Modelling strain localization in Ti-6Al-4V at high loading rate: a phenomenological approach.

A phenomenological approach, based on a combination of a damage mechanism and a crystal plasticity model, is proposed to model a process of strain localization in Ti-6AI-4V at a high strain rate of 103 s-1. The proposed model is first calibrated employing a three-dimensional representative volume element model. The calibrated parameters are then employed to investigate the process of onset of strain localization in the studied material. A suitable mesh size is chosen for the proposed model by implementing a mesh-sensitivity study. The influence of boundary conditions on the initiation of the strain localization is also studied. A variation of crystallographic orientation in the studied material after the deformation process is characterized, based on results for different boundary conditions. The study reveals that the boundary conditions significantly influence the formation of shear bands as well as the variation of crystallographic orientation in the studied material. Results also indicate that the onset of strain localization can affect considerably the material's behaviour. This article is part of the theme issue 'Modelling of dynamic phenomena and localization in structured media (part 2)'.

Thermoelectric properties of a Mn substituted synthetic tetrahedrite.

Tetrahedrite compounds Cu(12-x)Mn(x)Sb4S13 (0 ≤x≤ 1.8) were prepared by solid state synthesis. A detailed crystal structure analysis of Cu10.6Mn1.4Sb4S13 was performed by single crystal X-ray diffraction (XRD) at 100, 200 and 300 K confirming the noncentrosymmetric structure (space group I4[combining macron]3m) of a tetrahedrite. The large atomic displacement parameter of the Cu2 atoms was described by splitting the 12e site into a partially and randomly occupied 24g site (Cu22) in addition to the regular 12e site (Cu21), suggesting a mix of dynamic and static off-plane Cu2 atom disorder. Rietveld powder XRD pattern and electron probe microanalysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn(2+) at the Cu(1+) site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit ZT decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum ZT = 0.76 at 623 K is obtained for Cu12Sb4S13. The coefficient of thermal expansion 13.5 ± 0.1 × 10(-6) K(-1) is obtained in the temperature range from 460 K to 670 K for Cu10.2Mn1.8Sb4S13. The Debye temperature, Θ(D) = 244 K for Cu10.2Mn1.8Sb4S13, was estimated from an evaluation of the elastic properties. The effective paramagnetic moment 7.45 µB/f.u. for Cu10.2Mn1.8Sb4S13 is fairly consistent with a high spin 3d(5) ground state of Mn.

Combinatorial effect of rolling and carbonaceous nanoparticles on the evolution of crystallographic texture and structural properties of ultra high molecular weight polyethylene.

Ultra high molecular weight polyethylene (PE) is a structural polymer widely used in biomedical implants. The mechanical properties of PE can be improved either by controlled crystalline orientation (texture) or by the addition of reinforcing agents. However, the combinatorial effect has not received much attention. The objective of this study was to characterize the structure and mechanical properties of PE composites incorporating multiwall carbon nanotubes (MWCNT) and reduced graphene oxide (RGO) subjected to hot rolling. The wide angle X-ray diffraction studies revealed that mechanical deformation resulted in a mixture of orthorhombic and monoclinic crystals. Furthermore, the presence of nanoparticles resulted in lower crystallinity in PE with smaller crystallite size, more so in RGO than in MWCNT composites. Rolling strengthened the texture of both orthorhombic and the monoclinic phases in PE. Presence of RGO weakened the texture of both phases of PE after rolling whereas MWCNT only mildly weakened the texture. This resulted in a reduction in the elastic modulus of RGO composites whereas moduli of neat polymer and the MWCNT composite increased after rolling. This study provides new insight into the role of nanoparticles in texture evolution during polymer processing with implications for processing of structural polymer composites.

In situ production of low-modulus Ti-Nb alloys by selective laser melting and their functional assessment toward orthopedic applications.

This work aimed to manufacture Ti-28.5Nb and Ti-40.0Nb (wt%) alloys in situ via selective laser melting (SLM) from Ti and Nb elemental powders. X-ray diffraction analysis revealed complete ß-phase (cubic) in Ti-40.0Nb and a mixture of (α'' orthorhombic + ß cubic) phases in Ti-28.5Nb were formed, whereas few of the Nb particles remained only partially fused during manufacturing. The fraction of partially melted Nb particles was determined as ∼2 and ∼18% in Ti-28.5Nb and Ti-40Nb, respectively. Mechanical characterization revealed higher hardness and more strength in Ti-28.5Nb than in Ti-40.0Nb due to the presence of the α'' phase in the former. Tribocorrosion tests reveal a significantly better wear-corrosion resistance for Ti-40.0Nb, as determined from a lower total volume loss in Ti-40.0Nb (∼2 × 10-4 mm-3) than in Ti-28.5Nb (∼13 × 10-2 mm-3). The lower volume loss and better corrosion resistance behavior are attributed to the ß phase, which was dominant in Ti-40.0Nb. Cell studies reveal no toxicity for up to 7 days. Both the alloys were better at supporting cell proliferation than wrought Ti6Al4V. This study presents a route to preparing Ti-Nb alloys in situ by SLM that are promising candidates for biomedical applications.

A Review-Additive Manufacturing of Intermetallic Alloys Based on Orthorhombic Titanium Aluminide Ti2AlNb.

Titanium alloys based on orthorhombic titanium aluminide Ti2AlNb are promising refractory materials for aircraft engine parts in the operating temperature range from 600-700 °C. Parts made of Ti2AlNb-based alloys by traditional technologies, such as casting and metal forming, have not yet found wide application due to the sensitivity of processability and mechanical properties in chemical composition and microstructure compared with commercial solid-solution-based titanium alloys. In the last three decades, metal additive manufacturing (MAM) has attracted the attention of scientists and engineers for the production of intermetallic alloys based on Ti2AlNb. This review summarizes the recent achievements in the production of O-phase-based Ti alloys using MAM, including the analysis of the feedstock materials, technological processes, machines, microstructure, phase composition and mechanical properties. Powder bed fusion (PBF) and direct energy deposition (DED) are the most widely employed MAM processes to produce O-phase alloys. MAM provides fully dense, fine-grained material with a superior combination of mechanical properties at room temperature. Further research on MAM for the production of critical parts made of Ti2AlNb-based alloys can be focused on a detailed study of the influence of post-processing and chemical composition on the formation of the structure and mechanical properties, including cyclic loading, fracture toughness, and creep resistance.

Surface nanocrystallization enhances the biomedical performance of additively manufactured stainless steel.

Additive manufacturing enables the fabrication of patient-specific implants of complex geometries. Although selective laser melting (SLM) of 316L stainless steel (SS) is well established, post-processing is essential to preparing high-performance biomedical implants. The goal of this study was to investigate surface mechanical attrition treatment (SMAT) as a means to enhance the electrochemical, biomechanical, and biological performances of 316L SS fabricated by SLM in devices for the repair of bone tissues. The SMAT conditions were optimized to induce surface nanocrystallization on the additively manufactured samples. SMAT resulted in a thicker oxide layer, which provided corrosion resistance by forming a passive layer. The fretting wear results showed that the rate of wear decreased after SMAT owing to the formation of a harder nanostructured layer. Surface modification of the alloy by SMAT enhanced its ability to support the attachment and proliferation of pre-osteoblasts in vitro. The study of the response in vivo to the additively manufactured alloy in a critical-sized cranial defect murine model revealed enhanced interactions with the cellular components after the alloy was subjected to SMAT without inducing any adverse immune response. Taken together, the results of this work establish SMAT of additively manufactured metallic implants as an effective strategy for engineering next-generation, high-performance medical devices for orthopedics and craniomaxillofacial applications.

Anisotropy of Additively Manufactured Co-28Cr-6Mo Influences Mechanical Properties and Biomedical Performance.

Additive manufacturing (AM) of biomedical alloys such as Co-Cr-Mo alloys holds immense potential for fabricating implants with complex geometry and tailored to meet patient-specific needs. However, layer-by-layer fabrication in AM processes results in undesired anisotropy due to the solidification texture and grain morphology. The present study aimed to investigate the effect of build orientation on the mechanical properties and functional performance, including tribocorrosion behavior and cytocompatibility of an orthopedic Co-28Cr-6Mo alloy manufactured by selective laser melting. Although the fabricated alloy showed weak crystallographic texture due to the rotational scanning strategy, significant anisotropy was found in the tensile properties due to the grain size and morphology. The presence of larger, elongated grains along the build direction as compared to smaller, equiaxed grains perpendicular to the build direction imparted the observed tensile anisotropy. Quantitative analysis based on current models for strengthening mechanisms is insufficient to explain the observed anisotropy, which is ascribed to the possible role of the cellular dendrites and stacking fault strengthening in Co-Cr alloys. Unlike the electrochemical behavior, which was largely independent of the build orientation, the bio-tribocorrosion studies revealed an anisotropic wear rate under fretting conditions. Osteoblast attachment and proliferation were found to be higher on the plane perpendicular to the build direction, owing to the differences in grain size. This work provides novel insights into the role of the manufacturing parameters in a selective-laser-melted Co-Cr alloy and its potential application in engineering load-bearing orthopedic implants.

Laser Powder Bed Fusion Additive Manufacturing of a Low-Modulus Ti-35Nb-7Zr-5Ta Alloy for Orthopedic Applications.

Laser powder bed fusion (L-PBF) was attempted here to additively manufacture a new generation orthopedic ß titanium alloy Ti-35Nb-7Zr-5Ta toward engineering patient-specific implants. Parts were fabricated using four different values of energy density (ED) input ranging from 46.6 to 54.8 J/mm3 through predefined laser beam parameters from prealloyed powders. All the conditions yielded parts of >98.5% of theoretical density. X-ray microcomputed tomography analyses of the fabricated parts revealed minimal imperfections with enhanced densification at a higher ED input. X-ray diffraction analysis indicated a marginally larger d-spacing and tensile residual stress at the highest ED input that is ascribed to the steeper temperature gradients. Cellular to columnar dendritic transformation was observed at the highest ED along with an increase in the size of the solidified features indicating the synergetic effects of the temperature gradient and solidification growth rate. Density measurements indicated ≈99.5% theoretical density achieved for an ED of 50.0 J/mm3. The maximum tensile strength of ≈660 MPa was obtained at an ED of 54.8 J/mm3 through the formation of the columnar dendritic substructure. High ductility ranging from 25 to 30% was observed in all the fabricated parts irrespective of ED. The assessment of cytocompatibility in vitro indicated good attachment and proliferation of osteoblasts on the fabricated samples that were similar to the cell response on commercially pure titanium, confirming the potential of the additively manufactured Ti-35Nb-7Zr-5Ta as a suitable material for biomedical applications. Taken together, these results demonstrate the feasibility of L-PBF of Ti-35Nb-7Zr-5Ta for potentially engineering patient-specific orthopedic implants.

Wire Arc Additive Manufacturing of Zinc as a Degradable Metallic Biomaterial.

Wire arc additive manufacturing (WAAM) offers a high rate of material deposition among various additive manufacturing techniques with wire as feedstock material but has not been established for zinc alloys. Zn alloys can be used as degradable biomaterials, in contrast to conventional permanent metallic biomaterials. In this work, commercially pure Zn was processed by WAAM to obtain near-dense parts, and the properties obtained through WAAM-processed Zn were compared with wrought (WR) Zn samples. The microstructure and hardness values of the WAAM (41 ± 1 HV0.3) components were found to be similar to those of the WR (35 ± 2 HV0.3) components. Bulk X-ray diffraction texture measurements suggested that WAAM builds exhibit a heavily textured microstructure compared to the WR counterparts, with peak intensities around <3 3−6 2> or <0 0 0 2> in the directions parallel to the build direction (BD). The corrosion rates in simulated body fluid (SBF) were similar for WAAM (0.45 mmpy) and WR (0.3 mmpy) samples. The weight loss measurements in SBF were found to be marginally higher in the WAAM samples compared to the WR counterparts for a duration of up to 21 days. MC3T3-E1 preosteoblasts were found to be healthy and proliferating in the culture medium containing the degradation products from WAAM-Zn in a manner similar to WR-Zn. This work establishes the feasibility of processing Zn by WAAM for use in bioresorbable metallic implants.

Electrophoretic Deposition of Nanocrystalline Calcium Phosphate Coating for Augmenting Bioactivity of Additively Manufactured Ti-6Al-4V.

Additive manufacturing (AM) is being widely explored for engineering biomedical implants. The microstructure and surface finish of additively manufactured parts are typically different from wrought parts and exhibit limited bioactivity despite the other advantages of using AM for fabrication. The aim of this study was to enhance the bioactivity of selective laser melted Ti-6Al-4V alloy by electrophoretic deposition of nanohydroxyapatite (nanoHAp) coatings. The deposition parameters were systematically investigated after the coatings were deposited on the as-manufactured surface or after polishing the surface of the additively-manufactured sample. The surfaces were coated with nanoHAp suspended in either ethanol or butanol using different voltages (10, 30, or 50 V) for varied deposition times. The formation of the nanoHAp coating was confirmed by Fourier-transform infrared spectroscopy and X-ray diffraction. Microstructural analysis revealed that several conditions of the coating led to crack formation. The coated samples were subsequently heat-treated to improve the integrity of the coating. Heat treatment led to crack formation in several conditions due to thermal shrinkages. Coatings prepared using butanol were more uniform and had minimal cracks compared with the use of ethanol. Nanoindentation confirmed good stability and integrity of the nanoHAP coatings on the as-manufactured and polished surfaces. The coating on the as-manufactured sample exhibited higher hardness and lower elastic modulus as compared with the coating on the polished sample. In vitro study revealed that the nanoHAp coating markedly enhanced the attachment, proliferation, and differentiation of preosteoblasts on the alloy. These results provide a viable route to enhancing the bioactivity through deposition of nanoHAp with important implications for engineering additively manufactured orthopedic and dental implants suitable for better clinical performance.

Review of recent developments in surface nanocrystallization of metallic biomaterials.

Metallic materials are widely used to prepare implants for both short-term and long-term use in the human body. The performance of these implants is greatly influenced by their surface characteristics, which has motivated the development of several surface modification techniques. Surface severe plastic deformation (S2PD) techniques have emerged as promising strategies to enhance the performance of metallic biomaterials. They do not involve chemical modification of the surface and impart minimal changes to the surface topography. S2PD processes are based on the principle of generating nanocrystals at the surface, which can improve performance metrics, such as fatigue, wear, corrosion resistance, and biocompatibility through various mechanisms, such as surface hardening and alterations to the surface oxide layer. This review presents the state of the art on the development of different S2PD processes and their applications on metallic biomaterials. Brief descriptions of the different processes have been provided, followed by a discussion on the microstructural changes induced by these processes for different generations of biomaterials. The effect of S2PD on surface and bulk characteristics of the biomaterials and their performance is critically reviewed. As an emerging class of surface engineering techniques in biomaterials science, more work is needed to fully leverage their potential in this field, and these opportunities are discussed in this review.

Zinc and cerium synergistically enhance the mechanical properties, corrosion resistance, and osteogenic activity of magnesium as resorbable biomaterials.

Magnesium and its alloys have the potential to serve as a revolutionary class of biodegradable materials, specifically in the field of degradable implants for orthopedics. However, the corrosion rate of commercially pure magnesium is high and does not match the rate of regeneration of bone tissues. In this work, magnesium alloys containing zinc and cerium, either alone or in combination, were investigated and compared with commercially-pure magnesium as biomaterials. The microstructure, mechanical properties, corrosion resistance, and response of osteoblastsin vitrowere systematically assessed. Results reveal that alloying with Ce results in grain refinement and weakening of texture. The tensile test revealed that the ternary alloy offered the best combination of elastic modulus (41.1 ± 0.5 GPa), tensile strength (234.5 ± 4.5 MPa), and elongation to break (17.1 ± 0.4%). The ternary alloy was also the most resistant to corrosion (current of 0.85 ± 0.05 × 10-4A cm-2) in simulated body fluid than the other alloys. The response of MC3T3-E1 cellsin vitrorevealed that the ternary alloy imparts minimal cytotoxicity. Interestingly, the ternary alloy was highly efficient in supporting osteogenic differentiation, as revealed by the expression of alkaline phosphatase and calcium deposition. In summary, the extruded Mg alloy containing both Zn and Ce exhibits a combination of mechanical properties, corrosion resistance, and cell response that is highly attractive for engineering biodegradable orthopedic implants.

Enhanced biomechanical performance of additively manufactured Ti-6Al-4V bone plates.

As the global trauma fixation devices market expands rapidly, it is imperative to improve the production of fixation devices through enhanced design accuracy and fit for best performance and maximum patient comfort. Selective laser melting (SLM) is one of the mature additive manufacturing methods, which provides a viable route for the rapid production of such devices. In this work, the ability of SLM to produce near-net-shape parts, as desired for medical implants, was utilized for the fabrication of bone plates from Ti-6Al-4V alloy powder. Martensitic microstructure obtained after the printing of alloy resulted in poor ductility, limiting its application in the field of orthopedics. A specially designed repeated cyclic heating and cooling close to but below the ß-transus was used to transform from acicular to a bimodal microstructure without the need for plastic deformation prior to heat treatment for improving the ductility. Bone plates subjected to this heat treatment were mechanically tested by means of tensile and 3-point bend tests and demonstrated large improvement in ductility, and the values were comparable to those similar plates prepared from wrought alloy. Other important properties required for implants were assessed, such as corrosion resistance in simulated body fluid and cytocompatibility in vitro using MC3T3-E1 cells. These results for the bone plate after heat treatment were excellent and similar to those of the additively manufactured and wrought plates. Taken together, the performance of the additively manufactured bone plates after subjecting to heat treatment was similar to those of bone plate manufactured using wrought alloy. These results have important implications for the fabrication of patient-specific metallic orthopedic devices using SLM without compromising their biomechanical performance by subjecting them to a tailored heat treatment.

Preferential phonon scattering and low energy carrier filtering by interfaces of in situ formed InSb nanoprecipitates and GaSb nanoinclusions for enhanced thermoelectric performance of In0.2Co4Sb12.

Filling the voids of cage forming compounds with loosely bound electropositive elements and by incorporating nano-sized secondary phases are promising approaches to enhance the thermoelectric figure of merit of these materials. Hence, in this work, by combining these two approaches-inserting In into the voids of skutterudite Co4Sb12 as well as dispersing nanoparticles (GaSb)-we have synthesized various samples via ball-milling and spark plasma sintering. InSb as the secondary phase of the matrix, mixed with GaSb, forms the solid solution Ga1-xInxSb. Nanocrystalline grains together with a few larger grains (10-30 µm) are found to be spread in In0.2Co4Sb12. The former is comprised of either InSb, GaSb or Ga1-xInxSb. Because of their identical space group and similar lattice parameters, InSb, GaSb and Ga1-xInxSb could not be detected separately in EBSD. High resolution transmission electron microscopy (HRTEM) was used to resolve different phases, which showed GaSb grains of size ∼10-30 nm and InSb grains of size ∼30-100 nm. Scattering of charge carriers at the interfaces of InSb, GaSb and Ga1-xInxSb as well as the matrix phases increased both the electrical resistivity and Seebeck coefficient. The multi-scale size distribution of grains, of both the matrix phase and the secondary phases, scattered phonons within a broad wavelength range, resulting in very low lattice thermal conductivities. As a result, an enhanced figure of merit of 1.4 was achieved for the (GaSb)0.1 + In0.2Co4Sb12 nanocomposite at 773 K.

Surface mechanical attrition treatment of low modulus Ti-Nb-Ta-O alloy for orthopedic applications.

Surface mechanical attrition treatment (SMAT) is recognized as a surface severe plastic deformation (SPD) method that is effective in improving the surface-dependent mechanical and functional properties of conventional metallic biomaterials. In this study, we aimed to systemically investigate the effect of SMAT on the physical, electrochemical, tribological and biological performances of a newly developed low modulus ß Ti-Nb-Ta-O alloy with two different microstructures, namely, single phase ß-treated and dual phase ß + α aged. The microhardness results showed considerable hardening for the ß-treated condition due to formation of deformation substructures; that was associated with increased corrosion resistance resulting from a stronger and denser passive layer on the surface, as revealed by Tafel polarization, impedance studies and Mott-Scottky plots. The wear volume loss during fretting in serum solution was found to decrease by 46% while friction coefficient decreased only marginally, due to presence of a harder and more brittle surface. In the ß + α condition of the alloy, minimal hardening was observed due to coarsening of the precipitates during SMAT. However, this also reduced the number of α-ß interfaces, which in turn minimized the tendency for galvanic corrosion resulting in lower corrosion rate after SMAT. Wear resistance was enhanced after SMAT, with 32% decrease in wear volume loss and 21% decrease in friction coefficient resulted due to improved ductility on the surface. The attachment and growth of osteoblasts on the alloys in vitro were not affected by SMAT and was comparable to that on commercially pure Ti. Taken together, these results provide new insights into the effects of surface SPD of low modulus ß- Ti alloys for orthopedic applications and underscore the importance of the initial microstructure in determining the performance of the alloy.

Enhanced Thermoelectric Performance in the Ba0.3Co4Sb12/InSb Nanocomposite Originating from the Minimum Possible Lattice Thermal Conductivity.

The thermoelectric efficiency of skutterudite materials can be improved by lowering the lattice thermal conductivity via the uniform dispersion of a nanosized second phase in the matrix of filled Co4Sb12. In this work, nanocomposites of Ba0.3Co4Sb12 and InSb were synthesized using ball-milling and spark plasma sintering. The thermoelectric transport properties were studied from 4.2 to 773 K. The InSb nanoparticles of ∼20 nm were found to be dispersed in the Ba0.3Co4Sb12 grains with a few larger grains of about 10 µm due to the agglomeration of the InSb nanoparticles. The +2 oxidation state of Ba in Co4Sb12 resulted in a low electrical resistivity, ρ, value of the matrix. The enhancement of the Seebeck coefficient, S, and the electrical resistivity values of Ba0.3Co4Sb12 with the addition of InSb can be credited to the energy-filtering effect of electrons with low energy at the interfaces. The power factor of the composites could not be enhanced compared to the matrix because of the very high ρ value. A minimum possible lattice thermal conductivity (0.45 W/m·K at 773 K) was achieved due to the combined effect of rattling of Ba atoms in the voids and enhanced phonon scattering at the interfaces induced by nanosized InSb particles. As a result, the (InSb)0.15 + Ba0.3Co4Sb12 composite exhibited improved thermoelectric properties with the highest zT of 1.4 at 773 K and improved mechanical properties with a higher hardness, higher Young's modulus, and lower brittleness.

Role of aging induced α precipitation on the mechanical and tribocorrosive performance of a ß Ti-Nb-Ta-O orthopedic alloy.

A low modulus ß Ti-Nb-Ta-O alloy was subjected to heat treatment to investigate its phase stability upon aging. The resultant effect on the mechanical and functional properties was systematically evaluated. The aging of the ß-only microstructure, obtained by solutionizing and quenching, resulted in the formation of ultrafine α-precipitates with increasing order of size as the aging temperature increased from 400 °C to 600 °C. The variation in the size of α-precipitates effected the mechanical properties at the three different aging temperature. The highest hardening observed at 400 °C was associated with macroscopic embrittlement, whereas age softening was observed in samples aged at 600 °C due to coarsening of precipitates and softening of the ß-matrix. In contrast, aging at 500 °C resulted in about 32% increase in tensile strength from the ß-solutionized condition. As the samples aged at 500 °C showed optimum combination of mechanical properties among the aged samples, these were further characterized for their electrochemical, tribological and biological responses. The fretting wear studies showed that the wear rate of the solution-treated samples increased after aging due to the higher corrosion rate leading to a higher rate of tribocorrosive dissolution and formation of a transfer layer harder than that of solution treated sample. The Ti-Nb-Ta-O alloy supported the attachment and proliferation of osteoblasts similar to that on commercially pure Ti. Taken together, this work provides new insights into the preparation of next-generation Ti alloys for biomedical applications with high strength and low modulus through microstructural control induced by heat treatment.

Investigation on the structure and thermoelectric properties of CuxTe binary compounds.

Cu2Te is a superionic conductor that belongs to the Phonon Liquid Electron Crystal class of thermoelectric (TE) materials. Despite the simple chemical formula, the crystal structures and phases in the Cu2Te system have not been understood properly. In this work, we study the structural and TE properties of Cu2Te (CT2), Cu1.6Te (CT1.6) and Cu1.25Te (CT1.25). The samples were synthesized via a solid-state reaction method. Powder X-ray diffraction analysis revealed that the samples have different crystal structures depending upon the Cu : Te stoichiometry. The elemental compositional analysis showed that all the samples are copper deficient. This is due to the precipitation of metallic copper on the surface of the ingot arising from the thermal dissociation of Cu2Te. The transport properties were measured in the temperature range 300 K-600 K. The electrical conductivity (σ) decreases with an increase in temperature indicating a metal-like behaviour for all the samples. The positive Seebeck coefficients (S) for all the samples indicates that majority charge carriers are holes. The sample CT2 has a higher S (29.5 µV K-1 at 573 K) and a lower σ (2513 S cm-1 at 573 K) due to a lower carrier (hole) concentration compared to the other two samples. With the increase in Cu deficiency, the hole concentration increases, and this leads to higher electronic thermal conductivity in the samples CT1.6 and CT1.25. The maximum thermoelectric figure of merit of 0.03 at 524 K is achieved for the sample CT2 owing to its higher power factor (0.24 mW m-1 K-2) and lower thermal conductivity (3.8 W m-1 K-1). The present study bridges the gap between the theoretical predictions and experimental observations involving the various possible structures in this system. Furthermore, we have shown that the Cu vacancies are detrimental to the thermoelectric performance of Cu2Te.

Revealing the role of microstructure architecture on strength and ductility of Ni microwires by in-situ synchrotron X-ray diffraction.

Deformation mechanisms of cold drawn and electropolished nickel microwires are studied by performing in-situ monotonous and cyclic tensile tests under synchrotron radiation. X-ray diffraction tests allow probing elastic strains in the different grain families and establishing a link with the deformation mechanisms taking place within the microwires. The measurements were carried out on several microwires with diameters ranging from as-drawn 100 µm down to 40 µm thinned down by electropolishing. The as-drawn wires exhibit a core-shell microstructure with <111> fiber texture dominant in core and heterogeneous dual fiber texture <111> and <100> in the shell. Reduction of specimen size by electropolishing results in a higher yield stress and tensile strength along with reduced ductility. In-situ XRD analysis revealed that these differences are linked to the global variation in microstructure induced by shell removal with electropolishing, which in turn affects the load sharing abilities of grain families. This study thus proposes a new way to increase ductility and retain strength in nickel microwires across different diameters by tuning the microstructure architecture.

Engineering the next-generation tin containing ß titanium alloys with high strength and low modulus for orthopedic applications.

Metastable ß Ti alloys are the new emerging class of biomaterial for load bearing orthopedic applications. However, these alloys in the single ß phase microstructure have insufficient strength for use in load bearing applications. It is imperative to strengthen these alloys by carefully designed thermo-mechanical processing routes that typically involve aging treatment. In this investigation two newly designed Sn based ß Ti alloys of composition Ti-32Nb-(2, 4) Sn are evaluated. The effects of Sn content on the mechanical properties and biological performance of these alloys processed through designed thermo-mechanical processing route are investigated. The increase in the Sn content led to a reduction in the elastic modulus of the alloy. An increase in the Sn content increased the aspect ratio of the α precipitates, which led to a significant strengthening in the alloy while keeping the elastic modulus low. In addition, the corrosion behavior of the alloy was evaluated in simulated body fluid. The Sn containing ß alloys have an excellent corrosion resistance as desired for an implant material. The corrosion resistance improved with an increase in Sn content. These alloys were also observed to have excellent cytocompatibility as they not only supported the attachment and proliferation of human mesenchymal stem cells but also their osteogenic differentiation in vitro. The combination of high strength, low elastic modulus, superior corrosion resistance and biocompatibility underscores the promise of Sn containing ß Ti alloys for use in orthopedic applications.