Mitra, Amitava

This book highlights the qualitative and quantitative sequential changes in microstructure of P22 steel under various stress and temperature conditions. The P22 alloy is an established material used under elevated temperature and stress for the components of thermal power plants. Temperature and stress levels for laboratory experimentation have been selected based on the true operating condition of a boiler. This book describes both continuous as well as interrupted tests that were performed under given parameters. Subsequently, the microstructures, bulk hardness and NDE parameters (magnetic and non-linear ultrasonic) have been evaluated. For reliable data, the microstructures have been observed at different regions of creep exposed samples by different characterization techniques. This has been further followed by drawing correlation between specific features like precipitate size variation with creep strain / creep time and so on. Given the contents, this book will be a useful reference for researchers and professionals working in the area of materials especially in thermal power plants.

The progressive addition of P in Fe-rich FeSiB(P)NbCu nanocomposite alloy leads to grain refinement of α-Fe nanocrystals i.e, dense nucleation and grain growth inhibition. The P addition leads to linear reduction of α-Fe nanocrystallite size (D) and increases nucleation density (Nd) from ~ 30 nm and 2.5 × 1022 (0 at% P) to ~ 15 nm and 0.9 × 1023 for (8 at% P). The refined microstructure leads to coercivity reduction from 74 A/m (0 at% P) to 9.4 A/m (8 at% P), in agreement with Random-Anisotropy Model. The P assisted synergistic grain refinement mechanism is explained in the context of crystallization activation energy, selective solute re-distribution and enhanced stabilization of intergranular amorphous matrix. The 4 at% P alloy shows optimal soft-magnetic properties of 10.2 A/m and 1.64 T.

An investigation on quality assessment of cold rolled grain oriented (CRGO) steels is carried out in this research with the supports of microstructure, texture and magnetic property evaluation in one virgin and three service exposed CRGO sheets. The microstructure evolution reveals oxidation, precipitation and widening at grain boundary of service exposed samples, justifying structural deterioration with service exposure. Additionally, the misorientation of Goss grains is found with higher angle range for service exposed (7°–31°) than virgin (6°–17°) CRGO. The service exposed CRGO sheets are also examined with poor magnetic properties of high core loss compared to virgin one. The non-destructive evaluation explains the deterioration of magnetic anisotropy in service exposed CRGO sheets, which would be beneficial for the rapid inspection of CRGO quality. The quality degradation of CRGO sheet is justified through the lowering of magnetic anisotropy factor and Steinmetz exponent. This research outcome would be the baseline to transformer manufacturer for the separation of inferior quality CRGO sheets amongst good one.

The displacement reaction technique was used to produce a nanostructured core/shell FeCo/Cu composed of FeCo core and copper (Cu) shell. The formation of the core-shell structure and thickness of the Cu layer is established and determined by analyzing the X-ray diffraction (XRD) pattern. The transmission electron microscope and selected area diffraction pattern analysis corroborate the results of the XRD studies. Magnetic force microscopic studies reveal that the FeCo/Cu particle shows single-domain characteristics. Oxidation studies indicate that Cu serves as a protective layer and provides a better oxidation resistance to the FeCo core even at high temperatures. Heat-treated FeCo/Cu particles show the possibility of tuning the magnetic properties of this core-shell structure to fit specific application requirements.

The compositional effect of magneto-elastic and magnetostriction properties of Fe-rich Fe81B15−xPxSi2Nb1Cu1 (ii) Fe82B14−xPxSi2Nb1Cu1 and (iii) Fe83B13−xPxSi2Nb1Cu1 (x = 0, 4, 8) amorphous and annealed nanocrystalline alloy ribbons were investigated. The present study adds knowledge to the limited magnetostriction literature available for Fe-rich nanocrystalline alloys by systematically varying the Fe and P content. A combination of Becker-Kersten and small angle magnetization rotation (SAMR) techniques has been employed for the magnetostriction (λs) evaluation. Both the as-quenched and nanocrystalline ribbons exhibit large positive magnetostriction and show strong compositional dependence to the P content. In the as-quenched condition, 4 at% P addition shows maximum magneto-elastic response and magnetostriction constant, with Fe81B11P4Si2Nb1Cu1 alloy exhibiting a maximum of + 52 ppm and P-free Fe83B13Si2Nb1Cu1 alloy exhibiting a minimum of + 27 ppm. In the nanocrystalline state, a slight reduction of magnetostriction is seen for all alloys, with a maximum of + 32 ppm (4 at% P) and a minimum of + 22 ppm (P-free) in Fe83 at% alloys. The unusual large magnetostriction of optimally annealed samples is attributed to the relatively low crystal volume fraction (30–45%) of nanocrystalline ribbons. The lowest magnetostriction of Fe83B13Si2Nb1Cu1 alloy in both as-quenched and annealed state is explained based on ribbon structural heterogeneity consisting of crystal nuclei and textured α-Fe surface crystallization. The study reveals a contradictory response of magneto-crystal anisotropy (grain size reduction) and magneto-elastic anisotropy to the P addition and ribbon structural heterogeneity. The study discusses the implications of the large magneto-elastic anisotropy associated with Fe-rich nanocrystalline ribbons and the way forward for improving their magnetic softness.