JOURNAL OF NEW MATERIALS
Year:2015 | Volume:5 | Issue:2 (18)|Papers Count:12

In the present study, the uniaxial hot tensile tests were carried out in the strain rate range of 0.01-0.0001 S-1 in the temperature range of 25-350oC in order to evaluate the flow behavior of TX41 magnesium alloy sheet. The flow stress curves analyzed by Fields- Backofen constitutive equation can only give a good fit to the measured stress-strain relationship in the uniform deformation regime. A mathematical model containing the softening parameter s was used to calculate the flow stress curves in post-uniform deformation regime. The results indicate that this modeling approach can better predict the softening stage of the flow behavior in the uniaxial tensile tests at high temperatures.

Quasicrystals compounds have ordered structure, which is comparable with crystalline materials, but and are aperiodic. The formation of quasicrystalline structures was reported in Al-base, Ti-base, Zr-base and Mg-base alloys. One of the classes of these advanced materials is Al-Cu-Ni systems which have vacancy ordered phases (VOP) knowing as one-dimensional quasicrystals. Since there have been limited reports on the synthesis of the Al-Cu-Ni intermetallic by mechanical alloying (MA), the production of Al70Cu20Ni10 compound by MA and subsequent heat treatment was canied out in this work. Afterward, the stmctural properties of the prepared samples were conducted by x-ray diffraction (XRD) and the corrosion behavior of the compounds was studied by linear polarization test. It was found that by increasing the annealing temperatures, the amount of one-dimensional quasicrystals (t3) increases and in contrast, the Al2Cu (q) crystalline intermetallic compound quantity decreases, respectively. Moreover, the corrosion test results were revealed that the corrosion resistance behavior is improved by increasing the fraction of t3 phases.

The Accumulative Roll Bonding (ARB) process is used as a severe plastic deformation technique to produce aluminum composite sheets with ultra-fine structure, This method comprises the two basic steps of particles addition between sheets and the distribution of them and the production of ultrafine structure through repeating the passes. In the current study above technique is used to produce Al-Mg-SiC hybrid composite. In the first step the Al- %1 Vol Mg-%1 Vol SiC composite was fabricated. To provide the possibility of comparing the results of the produced composite sample with the pure one, pure 1050 aluminum nanostructure sample was also fabricated with the same method. The mechanical properties of the fabricated pure and composite samples have also studied by tensile, micro hardness tests .The results showed that the tensile strength and micro hardness of the processed samples increased with increasing the number of ARB cycle. However, elongation of the specimens dropped abruptly at the first cycle, afterwards, it remained nearly constant in the monolithic samples and increased slightly in the Al/Mg/SiC composites.

Production of large diameter aluminum billets with minimum defects is one of main goals of researchers in the relevant industries. In the current paper a model has been developed to simulate direct chill casting in the presence of low frequency electromagnetic fields. The model was verified using the results of temperature and magnetic field measurements. The results of solving of thermal, velocity, and pressure field as well as magnetic field with solidification approach showed that using of a simple coil back of the mold causes to push the melt to the center of billet. Also, parametric studies on this process illustrated frequency decrease and increase of electromagnetic intensity results in more uniform distribution of electromagnetic force and more severe agitation of the melt, respectively. Setting up the electromagnetic parameters in an appropriate range can improve surface quality and microstructure of the bulk near the surface of the large diameter billet. However there are slight modifications in the center of bilet.

Stress relieving is a common practice among building codes for welded quenched and tempered steel structures and pressure vessels to reduce the magnitude of residual stresses. The present work analyzes the effect of stress relieving at 480, 560, 620 and 680oC on microstructure and mechanical properties of 20 mm ASTM A517 Gr. B steel plate weldments produced by shielded metal arc welding process. For this purpose metallographic analysis, hardness measurement, charpy V-notched impact test, guided bend test and tensile test were performed and the results were compared with the non-heat treated samples (A W). In order to evaluate effects of stress relieving on corrosion resistance, cyclic potentiodynamic polarization test have been used. According to the result of tests, stress relieving can reduces impact toughness of all base metals but in general, stress relieving at 560oC represents better mechanical properties.

In this research, mechanical properties of two low-alloyed powder metallurgy steels with diffusion bands, Distaloy DH and Distaloy HP has been studied. Sintering process was done in temperature of 1120oC and in mixture of hydrogen and nitrogen gases for 35 minutes. Whole specimens have been cooled to room temperature with two different cooling rates in order to evaluate the outcomes on the microstructure futures. Density, hardness, tensile and fatigue tests were carried out on sintered samples for microstructural and fractographic evaluation. Results demonstrated that pores level and also its characterizations (size and shape) have critical impacts on mechanical properties of the studied PM steels. Also it showed that by increasing the density by 4.5% in Distaloy DH steel with 0.2% carbon, tensile and fatigue strength improve by 44% and 55%, respectively. The former and the later have been obtained for Distaloy HP as 26% and 47%, respectively. Results from microstructural assessments in accordance with comparing experimental data from mentioned steels showed that type of microstructure and distribution mood of metallurgical phases play critical role in mechanical properties especially on fatigue behaviour. The best properties were related to Distaloy HP steel with 0.5% carbon, density of 7.2 g/m3 and cooled at 1.2oC/s rate.

In this paper, boron carbide was produced by reduction of boron oxide with two types of carbon (carbon black and CMK-1 mesoporous carbon) by magnesiothermic process. The role of carbon was investigated in the synthesis of boron carbide. To do this, CMK1mesoporous carbon was synthesized with MCM-48 silicate precursor by used of molding method and consequently, samples with B2O3:C:Mg=11:1.5:12 weight ratio in the range of 700 to 750oC were used to synthesis boron carbide, by using carbon black or CMK-1 mesoporous carbon in magnesiothermic process. To remove additional phases obtained during the production process, the resulting powder was dissolved in hydrochloric acid. BET, FTIR, XRD, DSC-TGA and TEM analyses were used to characterize the precursor powder and resulting boron carbide. The results showed that the amount of CMK-1 mesoporous carbon contact area (659 m2/g) had an important role to facilitate synthesis of boron g carbide.

In this research, gel-casting method is used, to produce the porous body of WC. In order to achieve this, the Metacrylamid (MAM) and Methylen bore acrylamide (MBAM) monomer system is used as chain-maker and cross-linker monomers. First monomers were dissolved in the deionized water, then tungsten carbide powders was added to the solution, afterward ammonium per sulfate (APS) was added as initiator and Tetramethylethylenediamine (TEMED) was added as the catalyst to the suspension. Then, the suspension is immediately poured into the mold and the mold is exposed to 50oC for 30 minutes, so that the gel is formed. Then, the gelled green body were removed from the mold and dried at room temperature. Finally, the green bodies were de-binded and sintered at 1450DC under argon atmosphere. After investigation of rheological behavior of the suspension, it was found that the optimum amount of dispersant was about 0.2 %wt of ceramic powder. Also, results of Thermal analysis indicate the organic matter is removed at 600oC. After sintering of specimens, the effect of ceramic loading on the porosity and flexural strength was investigated and the results showed that by the increase of ceramic loading (from 10% to 30% Vol) both of these parameters are decreased (porosity decreased from 61.2% to 50.41 % and flexural strength decreased from 79MPa to 40.1MPpa). SEM image of the sintered specimen was prepared.

In this study, the effect of different microstructures on the fatigue strength and fracture of DIN 1.2210 cold work tool steel was investigated. For this purpose, different heat treatment cycles microstructures, including normalized (N), tempered martensite (TM), tempered bainite-martensite (TBM) and tempered bainite (TB) with approximately the same hardness of 32 RC were obtained. Specimens of these microstructures, tested by tension test, hardness and bending rotary fatigue test. It was found that by increasing martensite volume fraction in TB, TBM, and TM steels, the ductility reduced whereas yield strength and fatigue limit increased. In addition ultimate strength was the same in all microstructures. Fractography of the fracture surfaces by SEM microscope demonstrated with decreasing yield strength, average size of dimples increases and their number decreases and also the formation of cleavage plane which replaces dimple structure confirms the embrittlement behavior which causes the reduction of fatigue limit.

The mechanical properties and thermal stability are the main concerns for the commercial application of severe plastic deformed (SPD) metals. Microstructure of a material controls its final mechanical properties. The thermal stability of a microstructure can be defined as the resistance to the restoration processes of recovery and recrystallization. These thermally activated processes are two major phenomena which control microstructure evolution and restore the deformed microstructure. In this investigation, static recrystallization kinetics of copper, severely deformed by a novel SPD technique, known as simple shear extrusion (SSE), was studied. The micro hardness and microstructure of the deformed samples with different strains annealed at temperature range of 180oC to 290oC were analyzed. Softening fractions calculation at different times and temperatures resulted in investigation of JMAK equation constants. As a result, as the strain increases from 1.15 to 6.9 crystal size and thermal stability decrease, and also time for 50% recrystallization of the material at 210oC which is 864s for the first pass decrease to 299 and 207s for the second and sixth pass, respectively.

Aluminum matrix composites reinforced with nano MgO have been considered in specific industries due to its favorable properties such as strength and resistance to high temperatures. In this study, in addition to investigate the most effective parameters such as temperature and volume percent of reinforcement phase, aluminum foil method (in order to improve the existing problems in the production process) and use of MgO particles in nano scale can be as innovation in this paper. Al-nano MgO composites were manufactured via Vortex method. In order to produce these composites, MgO nanopartic1es with 60-80 nanometer diameter and A356 Al alloy were used as reinforcement and matrix, respectively. The molten composites were stirred for 13 minutes, and then cast into a metallic mold. The effects of MgO volume percent with 1.5, 2.5 and 5 %, and casting temperature (various temperatures, 800, 850 and 950oC) on microstructure of composites were investigated. The results revealed a homogenous distribution of MgO nanoparticles within the metal matrix. Although, MgO agglomerates are observed within the matrix, they are distributed homogenously.

Friction stir processing (FSP) is one the most recent grain size refining process. In this study, the effect of FSP parameters on microstructure and mechanical properties of Magnesium was investigated. The studied parameters was pin design and rotation speed. Three pin designs including conical, non-threaded and threaded was used. Rotation speed of FSP 400,500,630,800, 1000, 1250 and 1600 RPM was selected. The tunnel cavity in FSPed specimen was seen in advanced side of stir zone in low rotation speed condition. These defect was diminish with increasing of rotation speed. Although, in high rotation speed, the tunnel cavity was seen again due materials detachment form the advancing side. However, in threaded pin no defect was seen in medium to very high rotation speed. Grain size refining was seen in FSPed specimen due to dynamic recrystallization. In threaded pin due to higher materials flow, the lowest grain size was measured. However, a minor increase was seen in stir zone hardness due to dislocation removal in dynamic recrystallization. The texture of stir zone is consist of (0001) and (1013), slip and twin plane respectively.