Materials Science Forum Vols. 830-831

Abstract: The present paper describes a cost effective route to produce Al alloy-10 wt % TiB₂ metal matrix composites by in-situ molten flux assisted reaction synthesis. Now a days main focus is given to aluminium alloy as a matrix material due to its unique combination of good corrosion resistance, low density, superior mechanical properties, good vibration damping, higher wear resistance due to which alloy finds extensive applications in naval applications. With TiB₂ as particulate addition in Al-Alloy (Al-5Mg) matrix properties of alloy can greatly be improved.

Abstract: Al/2TiB₂ in-situ composite was fabricated using salt-melt reaction method. Subsequently, it was re-melted and treated with high intensity ultrasonic waves for various time intervals. Substantial reduction in TiB₂ particle size down to ~ 300 nm along with significant improvement in dispersion is achieved with the aid of ultrasonic treatment. Ultrasonic treatment has resulted significant increase in hardness of Al/2TiB₂ in-situ composite.

Abstract: In this investigation, the effect of nanoreinforcement particles such as Al₂O₃, tool rotational speed and traverse speed on microstructure and mechanical properties of Al/Al₂O₃ surface nanocomposites fabricated by friction stir processing was studied. The surface nanocomposites were produced by varying volume percentage of nanoreinforcement, tool rotational speed and traverse speed to attain the best outcome. The fabricated composites were characterized through microstructural evaluation, microhardness measurements and energy dispersive spectroscopy analysis. Microstructural evaluation of the composites revealed that the composites were produced finer grain structure in this stir zone and it is evident for, a dynamic recrystallization was taken place. Higher hardness values were found at the stir zone of the entire composite because of the equiaxed and well dispersion of reinforced particles. The energy dispersive spectroscopy analysis revealed the presence of various elements at the stir zone. A defect free parameter setting for friction stir processing of Al/Al₂O₃ was obtained at 1120 rpm and 16 mm/min.

Abstract: In this investigation, copper (Cu) based surface composites reinforced with silicon carbide (SiC) particles were fabricated using friction stir processing (FSP) route. FSP was carried out considering three-factor three-level Box-Behnken design to study the effects of process parameters on peak temperature and hardness of Cu/SiC_p surface composites. Microstructural evaluation using optical microscope (OM) revealed that SiC_p were uniformly distributed and well – bonded with copper matrix at an adequate heat input conditions. The microhardness of the surface composites were remarkably enhanced than that of base metal. Regression models have been developed for predicting peak temperature and microhardness of processed surface composites and the same were in good agreement with experimental results.

Abstract: Silica aerogels have been prepared through sol-gel process by polymerization of TEOS in the presence of NH₄F and NH₄OH as catalysts. The solvent present in the gel is replaced by ethanol followed by a non-polar solvent such as n-hexane prior to solvent modification step. Gels are made hydrophobic by treating them with HMDZ to prevent rupture during drying, which has been confirmed by FTIR. Gels are then washed and dried carefully in a PID controlled oven at atmospheric pressure. The ageing duration and solvent exchange combinations are optimized to yield crack-free gels prior to drying. Aerogels are characterized for density, specific surface area, pore volume, pore size, thermal stability and contact angle. Hydrophobic, high surface area (570 m2/g), low density (0.07 g/cm3) silica aerogels are synthesized by using optimized mole ratio of precursors and catalysts. Silica aerogel granules (1-3 mm) as well as monoliths (Ф~35 mm) could be produced through ambient pressure drying of gels.

Abstract: The widespread demand for light-weight materials in various emerging industrial sectors lead to the fabrication of aluminum- boron carbide composites. In this study, the B₄C particles were coated copper and Ni-B through electroless process using formaldehyde and sodium borohydride respectively as reducing agents under optimized condition. The microstructural and hardness behavior were investigated for powder metallurgy processed 20 vol. % of B₄C and coated B₄C particles in the aluminum matrix. Microscopic observation revealed that coating improved the dispersibility of B₄C particles in the matrix. The coated particles showed an increase in hardness and particle compaction with reduced porosity.

Abstract: A390 functionally graded material (FGM) pistons were fabricated by centrifugal casting, where the silicon particles were segregated in the head portion of the pistons by appropriate design and their density differences. Centrifugal casting offers casting of cylindrical structures with gradation in its properties. In centrifugally cast A390, a suitable die design can lead to the formation of hard primary Si particles gradually distributed towards the head region producing a particle rich zone, transition zone and matrix rich zone. Microstructure and chemical composition analysis confirms the composition gradation. Hardness and wear test results revealed that the gradation positively helps to improve the desired properties with the presence of in-situ primary silicon reinforcements.

Abstract: The objective of present investigation is to synthesize porous micro silica based ceramic preform with varying composition of particles using burn out technique and processing of Al-Micro silica metal-ceramic composites by squeeze infiltration method. Direct squeeze infiltration of 319 aluminum alloy on micro silica preform is successfully carried out with the controlled process parameters of initial preform temperature, liquid metal superheat, squeeze pressure and its rate of application, and die temperature. The preform and composites are characterized using optical microscopy, electron microscopy, hardness and compression strength testing. Porous ceramic preform with more than 70% porosity has been fabricated by PEG as pore former. The infiltrated composite have shown uniform and complete infiltration of aluminum alloy in between micro silica particles.

Abstract: Borosiloxane oligomers, which give ceramic residue in the range of 70-78% (at 900°C) in inert atmosphere, were synthesized through the condensation of boric acid with allyltriethoxysilane without using any catalyst. The effect of concentration of the reactants on the processibility, yield and thermal stability of the product obtained was studied. All oligomers were pyrolysed at 900°C followed by ceramic conversion after sintering at 1500°C and 1650°C in inert atmosphere and were characterized by XRD and spectroscopic techniques. Mixture of α and β-SiC are formed after sintering the samples at 1650°C for 3 hours in inert atmosphere.

Abstract: In the present work, Nd substituted Gd_5-xNd_xSi₂Ge₂ with x = 0.05, 0.1 and 0.2 have been synthesized by arc melting and the effect of Nd substitution on the structural and magnetocaloric properties of Gd₅Si₂Ge₂ has been investigated. The composition with x=0.05 undergoes a first order phase transition with a Curie temperature of 275 K. With the increase in Nd content to x=0.1 and x=0.2, the compounds undergo a second order magnetic transition at 300 K and 293 K respectively. Maximum entropy changes (-ΔS_M) for Gd_5-xNd_xSi₂Ge₂ alloys with x = 0.05, 0.1 and 0.2 are 12.8, 7.6 and 7.2 J/kg K, respectively, for 50 kOe. The critical exponent analysis of the paramagnetic to ferromagnetic transition and the scaling behaviour of field dependence of MCE for x=0.1 and 0.2 compositions is also carried out.