Srinivasa Rao Bakshi

Recent advances in alumina ceramics are focused toward innovative processing routes to improve their mechanical reliability while retaining their superior wear resistance, which might be possible if a thin layer of dense alumina can be formed on a metallic substrate such as Ti-6Al-4V with high mechanical strength. For this purpose, we propose a new two-step process in which a dense layer of Al deposited on the Ti alloy by cold metal transfer method, formed a dense Al3Ti gradient reaction layer at their interface to improve adhesion in a single step. Subsequent micro-arc oxidation treatment transformed Al layer to a graded alumina layer in which γ-alumina decreased and α-alumina increased with increasing depth. Abrasion of outer regions revealed underlying pure α-alumina regions with high Vickers hardness matching with that of sintered alumina. The designed alumina/Ti alloy hybrid can be a potential candidate for wear resistance applications.

Diborides of Zr, Ti and Hf have potential applications as ultrahigh temperature ceramics, armor material, MEMS, substrates for optoelectronic applications and solar absorbers. Previous reports suggest that crystallographically textured diboride compacts have better mechanical and functional properties. Current methods of fabricating textured compacts are either cumbersome or energy and time intensive. We demonstrate a novel method for fabrication of bulk textured TiB2 by reactive spark plasma sintering (RSPS) of Ti-B mixtures having graphene nanoplatelets (GNP) as reinforcement. GNP acted as a template resulting in formation of large TiB2 grains with plate type morphology. X-ray diffraction and Electron backscattered diffraction analysis revealed presence of basal texture in TiB2 – GNP compacts. In comparison to GNP, carbon nanotube addition is shown to result in randomly oriented equiaxed grains. The improvement in indentation fracture toughness indicates GNP is better toughening additive compared to CNT.

The present study investigates the oxidation behavior of nanocrystalline (NC) and microcrystalline (MC) Fe-20Cr-5Al alloys at 900 ºC. A protective layer of Al2O3 layer is formed at a relatively lower Al content in the case of microcrystalline Fe-Cr-Al alloys containing sufficient amounts of Cr (i.e. third element effect). The required critical content of Al for the formation of a full-fledged Al2O3 layer can further be reduced by the nanocrystalline structure. We demonstrate here that nanocrystalline Fe-20Cr-5Al alloy becomes capable of developing a protective layer of Al2O3. The results show that nanocrystallization presumably reduces the Al requirement below that is necessary for the “third element effect” of Cr. Consequently, the nanocrystalline Fe-20Cr-5Al alloy oxidized at an insignificant rate (c.f., its microcrystalline counterpart). Accordingly, the NC Fe-20Cr-5Al alloy follows logarithmic oxide growth kinetics, whereas the MC Fe-20Cr-5Al alloy follows parabolic kinetics. Additionally, the MC alloy exhibits four times higher weight gain than the NC alloy after 60 h of oxidation.

Four alloys with nominal compositions of TiAlNiCr, TiAlNiCrCo, TiAlNiCrFe and TiAlNiCrCoFe were synthesized through mechanical alloying followed by spark plasma sintering (SPS). After 8 h of milling, all the alloys developed a major parental BCC phase. SPS done on the alloys revealed that the BCC phase was retained after SPS. Sintering promoted the formation of one more BCC phase thus making the final microstructure comprised of two BCC phases. The phases were characterized using X-ray diffraction and scanning electron microscope. Backscattered electron images of the samples indicated that one BCC phase was rich in Ni and Al, whereas the second BCC phase was rich in Cr, Co and Fe. All the alloys showed good hardness and mechanical strength with values above 2000 MPa except TiAlNiCr.

Carbon nanotubes (CNT) have received huge attention from the scientific community in the last two decades due to their unique structure and properties. They have been considered for potential applications in various areas of science and technology. One of the major applications of CNT is as reinforcement for fabrication of light weight high strength composite materials for use in automobile and aerospace applications. Aluminium and its alloys are natural choices for such applications due to their low density, high specific strength and modulus. In the last decade, there have been significant advances in the processing of carbon nanotube reinforced aluminium matrix (Al-CNT) composites. New understanding has emerged due to research on several aspects such as damage to CNTs during processing, interfacial phenomena, novel methods of processing for improving CNT dispersion, tensile behaviour, numerical modelling and in situ tensile testing. This review summarizes the present status of the tensile properties of pure Al-CNT and Al alloy-CNT composites. The various processing routes for fabrication of Al-CNT composites have been compared in terms of the resulting microstructure, degree of CNT dispersion, extent of interfacial reaction and its effect on the tensile properties. Factors affecting strengthening efficiency and the strengthening mechanisms in Al-CNT composites are discussed.

A symposium entitled Advances in Surface Engineering: Alloyed and Composite Coatings, was held at the TMS 2012 Annual Meeting and Exhibition in Orlando, Florida, US, to address the latest advancements in alloyed and composite coatings. The symposium included 54 presentations with several invited talks from the US and international researchers. Talks concerning wear resistance, indentation behavior, development of residual stresses, and electrochemical behavior of composite and alloy coatings were presented over five sessions. The first paper by Boris D. Bryskin and co-workers entitled 'Chemical Vapor Deposition of Iridium and Rhodium Coatings from Hydridotetrakis Complexes' presented a study on iridium and rhodium coatings on molybdenum substrates by chemical vapor deposition. They showed the use of new organometallic complexes that resulted in pure coatings without contamination of carbon and oxygen.

Mechanically milled powder mixtures of Titanium and Boron containing 1, 2, 4 and 6 vol% multi-walled carbon nanotubes (CNT) were consolidated by reactive spark plasma sintering resulting in TiB2-TiC-CNT hybrid composites. Addition of CNT was found to affect the reaction rate for TiB2 formation which results in sudden volume shrinkage. Rod shaped TiC were formed due to reaction between Ti and CNT. TiB2-TiC-CNT hybrid composites having more than 96% relative density with nanosized TiB2 grains were obtained. XRD results revealed TiB2 and TiC as major and minor phases respectively. Unreacted CNT were observed in SEM and TEM. TiB2-TiC-CNT composites showed nanohardness of above 25 GPa and elastic modulus of 520 GPa. Indentation fracture toughness was improved by 55% (3.3±0.1 to 5.11±0.6) by the addition of 4 vol% of CNT. The effect of CNT addition on oxidation and co-efficient of thermal expansion is also presented.

Abstract: The formation of Mg17Al12 and Al3Mg2 is inevitable even in solid-state joining of Al and Mg alloys by friction stir welding (FSW). In the present study, the effect of a thin inter-layer of Sn (tin) and FSW parameters (pin length and tool rotation speed) on the microstructure and strength of friction stir lap welds of 3 mm thick AA6061 Al alloy and AZ31B Mg alloy sheets was investigated. A ~ 20 µm thick Sn layer was deposited on AZ31B alloy by displacement plating. The heat generated during the welding melted the Sn layer and the rotation action of the tool pushed the molten Sn to the hooks on the advancing side and retreating side. The extent of Mg17Al12 and Al3Mg2 intermetallic compound formation was reduced with Sn inter-layer with limited formation of Mg2Sn. The Sn inter-layer helped improve the lap shear strength at lower tool rotational speed (600 RPM) and pin lengths of 3.25 and 3.75 mm compared to joints without Sn inter-layer. Lap welds prepared with a 3.25 mm pin at 600 RPM had a lap shear strength of 245 N/mm which is the highest value reported so far and is 116% higher compared to joints without Sn inter-layer. This was due to elimination of hook at the advancing side and retreating side and disintegration of intermetallic compounds. Graphic abstract: [Figure not available: see fulltext.].

In this work, the rheological properties, thermal stability and the lap shear strength of epoxy adhesive joints reinforced with different carbon nano-fillers such as multi-walled carbon nanotubes (CNT), graphene nanoplatelets (GNP) and single-walled carbon nanohorns (CNH) have been studied. The nano-fillers were dispersed homogeneously using Brabender® Plasti-Corder®. The epoxy pre-polymer with and without the nano-fillers exhibited shear thinning behavior. The nano-filler epoxy mixtures exhibited a viscoplastic behavior which was analyzed using Casson's model. Thermo-gravimetric analysis indicated an increase in the thermal stability of the epoxy with the addition of carbon nano-fillers. Carbon nano-fillers resulted in increased lap shear strength having high Weibull modulus. The joint strength increased by 53%, 49% and 46% with the addition of 1 wt.% CNT, 0.5 wt.% GNP and 0.5 wt.% CNH, respectively. The strength of the joints having high filler content (>1 wt.%) was limited by mixed mode type of failure.

In this study, Titania (TiO2) based composites reinforced with 2 wt% of various carbon nanomaterials were prepared using spark plasma sintering (SPS). Prior to SPS, the samples were ball milled. The reinforcements used in the composites were graphene nanoplatelets (GNP), carbon nanotubes (CNT) and single walled carbon nanohorns (SWNH). The ball milled powders and SPS compacts were characterized using various techniques. Mechanical and photocatalytic properties of the SPS composites were evaluated and compared for different nano-carbon reinforced TiO2 composites. X-ray diffraction and Raman spectroscopy studies confirmed that the milled powders comprised of Anatase phase which transformed into Rutile phase during SPS. Nano transformation twins were observed in Rutile grains. Fractured surfaces showed that the reinforcements were well bonded with the TiO2 grains and the SWNH reinforcement resulted in comparatively finer grain size. Nanoindentation studies showed that the hardness and elastic modulus of GNP reinforced composites was 78% and 30% respectively higher compared to TiO2 matrix. The hardness and modulus of the CNT reinforced TiO2 increased by 22% and 5.4% respectively while that of the SWNH reinforced TiO2 increased by 11% and 23% respectively. GNP and CNT reinforced TiO2 exhibited superior photocatalytic activity for the degradation of methylene blue dye compared to pure TiO2.