پژوهشنامه ریخته گری
سال سوم شماره 3 (پیاپی 10، پاییز 1398)

Since the diffusion of alloying elements in nickel has been studied based on the diffusion couples and the presence of other alloying elements can alter the diffusion of a given element, there is need for a method for direct calculation of the diffusivities form the superalloys. In the present work, the as-cast microstructure of Inconel 718 superalloy was studied, where this microstructure was composed of austenitic dendrites that rejected alloying elements such as Nb, Mo, and Ti to the interdendritic regions. Moreover, the Laves phase/austenite eutectic structure and NbC and TiC phases were characterized in the interdendritic regions. Subsequently, the microsegregation of Nb, Mo, and Ti in the as-cast ingot and its amendment during homogenization heat treatment was studied. Based on the concept of residual segregation index, during homogenization treatment, the diffusivities and the corresponding activation energies for diffusion of these elements in the austenitic matrix were obtained. The activation energy for the interdiffusion of Nb, Mo, and Ti was determined as 232, 286, and 256 kJ/mol, respectively. The outcome of these calculations is important for the precipitation reactions, high-temperature creep, and thermomechanical processing of superalloys.

The oxide/metal/oxide sandwich method is one of the techniques for investigating the dynamic oxidation of the melt. For preparation of the oxide/metal/oxide sandwich samples air bubbles, with 0.8-atmosphere pressure, were blown into the melts and the intersection of the entrapped bubbles was taken for the study. The characteristics of surface oxide films in different amounts of zinc (3, 5, and 7 weight percent), including their morphology and thickness, were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). There was no significant difference in the morphology of oxide films in the investigated alloys. However, in higher Zn content, more cracks and locations of stress concentration were found on the oxide film. The thickness of the oxide films of Al-Zn alloys were estimated between 95 to 1070 nm. By adding zinc up to 3% wt., the thickness of the oxide film is increased while adding more than 3% zinc causes decrement in the thickness of the film. Reducing the thickness of oxide films is possibly an indication of the increase in oxidation resistance of the Al-Zn alloys

In this study, wear behavior of the pearlitic ductile cast iron samples with the same hardness achieved by applying two different ‎heat treatment cycles have been studied. A number of these samples after austenitzing at 900°C were quenched into a molten salt ‎bath of 290°C for holding time of 120 minutes followed by cooling in air. The rest of the samples after being austenitized under ‎the same conditions were quenched in oil and then were tempered in 505°C for 75 minutes. To compare wear behavior of the ‎aforementioned cast irons, the “pin on the disk” method based on the ASTM-G99 standard was used. In this method, the ‎samples were prepared in the form of disks and were worn by 52100 steel pins for a constant distance of 1000 meters and the ‎sliding speed of 0.08 m/s at the applied loads of 25 N and 75 N. Based on the results obtained from the wear surface ‎observations, mass reduction and friction coefficient measurements of the worn samples, it was revealed that at the lower ‎applied load, the quenched and then tempered samples exhibited a higher wear resistance comparing to the austempered samples. ‎Whereas at the higher applied load (75 N) the inverse result was achieved.‎

Properties of hypereutectic Al-Si alloys depend on the size and morphology of primary Si particles. Different methods were developed to modify the morphology of primary Si particles including semi solid processes. In the present study, mechanical stirring, as a semi solid technique, was used to prepare the samples. Isothermal stirring was carried out for 10 minutes with 1600 rpm at five different temperatures. Microstructural analysis revealed that the morphology and distribution of the primary Si particles were by increasing the stirring temperature. The most prominent microstructure was achieved by stirring at 660C, the highest temperature used for stirring. With decreasing the stirring temperature, and increasing the solid fraction, the extent of agglomeration of primary Si particles increased. This in turn causes very large Si particles to form which distributed non-uniformly in the microstructure. It was also observed that the hardness of the stir cast samples depends on the stirring temperature, morphology and distribution of primary Si particles. By increasing the stirring temperature hardness increases from 54 to 76 HB and wear rate decreased from 0.0015 to 0.0008 mm3/m as a result of the refinement and uniform distribution of primary Si particles

In the present study, the service-exposed gas turbine blade was rejuvenated after 80000 hr of service. In this report, the effect of cooling on liquid nitrogen and cryogenic after two stages of solution (full and partial) was investigated. Microstructural studies by scanning electron microscopy (SEM) showed that increasing of the cooling rate in full solution resulted decrease in the size and percentage of volume fraction of γ´ precipitates after aging. whereas the cryogenic effect at full solution stage was less than the cooling rate. The effect of high cooling rate on partial solution was a decrease in γ´ precipitates volume fraction after aging compared to similar effect on full solution stage. Cryogenic treatment after partial solution increased the secondary γ´ precipitates volume fraction and size till 230 nm. High cooling rate of partial solution resulted in the maximum of hardness after aging and high cooling rate of full solution resulted in minimum of hardness after aging.

Microstructure and mechanical properties of a PWA 1483 superalloy directionally solidified in a Bridgman furnace are investigated. The effects of sample diameter, i.e. 13, 33 and 55 mm, and withdrawal rate, i.e. 1.5, 3 and 4.5 mm/min, on primary dendrite arm spacing were evaluated using standard metallographic techniques. Morphology and size of different microstructural constituents were studied by the scanning electron microscopy. Hot tensile tests at 850 °C was utilized to evaluate the mechanical properties of the heat treated samples. While primary dendrite arm spacing decreased with increasing withdrawal rate, it increased with increasing sample diameter. The cubic gamma prime precipitates formed in the interdendritic region were smaller than those formed in the dendrite core. The hot tensile strength of aged samples increased with reduced sample diameter and increased withdrawal rate. In addition, the reduction of sample diameter resulted in increasing of the hot ductility. However, the tensile ductility did not change significantly with the withdrawal rate.