مجله مهندسی متالورژی
پیاپی 61 (بهار 1395)

Hot deformation of an extruded Mg–10Li–1Zn alloy was studied by compression testing in the temperatures range of 250- 450 ˚C and strain rates of 0.001–0.1 s−1. During the hot compressive deformation of the Mg-10Li-1Zn alloy, flow stress curves reach a maximum value and then reach a steady state which is indicative of the occurrence of dynamic recrystallization. Because of the activation of softening mechanisms at higher temperatures and lower strain rates, this phenomenon is more pronounced at lower temperatures and higher strain rate.
The flow stress of the Mg–10Li–1Zn alloy at elevated temperatures was modeled via an Arrhenius-type constitutive equation. The values for the activation energy of about
103 kJ mol–1 and the power-law stress exponents in the range of 5.2–6.0 obtained from the Arrhenius-type model indicate that the dominant mechanism during hot deformation of the Mg–10Li–1Zn alloy is dislocation climb which is controlled by the lattice
self-diffusion of Li atoms.

In this paper, the effects of pearlitizing on the as cast and as heat-treated microstructures of austenitic manganese steel (named Hadfield steel) were evaluated. The samples with different contents of carbon and manganese were produced using induction furnace and precision casting method. The as-cast, annealed, and solution treated microstructural studies were performed by optical microscopy, X-ray diffraction, ferritescopy and field emission scanning electron microscopy. The microstructural results showed that the as cast and annealed microstructures of the steel in room temperature consist of austenite matrix, carbide and ferrite. Fine examination by scanning electron microscope shows that the ferrite and carbide phases are formed as pearlitic colonies. It was also observed that the increase in carbon content and annealing heat treatment are more facilitated the pearlitizing. By applying the final heat treatment (solution and quenching in water), pearlite colonies convert to new fine austenite grains and then the austenite grains sizes are decreased.

Microstructural evolution during annealing of AISI 304 austenitic stainless steel following cold rolling was evaluated. It was revealed that three annealing stages are available: Reversion of martensite to austenite, recrystallization of the retained austenite, and grain growth. Reversion of martensite to austenite resulted to the development of an ultrafine grained microstructure. However, the recrystallization of the retained austenite postponed the formation of an equiaxed microstructure, during which, the fine austenite grains become coarser and a microstructure with average grain size between 1 and 2 µm was obtained. Evaluation of mechanical properties revealed that the reversion of martensite to austenite and the resultant grain refinement enhanced the tensile strength by 40%. Moreover, further annealing for the occurrence of the recrystallization in the retained austenite to obtain an equiaxed microstructure resulted in the decline of tensile strength and enhancement of ductility. In this way, it is possible to control the microstructure and mechanical properties of austenitic stainless steels.

Rhodium pertains to the platinum-group metals (PGM) and has a special catalytic activity to reduce nitrogen oxides (NOx) to N2. Hence, more than 80% of total rhodium demand is now used for the production of autocatalysts. As regards, rhodium is one of the rarest elements in the earth's crust and its natural resource deposits are strictly limited, so, their recycling process from secondary resources must be considered. In this study, car exhaust catalyst, in the presence of HNO3 and HCl was dissolved by the leaching process. All species and rhodium complexes formation were analyzed by Phreeqc software. The recovery of rhodium from leach solution containing Mg, Mn, Pb, Ni, Fe and Al was done through cementation with metallic copper powder. At an ambient temperature, a stirrer speed of 150 rpm, a stoichiometric ratio of Cu to Rh 15 and at a time of 240 minutes about 70% of rhodium on copper powder particles were recovered. By increasing the temperature to 60 ° C, the rate of recovery increased up to 95 percent and cementation of rhodium was shown to be a feasible process to achieve a high degree of rhodium recovery. Analysis SEM / EDAX confirmed rhodium formed in the presence of copper by the dendritic and multi-layer structures with a purity of 87 percent rhodium.

In this work, the martensitic transformation and mechanical response of a near equiatomic NiTi Shape Memory Alloy (SMA) subjected to different thermo-mechanical treatments has been studied. The NiTi wire in as drawn condition was subjected to seven heat treatment cycles in order to prepare the wires with different transformation characteristics. The transformation temperatures have been measured using DSC method. The wires were then subjected to tensile test at the temperature of 20, 80 and 90 C, in order to study the stress induced martensite.
According to the results, with increasing the heat treatment temperature or time, the Ms increases and the Rs decreases in such a way that the R-phase disappears when the temperature achieves to 600 C.
The stress at which, the martens tic reorientation takes place is independent of the heat treatment conditions. The stress induced martensite, however, forms under different stresses, depending on the heat treatment temperature and time. These samples show the superelastic effect at high temperature; but in the case of the samples with martensitic reorientation, the alloys are not able to present this effect. Therefore, it is possible to determine the mechanical response of the material with respect to the given applications.

In this study, ductile iron-steel hollow sphere syntactic foams were manufactured using sand mold casting technique. The steel hollow spheres were synthesized through fluidized bed and in this process, commercial iron powder was coated on the expanded polystyrene spheres. After coating, De-bonding and sintering processes were carried out and the steel hollow sphere produced with 4, 6, and 8 mm in diameter. Then, the steel hollow spheres were randomly filed in the cavities of prepared mold and ductile iron infiltrated the interstitial space between the spheres. Thus, ductile iron- steel hollow sphere syntactic foams with 4, 6, and 8 mm spheres sizes. Microstructural properties of manufactured foams were investigated by optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). In addition, cooling rates in the syntactic foams with different sphere sizes were calculated and effects of cooling rate on the graphite morphology and microstructure were studied. The results revealed that decreasing the spheres size caused to increasing the cooling rate. Furthermore, by increasing the cooling rate, surface fraction of graphite, graphite circularity, and surface fraction of iron carbide decrease and graphite nodule count increases.

In this study the structure of Cu-Ni3Al-MoS2 composite coating using physical vapor deposition have been investigated. It is expected, the use of this coating for operation in dry conditions and different temperatures, provides good abrasion resistance and lead to increase the life time of components while maintaining the economic efficiency. Because of the presence of the employed elements, Cu-Ni3Al-MoS2 coating has a good lubrication and conductivity properties with hard structure. This coating is used in aviation industries in cases, where surfaces slip to each other. For this purpose Target tablets containing Ni3Al-wt.30% MoS2 with a copper substrate were prepared using magnetron dispersion of composite coating Cu-Ni3Al-MoS2 structure was created on 4340 steel substrate. Coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy depressive spectroscopy (EDS) and roughness measurement test (RT). The hard cover was calculated, by the Vickers microhardness measurement of Cu-Ni3Al-MoS2 about 380 HV over 25 g load. During the study, it was demonstrated that this coating has a good adhesion and morphology with substrate.