Department of Metallurgical & Materials Engineering

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The origin of ferromagnetism in ZnO doped with transition metal impurities has been discussed extensively and appeared to be a highly controversial and challenging topic in today's solid state physics. Magnetism observed in this system is generally weak and soft. We have grown Co:ZnO up to 30 at% Co in single crystal thin film form on c-plane sapphire. A composition dependent coercivity is observed in this system which reaches peak value at 25 at% Co, the values are 860 Oe and 1149 Oe with applied field along parallel and perpendicular to the film substrate interface respectively. This giant coercivity might pave the way to exploit this material as a magnetic semiconductor with novel logic functionalities. The findings are explained based on defect band itinerant ferromagnetism and its partial interaction with localized d electrons of Co through charge transfer. Besides large coercivity, an increase in the band gap with Co concentration has also been observed along with blue emission peak with long tail confirming the formation of extended point defect levels in the host lattice band gap. © 2013 Elsevier B.V. All rights reserved.

World University root node

Journal of magnetism and magnetic materials

Giant coercivity in ferromagnetic Co doped ZnO single crystal thin film

Compressive flow behavior of hot-rolled (64% reduction in thickness) Al and Al-15% B4C composite was studied over the temperature and strain rate ranges of 25-500 °C and 10-4-1 s-1, respectively, in longitudinal (LT) and transverse (TD) directions. It was found that athermal region (i.e. temperature-independent region) in aluminum becomes less prominent upon reinforcing with 15% B4C particulates. The apparent stress exponent n' was found to vary between 10 to 30. It was also found that the stress exponent values could reduce to n = 5 upon subtracting the threshold stress from applied stress. Microstructural variation in aluminum was studied by EBSD and correlated with its flow behavior. © QinetiQ Ltd 2013.

ICCM International Conferences on Composite Materials

Flow behavior of alumnium-15% boron carbide composite by differential strain rate compression test

A 50:50 atomic ratio of elemental Ni and Al powders were subjected to mechanical alloying (MA) in a toluene medium and were consolidated via spark plasma sintering (SPS). The transmission electron microscopy (TEM) results indicated that the ultra-fine grains (226±107nm and 307±150nm) and nanometre-sized Al2O3 and Al4C3 particles dispersed in the NiAl samples sintered at 900 and 1000°C. A high hardness of 670 HV and a compressive yield strength of 2251MPa were obtained in the NiAl sintered at 900°C. A maximum plastic strain of ~16.5% with a compressive yield strength of 2041MPa was realised for the NiAl sample sintered at 1000°C. The contributions of the grain size and particles to the strength were calculated using the appropriate strengthening models, and the calculated theoretical yield strength of the synthesised NiAl was found to closely match the experimental values. © 2013 Elsevier B.V.

Materials Science and Engineering A

Ultrafine-grained, high-strength NiAl with Al2O3 and Al4C3 nanosized particles dispersed via mechanical alloying in toluene with spark plasma sintering

We have investigated the non-equilibrium solubility of Mn and Co in Zn 1-xTMxO (x = 5-30 at.% TM = Co, Mn) single crystal thin film and the role of Co/Mn concentration on the structural and magnetic properties. The films have been grown by pulsed laser deposition and the structural and magnetic characterization has been performed using high resolution transmission electron microscopy and superconducting quantum interference device respectively. Cobalt shows high solubility in single crystalline ZnO films with no tendency to form incoherent secondary precipitates up to 30 at.%. Whereas for Mn:ZnO, secondary precipitate started forming from Mn concentration of 25 at.% and above. HREELS of both Co and Mn L2,3 absorption edges shows that the Co and Mn atoms have substituted the Zn positions in the lattice. ZnO doped with 30 at.% Mn forms single crystalline spinel Mn3O4 phase on the sapphire substrate in the form of an interlayer which is responsible for aligning subsequent Mn:ZnO film with respect to sapphire as [1 1 -2 0]Al2O3 || [1 1 -2 0] ZnO compared to the usual [1 1 -2 0]Al2O3 || [0 1 -1 0] ZnO orientation. Saturation magnetization remained almost the same (∼10 20-1021 μB/cm3) for Co:ZnO but a composition dependent coercivity has been observed. For Zn0.75Co 0.25O film, the coercivity is ∼922 (769) Oe corresponding to applied field perpendicular (parallel) to the c axis, which is the highest value reported in this system. For Mn:ZnO ferromagnetism disappears beyond Mn concentrations of 15 at.%. © 2013 Elsevier Ltd. All rights reserved.

Superlattices and Microstructures

Characterization of structure and magnetism in Zn1− x (Cox/Mnx) O epitaxial thin films as a function of composition

The kinetics of dynamic recrystallization (DRX) during hot compression of 304 austenitic stainless steel was studied over the temperature range of 900-1200 C and strain rate range of 0.002-0.1 s-1. The initiation and evolution of DRX were investigated using the process variables derived from flow curves. By the regression analysis for conventional hyperbolic sine equation, the activation energy for DRX was determined as Q = 475 kJ mol -1. The temperature and strain rate domain where DRX occurred were identified from the strain rate sensitivity contour map. The critical stress (and strain) for the initiation of DRX was determined from the inflection point on the work hardening rate (θ = dσ/dε) versus flow stress (σ) curve. The saturation stress of the dynamic recovery (DRV) curve was calculated from the θ-σ plot at the same condition at which DRX occurred. Progress of fraction recrystallization was determined from the difference between the generated DRV curve and the experimental DRX curve. In addition, the microstructural evolution at different strain levels during DRX was characterized and compared with the calculated fraction recrystallization. © 2013 ASM International.

Journal of Materials Engineering and Performance

Dynamic recrystallization during hot deformation of 304 austenitic stainless steel

The hot working behavior of near-α titanium alloy IMI 834 with a duplex starting microstructure was studied using the technique of processing map. The processing map was interpreted in terms of the microstructural processes occurring during deformation, based on the values of dimensionless parameter η which represents the energy dissipation through microstructural processes. An instability criterion (ξ&lt;0) was also applied to demarcate the flow instability regions in the processing map. Both the parameters (η and ξ) were computed using the experimental data generated by carrying out hot compression tests over a range of temperatures (850–1060°C) and strain rates (3×10−4−1/s). The deterministic domains observed under the investigated temperature and strain rate conditions were attributed to continuous dynamic recrystallization or globularisation of α lamellae, dynamic recrystallization and growth of β grains through microstructural observations. An unified strain compensated constitutive equation was established to describe the hot working behavior of the material in the selected temperature–strain rate range. The established constitutive equation was validated using standard statistical parameters such as correlation coefficient and average absolute relative error. © 2013 Chinese Materials Research Society

Fulltext

Progress in Natural Science: Materials International

Modeling the hot working behavior of near-α titanium alloy IMI 834

Microstructural evolution and related dynamic recrystallization phenomena were investigated in overlapping multipass friction stir processing (FSP) of hypereutectic Al-30 pct Si alloy. FSP resulted in the elimination of porosities along with the refinement of primary silicon particles and alpha aluminum grains. These alpha aluminum grains predominantly exhibit high angle boundaries with various degrees of recovered substructure and dislocation densities. The substructure and grain formation during FSP take place primarily by annihilation and reorganization of dislocations in the grain interior and at low angle grain boundary. During multipass overlap FSP, small second phase particles were observed to form, which are accountable for pinning the grain boundaries and thus restricting their growth. During the multipass overlap FSP, the microstructure undergoes continuous dynamic recrystallization by formation of the subgrain boundary and subgrain growth to the grain structure comprising of mostly high angle grain boundaries. © 2012 The Minerals, Metals &amp; Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Recrystallization phenomena during friction stir processing of hypereutectic aluminum-silicon alloy

Enhanced electrical transport and ultra low thermal conductivity resulted in a high thermoelectric figure of merit, ZT, of ∼1 and ∼1.15 at ∼680 K in 4 mol% Pb and 2 mol% Bi doped AgSbSe2, which are 150 and 190% higher compared to that of the pristine sample, respectively. With this excellent thermoelectric performance, p-type AgSbSe2, constituting earth abundant Se, offers promise to replace traditional metal tellurides containing expensive and scarce Te for mid temperature (350-700 K) thermoelectric applications. © 2013 The Royal Society of Chemistry.

Energy \& Environmental Science

High thermoelectric performance in tellurium free p-type AgSbSe 2

Aluminum-Boron carbide (Al-B4C) composites containing Al-0, 5, 10 and 15 wt% B4C were produced by flux assisted reaction method at 900°C. In order to study the effects of temperature on flow property and microstructural evolution, compression tests were done over the temperature range of 25° to 500°C at an initial strain rate of 10-2 s-1 and the microstructures were examined in Scanning Electron Microscope. The stress-strain curves exhibit an increase in flow stress with the B4C content, except that the same decreased for Al-15 wt% B 4C composite below that of Al-10 wt% B4C composite. The flow stress was found to decrease with the increase in test temperature above ∼200°C, but it exhibited nearly athermal behavior below this critical temperature. The flow property of the composite is discussed in terms of the flow of the matrix and particulate, and the concurrent microstructural evolution that occurs during deformation.

TMS Annual Meeting

Compressive properties of Al-B4C composites over the temperature range of 25-500°C

As-cast AZ80 Mg alloy contains α-Mg, partially divorce eutectic of α and γ (Mg 17Al 12), fully divorce eutectic of α and γ, and lamellar eutectic of α and γ phases. During homogenization, second phase (γ-Mg 17Al 12) gets dissolved can change the mechanical properties. Therefore, the aim of the present work is to bring out the kinetics of dissolution of γ phase and evaluate its effect on mechanical properties. Microstructure evolution during homogenization was investigated as a function of time for 0.5 to 100 h and at the temperatures of 400° and 439°C. In as-cast state, this material was found to contain 70% α-Mg and 30% eutectic phase. With increasing homogenization time, dissolution of lamellar eutectic occurs first which is followed by dissolution of fully divorce eutectic and partially divorce eutectic. The dissolution kinetics of γ phase was analyzed based on the decrease in its volume fraction as a function of time. The time exponent for dissolution was found to be 0.38 and the activation energy for the dissolution of γ phase was found to be 84.1 kJ/mol. This dissolution of γ phase leads to decrease in hardness and tensile strength with increase in homogenization time.