained by heating at 655˚C for 15 hours. The crystallization process of glasses was investigated by DTA, XRD and SEM techniques. Transparent glass-ceramics in which a large amount of lithium-mica with the particle size of less than 50 nm was separated, was prepared using the optimum conditions of heat treatment.

Synthetic slag process decreases sulfur and phosphorus contents of steel, improves inclusion removal, protects steel from atmosphere and prevents temperature drop of the liquid steel in the ladle. In this study, the synthetic slags of ternary CaO-Al2O3-SiO2,and quaternary CaO-Al2O3-SiO2-Na2O and CaO-Al2O3-SiO2-SrO systems were pre-melted and then were kept in contact with the liquid steel in a high frequency induction furnace. The results showed that the optimum reaction time for desulfurization is 15 minutes and the desulfurization efficiency for ternary slag is 33% whereas for the SrO- and Na2O-containing quaternary slags are 34% and 7%, respectively. Furthermore, it was shown that the desulfurization efficiency for SrO-containing slag can be obtained more than doubled (from 34% to 77%) by employing covered crucibles.

Deterioration of soft magnetic properties of amorphous ribbons happens when oxygen atoms diffuse in matrix during heat treatment. In this research, surface oxidation of Co67Fe4Cr7Si8B14 amorphous ribbons prepared by Planar Flow Melt Spinning (PFMS) process during crystallization was investigated. In the first step, thermal stability of the ribbons was studied by Differential Thermal Analysis (DTA). Amorphous samples were then annealed isothermally in argon atmosphere with 0.2% oxygen for 30 minutes at the temperature range of 350-650 ºC with 50 ºC interval. Heat treated samples were studied by X-Ray Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis. According to the results, surface oxidation started around 550ºC. The oxide was found to be SiO2 with amorphous structure. An increase in the temperature up to 600ºC, no crystalline phases was recognized by XRD in the sample annealed at this temperature, which can be attributed to increasing the thickness and continuity of the oxide layer. For the temperatures above 600ºC, the instability and discontinuity of the oxide layer were observed in some areas. In these areas, oxygen diffused in the matrix and combined with cobalt as CoO, was also observed.

In this study, nickel-phosphorus coating was prepared by electrodeposition technique on the 304 stainless steel substrate. The effects of current density, phosphorous acid concentration, time and pH on the phosphorous content was investigated using Taguchi method. A uniform amorphous-nanocrystalline structure (Ni-12%P) was obtained. SEM, EDS, TEM and XRD were used in order to study the surface morphology, chemical composition and phase structure. Based on the DSC results, the thermal behaviour of coatings with different phosphorus contents was analyzed and the relationship between the structure and temperature was determined. The results showed that at the temperature of 409.1 °C, the structure of coating (Ni-2.51%P) changed from amorphous to crystalline with a greater thermal stability. The transformation temperature decreased to 328° C by an increase in the phosphorous content.