Papers by Author: Sidananda Sarma

Abstract: Polycrystalline ingots of Co70–xNixGa30 (20 ≤ x ≤ 26) ferromagnetic shape memory alloy (FSMA) were prepared by arc melting elemental powders followed by homogenization at 1230 °C for 24 hrs and quenching in liquid nitrogen. Room temperature X-Ray diffraction (XRD) patterns of as-quenched samples exhibited single-phase tetragonal structure for alloy compositions with x = 21 to 26, and a two-phase structure (cubic A2-phase along with weak tetragonal phase) for the alloy with x = 20. Rietveld refinement was performed on the X-ray diffraction patterns to obtain the refined structural parameters. Differential Scanning Calorimeter (DSC) curves recorded from 30 °C to 250 °C revealed martensite-austenite and austenite-martensite transformations in all alloys except the alloy with composition x = 20. Low temperature ac magnetic susceptibility measurements confirmed the existence of martensitic transformations in the alloy with x = 20. The structural transformation temperatures show a linear variation with e/a ratio. All the alloys were ferromagnetic at room temperature. Curie temperature was determined using a high temperature ac magnetic susceptibility measurement set-up.

Abstract: Co-Ni-Ga and Co-Ni-Al alloys are expected to be good ferromagnetic shape memory alloys (FSMAs) owing to their higher ductility resulting from the presence of γ-phase precipitates and higher stability in preparation since they do not contain the highly volatile element Mn. Co-Ni- Ga alloys have a wide range of martensitic transformation and Curie temperatures. In order to explore the possibility of obtaining Co-Ni-Ga alloys with room temperature ferromagnetic martensitic phase, two series of compositions, viz., Co70-xNixGa30 (20 ≤ x ≤ 26) and CoxNi25Ga75-x (43 ≤ x ≤ 50) were taken up for investigation. Polycrystalline ingots of these alloys were prepared by arc melting followed by homogenization and quenching at ice water. Analysis of room temperature X-ray diffraction patterns revealed that most of these alloys had a single-phase (tetragonal) structure typical of a martensitic phase, while some of the alloys exhibited a two-phase (cubic and tetragonal) structure due to the presence of both martensite and austenite phases. All alloys having single martensite phase at room temperature showed martensitic transformation at elevated temperature as well as a linear change of the characteristic martensitic transformation temperatures (As, Af, Ms and Mf) with the number of valence electron to atom ratio (e/a). As, Af, Ms and Mf showed distinctive variations when aged in the martensite phase and austenite phase. All the alloys were ferromagnetic at room temperature and the Curie temperature was determined by high temperature ac magnetic susceptibility and magnetization measurements. The typical twin lamellar structure of martensite phase was observed by optical microscope and the development of the cubic γ-phase along with the parent β′-phase was investigated for different ageing temperatures and annealing temperatures. These studies provide useful information about the potential of these alloys for actuator applications.

Abstract: Polycrystalline ingot of Co47Ni23Ga30 alloy was prepared by arc melting constituent elemental powders under argon atmosphere. The alloy ingot was then vacuum sealed in a fused silica ampoule, homogenized at 1230 °C for 24 hours and quenched in liquid nitrogen. X-Ray diffraction patterns of the as-quenched samples revealed single-phase tetragonal structure. The quenched alloy was then separately annealed at 900 °C, 1000 °C and 1150 °C for 6 hours and subsequently quenched in ice water. The alloys annealed at 1150 °C and 1230 °C exhibited a singlephase martensite structure (β′-phase) at room temperature, whereas, presence of a face centred cubic (γ) phase along with the martensite phase was observed in alloy pieces annealed at 900 °C and 1000 0C, respectively. The martensite-austenite structural phase change in this alloy was observed using a Differential Scanning Calorimeter. It was found that the martensite-austenite and austenitemartensite transition temperatures (As, Af, Ms and Mf) shifted to higher temperatures when the annealing temperature was increased. The Curie temperature shifted towards lower temperatures as the percentage of γ-phase increased in the alloy. The saturation magnetization did not show any appreciable change when the annealing temperature was changed. Presence of the additional γ- phase in the alloy annealed below 1150 °C was confirmed by Optical Microscopy and Scanning Electron Microscopy analysis. The influence of the annealing temperature on the properties of this ferromagnetic shape memory alloy composition is discussed in the paper.

Abstract: Polycrystalline ingots of Co70-xNixGa30 (22 ≤ x ≤ 25) alloys were prepared by a sequence of arc melting high purity Co, Ni and Ga in argon atmosphere, followed by homogenization at 1150°C under a pressure of 10-3 Pa, and quenching in ice water. Structural characterisation of the quenched alloys was carried out to verify the presence of the martensite phase at room temperature. The martensite start (Ms), martensite finish (Mf), austenite start (As) and austenite finish (Af) temperatures for the alloys were determined using a differential scanning calorimeter. The ferromagnetic to paramagnetic phase transition temperature (TC) of the alloys was determined using an indigenously developed ac susceptometer. All the alloys are FSMAs with Ms, Af and TC above room temperature. The composition dependence of the properties of these alloys could be understood on the basis of the e/a (electrons to atom) ratio and the Co/Ni ratio. Presence of γ-phase precipitates along with the β-phase in these alloys enhances the ductility as well as influences the physical properties of these alloys.