Journal of Metallurgical and Materials Engineering
Year:2014 | Volume:25 | Issue:1|Papers Count:8

In this article, the effects of adding Zn and conducting age hardening treatment on impression creep properties of Mg-5%Sn alloy have been investigated. The creep tests were carried out using impression creep method in which a cylindrical punch was used to apply the stress in range of 450 MPa < simp < 270 MPa and at the test temperatures of 150, 175 and 200oC. The results showed that age hardening treatment can make the distribution of Mg2Sn precipitates more uniform within the grains. Additions of 0.6% Zn to the alloy improved its creep properties and this effect was attributed to the greater amount of precipitates formed on the non-basal planes and to their homogenous distribution in the microstructure. The values of stress exponent and activation energies determined in this investigation indicated that the pipe diffusion controlled by dislocation climb is the dominant creep mechanism for both Mg-5%Sn and Mg-5%Sn-0.6%Zn alloys under the conditions used in this study.

In this research, the effects of temperature, time and grinding on the types of phases in the calcined kaolin and calcite mixture was investigated. Differential thermal analysis (DTA) and X-Ray diffraction of samples showed that gehlenite is formed at 990oC as the main phase which transforms into anorthite with an increase in the temperature to 1200oC. After grinding the mixture and with increasing the grinding time at the calcination temperature of 900oC, anorthite was recognized to be the main phase.

The main aim of this research was to investigate the effects of plasma nitriding parameters including frequency and duty cycle for a seris of test samples with different grooves during conventional plasma nitriding (CPN) and active screen plasma nitriding (ASPN) treatments. The sample assemblies were consisted of rectangular grooved steel blocks with different groove dimensions of 2, 4, 6, 8 and 10 millimeters (width), 40 millimeters (height), and 20 millimeters (length) and AISI H13 steel plates (as the substrate material) with dimensions of 10 ´40´60 mm3. The sample assemblies were processed in conventional and active screen plasma nitriding chamber at the atmosphere of 75%H2-25%N2 with the temperature of 500oC, the duty cycles of 40, 60, 80%, and the frequencies of 8, 10 kHz for 5 hours. The properties of processed substrates were investigated through the evaluation of the compound layer thickness, case depth, phase composition as well as the hardness profile. The results showed that the hallow cathode effect occurs during CPN method for a sample with 2 mm width groove and the duty cycle of 80% leading to the overheating of the sample. The surface morphology of the CPN treated samples was consisted of cauliflower shaped surface nitrides while the surface of the ASPNed samples were covered by the hexagonal particles with a uniform distribution.

In this work, a new aluminum matrix nanocomposite reinforced with beta-magnesium aluminide nanoparticles was produced by powder metallurgy technique. The prealloyed Al-40 wt.% Mg intermetallic ingot was milled for 100 hours to synthesize b-Al3Mg2 nanoparticles with the average size of 25 nm. Different amounts of nanoparticles were added to Al matrix powder and the mixture was then milled for 10 hours. Al/ b-Al3Mg2 nanocomposite samples were prepared by hot pressing of the composite powders in a uniaxial die at 400oC under the constant pressure of 600 MPa for 2 hours. The results of density measurements revealed that both the density and relative density of nanocomposite samples decrease due to the usage of superlight b-Al3Mg2 reinforcement. Mechanical properties of the nanocomposites were significantly improved with increasing the reinforcement content. Increasing the amount of b-Al3Mg2 nanoparticles continuously improved the hardness and yield strength of composites from 35 BHN and 87 MPa for pure aluminum to approximately 176 BHN and 625 MPa for the sample containing 20 wt.% b-Al3Mg2 nanoparticles, respectively. However, the addition of more reinforcement particles up to 20 wt.% resulted in a decrease in the ultimate strain of the samples, and hence, 10 wt.% nanoparticles was considered to be the optimum content of the reinforcement component.

Accumulative Roll Bonding (ARB) process was used as a severe plastic deformation technique to produce aluminum composite sheets with ultrafine grain structure. In this study, the simultaneous effects of Mg and SiC particles as reinforcing constituents on tensile properties of Al-Mg-SiC hybrid composite have been studied. Al-Mg-SiC hybrid composite samples with different amounts of Mg and SiC were fabricated and the variation of their mechanical properties was examined by tensile test. Results showed a simultaneous increase in both the strength and ductility of composite samples with the addition of SiC and Mg particles. In addition, for the same volume fraction of particles, Al-Mg- SiC composites showed a higher strength and ductility as a comparison with Al-Mg and Al-Si composites. The examination of fracture surfaces of the tensile samples by scanning electron microscope showed a ductile shear fracture mode with shallow and elongated dimples formed on the fracture surfaces.

In order to increase the life span of implants, coatings such as bioglass are applied on implants' surfaces to prevent their corrosion in biological fluids. In this research, 45S5 bioglass coatings were applied on 316L stainless steel substrate by sol- gel method and coating's properties were examined by different analyses. According to the XRD results, an amorphous compound containing Na2Ca2Si3O9 crystalline phase is obtained by sintering the coated substrates at 600oC for 5 h,. The results of SEM examination showed that cracks appear on the coat by increasing the number of dipping stages. Furthermore, the results of potentiodynamic polarization tests in SBF solution at 37oC showed that 316L stainless steel substrate coated by 45S5 bioglass has a higher corrosion resistance.

In the present work, the AA2124/25vol.%MoSi2 composite was produced by powder metallurgy technique. The aluminum alloy 2124 powder was produced by gas atomization method and the MoSi2 intermetallic particles were produced through combustion synthesis. The AA2124 matrix powder was mechanically blended with 25vol.%MoSi2 using a ball milling mixer. The blended powder mixture was encapsulated and then hot extruded to composite bars. The heat treatment T6 was carried out on some of the composite samples. Wear tests were performed on both as-extruded and heat-treated samples using a 'pin on disk' apparatus with a tool steel counterpart, a track length of 1000 m, and a speed of 0.1 m/sec. The friction coefficient was measured and the wear mechanism was determined through the evaluation of weight loss of the composite samples. A complete study was done using the scanning electron microscopy (SEM) and EDS analyzing of the worn surfaces. The results indicated that the main wear mechanism is the adhesive wear, however, the oxide and abrasive wear mechanisms were observed to slightly happen. The heat treatment improved the wear resistance of the composite samples for 25 percent in comparison with the as-extruded samples.

The flow stress of steel depends significantly on microstructural changes taken place during hot deformation processes. In hot deformation processes, the flow stress of steel is influenced by two main metallurgical phenomena, i.e. recovery and recrystallization, along with its chemical composition and thermo-mechanical parameters. The final mechanical properties of steel are also determined by microstructural evolution during hot deformation process. In this paper, dynamic softening and flow behaviour of a Nb-V-Ti microalloyed steel during hot deformation process was investigated. The effects of temperature, strain and strain rate on hot flow stress of this steel and onset of dynamic recrystallization were studied during hot compressive loading. In addition, the semi-empirical models of dynamic recovery and recrystallization were developed for the investigated steel.