Journal of Metallurgical and Materials Engineering
Year:2016 | Volume:28 | Issue:1|Papers Count:9

The purpose of this study was to investigate the effect of heat treatment including solution treatment and aging on the microstructure of cast nickel-based IN100 super alloy including size, volume fraction and morphology of primary g' and different kinds of carbides. For this purpose, different solution heat treatment cycles and a constant aging cycle were implemented on cast ingot samples and the microstructure of the samples were analyzed by scanning electron microscopy and optical microscopy. It was found that, the alloy in the full + partial + aging treated condition had the optimal combination of g′ morphology, volume fraction and size. In this condition, the alloy possesses a cubic primary g′ with 45% volume fraction which is a significant increase as compared to other cycles. Discrete M23C6 carbides were formed at the grain boundaries and the morphology of the cubic MC carbide was changed to the spherical shape.

In this research, the effect of 2.3 wt.% aluminum addition on the microstructure and tensile properties of high manganese austenitic steel Fe-18Mn-0.6C was studied. For this purpose, the samples were investment cast, homogenized and hot rolled. Tensile testing was carried out at ambient temperature. Microstructural investigations and fractography on the steel samples were carried out using an optical microscope and a scanning electron microscope. The results showed that the aluminum addition increases the austenite grain size. In addition, 2.3 wt.% aluminum addition increases the yield strength and plastic strain, decreases the ultimate tensile strength and eliminates the serrated flow during plastic deformation.

In this study, the fatigue behavior of alloy steels with different hardness values was tested. Results showed that the fatigue limit increases by the increment of hardness up to 400 Vickers whereas it drops when the hardness value exceeds 400 Vickers. Fractographic observations using scanning electron microscopy revealed non-metallic inclusions as the dominant fatigue crack initiation sites. By predicting the threshold stress intensity, with the aid of scanning electron microscope observations of the fracture surface and comparing the fatigue limit of the steels, the appropriate effect of hardness on the fatigue behavior of the steel can be presented. Finally, simple and accurate models were proposed for the estimation of the fatigue limit by means of hardness.

Equal channel angular pressing (ECAP) is one the most important severe plastic deformation techniques with many privileges such as extensive grain refinement. In the present study, the ECAP process of AA6061 is simulated using ABAQUS software and the strain distribution is evaluated. The microstructure of the pressed sample after annealing is predicted and the average grain size is calculated using Monte Carlo algorithm with the aid of MATLAB software. The results indicate that as the strain inhomogeneity increases, the grain structure uniformity after annealing treatment reduces. Moreover, as the channel intersection angle increases, the average grain size is enhanced.

The micro-segregation in binary Al-Cu alloys containing 2.2, 3.7, and 4.8 wt.% copper was investigated by differential thermal analysis (DTA) and numerical modeling. For this purpose, the alloys were melted using a DTA furnace with the capability of quenching samples during solidification. The melted samples were cooled at 0.008 and 0.083 K/s and subsequently quenched from predetermined temperatures during cooling. The evolution of the microstructure and micro-segregation was modeled with and without considering excess vacancies formed during solidification. It was found that the experimental and modeling results show a better correlation when excess vacancies are considered.

Joining of titanium alloys to other materials especially steels has attracted much attention in recent years due to the exceptional properties of titanium such as excellent corrosion resistance and mechanical properties and its increasing application in various industries. According to the Fe-Ti binary phase diagram, these two elements do not have complete solubility. This leads to difficulties during fusion welding of these two alloy. One of the best methods for joining dissimilar alloys is brazing. In this research, the metallurgical and mechanical properties of brazing lap joints of steel to commercially pure titanium using a commercial filler metal (BAg-8) made under different time and temperature conditions were investigated. The study of the microstructure interface shows the formation of a chemical layer close to the titanium side of the joint, while no chemical compounds were created on the steel side. A coarse structure is formed at the interface between the steel and silver solder alloy. The observed coarse grain structure is related to the grain growth along with recrystallization in the steel substrate at high temperatures. Analysis of the joints was carried out by optical microscopy, scanning electron microscopy, X-ray diffraction and hardness measurements. The results show that with increasing temperature, as well as brazing time, the average shear strength decreases due to the increased thickness of the intermediate layer.

AA8090 is an extra light and deformable group of lithium containing aluminum alloys which have recently replaced some aerospace aluminum alloys, such as 2000 and 7000 series, due to their higher elastic modulus and lower specific weight. In this study, AA8090 matrix composites containing 3, 6 and 9 vol.% SiC were cast using a modified stir casting method. The casting billets were deformed though a hot extrusion operation, followed by a precipitation hardening treatment (T6) with directional quenching and aging at 190°C. Wear tests were conducted before and after the precipitation hardening (at the peak hardness aging time) using the pin-on-disc method at loads of 10, 20 and 30 N. All samples exhibited a mild wear at 10 and 20 N wear loads, which turned into a sever wear at 30 N. It was shown that increasing the SiC content in the composite samples reduces the wear rate. Also, the wear results after precipitation hardening indicated a decrease in the wear rate only at the wear loads of 10 and 20 N. At the wear load of 30 N, the wear rate increased after precipitation hardening. This behavior was attributed to a transition in the wear mechanism from abrasive to adhesive by increasing the load.

In this investigation, continuous cooling transformations and microstructures have been studied in detail in a low carbon Ti-Nb bearing microalloyed steel by means of dilatometry, metallography and hardness measurements. The results indicate that when the cooling rate is greater than 1°C/s, the dilatation curve is associated with a major dilatation at relatively low transformation temperatures, which is consistent with the martensitic phase transformation. For a cooling rate between 0.2-0.8°C/s, the dilatation curve is also characterized with a uniform single transformation over the martensitic and bainitic phase transformation temperature range. However, as the cooling rate is reduced to less than 0.2°C/s, the dilation curve is characterized with a quite variable double stage transformation which is associated with the ferrite and bainite transformation products. These results suggest that the bainitic and martensitic phase transformations are developed with a similar displacive mechanism, while ferrite is formed by a diffusional mechanism.

The monolithic oxynitride glasses in SiO2-Al2O3-B2O3 system were synthesized using sol-gel method. The porous, dried gels were heated in flowing ammonia for different times and varying temperatures. The nitrogen content of the oxynitride glasses was determined using an oxygen-nitrogen analyzer. The chemical bonding state and properties of glasses were investigated by means of Fourier transform infrared spectroscopy, dilatometry, and Vickers hardness methods. The crystallization behavior of glasses was studied with X-ray diffraction. Spectroscopic evidence indicated that nitrogen was chemically dissolved in the glass network. The results showed that the chemical composition of the glass has an effect on the nitridation processes. The nitrogen content of the glasses was increased to 3.8% wt. by increasing the soaking time of nitridation from 2 to 15h. The dilatometric softening point temperature of the glasses was increased approximately 25°C, the micro-hardness value reached 10.89 GPa and thermal expansion coefficient was decreased from 3.88×10-6 to 3.39×10-6 after the nitridation process at optimal composition. The X-ray diffraction pattern of the glasses confirmed that nitrogen does not enter the crystalline phase.