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Ferroelectric (piezoelectric) and ferromagnetic materials have extensively permeated in modern industry. (Na1/2Bi1/2)TiO3-BaTiO3 (NBT-x%BT) single crystals and K1/2Na1/2NbO3 (KNN) textured ceramics are top environment-friendly candidates which have potential to replace the commercial lead zirconate titanate or PZT. High magnetostrictive strain (up to 400 ppm) of Fe-xat.%Ga makes this alloys promising alternatives to existing magnetostrictive materials, which commonly either contain costly rare-earth elements or have undesirable mechanical properties for device applications. These systems have common characteristics: compositional/thermal/ electrical dependent structural heterogeneity and chemical disorder on sub-micron or nano scale, resulting in diverse local structures and different physical properties. In this work, I have investigated domain and local structures of NBT-x%BT crystals, KNN ceramics and Fe-xat.%Ga alloys under various conditions, mainly by scanning probe and electron transmission techniques. In NBT-x%BT single crystals, polarized light, piezo-response force (PFM) and transmission electron (TEM) microscopies were used to study domain structures and oxygen octahedral tiltings. Hierarchical domain structures were found in NBT: a high-temperature tetragonal ferroelastic domain structure is elastically inherited into a lower temperature rhombohedral ferroelectric phase. Nanoscale domain engineering mechanism was found to still work in NBT-x%BT system and a modified phase diagram was proposed based on domain observations. An increased intensity of octahedral in-phase tilted reflections and a decrease in the anti-phase ones was observed, with increasing x as the morphotropic phase boundary (MPB) is approached. It was also found that Mn substituents favor the formation of long range ordered micro-sized ferroelectric domains and octahedral in-phase tilted regions near the MPB. Nano-size heterogeneous regions were observed within submicron domain structure, indicating that the nanoscale polarization dynamics are not confined by domain boundaries, and the high piezoelectricity of NBT-x%BT is due to a polarization dynamics with high sensitivity to electric field and a broadened relaxation time distribution. In KNN textured ceramics, an aging effect was found to exist in the orthorhombic single phase field, not only in the orthorhombic and tetragonal two-phase field as previously reported. No variation of phase structure was revealed between before and after aging states. However, pronounced changes in domain morphology were observed by both PFM and TEM: more uniform and finer domain structures were then found with aging. These changes were even more pronounced after poling the aged state. A large number of sub-micron lamellar domains within micron-domains were observed: suggesting a domain origin for improved piezoelectric properties. In Fe-xat.%Ga alloys, an underlying inhomogeneity from Ga atoms embedded into the α-Fe matrix was believed to be the origin of giant magneostrictive properties. I have systematically investigated the phase structure and nano-size heterogeneity of Fe-xat.%Ga alloys subjected to different thermal treatments using standard TEM and high resolution TEM for 10<x<30. Nano-precipitates were observed in all specimens studied: A2, D03 and B2 phases were found depending on x. Nano-precipitates of D03 were observed to be dominant for compositions near the magnetostriction peaks in the phase diagram. Quenching was found to increase the volume fraction of nanoprecipitates for x=19, near the first magnetostriction peak. With increasing x to 22.5, nanoprecipitates were observed to undergo a D03 – B2 transformation. A high density of D03 precipitates of nanoscale size was found to be the critical factor for the first maximum in the magnetostriction. / Ph. D.

Giant magnetostrictive RFe2 type (R represents rare earths) intermetallics form an important class of magnetic materials keeping in view of their potential applications as sensors and/ or actuators. In this thesis, one such mixed rare earth compound (Tb,Dy)Fe2 has been chosen for investigations. Being a technologically important material system, several investigations concerning physical and magnetic properties of the material and effect of processing parameters on magnetic properties have been reported in the available literature. However, existing literature does not provide a clear insight into some important aspects such as phase equilibria, evolution of texture and microstructure of directionally solidified Tb-Dy-Fe alloys. Therefore, the present work was undertaken to bring out tangible process-structure-property correlations with an emphasis to clarify the grey areas in the available literature. The investigation on the nature of ternary phase equilibria of Tb-Dy-Fe was taken up with an aim to understand the effect of Tb/Dy ratio on phase equilibria and magnetic properties of TbxDy1-xFe1.95 (x=0-1) alloys. Microstructural and micro-chemical analysis along with study of lattice parameter has been used to predict the nature of phase equilibria and the deviation from the assumed pseudo-binary behaviour. Further, from the microstructural investigations and study of lattice parameter and Curie temperature, a schematic sketch of a section of the ternary diagram, where (Tb,Dy) / Fe =1.95, was formulated and presented. Directional solidification technique is the most widely adopted method for processing the (Tb,Dy)Fe2, to impart grain orientation for practical applications. Therefore, it was aimed in the present study to understand the evolution of texture and microstructure in directionally solidified Tb0.3Dy0.7Fe1.95 alloy by modified Bridgman and zone melting techniques. The alloy was directionally solidified by modified Bridgman technique with a series of growth rates 5 - 100 cm/h, at a constant temperature gradient of 150oC/ cm. Microstructural investigation revealed formation of island banding at lower growth rate and peritectic coupled growth at higher growth rates. The texture study indicated a transition of growth texture from <113> to <110> and finally to <112> with increase of growth rate. A mechanism based on atomic attachment kinetics is proposed to explain the orientation selection with growth rate. The texture and microstructure have been correlated with magnetostriction and static strain co-efficient (dλ/dH) of the Bridgman solidified alloys. The solidification morphology observed in Bridgman solidified samples was found to be mostly plane front. Therefore, in order to understand the microstructure and texture evolution in cellular/ dendritic regime, directional solidification of Tb0.3Dy0.7Fe1.95 was attempted by zone melting technique with a lesser temperature gradient of 100oC/cm. A detailed texture study indicated a transition in preferred growth direction from <110> to <112> with increase of growth rate. In this case of cellular/ dendritic growth regime, a mechanism based on atomic attachment kinetics has been proposed and the preferred morphologies of the solid-liquid interface for <110> and <112> growth have been modelled. The modelled interfaces have been correlated to the shape of cell/ dendrite cross-section observed for the growth rates adopted in this study. Apart from the investigation carried out on the (Tb,Dy)Fe2 alloys, attempts have been made to understand the role of microstructure, especially the ordered phases on the magnetostriction of an emerging magnetostrictive material Fe-Ga. A series of alloy compositions of Fe-x at % Ga (x=17, 20, 23 and 25) were prepared and subjected to different thermal treatments and characterized for microstructural features and magnetostriction. Microstructure investigation of slow cooled, quenched and quenched + aged alloys reveals formation of ordered DO3 phase from disordered A2 phase by first order transformation in 17 and 20 at% Ga alloys, whereas for 23 and 25 at% alloys, the transformation takes place by continuous ordering. It could be observed that large magnetostriction arises owing to the presence of disordered A2 phase or ordered DO3 phase alone. The magnetostriction however decreases substantially when these two phases are co-existing.