Iranian Journal of Materials Science and Engineering

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In the present work microstructural evolution of A356 Aluminum alloy using an inclined cooling plate casting process for thixoforming feedstock production is investigated. The resultant microstructure was evaluated and compared with those of the same alloy produced by the conventional casting process, i.e. directly cast in the same mold without using an inclined cooling plate. It was found that when alloy melt poured over an inclined cooling plate and subsequently cast in semisolid condition into a metallic mould resulted in fine rosettes and nearly globular α-Al primary phase uniformly distributed in an Al +Si eutectic matrix. The effect of the processing parameters such as the lengths and angles of the inclined cooling plate and their combinations were identified to produce alloy ingot with the most suitable microstructural constituent for thixoforming process

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In the present work, one batch of prealloyed 6061Al powder was processed by mixing and another one was ball milled with varying amount of lead content (0-15 vol. %). These powders were compacted at 300MPa and sintered at 590˚C under N₂. The instrumented hardness and the young’s modulus of as-sintered 6061Al-Pb alloys were examined as a function of lead content and processing route. The wear test under dry sliding condition has been performed at varying loads (10-40 N) using pin-on-disc tribometer. The microstructure and worn surfaces have been investigated using SEM to evaluate the change in topographical features due to mechanical alloying and lead content. The mechanically alloyed materials showed improved wear characteristics as compared to as-mixed counterpart alloys. Delamination of 6061Al-Pb alloys decreases up to an optimum lead composition in both as-mixed and ball-milled 6061Al-Pb alloys. The results indicated minimum wear rate for as-mixed and ball-milled 6061Al alloy at 5 and 10 vol. % Pb, respectively.

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In this research, nanocrystalline Co-Fe coatings were electrodeposited on copper substrate. The influence of current density on different properties of the films at two pH levels was investigated. All the coatings showed nodular structure with rougher morphology at higher current densities. Due to anomalous deposition at higher current density, the amount of iron content increased and reached its maximum value at about 50 wt.% for the coating obtained from pH 5. X-ray diffraction patterns showed hcp structure as the dominant phase. However, by increasing current density at lower pH value, a double phase structure containing fcc+hcp phases was detected. It was observed that current density has a positive effect on grain refinement. However, coarser grains would obtain at lower pH value. Microhardness measurements showed that, there is a direct relationship between grain size and microhardness. Moreover, microstructure in double phase structure films can influence microhardness more dominantly. Vibrating sample magnetometer (VSM) measurements indicated that the saturation magnetic is proportion to deposited iron content and reached its maximum value at about 1512 emu/cm³. It was cleared that grain size, phase structure and chemical composition can affect coercivity of the films effectively.

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In this research, the effect of annealing and aging temperature as well as the effect of quenching media on the microstructure, hardness and dry sliding wear behavior of Ti-6Al-4V alloy has been studied. Cylindrical samples with the diameter of 10 mm and the height of 20mm were solutionized at 930˚C and 1060˚C for 600 seconds and then were quenched in the cold water and in the air. The samples were aged at different temperature of 480˚C, 550˚C and 610˚C for 360 s to increase the hardness. Heat treated samples were tested using standard pin-on-disc test machine at the applied loads of 100, 150 and 200 N. Microstructural investigations using scanning electron microscope revealed that for the samples solutionized at 930˚C and quenched in the water, the microstructure is composed of primary α and high volume fraction of martensitic α΄ phase with fine precipitated of β between martensitic lathes. In the case of air cooled samples, transformed β has also been  appeared in the vicinity of primary α. For the samples solutionized at 1060˚C and quenched in the water, fully martensitic micro structure with fine β precipitates was observed. For the sample solutionized at 1060˚C and air quenched, plate like α and lamellar grain boundary β were detected. The maximum hardness value relates to the sample solutionized at 1060˚C and quenched in water which is equal to 433 HV. Different wear mechanisms, including oxidative wear, scratch, and delamination occurred at the worn surfaces at different applied loads. For the samples quenched in the water, the oxidative wear mechanism governing at low applied load and oxide debris was observed as separate or compacted particles which formed in the contact between pin and steel counter face detached from the sample. At higher applied loads, delamination and scratch mechanism was also observed and metallic plate like debris was detached from the sample. Mechanically mixed layer (MML) was formed on the surface of the pin at high applied loads and for the samples with low hardness value adhesion marks were also revealed on the steel disc. The minimum weight loss in the wear test is related to the sample quenched from the 1060ºC in the cold water and aged at 550 ºC .  

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The densification behavior, structural and microstructural evolution and microwave dielectric properties of Li₂TiO₃ + xZnO (x = 0, 0.5, 1, 1.5, 2, 3, and 5 mol%) ceramics have been investigated using X-ray diffraction, Field Emission Scanning Electron Microscopy, Raman spectroscopy and microwave resonant measurement. The Maximum density of 3.33 g/cm³ was obtained in Li₂TiO₃ + 2ZnO ceramic at low sintering temperature of 1100˚C. SEM investigations revealed good close packing of grains when x = 2 and preferential grain growth when x ≥ 3. The maximum values of Q × f = 31800 GHz and ε_r = 22.5 were obtained in Li₂TiO₃ + 3ZnO and Li₂TiO₃ + 2ZnO compositions, respectively. The observed properties are attributed to the microstructural evolution and grain growth (first case) or high density of the obtained ceramic (second case).
 

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Relations between the microstructure, mechanical properties and machinability of as-cast 65Cu-35Zn brass with various amounts of Al from 0 to 4.72 and Si from 0 to 3.62 wt% were investigated. Both Si and Al initially enhanced the UTS and toughness of the brass samples, which led to improvement in machinability due to a reduction in the main cutting force. A duplex brass with random oriented α plates in β’ matrix was found to have the best machinability among the other microstructures. It was found that beside the presence of brittle phases, such as β’ phase in the microstructure, the morphology and hardness of the phases involved had significant influence on machinability.

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In this study, the effects of the second pulse resistance spot welding on the microstructure and mechanical properties of TRIP1100 steel were evaluated. The thermal process after welding was designed to improve metallurgical properties with pulse currents of 6kA, 9kA and 12kA after initial welding with 10kA current. The effect of the second pulse on mechanical and microstructural properties was investigated. The fracture of the welds was for pulsed samples of 6kA and 9kA PO with CTS test. Due to existence of the microstructure including the equaxial dendritic and finer in FZ in the pulsed current 9kA, the maximum fracture energy and maximum force were observed. A significant decrease in the FZ hardness in 6kA current was observed in the nanohardness results, which was attributed to existence martensitic and ferrite temper. The highest ratio of CTS / TSS was obtained for 6kA and 9kA, respectively, and force displacement rate was maximum in 9kA. The fracture surfaces included dendrites and dimples. The results of partial fracture revealed separation in the coherent boundaries of the coarse grain of the annealed region.

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The combined influence of both melt filtration and cooling rate on the microstructure features and mechanical properties of A356 cast alloy was studied. A step casting model with five different thicknesses was used to obtain different cooling rates. The effect of melt filtration was studied by using of 10 and 20 ppi ceramic foam filters in the runner. Results showed that secondary dendrite arm spacing decreased from 80 μm to 34 μm with increasing cooling rate. Use of ceramic foam filters in the runner led to the reduction of melt velocity and surface turbulence, which prevented incorporation of oxide films and air in the melt, and consequently had an overall beneficial effect on the quality of the castings. A matrix index, which is the representative of both SDAS and microporosity content, was defined to consider the simultaneous effect of melt filtration and cooling rates on UTS variations. Also, the fracture surface study of test bars cast using 10 and 20 ppi ceramic foam filters showed features associated with ductile fracture.
 

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In the present work, accumulative roll bonding (ARB) was used as an effective method for processed of nano/ultrafine grained AA6063 alloy. Microstructural characteristics indicate considerable grain refinement leading to an average grain size of less than 200 nm after 7 ARB cycles. Texture analysis showed that 1-cycle ARB formed a strong texture near Copper component ({112}<111>). However, texture transition appeared by increasing the number of ARB cycles and after 7-cycle of ARB, the texture was mainly developed close to Rotated Cube component ({100}<110>). The results originated from mechanical properties indicated a substantial increment in strength and microhardness besides a meaningful drop of ductility after 7 ARB cycles.

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Mechanical properties of metals are substantially dependent on the microstructure, which can be controlled by thermo-mechanical parameters such as the temperature, strain and strain rate. Hence, understanding the microstructural evolution of alloys during the hot deformation is crucial for engineering the metal forming processes. The main objective of this work is to present an overview of Cellular Automaton (CA) modeling for predicting the microstructure of alloys during the dynamic recrystallization (DRX) phenomenon. In this review paper, first, overall descriptions about the DRX phenomenon and CA modeling were presented. Then, the CA modeling procedure was compared with similar methods. Meanwhile, related studies in the field of the DRX simulation by using the CA modeling were evaluated. Four main stages of the model were analyzed in terms of the “nucleation”, “growth”, “topological changes” and “texture evaluation” steps. Most important limitations including the calibration sensitivity, limitations in continuous DRX modeling, ignoring microstructural effects on the deformation behavior, limited applications and database as well as repeated results were discussed and then objective suggestions for the further development were provided. Finally, future prospects in CA modeling of DRX were presented in the last section.
 

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The effect of cooling rate after annealing at 900 °C on the microstructure and hardness of high entropy alloys was investigated using two typical samples with the chemical composition of Co₁₆Cr_14.5Fe₂₉Mn_11.5Ni₂₉ and Co_11.5Cr₇Fe₂₇Mn₂₇Ni₂₇(Nb_0.08C_0.5) (at%). The microstructural characterisation and hardness measurements were carried out by optical microscopy, scanning electron microscopy, wavelength-dispersive X-ray spectroscopy, electron back scattered diffraction, X-ray diffraction technique and Vickers hardness testing. A face centred cubic crystal structure matrix was observed in both alloys before and after annealing and regardless of cooling conditions. SEM analyses revealed an extensive precipitation in Co_11.5Cr₇Fe₂₇Mn₂₇Ni₂₇(Nb_0.08C_0.5) alloy after annealing. It was also found that air/furnace cooling can enhance grain growth-coarsening just in Co₁₆Cr_14.5Fe₂₉Mn_11.5Ni₂₉. However, the hardness results generally showed insignificant hardness variations in both alloys after water-quenching, air-cooling and furnace-cooling. The results suggested that the hardness is mainly controlled by solid solution strengthening.

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This study deals with the effects of erbium (Er) addition on the microstructural evolution and tensile properties of Al-Mg₂Si in-situ metal matrix composites. The morphology of primary Mg₂Si and eutectic phases were observed in details using optical microscope and scanning electron microscopy (SEM). The results showed that the increase of Er content has a slight effect on the size and morphology of primary Mg₂Si phases, but the eutectic structure evolves from the coarse structure into the fine one. Also, with Er addition the eutectic mixtures of Al and Mg₂Si with fibrous morphology has been developed instead of the flake like Al-Mg2Si eutectic microstructure. Meanwhile, Al₃Er phase was observed in the samples containing Er. The ultimate tensile strength (UTS) of the composite changes under the various content of Er. The maximum strength was found at the 0.6 wt% Er with the fine eutectic microstructure. The study of SEM micrographs from the fracture surface of composites revealed that Er addition changes the fracture mode from brittle to ductile one with fine dimples. The mechanism of microstructural evolution was discussed in details.

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Niobium addition is an appropriate approach for improvement of superalloy’s operation. The purpose of this study is twofold: (1) to investigate on the η and γ^/ phase precipitations along with (2) to identify the high-temperature tensile properties in A286 and Nb-A286, as a modified type. The heat treatment of both alloys was carried out in a two-stage aging procedure at 760°C for 16 h and 820°C for 2 to 30 hours, following characterized by optical and Scanning electron (SEM-EDS) microscopies, differential thermal analysis (DTA) and high temperature tensile tests. The results showed that niobium addition was increased the volume fraction of γ^/ phase, from 10.7% to 12%, decreased its size, from 94 to 71 nm, and rising the γ^/-dissolution temperature from 987°C to 1007°C. Moreover, the γ^/ to η phase transformation was sluggishly occurred in Nb-A286 due to more stable of γ^/ precipitations. Furthermore, the Nb-A286 alloy demonstrates higher mechanical properties than A286 one, approximately 100MPa improvement, which it was contributed to the much large volume fraction and finer size of more stabilized γ^/ phase.