Search

Single phase novel (Ti0.2W0.2Ta0.2Mo0.2V0.2)C0.8 high entropy carbide (HEC) compacts were successfully synthesized by reactive spark plasma sintering of ball milled metal-carbon elemental mixture at temperatures of 1400−1800 °C. X-ray diffraction and element distribution maps indicated single phase carbide formation with lattice parameter ranging from 4.307 A to 4.312 A with small amount of TiO2. X-ray energy dispersive spectroscopy (EDS) mapping showed uniform distribution of the transition metals in the carbide phase. The microhardness, elastic modulus, fracture toughness, electrical resistivity and thermal expansion coefficient (25 °C–600 °C) of the compact sintered at 1800 °C were found to be 25.8 ± 2.8 GPa, 461 ± 36 GPa, 3.7 ± 0.4 MPa.m1/2, 7 × 10−4 Ω/m2 and 7 × 10-6 K−1 respectively.

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.

Sintering of pure B4C requires extremely high temperatures of more than 1800 °C, even with techniques such as spark plasma sintering (SPS). In this study, sintering of B4C was achieved at low temperature using reactive Ti–B mixture as a sintering additive. Ti and B powders (1:2 atom ratio) were milled for 8 h and added to B4C powder in proportions of 5, 10, and 20 wt% and milled for 4 h to achieve good dispersion. Dense B4C composite compacts were obtained using SPS at 1400 °C. A pore free microstructure consisting of B4C grains surrounded with fine grains of (Ti0·9W0.1)B2 and W2B5 was obtained due to WC contamination. The activation energy for sintering reduced from 234 to 155 kJ/mol with the increase in the amount of Ti–B mixture from 5 to 20 wt%. The B4C composite with 5 wt% Ti–B mixture had the maximum hardness of 3225 ± 218 H V.

In the present work, friction stir welding of 3-mm-thick AA6061-T6 Al alloy and AZ31B-H24 Mg alloy sheets was carried out in lap configuration with Al alloy on top. Experiments were carried out by varying the pin length (3.25, 3.75, and 4.25 mm) and tool rotation speed (600, 800, and 1000 RPM). Microstructures of the joint cross-section revealed the presence of a compound intermetallic layer along the interface consisting of Al-rich (Al3Mg2) intermetallic compound near the Al alloy side, and Mg-rich intermetallic compound (Mg17Al12) near the Mg alloy side. The formation of a eutectic mixture of Mg solid solution and Mg17Al12 was observed at the hook on either side. Two modes of failure were observed; Mode 1 through the interface and Mode 2 due to failure of the sheet. The Mode 1 failure strengths obtained were the highest (148 ± 6 N/mm) compared to the literature and were obtained for the 3.25 and 3.75 mm pin at 800 RPM. A lap shear strength of 212 ± 6 N/mm was obtained with 4.25 mm pin length at 600 RPM, which is the highest reported so far and was attributed to Mode 2 failure. The effect of interface pull-up, angle of the interface at the advancing side, and distribution of intermetallic compounds on the lap-shear strength are discussed.

W-Cu functionally graded composites (FGCs) up to six layers have been developed using high energy ball milling and spark plasma sintering (SPS) at a lower temperature of 900 °C. The relative density of W-Cu composites increased from 85.4% (W80Cu20 layer) to 95.7% (W20Cu80 layer) with increasing Cu content. All the W-Cu FGCs exhibited a graded structure even after SPS and showed a gradual change in hardness, Young’s modulus, and coefficient of thermal expansion (CTE). Furthermore, W-Cu composites showed a CTE and modulus between those of W and Cu and could be used as an intermediate layer between W and Cu in plasma facing components. The thermal cycle testing at 800 °C has confirmed that the W-Cu FGCs developed in this study can withstand thermal shock and showed a superior performance over directly bonded W-Cu sample. The W-Cu FGCs developed in the present study are not only suitable for plasma facing components but can also be used where the thermal stresses are introduced due to the large mismatch in CTE or elastic modulus.

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.

The effect of isothermal post-weld heat treatment on keyhole plasma arc welded DP600 steel was investigated using optical microscopy, scanning electron microscopy and analytical transmission electron microscopy. The isothermal heat treatment had no effect on the allotriomorphic, Widmanstatten and acicular ferrite in the fusion zone (FZ). The martensite lath structure showed complete recovery in FZ, base metal (BM) and heat-affected zone (HAZ) with the formation of coarse cementite particles of up to 200 nm size in subcritical HAZ. The coarse grain HAZ showed little change in bainite and had tempered martensite and transformed retained austenite. The fine grain HAZ consisted of bainite, tempered martensite and coarsened preexisting cementite. The recovered BM had tempered martensite and reduced dislocation density. Hardness was found to be the lowest in subcritical HAZ, which is attributed to complete tempering of martensite, formation of large cementite particles and disappearance of dislocations.

The present study reports the microstructural and morphological changes during high energy ball milling of Cu with Ag and Graphite flakes. XRD patterns of ball milled Cu-Ag showed a reduction in the intensity of Ag peaks (1 1 1) and an increase in the lattice parameter of Cu. With an increase in milling time, the formation of metastable Cu-Ag solid solution was observed. Lattice parameter values for 40 h milled Cu (3.6169 A) and Cu-GF composites (3.6166 A) indicated that C does not dissolve in Cu. The lattice parameter of Cu in milled Cu-Ag-graphite flake was lower compared to milled Cu-Ag mixture indicating that graphite flakes inhibit solid solution formation. Raman spectra revealed that graphite flakes were converted into multilayer graphene after 10 h of milling. The crystallite size of Cu in the milled powders decreased with increase in milling time and reached a value of ∼25 nm after 35 h of milling. The lattice strain also increased with milling time. The D10, D50 and D90 size reduced appreciably after 5 h of milling.

The effect of graphene nanoplatelets (GNPS) on the grindability of GNP reinforced ZrB2 was studied using a resin bonded diamond grinding wheel under dry and wet conditions. A comparative study of grinding forces was performed at selected wheel surface speeds and depths of cut for surface grinding. ZrB2-GNP showed lower normal grinding forces due to the improved fracture toughness and reduced hardness. The presence of GNP reinforcement in ZrB2 resulted in lower tangential forces and reduced specific grinding energy due to the role of GNP as a solid lubricant. The measured forces showed a good correlation with the micro cutting model for ZrB2 and ZrB2-GNP under dry conditions. The tangential forces showed the same trend as normal forces at different depths of cut and wheel surface speeds for ZrB2 and ZrB2-GNP with average force ratios of 0.3 and 0.35, respectively. The presence of porosity in ZrB2 increased the normal grinding forces during wet grinding. Scanning electron microscope (SEM) images of the grinding chips indicated a mixture of both the ductile mode and the brittle mode of material removal with predominantly brittle fractured chips. Energy-dispersive spectroscopy (EDS) confirmed the presence of GNPs in ZrB2-GNP grinding chips. The topography of the grinding wheel showed higher wheel loading after the dry grinding than that of wet grinding. The wet grinding resulted in relatively lower surface roughness (Ra values) than dry grinding.

Microstructural characteristics of keyhole plasma arc-welded DP600 steel were analyzed using optical microscopy, scanning electron microscopy, and analytical transmission electron microscopy. The fusion zone (FZ) was observed to consist of allotriomorphic, Widmanstatten, and acicular ferrite along with bainite and martensite leading to enhancement in the hardness compared with the base metal. The coarse-grain HAZ consisted of bainite and martensite, while the fine-grain HAZ consisted of bainitic ferrite and tempered martensite. The sub-critical HAZ was found to consist of tempered martensite with reduced density of dislocations and carbide precipitation. This softening resulted in yield point phenomena and failure in the sub-critical HAZ during the transverse tensile test of welds. The non-isothermal tempering resulted in retarded cementite precipitation due to inadequate time for diffusion of carbon, which led to the lowest hardness of 168 HV0.5 in the sub-critical HAZ. The different features in the FZ and HAZ and their correlation with mechanical properties are discussed.

A comparative study of the Stellite 21 hardfacing on H13 steel produced by cold metal transfer (CMT) process and conventional plasma transferred arc welding (PTAW) process was carried out. The lower heat input of the CMT process resulted in finer microstructure and lesser dilution in the Stellite 21 coating than the PTAW process. The volumetric dilution values showed much scatter across the thickness of the PTAW coating than CMT. The effect of lower dilution and finer structure of the CMT coating resulted in a marginal increase in the microhardness. The room temperature (RT) and high temperature (600 °C) wear performance of the coatings were studied using a ball-on-disc tribometer using alumina ball. The wear tracks were analyzed using an optical profilometer and the volume loss was calculated. The CMT deposited coating showed better wear performance than PTAW coating. X-ray diffraction (studies of the room temperature wear tracks showed the transformation of γ-fcc phase in the as-deposited coating to e-hcp phase after the wear. The volume percentage of the e-hcp was observed higher for CMT deposited coating than the PTAW coatings, which can be attributed to lower dilution and finer microstructure of the former.

In this work, key hole mode plasma arc welding of 6 mm thick DP600 steel plates have been carried out and the effect of post-weld heat treatment is presented. The microstructures of the fusion zone and heat affected zones are studied. The tensile strength of the as-welded joint is found to decrease by 8%, while the post-weld heat treated weld joint and heat treated base metal showed a 15% reduction compared to as-received DP600. The failure occurred in the far heat affected zone, where softening occurred due to martensite tempering. The impact properties at room temperature were similar for all samples, whereas at −29 °C, the base metal impact property did not decrease while that of weld metal decreased considerably. Bend tests indicated good ductility and formability.

The present work investigates the effect of dynamic correction (DC) and the arc length correction (ALC) parameters on the deposition of Stellite 6 filler wire on AISI H13 steel by Cold Metal Transf...

In this study, Ti powders were ball-milled with 2 wt% graphene nanoplatelets (GnP) and graphitic carbon nitride (g-C3N4) and subjected to spark plasma sintering (SPS) in order to prepare Ti-TiCx and Ti–Ti(C,N) composites. Dispersion of GnP and g-C3N4 in Ti matrix after milling was analysed. The effect of GnP and g-C3N4 on microstructure, phase evolution and the mechanical properties of the resultant composites were studied. Lattice parameter evaluation indicated the presence of TiC0.40 phase in the sintered compact of Ti-2GnP whereas Ti(C, N) was found to be formed in Ti-2g-C3N4 compact. Microstructural studies indicated finer secondary phase in Ti-2g-C3N4 compared to Ti-2GnP due to well dispersion of g-C3N4 in Ti during milling. Ti-2g-C3N4 showed superior Vickers hardness (820 ± 28 VHN), elastic modulus (168 GPa), nanohardness (12 GPa), room temperature (2.9 GPa) and high temperature compression strength (120 MPa) compared to other composites and pure Ti.

In this work, the solidification and liquation cracking behavior of a dual-phase steel DP600 was examined using longitudinal Varestraint testing and hot ductility testing. Varestraint tests showed only limited cracking even at 4 pct augmented strain. Hot ductility tests showed that the nil-strength temperature of the steel was 1459 °C, while the nil-ductility and the ductility-recovery temperatures were 1435 °C and 1420 °C, respectively. The brittleness temperature range was found to be 39 °C, and the DP600 steel was highly resistant to both weld metal solidification cracking and heat-affected zone liquation cracking. The reasons for the high hot cracking resistance of dual-phase steel DP600 are discussed.

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.

Nano crystalline High Entropy Alloys (HEA) with equi-atomic compositions were synthesized through mechanical alloying followed by Spark Plasma Sintering (SPS). Alloys with nominal compositions of TiAlNiCo, TiAlNiFe and TiAlNiCoFe were selected for the study. Microstructure, hardness and room temperature compressive strengths of the alloys were studied. All the alloys had a single phase BCC microstructure along with TiCx nanoparticles dispersed uniformly in it. The alloys exhibit good hardness of more than 700 HV1.0 and a compressive strength more than 2.5 GPa.

The present article presents the effect of Sc addition on the secondary dendritic arm spacing, eutectic Si modification, intermetallic phase modification, aging behavior and subsequently the mechanical behavior in A356-5TiB2 in-situ composite. The A356-5TiB2 in-situ composite showed a 40% reduction in SDAS compared with A356 alloy, whereas addition of 0.4 wt% Sc and 5 wt% TiB2 in A356 alloy resulted in a 65% reduction in SDAS. The eutectic Si in A356-5TiB2 composite had a plate like morphology similar to A356 alloy but showed a reduction in length, whereas the addition of Sc resulted in a fibrous and particulate morphology of eutectic Si. The needle like β-Al5FeSi phase in A356-5TiB2 composite changed to Al5Fe(Si,Sc) phase having smaller size and irregular morphology with the addition of Sc. The intermetallic phase formation and phase modification was confirmed and characterized using X-ray diffraction, Transmission electron microscopy - Energy dispersive spectroscopy and Scanning electron microscopy. Addition of Sc to A356-5TiB2 in-situ composite resulted in improved mechanical properties. Further, T6 heat treatment revealed a significant improvement in mechanical properties for A356-5TiB2 in-situ composite with and without Sc addition.

Simultaneous synthesis and densification of boron carbide and boron carbide- graphene nano platelets (GNP) were carried out by reactive spark plasma sintering of amorphous boron and graphene nano platelets at temperature ranging from 1200 to 1600 °C, pressure of 50 MPa and heating rate of 50 °C/min and 100 °C/min. X-ray diffraction and Raman spectroscopy confirmed the formation of required phases. Electron microscopic images revealed the formation of sub-micron and nano sized grains of plate like morphology. Sintered product with high relative density of 96%TD was achieved at a temperature of 1600 °C and heating rate of 50 °C/min for B4C stoichiometric composition and also exhibited maximum hardness of 21.10 GPa.

Equiatomic CoCrFeMnNi high entropy alloy has been very well-known in scientific community as a material offering outstanding structural properties at a wide range of temperature. To realize this potential, interstitial alloying with various elemental additions has been employed in the past. Carbon has been the primary and the most popular interstitial addition to CoCrFeMnNi so far. This review summarizes the large body of work which has been done to establish the structural benefits of carbon addition to CoCrFeMnNi. The effect of carbon addition to CoCrFeMnNi is discussed at different length scales starting from the lattice level to the microstructural level and finally at the macro-mechanical level. At the lattice level, the effect of carbon addition on stacking fault energy and its effect on deformation mechanisms like TWIP and TRIP are discussed. The effect of carbide precipitation on the microstructure is reviewed at the microstructural level, along with the concurrent changes in mechanical properties like strength, hardness, and strain hardening rate at the macro level. In the second part of this review, the effect of 2 wt.% graphite flake addition on the microstructure of CoCrFeMnNi synthesized by mechanical alloying and spark plasma sintering is presented.

This paper presents the results of an experimental study on evolution of the nanocrystalline microstructure in a mechanically alloyed Oxide dispersion strengthened (ODS) 18Cr ferritic steel powder during densification by spark plasma sintering (SPS) in the temperature range of 1273 K (1000 °C) to 1423 K (1150 °C). Systematic Electron Back-Scatter Diffraction analysis has been carried out to study the grain size distribution and texture as a function of consolidation temperatures. Based on the kinetics of the densification process and resultant microstructure/microtexture, a sintering temperature slightly above 1323 K (1050 °C) within a range of 50 K was found to be optimum. The 18Cr-ferritic steel powder consolidated at 1323 K (1050 °C) was also studied to understand the role of dispersoids on microstructure. The dispersoids exerted a profound influence on the strength as well as toughness of the steel by restricting the grain growth at high temperatures. Further, a signature of (1 1 0) grain cluster is observed during consolidation and its preferential growth with increase in sintering temperature is noticed which lead to the alignment of the (1 1 0) plane in the direction of applied pressure. The minimum creep rate of the consolidated steel under a load of 300 MPa was found to be 5E-7 h−1 and 1E-4 h−1 at 873 and 973 K (600 and 700 °C) respectively. The apparent activation energy for creep deformation was estimated as ~ 402 kJ/mol, which is typical of lattice diffusion assisted general climb mechanism of dislocations over the barriers such as present dispersoids.

NiTiCuFe multi-component alloy was synthesized using vacuum induction melting under argon atmosphere. As-cast structure shows a mixture of phases with FCC as the major phase. Microstructure of the alloy exhibits coarser non-equi-axed dendritic grains with spike-shaped inter-dendritic phases. Scanning electron image with an energy-dispersive X-ray spectrograph line scan taken across dendrites shows Cu segregation in the inter-dendritic region and other elements in the dendritic region. The result shows the presence of Fe2Ti Laves phase. Also the major phase is observed to be a Cu–Ni FCC phase. XRD analysis also confirms the presence of Fe2Ti Laves phase and Cu–Ni FCC phase. The presence of phases is confirmed using a transmission electron microscope (FEI Tecnai T20) operating at 200 kV. The alloy also exhibits good mechanical properties.

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.

Boron carbide (B4C) based composites with graphene nano-platelet (GNP) as reinforcement have been prepared by hot pressing. Effect of GNP addition on mechanical properties of the composite, like, hardness, fracture toughness, flexural strength was evaluated. Phase evolution was studied by X-ray diffraction (XRD) and Raman spectroscopy. 2 vol% addition of GNP improved the density of the composite, which acted as a sintering aid. Further addition of GNP resulted in formation of the interconnected networks, agglomerates and interfacial porosities. Improved fracture toughness (KIC) but decreased flexural strength was observed with addition of GNP. Maximum fracture toughness of 5.41 ± 0.55 MPa m1/2 (by indentation method) and 4.52 ± 0.15 MPa m1/2 (by single edge notch bend (SENB) method) was obtained for 10 vol% GNP-B4C composite. Hardness and elastic modulus were also improved with 2 vol% addition of GNP. The observations have been explained with suitable mechanism and model, supported by microstructural and mechanical property data.

In this study, the effect of adding different forms of carbon such as graphite flakes (GF), graphene nano-platelets (GNP) and carbon nanotubes (CNT) on the microstructure evolution and wear behavior of CoCrFeMnNi multi-component alloy fabricated by mechanical alloying and spark plasma sintering were investigated. Carbon addition of 2 wt% decreased the particle size and the amount of WC contamination due to milling. Post sintering, the phases observed in all the compacts were Ni-rich FCC, Cr-rich M7C3, Cr3C2 and MnO. Carbon addition resulted in coarser microstructure and higher amount of secondary phases like M7C3, MnO, and Cr3C2 resulting in higher hardness. Ball-on-disk wear tests at room temperature showed that the GNP added alloy showed the highest specific wear rate (3.26 mm3/Nm) in spite of highest hardness. The predominant wear mechanism observed for all the alloys were oxidative wear and abrasive wear which were more prominent for the FCC phase. The CNT and GF added CoCrFeMnNi alloy exhibited the least specific wear rates of 0.85 and 0.96 mm3/Nm, respectively compared to 2.77 mm3/Nm for the base alloy. The wear behavior was explained based on microstructure and composition of the phases.

Cold metal transfer (CMT) welding is a new gas metal arc welding process having several advantages such as low heat input and spatter free welding. This makes it of great advantage in weld cladding applications. In this study, hardfacing of AISI H13 die steel with Stellite 21 alloy has been carried using CMT process. Coatings were deposited on H13 substrate in annealed as well as quenched & tempered (Q&T) condition at room temperature as well as with a preheat of 400 °C. The Q&T substrate with and without preheat and the annealed substrate without preheat were found to be susceptible to underbead cracking upon Stellite deposition. The cracking in the heat affected zone (HAZ) was due to formation of brittle martensite upon rapid cooling which is associated with formation of high tensile residual stresses at the bead toe. The annealed substrate with preheat of 400 °C showed the least cracking tendency. The cracking tendency was investigated by studying the variation of the microstructure and microhardness along the depth of HAZ. The dilution levels based on Fe content was found to be 3–4%, which was considerably lower than that of conventional arc welding deposits. The Stellite coated H13 plate (annealed with preheat) could be successfully subjected to quenching & tempering heat treatment to restore the properties of the substrate without introducing any defects.

In this study, the effect of hygrothermal aging on the strength of pure and carbon nano-filler reinforced epoxy adhesive joints is presented. Epoxy adhesives containing three different carbon nano-fillers, namely, multi-walled carbon nanotubes (CNT), graphene nanoplatelets (GNP) and single walled carbon nanohorns (CNH) were prepared by using a high shear rate mixer. Thermogravimetric analysis (TGA) of the nano-filler reinforced epoxy indicated an improvement in the thermal stability compared to pure epoxy. Aging studies were carried out on single lap-shear specimens as per EN ISO 9142:2003 standard (cycle D3). The strength of the joints was found to decrease with hygrothermal aging time. The nano-filler reinforced joints showed less reduction in joint strength with aging compared to pure epoxy joints. The GNP/epoxy joints showed the best resistance to aging followed by CNT and CNH reinforced joints. Fourier Attenuated Total Reflectance Transformation Infrared Spectroscopy (ATR-FTIR) analysis of the hygrothermal aged samples indicated lower degradation in terms of hydrolysis of epoxy groups for nano-filler reinforced epoxy. Gravimetric analysis of samples immersed in water showed lower weight gain for nano-filler reinforced epoxy composites. Fracture surface analysis was carried out to study the change in failure mechanism with aging time.

In this study, powder mixtures of Ti with 5 and 10 wt% carbon nanotubes (CNT) were ball milled and subjected to spark plasma sintering (SPS) with an aim to prepare Ti-25 wt% TiC and Ti-50 wt% TiC composites. Changes in the powder particle size and the phase evolution were studied as a function of milling time. The effect of SPS temperature (800 °C, 1000 °C and 1200 °C) on the microstructure, phase evolution and the properties are presented. Reaction between Ti and CNT lead to formation of non-stoichiometric TiC x (x x was observed at 1200 °C temperature for the Ti-CNT mixtures. CHN measurements were carried out on the powder and the compact to study the source of additional carbon in the samples. Microstructural studies indicated that completely dense TiC x was produced at 1200 °C from Ti-CNT powders, while the milled Ti powders resulted in fully dense Ti-TiC x composite. The effect of C/Ti ratio on the lattice parameter of TiC x was studied. Mechanical properties were measured using Vickers microhardness and Nanoindentation.

In this study, dense fine-grained ZrB2 and TiB2 were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB2 and TiB2 compacts having relative density of similar to 48 and similar to 65%, respectively. The second stage of densification occurred at temperatures more than 1100 degrees C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.

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.

A Copper (Cu) matrix composites reinforced with 0.2, 5 and 10 vol% single walled carbon nanotubes (SWCNT) and 5 and 10 vol% multi-wall carbon nanotubes (MWCNT) was processed by high energy attritor milling of pure copper powder with carbon nanotubes (CNT) and subsequently consolidated by spark plasma sintering (SPS). Microstructural characterization shows a network of CNT along the grain boundaries and the presence of porosities at grain boundaries as well as triple junctions. By covering the particle boundaries, the higher volume fraction of CNT makes the sintering difficult as compared to single phase copper or low volume fraction CNT composites. Raman spectroscopy indicates that there is an increase in number of defects in the nanotube after milling and sintering of the composite. Mechanical properties evaluation shows that SWCNT composites results in higher strength and deformability compared to MWCNT. The failure strain decreases with increase in volume percent of CNT due to clustering of CNTs.

A novel low carbon Ti–Nb microalloyed hot rolled steel with minimum yield strength of 700 MPa and good balance of stretch-flangeability and impact toughness has been developed by controlled thermo-mechanical processing following thin slab direct rolling route. In the present work, the effects of two coiling temperatures on the resulting microstructure, micro-texture and mechanical properties on this Ti–Nb microalloyed steel have been studied. It is observed that increase in coiling temperature from 520 to 580 °C significantly affects the mechanical properties. Higher dislocation density and increased precipitation along with slightly smaller grain size is observed for 580 °C coiling temperature resulting in about 50 MPa increase in yield and tensile strengths as compared to 520 °C coiling temperature.

This study is focused on the evaluation of the thermal stability of a nanocrystalline Mg alloy, a relatively less explored topic. Guided by Darling et al. model which advises choosing of alloying elements based on their enthalpy of mixing and elastic enthalpy with respect to the parent element, Mo was chosen for stabilizing the nc Mg structure. High energy ball milling experiments were conducted on powders of Mg and Mo to achieve a composition of Mg–2at%Mo. Characterization of the ball milled powders using XRD and TEM indicated a lack of mixing of Mg and Mo and the microstructure was found to bear a mix of Mg and Mo phases. Kissinger's analysis using DSC yielded the activation energy of grain growth in the ball milled material as 73 kJ/mol which was similar to that of pure Mg, indicating that Mo did not alter the grain growth kinetics of Mg. The poor grain size stability of the nanocrystalline Mg–2Mo composition was also observed during spark plasma sintering studies conducted at 673, 723 and 773 K. The obtained results were evaluated in the light of the Murdoch and Schuh model and this revealed that the Mg–Mo alloy is not expected to be thermally stable due to the tendency of Mo to exist as a separate phase. This analysis appears to suggest that the Murdoch and Schuh model is more suitable for identifying thermally stable nanocrystalline compositions compared to the Darling et al. model.

In this study, Titania (TiO 2 ) 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 TiO 2 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 TiO 2 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 TiO 2 matrix. The hardness and modulus of the CNT reinforced TiO 2 increased by 22% and 5.4% respectively while that of the SWNH reinforced TiO 2 increased by 11% and 23% respectively. GNP and CNT reinforced TiO 2 exhibited superior photocatalytic activity for the degradation of methylene blue dye compared to pure TiO 2 .

Effect of Sc addition and T6 aging treatment on the secondary dendritic arm spacing (SDAS), modification of eutectic Si morphology, β-Al 5 FeSiand π-Al 8 Mg 3 Si 6 Fe 1 phases and its effect on mechanical properties in A356 alloy has been investigated. Addition of 0.4 wt%Sc in A356 alloy resulted in a 50%reduction in the secondary dendritic arm spacing (SDAS). Sc addition changed the morphology of eutectic Si from plate like to fibrous and globular. The needle like morphology of β-Al 5 FeSi phase in A356 alloy changed to Al 5 Fe(Si,Sc) phase having smaller size and irregular morphology. Transmission electron microscopy (TEM) diffraction pattern and Energy dispersive spectroscopy (EDS) analysis revealed the presence of β-Al 5 FeSiand π-Al 8 Mg 3 Si 6 Fe 1 phases in A356 alloy which changed to β-Al 5 Fe(Si,Sc), π-Al 8 Mg 3 (Si,Sc) 6 Fe 1 and additional V-AlSi 2 Sc 2 phase was observed in Sc containing alloys. Addition of 0.4 wt%Sc to A356 alloy improved its Vickers hardness, Ultimate tensile strength (UTS), Yield strength (YS) and ductility by 20%, 25%, 20% and 30% respectively. Artificial aging treatment resulted in significant improvement in the tensile properties for both A356 and Sc added A356 alloys.

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 TiB 2 -TiC-CNT hybrid composites. Addition of CNT was found to affect the reaction rate for TiB 2 formation which results in sudden volume shrinkage. Rod shaped TiC were formed due to reaction between Ti and CNT. TiB 2 -TiC-CNT hybrid composites having more than 96% relative density with nanosized TiB 2 grains were obtained. XRD results revealed TiB 2 and TiC as major and minor phases respectively. Unreacted CNT were observed in SEM and TEM. TiB 2 -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.

Dense (>98%) submicron sized monolithic zirconium diboride (ZrB 2 ) was fabricated by reactive spark plasma sintering (RSPS) of 8 h ball milled Zr and B elemental mixtures at 1200 °C. During RSPS, ZrB 2 formed in-situ and the densification takes place by a two-step process. Plastic flow of ZrB 2 resulting from application of higher pressure enhanced densification significantly. Improvement in nanohardness, elastic modulus and indentation fracture toughness were observed with increased sintering pressure.

Yttria dispersed Fe3Al alloy with a nominal composition of Fe–13Al–5Cr-0.18Ti-0.35 Y2O3 was produced by high energy ball milling followed by hot extrusion. The extruded and heat treated ODS Fe3Al exhibited both elongated and fine equi-axed grain structure in longitudinal direction, whereas, equi-axed grain structure was observed in transverse direction. Even though, both prealloyed as well as milled powder showed only FeAl phase, D03 structured Fe3Al along with Y3Al5O12 and TiO phases existed in ODS Fe3Al. Presence of fine (7–25 nm corresponding to Y3Al5O12) and coarse (>25 nm, TiO) particles were observed in ODS Fe3Al. The material exhibited yield and ultimate tensile strengths of 1104 and 1308 MPa with 8% elongation. The improved strength and elongation are due to the presence of dispersoids and fine grained structure.

In this work, wire arc additive deposition of Stellite 6 alloy was carried out using cold metal transfer (CMT) process and the microstructural features, microhardness, and high temperature mechanical properties were studied. The as-deposited multi-layer wall was free of any defects. High temperature compression test samples were extracted parallel to the build direction, and the tensile samples were prepared along parallel and perpendicular orientation to the build direction. The hot compression test data was used to predict the flow behavior of the additively deposited alloy using an Arrhenius type equation at a strain of 0.6. The flow behavior of the additively deposited Stellite 6 alloy was similar to the conventional Co-based alloys in spite of the layer-by-layer macrostructure and microstructural variations. Microstructure, hardness, and phase analysis studies were carried out for the deformed samples after hot compression. The high temperature tensile properties were found to be anisotropic and location-specific, which were correlated with the columnar dendritic microstructure, alignment of the layer interfaces and variation in the secondary dendrite arm spacing due to repeated thermal cycles and difference in the cooling rate.