Iranian Journal of Materials Science and Engineering

Mullite–zirconia composites were prepared using lanthanum oxide (La₂O₃) additive which three different mole ratio by the reaction sintering (RS) route of alumina, kaolinite and zircon. Starting materials were planetary milled, shaped into pellets and bars and sintered in the temperature range of 1450–1550 ⁰C with 5 h soaking at peak temperature. In this work, the mullite-zirconia composites were characterized by thermal expansion coefficient, physical, microstructures and mechanical properties. The XRD method were employed for determining the crystalline phase composition of these composites. Microstructure of the composites was examined by SEM. ZrO₂ takes part in both the intergranular as well as intragranular positions. However, intragranular zirconias are much smaller compared to intergranular zirconias.

An experimental–numerical methodology was used in order to study the microstructural effects on stress state dependency of martensitic transformation kinetics in two different TRIP800 low alloy multiphase steels. Representative volume elements extracted from actual microstructure have been utilized for simulating the mechanical behavior of mentioned steels. The mechanical behavior for each constituent phases required in the model has been taken out from those reported in the literature. A stress invariant based transformation kinetics law has been used to predict the martensitic phase transformation during deformation. Crystallographic and thermodynamic theories of martensitic phase transformation have been utilized for estimating the constant parameters of the kinetics law, in a recently performed investigation, but the sensitivity of the transformation to the stress state remained as an adjusting parameter. The results of the current work show that the stress state sensitivity of martensitic phase transformation in the investigated steels is microstructure-dependent and the value of this parameter is almost equal to half of the bainite volume fraction. Therefore, the volume fraction of bainite in the low-alloy multiphase TRIP800 steels can be used as a first postulation for the value of the martensitic phase transformation sensitivity to the stress state and the microstructure based model previously developed for calculating the mechanical behavior of the TRIP800 steels can be utilized as a virtual design tool for development of TRIP steels having specific mechanical properties.

In the present investigation, A356-TiB₂/TiC composites with a various weight fractions (0, 2.5, 5 and 7.5%) were synthesized through a K₂TiF₆-KBF₄-Graphite (C) reaction system. Formation of TiB₂ and TiC particulates and their distribution are confirmed by various characterization techniques. The tensile properties such as ultimate strength, yield strength, young's modulus and percentage of elongation in addition to their failure behavior of these composites were studied at ambiance and high temperatures (100, 200 and 300⁰C). The increment in the volume fraction of the composite raises the hardness and the enhancement of hardness was reported up to 49% at 7.5% reinforced composite due to the strengthening effect. The density and porosity of fabricated composites were investigated. The rise in volume fraction of reinforcement phase declines the density and increase the porosity of composites. Further, the ultimate strength, yield strength, young's modulus is declining by raising the temperature. Result analysis illustrates that the 7.5% reinforced composite retaining the ultimate strength up to 84.4% and the ductility is raised by 27% at 300⁰C. Yield strength and young's modulus are also retained 74.31% and 71.09% respectively at the similar material and experimental conditions. The fracture surface analysis of the composites illustrates that, the ductile nature of failure appearance microscopically with the formation of fine dimples and voids on fracture surface at elevated temperatures. Cleavage facets and tear crumples observation indicates the brittle kind of failure at the ambient temperature. Findings from the experimental study provide the tensile behavior of the composites at the regular working temperature of the automobile engine piston.

Overusing nitrogen fertilizer causes some serious problems for water resources, soil, and agriculture products. Researchers have been trying to develop effective means which may use less amount of fertilizers containing nitrogen. In this work, cost-effective ceramic granule adsorbent was prepared to be used as a fertilizer carrier of controlled release behavior. A mixture of 70 wt.% domestic kaolin and 30 wt.% gibbsite was used to produce the granules. By utilizing thermal analysis of raw granule, the calcination temperatures were obtained and the effect of various calcination temperatures of 500, 600, and 700˚C on the water adsorption was studied. The characteristics of granules were investigated by XRD, BET, FTIR, and SEM analyses. The results showed that by increasing the calcination temperature, the crystal structure of the granules was transformed into a dehydrated form and by calcination at 600°C the specific surface area of granules increased from 7.50 to 53.45m²/g. The granules were soaked in a 500g/lit solution of urea, where they adsorbed about 10wt.% urea. The dried urea-loaded granules were placed in water where the release of urea was measured by UV-vis spectrophotometry. Finally, different portions of urea-loaded granules were evaluated as fertilizer in the growing bed of corn plant where the height and the stem diameter of samples were compared with a control sample as well as a sample fertilized by urea directly. The results showed that by using the loaded granules, the urea consumption can be reduced by 50%.
 

    The structural and dielectric properties of iron and bismuth co-substituted BaTiO₃ ceramic with the formula: B_0.95Bi_0.05Ti_1-xFe_xO₃ for x=0.00 to 1.00, synthesis with solid state route, were characterized.     The X-ray diffraction results show a tetragonal phase for x=0.00. While for x=0.40 to 0.80 we observed a coexistence of tree phase tetragonal, hexagonal and pseudo-cubic. And at x=1.00 only the pseudo-cubic phase is present and the other phase disappeared. The Raman results indicate the existence of tetragonal band for x≤0.40, and an appearance of characteristic bands of Fe³⁺ ions for more than 0.40 of Fe content. The SEM micrographs show an increase in grain size with the increase of Fe content and it reaches a maximum at x=0.40.  And the Mossbauer spectroscopy indicates that our samples is paramagnetic at room temperature and that the Fe is   oxidized under Fe³⁺ with no existence of Fe²⁺ and Fe⁴⁺ ions. The temperature dependence of dielectric permittivity was investigated in the frequency range from 20 Hz to 2MHz. The results show three dielectric relaxation phase transitions from a rhombohedral ferroelectric to orthorhombic ferroelectric (T_R-O) then to a tetragonal ferroelectric phase (at T_O-T), and finally to cubic paraelectric at the Curie temperature (T_C).  In addition, the temperature of phase transition shifted to the lower temperature with the increase of Fe content for all the phase transitions. And the maximum of dielectric permittivity increases for T_R-O while for T_T-O and T_m phases transitions, it reaches a maximum at x=0.60 and x=0.80 respectively and then decreases.

The influence of melt superheating treatment on the solidification characteristics and microstructure of Al–20%Mg₂Si in-situ composite has been investigated. The results revealed that melt superheating temperature has a significant effect on solidification parameters and morphology of primary Mg₂Si particles. Solidification parameters acquired using cooling curve thermal analysis method, indicate that both nucleation temperature and nucleation undercooling of primary Mg₂Si particles increase by increasing melt superheating temperature, while recalescence undercooling decrease under the same condition. Also, based on the microstructural evaluations, melt superheating treatment can refine primary Mg₂Si particles and alter their morphology from dendritic shape to more spherical shape and the eutectic microstructure of a-Al + Mg₂Si becomes finer and the distance between eutectic layers becomes smaller.
 

Coatings based on chromium borides and chromium carbides are commonly employed in applications requiring mechanical performance, such as high hardness and low friction coefficient, as well as corrosion resistance. In this work, we made layers of chromium borides and chromium carbides on the surface of low carbon steel through some specific treatments. For chromium borides, the boriding treatment in a solid medium at 900 °C for 4 hours followed by chromium electroplating on the steel surface and finally the application of annealing treatment at temperatures at 950 °C for 1 and 2 hours. For chromium carbides, the cementation in a solid medium followed by electroplating of chromium on the surface and finally the application of annealing treatment at temperatures between 500 and 1100 °C for 1 hour. The obtained results show that, in the first case, boron diffusion and chromium deposition lead to chromium borides on the treated surface. Similarly, for the second case, the cemented layer and the chromium deposited on the surface combine to form chromium carbides on the treated surface after annealing. The characteristics of the chromium borides and chromium carbides obtained are very similar to those of chromium borides and chromium carbides obtained by other processes.

Hybrid composites consisting of AA6061 matrix reinforced with TiB₂ (2, 4, 6, and 8 wt. %), Al₂O₃ (2 wt. %) particles were produced by the sintering process. In comparison to the base material AA6061, the composite produced had improved mechanical properties. The sintered composites' mechanical properties, such as tensile strength and hardness, are measured and compared to the wear-tested specimen. Optical micrographs reveal that composites were riddled with defects like blowholes, pinholes, and improper bonding between the particulates before sintering. However, the post-sintered optical micrograph showed that the defects were greatly suppressed. Micrographic images revealed the changes in surface characteristics before and after wear. Until a sliding distance of 260 m, the wear rate of the hybrid composites was kept lower than that of the base material. The coefficient of all the composite materials produced for this study was noted to be less than that of the base material. The results reveal that the hardness of hybrid composites having 4 wt. % and 6 wt. % of TiB₂ particulates increased by 5.98 % and 1.35 %. Because of the frictional heating during the wear test, the tensile properties lowered by up to 49.6%. It is concluded that the hybrid composites having 4 wt. % and 6 wt. % of TiB₂ particulates exhibited less wear rate for extended sliding distance, good hardness, moderate tensile strength, and decent elongation percentage compared to its counterparts.