seyed mahdi abbasi

The as-cast and homogenized microstructures of Al10Co25Cr8Fe15Ni36Ti6 high entropy alloy were investigated in this study. To this end, the high entropy alloy was produced using vacuum induction melting and electro slag remelting processes, and the ESRed block was homogenized at the temperature of 1220 °C for 17 hr. The as-cast, as ESRed and homogenized microstructures were then investigated using optical microscopy, scanning electron microscopy, and X-ray diffraction. On the basis of theoretical measurements, the mixing entropy, atomic size differences, and Valence electron concentration were obtained as 13.05 kJ/mol, 8.8, and 7.97, respectively. These values can predict the formation of a solid solution matrix as the type of BCC + FCC and intermetallic phases. The as-cast microstructure included γ+ γʹ in the dendritic zones and γʹ+ NiAl in the interdendritic areas. After homogenization, the dendritic structures were almost eliminated, and they became discontinuous which is an indication of elements distribution homogenization. NiAl phase was also omitted after the homogenization process.

Grain refinement through dynamic recrystallization is one way to improve the mechanical properties of high nitrogen austenitic stainless-steel ingots during hot rolling. In this paper, by using simulation based on the finite element method and experimental results, the effect of conventional rolling and gradient temperature rolling on the microstructural evolution of Nitronic 50 austenitic stainless-steel ingot during hot rolling is investigated. The results of simulations showed that strain in the center of the ingot be increased under a gradient temperature, while the maximum strain value in the conventional rolling is on the surface. The microstructural results obtained from optical microscopy for gradient temperature rolling showed strain value in the center is more than the surface. The temperature difference between the center and the surface leads to a significant reduction in the strength center of the ingot against the surface. The difference in average grain size of dynamic recrystallized for gradient temperature in the surface and center was less than 3μm. The existence of primary grains (68μm average grain size) and bulged grain boundaries in the center of ingot and refinement and small grains in the surface area (8.5μm) in conventional rolling (1000℃) showed that dynamic recrystallization, due to low effective strain and temperature, has not occurred in the center of the ingot. The almost uniform distribution of austenite grains in the direction of ingot thickness for gradient temperature rolling showed that this method is an appropriate method for hot rolling of austenitic steel ingots compared to conventional conditions.

The effect of ageing time and temperature on the microstructure and hardness of Al10Co25Cr8Fe15Ni36Ti6 high entropy alloys is investigated in this article. The alloy melted in vacuum induction melting furnace and purified using vaccum arc remelting process. Then homogenized at 1220°C for 17 hours. Then sample aged at 850, 900 and 950°C for 1-16 hours and assessed using XRD and SEM. The microstructure is involved ′γand NiAl precipitates and γ matrix. With increasing ageing temperature, the volume fraction and size of ′γ decreased from 60 Vol% and 338 nm at 850°C to 31 Vol% and 123 nm at 950°C, respectively. The ′γprimary morphology is cubic which changed to square and semi-square like. The hardness of alloy is 314, 329 and 327 Hv at 850, 900 and 950 °C, respectively. According to these results, 850°C for 8 hours is the best ageing conditions. The best high temperature tensile properties of 840.9 MPa ultimate tensile strength and 17.9 elongation obtained at 850 °C.

The purpose of this research is to investigate the possibility of reducing the solutionizing heat treatment duration of CMSX-4 single-crystal superalloy, which normally takes about 18 to 20 hours according to the standard. In this research, three solution heat treatment cycles with reduced time on single crystal samples of CMSX-4 superalloy made by the vertical Bridgman method, has been done and its results have been analyzed. The separation of alloying elements, the percentage of eutectic pools and the amount of porosities have been accurately evaluated based on microstructural measurements with optical microscope (OM), scanning electron microscope (SEM) and energy diffraction spectroscopy (EDS). The results showed that the solutionizing treatment performed with a reduced duration compared to the standard, caused the complete removal of inter-dendritic eutectic pools and the segregation of alloy elements has been reduced by about 30% on average compared to the as-cast sample. But the results have shown that even the longest suggested heat treatment duration was not enough to remove the porosity formed during the solutionizing process and the percentage of porosity has been reached from 0.83% in the as-cast sample to 1.01, 1.75 and 1.41% in heat treated samples for 8.5, 9.5 and 10.5 hours respectively.

This study investigates the low-cycle fatigue of Nimonic105 alloy with boron and zirconium of 0.003-0.013 wt.% and 0.0-0.16 wt.%, respectively at 750 °C. The fatigue test results indicated that the alloy with boron of 0.013 wt.% had the highest fatigue life of 400 cycles, while the base alloy showed the lowest fatigue life of 21 cycles at 2% strain amplitudes. For the alloy with 0.16 wt.% Zr, and the alloy with 0.08 wt.% Zr and 0.006 wt.% B, cyclic-hardening occurred at a constant slope. Then, hardening followed a nonlinear procedure at a reducing rate. Finally, softening and fracture happened. For 0.013 wt.% Zr alloy, however, the diagram reached a stable state or slow cyclic-softening and failed after a relatively short period of cyclic - softening. The Coffin-Manson equations’ parameters verified the increased flexibility due to the addition of B. to be a factor in improving high-temperature LCF strength. The investigation of the samples’ fracture surfaces indicated that the intergranular fracture of the base alloy with the lowest fatigue life became intergranular and transgranular fracture in the alloy with 0.16 wt.% Zr content and the alloy with 0.08 wt.% Zr content and 0.006 wt.% B contents. Also, 0.013 wt.% B alloy with the highest fatigue life showed a completely transgranular fracture.

The purpose of this study is to investigate the effects of withdrawal rate on the dendrite microstructure and its formation mechanism, the porosity, and the interaction between them in Rene 80 superalloy. So, Rene 80 Ni-base superalloy was directionally solidified on a laboratory scale using the Bridgman method. The cylindrical rods were grown at withdrawal rates of 2, 4, 6, 8, and 10 mm.min-1. Dendritic structure and solidification microporosities were evaluated in transverse and longitude sections. The results showed that when the withdrawal rate was increased, the primary and secondary dendritic arm spacing decreased. With an increasing withdrawal rate, which causes to decrease in the dendritic arms spacing, the volume fraction of inter-dendritic gamma prime was first decreased until the rate of 6 mm.min-1, and after that, its volume fraction increased. This structure results from peritectic and eutectic transformations with checkerboard-like and fan-like morphology, respectively. Moreover, the volume fraction of microporosities was minimal at the rate of 6 mm.min-1, while their average size decreased from 13.2 to 8.7 μm. The specimens were given a two-stage heat treatment followed by a stress rupture test at 191 MPa and 980˚C. It was shown that at R=6 mm.min-1, directionally solidified rods with a less solidification microporosity and well-orientated dendritic structure give higher rupture life of 25.43 hrs.

Rene 80 is a cast alloy which widely used in turbine blade industry. The purpose of this investigation is to study the effect of crucible type (Zirconia and Alumina) in vacuum induction melting furnace (VIM) with alumina rucible on the amount of gases, γ' precipitates and stress rupture properties of Rene 80 superalloy. For this purpose, the amount of O, N gasses and chemical composition were analyzed. The microstructure was assessed by scanning electron microscopy (SEM) and stress rupture properties was performed at 980˚C. The results showed that Crucible type has no considerable effect on the γ' size and morphology but γ' volume fraction increased about 3% in the alloy melts in Zirconia crucible in comparison to Alumina one. The result of mechanical test showed that the stress-rupture life of sample melts in Alumina and Zirconia crucibles are 22.6 and 24.7 respectively because of reducing γ' volume fraction in the alloy melt in Alumina crucible.

The aim of this study is the improvement of fatigue properties via controlling the vacuum pressure and melt holding time. The vacuum pressure of 10-1 and 10-3 mbar were applied during melting in VIM and the melt were kept for 3 and 30 minutes. The results show that with increasing the pressure from 10-1 to 10-3 mbar at 3 and 30 min, the grain size increase from 44 to 66 µm and from 71 to 78 µm, respectively. Also, the volume fraction of carbides at 3 min decrease from 9.6 to 8.05 vol% with increasing the pressure and it also decrease with increasing time as well. High temperature low cycle fatigue test was carried out at strain of 0.6% and temperature of 750°C. At 3 and 30 min with increasing pressure, the cycling life increase 26% and 15%, respectively. At constant pressure, with decreasing time, the cycling lives also increase.

The effects of 0.05, 0.1 and 0.15 wt% C on the microstructure, hardness and tensile properties of annealed Haynes 25 were investigated. The alloys were melted in a vacuum induction melting furnace and purified through electro slag remelting process then solution annealed at 1200˚C for 30 min. The results indicated that W-riched M6C carbides developed in the microstructure and its amount increase with increase of carbon. By increase of Carbon from 0.05 to 0.15wt%, grain size decrease from 44 to 35 µm. The carbides act as nucleator and also retard grain growth that cause to reduce grain size. Mechanical properties investigation shows that with increasing carbon, as a result of grain refinement and the proper size and morphology of carbides, hardness increases from 255 to 290 Hv, yield strength from 450 to 510 MPa and ultimate tensile strength from 946 to 1088 MPa but ductility decreases slightly.

In this article, the effects of VIM and VAR remelting processes are investigated on the microstructure and absorption/desorption characteristics of MmNi4.8Al0.2 hydrogen storage alloy. The main alloy prepared in a Vacuum induction furnace and remelted in VIM and VAR. The microstructures and phases were analyzed with SEM and XRD. Hydrogen absorption/desorption characteristics is performed on Sievert apparatus. The results showed that the microstructure is consisting of matrix, second phase as a result of Al segregation, porosities and cracks. The second phases in the main alloy and VAR remelted have low content of La and Ce. This phase is solutionized or decreased to low level in VIM remelted alloy. Remelting, also, declined the absorption pressure to 21.55 and 18.17 bar and the desorption pressure to 7.13 and 5.49 bar in VAR and VIM remelted alloy, but increased the hydrogen storage capacity increased to 1.42 and 1.46 wt%  respectively. The more the homogeneity degree, the less the absorption/desorption pressure and the more the hydrogen storage capacity.

In order to investigation of dynamic and static restoration of SP-700 alloy, in this study continuous and interrupted hot torsion tests carried out at 850 and 1000ºC at different pass-strains and inter-pass times. The dominant mechanism in hot deformation at 1000°C is dynamic recrystallization (DRX) and consequently the entire microstructure comprises equiaxed grains, whereas at 850°C serration and tanglement of the grain boundaries were observed. Nevertheless, the microstructure of sample twisted at 850°C, indicates the occurrence of DRX and the formation of very fine grains. The mechanism of the formation of recrystallized grains in the vicinity of grain boundaries and triple points is bulging. With an increase in pass-strain (ε=0.5) at 1000°C, due to the increase in driving force for nucleation and growth of new grains, the kinetics of static restoration increases. In fact, at 850°C, in addition to static restoration there is another factor contributing in fractional softening which is β to α phase transformation.

The purpose of this investigation, is study the effect of melt holding time (3 , 20 min) in vacuum induction melting furnace (VIM) with alumina crucible on the amount of gases, γ' precipitates and stress rupture properties of Rene 80 superalloy. For this purpose, the amount of O, N gasses and chemical composition were analyzed. The microstructure was assessed by scanning electron microscopy (SEM) and stress rupture properties was performed at 980˚C. The results showed that with increasing the melt holding time from 3 to 20 min, γ' volume fraction reduced from 46 to 37% and also, oxygen level of the alloys increased from 79 to 165 ppm because of the alumina reduction. The morphology of γ' changed from semi-cubic to cubic one. The result of mechanical test showed that the stress-rupture life of samples reduced from 22.6 to 19 hours by increasing melt holding time because of reducing γ' volume fraction.

The creation of γ' phase uniform distribution is very important on the performance of superalloys. The addition of secondary solution annealing stage to the conventional heat treatment process of the GTD-111 polycrystalline superalloy which is one of the useful industrial superalloys can lead to improving the microstructure of this alloy by decreasing the amount of eutectic phase and increasing the volume percent and modifying distribution of γ' phase. In this article, the effects of temperatures of 960, 980 and 1000 °C and times of 1, 2, 4 and 8 hrs on the microstructure of GTD-111 superalloy are investigated. Microstructural investigations by OM and SEM methods show the secondary solution heat treatment at temperature of 980˚C for 4 hrs leads to improving the microstructure with respect to decreasing the eutectic phase amount to 4 volume percent and increasing the γ' strengthening phase volume percent to 54%. Also, the morphology of γ' phase after this heat treatment is cuboidal with unimodal and homogenous distribution in the matrix phase. The microstructures of GTD-111 heat treated at other temperatures and times are discussed in the article.

Recycling of industrial scrap of IN317LC superalloys via ESR process is investigated in this article. The purpose of this study is reach to the best chemical composition, microstructure and mechanical properties according to AMS5377E standard. Different levels of TiO2 (0, 3, 6 wt %) were added to 70CaF2-30Al2O3 ESR slag. The results show that in slag wih 3 wt % TiO2, Ti loss compensate by Oxidation-Reduction reaction between slag and melt. As a result of the variation of slag activity, oxygen and nitrogen of the recycled ingot reach to 14.3 and 16 ppm, respectively. In addition, this ingot has the maximum level of γ' particle with minimum size because of high level of (Ti) of this recycled alloy, the good microstructure and the stress rupture life of 47 hr obtained. In the recycled ingot by 6 wt % TiO2, despite of compensation of Ti loss and increase of Ti level, the mechanical properties reduced as a result of reduction of γ' volume fraction.

In this research, the effect of homogenizing treatment on the microstructure and hardness of polycrystalline GTD 111 Ni-based superalloy was assesed. Homogenizing is one of the key parameters on the efficiency of annealing and ageing of the alloy. The superalloy was melted in a vacuum induction melting furnace and homogenized at 1200 and 1220C for different times. Microstructure evolution by optical and scanning electron microscopy showed that the segregation of Al and Ti at interdendritic region and W at the core of the dendrite eliminates after homogenizing at 1220C for 2 hr. volume fraction of carbides, eutectic and gamma prime of the cast alloy was 6%, 10% and 28% respectively that reach to 4%, 7% and 41% after homogenizing at 1220C for 2 hr. The hardness of the alloy at this condition was 440Hv which is favorable for the homogenized alloy.

In this research of hot tensile temperature and strain rate on the microstructure and hot tensile properties of Ti2448 alloy has been investigated. For this purpose, tensile test has done as a function of temperature (700- 900˚C) and strain rate (έ=0.01, 0.1 and 1 s-1). With increasing hot tensile temperature, ductility has been in beta phase. During hot tensile in 850 and 900˚C, homogenous deformation has been observed as result of counterbalancing between dynamic recovery and strain hardening. Similarity of microstructure and equal volume fraction of alpha phase resulting equal ductility in the range of 700-800˚C. Microstructure examination shows that β→α transformation has been increased. During αβ deformation, β→α transformation has been accelerated as a result of high volume fraction of preferred nucleation sites.

The age-hardening curves of hardness measurements obtained for Ni-Span C 902 superalloy under different amounts of cold work, aging temperatures and times showed leveling and pronounced oscillations, indicating instability and reflecting a competition between the effect of sub-structure coarsening and the effect of solute drag and precipitation hardening. An artificial neural network (ANN) was used to model the nonlinear relationship between the parameters of the aging process and the corresponding hardness measurements. The predicted values of the ANN are in accordance with the experimental data. Results showed that the non-deformed and 50 pct cold rolled alloy exhibited a maximum hardness at a tempering parameter of 22 and 21, respectively.

The hot ductility of IMI834 alloy in two conditions of first hot rolling and second hot rolling has been studied by conducting tensile tests with a strain rate of 0.1 s−1 and temperature range of 800–1000 °C. In first hot rolling condition, IMI834 alloy showed little hot ductility in the temperature range 800–900 °C and ductility loss occurred at 850 °C. In second hot rolling condition, hot ductility increased at all temperatures and ductility loss is not occurred. Especially at 900 °C, hot ductility increased from 31% in first hot rolling to 95% in second hot rolling.