پژوهشنامه ریخته گری
سال چهارم شماره 4 (پیاپی 15، زمستان 1399)

The aim of this study was to evaluate the effect of aging on the stress rupture and hot pressure of novel cobalt base superalloy of Co-7Al-7W-4Ti-2Ta composition. After melting the superalloy by VIM method and remelting by VAR method, the ingot was homogenized at 1250 °C for 10 hours. Then, aging treatment was performed on homogenized samples at 800 °C for 8, 16 and 24 hours. After microstructural and phase analysis by SEM and X-ray diffraction pattern testing, the high temperature mechanical properties including hot pressure and stress rupture tests were performed. The results showed that the alloy in the cast condition contains ɣ, ɣ‘, a and β phases in which the ɣ‘, a and β phases are dissolved in ɣ phase by homogenization treatment. By aging of the studied alloy, ɣ‘ precipitates nucleate and grow, where in by increasing aging time the volume fraction and size of these precipitates increases. The results of pressure tests in the temperature range of 500 °C to 1000 °C showed that the strength of the alloy increased at a temperature of 700 °C, which indicates the occurrence of the stress anomaly phenomenon as a consequence of ɣ‘precipitation. Besides, the highest resistance to stress rupture at 770 °C under 300 MPa for 22 hours was achieved for the specimen aged at 800 °C for 16 hours.

In this research, the effect of manganese addition on the as cast microstructure of nodular ‎cast iron was investigated using experimental and simulation methods. For this purpose, ‎required cast sample with different manganese contents 0.5, 5, 11 and 17 wt.% were ‎produced in sand mold. The as cast microstructures were examined by optical microscopy ‎equipped by image analysis (MIP4 software) and hardness measurement. Then, using JMatPro ‎simulation software, the manganese segregation, the fraction, analysis and stability of the phase’s ‎at different temperature were predicted based on thermal analysis. The results showed that ‎the addition of manganese with 5, 11 and 17% weight percent has strongly affected the ‎solidification characterization, hardness, and morphology of graphite. A quantitative ‎evaluation of the graphite morphology showed that with increasing manganese content to ‎‎17 weight percent, the number of nodal counts and nodularity decrease 60 and 28 percent ‎respectively. The results of the JMatPro software showed that the M7C3 carbide is formed ‎alongside the ledeburitic composition. Also, the thermodynamically analysis of this software ‎revealed that manganese reduces the eutectic temperature and increases the solidification range.

This study was conducted to investigate the effect of Si addition (1, 2, 3, and 5 wt. %) on the microstructure and castability of hypoeutectic Al-4.5Cu-Si alloys. According to the results, Si addition increased the average size and volume fraction of eutectic Si platelets in the microstructure. Adding Si also improved the fluidity and decreased the porosity content of the alloy, and enhanced its resistance against hot tearing. According to the constrained rod test results, the hot tearing sensitivity index (HTS) of the alloys containing 1, 3, and 5 wt. % Si, was decreased by 48, 78, and 88%, respectively. In agreement with hot tearing testing results, the extensive existence of dendrites and shrinkage micropores on the hot torn surface of the alloy without Si implies on its low potential in feeding the solidification shrinkages and healing hot cracks. Adding Si decreased the amount of shrinkage micropores on the hot torn surface and due to improving the fluidity as well as increasing the amount of Al-Cu-Si ternary eutectic at the last stage of solidification, improved the feeding characteristics of alloy. Therefore, the amount of free dendrite arms was significantly decreased on the fracture surface. However, adding 5 wt. % Si increased the amount of eutectic Si on the hot torn surface giving rises to a more brittle fracture mode.

Melting temperature is one of the most important size-dependent properties in nanoparticles. The experimental ‎study of nanoparticle melting is so complicated. So, many mathematical models have been proposed to ‎predict the melting temperature of free and embedded nanoparticles. However, most of these models are based ‎solely on solid phase properties and does not take the properties of the liquid state into account. This causes ‎inaccuracies in the results of these models. Therefore, development of new models with more accuracy is ‎essential. Based on the calculation of Gibbs free energies of solid and liquid phases, in the present study, a ‎thermodynamic model is proposed to calculate the melting temperature of embedded nanoparticles. The ‎proposed model used to calculate the melting temperature of silver nanoparticles embedded in nickel matrix ‎and Pb nanoparticles embedded in Cu and Zn matrices. The results showed that in the studied systems, ‎reducing the particle size will increase the melting temperature of the nanoparticles. Comparing the calculated ‎results with the available experimental data as well as the results of the previous models confirms the higher ‎accuracy of the proposed model. In addition to the particle size, the effect of nanoparticle shape on the melting ‎temperature is investigated. The results showed that the change in melting temperature with the change in ‎nanoparticles shape is evident only in nanoparticles smaller than 10 nm.‎

Effects of withdrawal rate on the structure of directionally solidified GTD-111 Ni-based superalloy were investigated. To this end, DS specimens were first obtained by the Bridgman Furnace (equipped with a graphite cooling zone) at withdrawal rate of 1 and 10 mm/min. Then, structural investigation was carried out by optical microscopy and field emission scanning electron microscopy at longitudinal and transverse sections with respect to solidification direction at the bottom (location of starting solidification) and top (location of finishing solidification) zone of specimens. The results showed that the polycrystalline grain zone at bottom of the specimen enlarged with increasing the withdrawal rate. Furthermore, with increasing withdrawal rate from R=1mm/min to R=10mm/min, the primary and secondary arm spacing decreased from 445μm to 252μm and from 110μm to 60μm, respectively. The γʹ size also decreased from 468μm to 421μm at top of specimens and from 711μm to 604μm at bottom of specimens with increasing the withdrawal rate from R=1mm/min to R=10mm/min. In addition, due to the direction of heat transfer during directional solidification at a constant withdrawal rate, the bottom zone of specimen was exposed to heat output from top zone. This led to increase the γʹ size at bottom zone (711μm at R=1mm/min and 604μm at R=10mm/min) compared to the top zone (468μm at R=1mm/min and 421μm at R=10mm/min) of specimen. The best microstructure belonged to the top zone of DS specimen with withdrawal rate of 10mm/min which has a small gamma-prime average size, homogenous distribution, and regular morphology.