Department of Metallurgical & Materials Engineering

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Thermoelectric "waste heat-to-electrical energy" generation is an efficient and attractive option for robust and environmentally friendly renewable energy production. Simultaneous tailoring of interdependent thermoelectric parameters, i.e. electrical conductivity, thermopower and thermal conductivity, to improve the thermoelectric figure of merit is the utmost challenge in this field. Another important aspect is to develop high performance materials based on cheap and earth abundant materials. We have chosen AgSbSe2, a homologue of AgSbTe2 containing earth abundant selenium, as a model system for thermoelectric investigation due to its low thermal conductivity and favourable valence band structure. Herein, we show that by integrating different but synergistic concepts: (a) carrier engineering, (b) second phase endotaxial nanostructuring and (c) bond anharmonicity, we can achieve a maximum ZT of ∼1.1 at 635 K in AgSbSe2-ZnSe (2 mol%), which is significantly higher than that of pristine AgSbSe2. The above system therefore offers promise to replace traditional metal tellurides for mid-temperature power generation. We demonstrate a design strategy which provides simultaneous enhancement of electrical transport through optimized doping, superior thermopower by the convergence of degenerate valence bands, and glass-like thermal conductivity due to the effective scattering of phonons by nanostructuring, bond anharmonicity and a disordered cation sublattice. © 2014 The Royal Society of Chemistry.

World University root node

Journal of Materials Chemistry A

Nanostructuring, carrier engineering and bond anharmonicity synergistically boost the thermoelectric performance of p-type AgSbSe 2–ZnSe

The present study highlights the changes in morphology in a Sn matrix during pulsed electrodeposition of a Sn-Cu near-eutectic alloy with varying current density in an acidic electrolyte. 3D image construction has been employed using SHAPE V7.3; Shape Software: Kingsport, Tennessee, USA to replicate the shape of grown crystals and analyze the results. It is observed that the growth morphology changes with the introduction of high index planes at higher current densities. Furthermore, a growth pattern occurs with high index planes formed from a stepped structure in low index planes. There has been a change in growth direction from 〈001〉 to 〈110〉 at higher current densities. At very high current densities, sympathetic or secondary nucleation is observed. © 2014 American Chemical Society.

Crystal Growth and Design

Effect of current density on the nucleation and growth of crystal facets during pulse electrodeposition of Sn-Cu lead-free solder

Accelerated rolling contact fatigue (RCF) experiments on M-50 NiL case carburized bearing steel were conducted using a three-ball-on-rod tester at high contact stress levels (5.5 GPa) and over several hundred million contact cycles to investigate the evolution of plastic deformation zone characteristics. A microindentation technique was used to measure the changes in the material hardening and softening characteristics within the RCF affected zone and represent the changes in the form of microhardness maps. These maps revealed regions of considerable material hardening inside the plastic zone and softening of material outside the hardened region. The results indicate large amount of plastic strain accumulation beneath the contact surface via ratcheting due to RCF cycling. The hardened and softened regions continue to evolve in severity and size with increasing number of RCF cycles. Using the Hertzian contact theory, a rationale for the evolution of hardened and softened regions during RCF loading is provided. The entire plastic zone is shown to be dominated by four different principal stress states which contribute to the evolution of these regions during RCF. © 2013 Elsevier Ltd. All rights reserved.

International Journal of Fatigue

Evolution of subsurface plastic zone due to rolling contact fatigue of M-50 NiL case hardened bearing steel

High temperature creep is an important property of titanium alloys used in aeroengines. Creep resistance of titanium alloys generally varies with heat treatment, temperature and cooling rate. Both the parameters affect the morphology and topology of the α (HCP) and β (BCC) phase present in the material. Various theories have been proposed in the literature to explain (i) the increase in creep strain with decreasing solution treatment temperature and (ii) the U-shaped variation of creep strain with cooling rate. Some of these theories are quite contradictory. An attempt is made here to systematically (a) evaluate and establish a direct microstructure-mechanical property correlation and (b) to explain the observed variation in the creep behaviour of a near-α titanium alloy IMI 834. The results obtained indicate that the observed U-shaped variation of creep curve is due to the counter acting nature of various microstructural features present in the material. © 2014 Elsevier B.V.

Materials Science and Engineering A

Correlation between microstructural features and creep strain in a near-α titanium alloy processed in the α+β regime

Recent techniques of severe plastic deformation including variants of rolling, equi-channel angular pressing and friction stir processing were used to enhance tensile properties of single phase, quasi-single phase and two phase alloys by grain refinement and substructural development. This led to the feasibility of mechanical processing of not only brittle material like hypereutectic Al-Si alloys, composites, and Mg alloys but also improved strengthening of ductile materials like Type 304 stainless steel. Presented here is a review of the recent works done in the author's group along with that emerged from collaboration elsewhere. © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.

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Procedia Engineering

Enhancement of flow properties by grain refinement and other structural modification

It is thought that the proposed new family of multi-functional materials, namely, the ferroelectric thermoelectrics may exhibit enhanced functionalities due to the coupling of the thermoelectric parameters with ferroelectric polarization in solids. Therefore, the ferroelectric thermoelectrics are expected to be of immense technological and fundamental significance. As a first step towards this direction, it is most important to identify the existing high performance thermoelectric materials exhibiting ferroelectricity. Herein, through the direct measurement of local polarization switching, we show that the recently discovered thermoelectric semiconductor AgSbSe2has local ferroelectric ordering. Using piezo-response force microscopy, we demonstrate the existence of nanometer scale ferroelectric domains that can be switched by external electric field. These observations are intriguing as AgSbSe2crystalizes in cubic rock-salt structure with centro-symmetric space group (Fm-3m), and therefore, no ferroelectricity is expected. However, from high resolution transmission electron microscopy measurement, we found the evidence of local superstructure formation which, we believe, leads to local distortion of the centro-symmetric arrangement in AgSbSe2and gives rise to the observed ferroelectricity. Stereochemically active 5S2lone-pair of Sb may also give rise to local structural distortion thereby creating ferroelectricity in AgSbSe2. © 2014 AIP Publishing LLC.

Applied Physics Letters

Direct evidence of strong local ferroelectric ordering in a thermoelectric semiconductor

Mg-4Li-1Ca (LC41) alloy was hot rolled at 300 °C, which resulted in refinement of the grain size from 100 μm to 5 μm by recrystallization in the presence of fine uniformly dispersed eutectic (α-Mg + Mg 2Ca). Evidence of profuse deformation twinning in the microstructure suggested twinning to be the important mode of deformation in the present material. Room temperature tensile tests of the alloy at a strain rate of 10-4 s-1 exhibited substantially high specific strength of 142 kN m/kg owing to its low density and combined effect of grain boundary strengthening and dispersion strengthening in the presence of Mg2Ca phase. LC41 alloy also showed an increase in the overall bio-corrosion resistance (Rp = 0.266 kω cm2) in simulated body fluid (SBF) as a result of a change in the corrosion mechanism from localized to uniform corrosion. The reported combination of excellent properties is a clear indication that the new ternary alloy LC41 has a significant potential for use in lightweight structural and biomedical applications. © 2014 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Microstructure refinement and its effect on specific strength and bio-corrosion resistance in ultralight Mg-4Li-1Ca (LC41) alloy by hot rolling

Zr-1Nb alloy uniaxially compressed at room temperature at 10-2 and 103 s-1 exhibited twinning and a three-stage strain-hardening behavior. At 103 s-1 the twin fraction initially increased to 0.3, decreasing to 0.02 at higher strains. Despite the difference in texture at intermediate strains, the final texture was similar at both strain rates. The increasing strain-hardening rate in the second stage was attributed to strengthening from grain boundaries and dislocations, and softening from reorientation due to twinning. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Scripta Materialia

Effect of strain rate on twinning in a Zr alloy

Flow behavior of aluminum-5 wt.% boron carbide (Al-B4C) composite was investigated by carrying out compression tests over a range of strain rates (10-4-100 s-1) and temperatures (200-500 °C). The flow stress data obtained from these tests at true strain 0.5 were used to develop processing map. The stable and instable flow regimes in the map were characterized by the microstructural examination using Scanning Electron Microscopy and Electron Backscattered Diffraction. The optimum condition for processing of Al-5%B4C composite was found to lie between 425 and 475 °C at the strain rate of around 10-4 s -1. A strain-compensated Sellars-McG Tegart constitutive equation was established to model high-temperature deformation behavior of the material. © 2014 ASM International.

Journal of Materials Engineering and Performance

Hot workability and flow characteristics of aluminum-5 wt.% B4C composite

The flow properties of thermo-mechanically processed Ti-6A1-4V alloy were investigated in this study. Two samples from as received Ti-6Al-4V alloy plate with consisting coarse β grains of ∼ 170 μm size were rolled at 940°C and 550°C respectively. In hot rolled (940°C) sample fully equiaxed microstructure with average grain size ∼ 2.3 μm was produced. In warm rolled (550°C) sample, heterogeneous microstructure was produced, which consists of elongated and partially equiaxed grains. To investigate flow properties of hot and warm rolled Ti-6Al-4V alloy samples, differential strain rate tests were performed. Strain rate sensitivity (m) values and apparent activation energy (Q) for were calculated for both hot and warm rolled samples.