نشریه مهندسی متالورژی و مواد
سال بیست و دوم شماره 1 (پاییز و زمستان 1389)

In aluminum foams, bubbles are formed as a result of the decomposition of TiH2 powder in contact with the molten aluminum. Hence, the study of these reactions has an important effect in determining the mechanism of decomposition of TiH2 inside the molten aluminum. In this research, TiH2 powder was put in contact with the molten aluminum at various temperature ranges, i.e. a)700-750ºC, b)750-800ºC and c)800-1000ºC. After solidification of the melt, the solidified samples were taken for the SEM and EDAX examinations. The results showed that as soon as the TiH2 particles touch the melt, a thin layer of TiAl3 is formed around the particles. At the temperature range of (a), the TiAl3 layer formed was almost continuous, hence, the decomposition process was controlled by internal diffusion (diffusion of atoms within the TiAl3 layer). At the temperature range of (b); the TiAl3 layer had no sufficient continuity and its overall thickness was greater than that for the range (a). In this case, the powder particles had a semi-collapse shape. Consequently, the decomposition process was under the control of both the internal diffusion and the chemical reaction (reaction between the TiH2 powder and the melt). At the temperature range of (c), the TiAl3 layer formed around the TiH2 particles was shattered and finally dispersed within the melt. Therefore, the decomposition process in this case was controlled by the chemical reaction. The amount of remaining powder particles decreased with an increase in the melt temperature. The rate of decomposition process increased with increasing the melt temperature, due to the decrease of the thickness of TiAl3 layer formed around the particles, as well as its non-continuous character.

In this research, a mathematical model based on Bezier equations was developed in order to predict the admissible velocity field during the deformation of the aluminum-copper-aluminum laminated composite in a plane strain equal channel angular extrusion (ECAE) process. The strain and strain rate fields were evaluated using the fixed Taylor factor based on the proposed streamline and the modified ETMB microstructural model for the nanostructural changes in aluminum-copper layers. The results showed that the size of the dislocation cells formed in copper layer was smaller than that of the Al layer, especially when the aluminum layer was located adjacent to the outer corner angle and when a lower die angle was used. Furthermore, the results predicted by the model regarding the different passes of ECAE process were in good agreement with the results obtained from the microstructural and textural examinations performed using TEM and EBSD techniques.

In the present study, copper matrix composite samples with 53 volume percent WC reinforcement particles were produced using the infiltration technique. The density and electrical conductivity of the composite samples produced by this technique were measured. Moreover, the wear behavior of WC/Cu composite was investigated using the pin-on-disk method with an abrasive disk made up of SiC. It was found that the final density approaches the theoretical density with an increase in the temperature and time of the infiltration process, and for a given temperature, the density increases as the time increases. For an applied normal load of 9 N, the wear rate of WC/Cu composite increases linearly with the applied load. The presence of porosity increased the wear rate of the composite due to a decrease in the effective contact surface and induced notch effect on the composite surface.

Plasma nitriding is a thermochemical process that is used for surface hardening of many industrial steel components. In this study, hot work tool steel samples were plasma nitrided at 450, 500 and 550 oC temperatures, for 2.5,5,7.5 and 10 hours in a 75%N2-25%H2 gas atmospheve inside a pulsed DC vessel in order to determine the growth mechanism of surface layers. Diffusion zone, compound layer and surface morphology were studied by using optical and scanning electron microscopes. The phases formed in the nitriding process were determined by XRD method. In addition, the microhardness changes from surface to the centre of the samples were measured. Results showed that nitrogen has diffused in the substrate material via grain boundary diffusion mechanism.The surface hardness and the depth of hardened layer increased with an increase in the nitriding temperature and time. Moreover, the thickness of the compound layer increased by increasing the temperature and time. It seems that the radius of nitride particles on the surface is direethy related to the processing time. By increasing the nitriding temperature and time, surface morphology was changed and the radius of nitride particles on the surface also increased.

In this article, the influence of martensitic and austenitic structures on two-way shape memory strain in two Ni-Ti alloys with 49.9 and 50.5 at% Ti trained by bending were investigated. The samples prepared with VIM (Vacuum Induction Melting) were trained in martensitic and austenitic conditions by applying bending deformation. The effects of pre-strain level and number of training cycles in martensitic and austenitic conditions were also studied. The results showed that the two-way shape memory strain in the Ni-rich alloy is greater than that of the Ti-rich alloy. In addition, it was observed that training the martensitic microstructure results in higher two-way shape memory strain when compared to the austenitic structure. The pre-strain values of 16.7 and 12.5 % and the optimum training cycles of 10-15 and 15-25 were obtained for the cases of martensitic and austenitic structures, respectively.

Forming limit diagrams (FLDs) are very powerful tool for prediction of the formability of sheet metals. However, it has been shown that FLD diagrams are valid only for proportional loadings during which the ratio between the principal stresses remains constant throughout the forming process. Therefore, forming limit stress diagrams (FLSDs), which are independent of the straining path, have been developed. In this work, the forming limit diagram for aluminum alloy 3105 was first obtained experimentally using the Nakazima out-of-plane test. The forming limit stress diagram was then calculated by the application of plasticity theory. In addition, the FLDs and FLSDs for AA3105 were simulated using ABAUUS finite element software through the application of a ductile fracture criterion. The results showed that the diagrams obtained by the application of the ductile fracture criterion are relatively in good agreements with the experimental results, and hence, can be successfully used for the prediction of forming limit diagrams of the aluminum alloys.

The MgO-Cr2O3 refractories are amongst the most important refractory materials that are used in rotary furnaces in the cement production line. In this research, the corrosion behavior of these refractories was studies under the influence of clinker. A number of refractory samples with clinker were first heated at 1450C for 3 hours. The sample's cross section were then examined. The XRD technique was used for the phase analysis and the scanning electron microscope (SEM) equipped with the EDX facility was employed for the microstructural analysis. The results showed that the chemical composition (the amounts of oxides) was changed in the brick's surface layers when the MgO-Cr2O3 refractory material with the clinker was heated. In addition, the detrimental effect of clinker on the MgO-Cr2O3 refractories was attributed to the diffusion of the liquid phases through the grain boundaries and the pores which were present in their surface layers.