Puspendu Sahu | Jadavpur University (original) (raw)
Papers by Puspendu Sahu
Vacuum, 2015
The present study investigates the microstructure and grain boundaries evolutions during recrysta... more The present study investigates the microstructure and grain boundaries evolutions during recrystallization of commercial FeCo-~7 wt.% V. The resulting microstructures were characterized using electron back scatter diffraction after annealing at different temperatures. At low temperatures (~400 C), where only recovery occurs during annealing, the fraction of low angle boundaries was increased. But, at high temperatures (~850 C), where recrystallization occurs during annealing, the fraction of high angle boundaries and low-S coincidence site lattice (CSL) boundaries (especially S3) were increased.
Physical Review Letters, Feb 1, 1997
High-pressure x-ray diffraction studies of the high temperature structural intermetallic, Ti3Al (... more High-pressure x-ray diffraction studies of the high temperature structural intermetallic, Ti3Al ( DO19 structure at NTP), were carried out with the motivation of finding clues for identifying the alloying elements for stabilizing the cubic L12 structure, which is expected to have better ductility. The present investigation revealed a structural transition from DO19 to DO24 structure in the pressure range 10-15 GPa. The geometrical relationship between these two structures and the possible mechanism of the transition have been elucidated. From this investigation follow two important results that may have significant bearing in the search for the cubic Ti3Al.
Microstructure and Texture in Steels, 2009
Page 1. 361 A. Haldar, S. Suwas, and D. Bhattacharjee (eds.), Microstructure and Texture in Steel... more Page 1. 361 A. Haldar, S. Suwas, and D. Bhattacharjee (eds.), Microstructure and Texture in Steels, © Springer 2009 Chapter 21 Evolution of Recrystallization Texture in AISI300 Series Austenitic Stainless Steels After Cold Rolling to Large Strain ...
A novel, low temperature synthesis technique is developed for fabrication of nanocrystalline CaCu... more A novel, low temperature synthesis technique is developed for fabrication of nanocrystalline CaCu 3 Ti 4 O 12 ceramic powders, using inexpensive and easily available reagents. Structural and microstructural characterization was undertaken by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The confirmation regarding the phase formation was done using the Reitveld analysis. The compound belongs to the cubic system with the lattice parameter a = 7.3985 Å, which agrees well with the values reported in the literature. The particles formed were spherical in shape, with the average size of 70 nm.
Controlled martensitic reversion annealing was applied to a heavily cold-worked metastable austen... more Controlled martensitic reversion annealing was applied to a heavily cold-worked metastable austenitic low-Ni Cr-Mn austenitic stainless steel (Type 201) to obtain different ultrafine austenite grain sizes to enhance the mechanical properties, which were then compared with the conventional coarse-grained steel. Characterization of the deformed and reversion annealed microstructures was performed by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and light and transmission electron microscopy (TEM). The steel with a reverted grain size $ 1.5 μm due to annealing at 800 1C for 10 s showed significant improvements in the mechanical properties with yield stress $ 800 MPa and tensile strength $ 1100 MPa, while the corresponding properties of its coarse grained counterpart were $ 450 MPa and $ 900 MPa, respectively. However, the fracture elongation of the reversion annealed steel was $ 50% as compared to $ 70% in the coarse grained steel. A further advantage is that the anisotropy of mechanical properties present in work-hardened steels also disappears during reversion annealing.
A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low str... more A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low strain rate tensile deformation of a Fe-27Mn-2.5Si-3.5Al austenitic steel at room temperature by using X-ray diffraction analyses and transmission electron microscopy (TEM) observations. The effective SFE of austenite increased with strain from 18to40mJmAˋ2,anditwasreasonedthatthiswastheresultoftheincreaseinstrainenergyofthestackingfaults(SFs)perunitareaduetoitsdependenceonthedislocationcharacteranddensity.Twinningwasobservedat218 to 40 mJ m À2 , and it was reasoned that this was the result of the increase in strain energy of the stacking faults (SFs) per unit area due to its dependence on the dislocation character and density. Twinning was observed at 2% strain and confirmed to occur by a glide mechanism of a 6 h1 2 1i Shockley partial dislocations, leading to the formation of overlapping intrinsic-extrinsic SF pairs representing a three-layer twin embryo, revealing periodic dislocation contrast in TEM. The early onset of twinning is attributed to the unusually low critical twinning stress of the steel, 18to40mJmAˋ2,anditwasreasonedthatthiswastheresultoftheincreaseinstrainenergyofthestackingfaults(SFs)perunitareaduetoitsdependenceonthedislocationcharacteranddensity.Twinningwasobservedat2200 MPa. In spite of twinning, the stacking fault probability (P sf ) of twinned austenite was remarkably low ($10 À4 ) at low strains, but increased moderately (to 10Aˋ3)uptofailurestrain.Attheemergenceoftwinning,thecorrespondingperfectdislocationdensitywaslow(10 À3 ) up to failure strain. At the emergence of twinning, the corresponding perfect dislocation density was low (10Aˋ3)uptofailurestrain.Attheemergenceoftwinning,thecorrespondingperfectdislocationdensitywaslow(10 14 m À2 ) but was well above the critical dislocation density required for twinning occurrence. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character from edge to screw type. The microstructural parameters of the steel estimated in direct or indirect relation to its SFE could explain its flow stress and strain hardening behavior.
Structure and microstructure evolution under various cooling rates of a wrought austenitic steel,... more Structure and microstructure evolution under various cooling rates of a wrought austenitic steel, Fe-26Mn-0.14C (composition in mass %), were studied by the Rietveld method of X-ray diffraction pattern fitting, grain boundary characterization by electron back-scattered diffraction (EBSD) and optical microscopy. Cooling rate, density of stacking faults, and austenite grain size and grain boundaries influence the observed fcc ! " hcp transformation and lead to significant anisotropic X-ray line broadening. Depending on the cooling conditions, the grain boundaries are misoriented at both lower and higher angles. In the "-martensites, the dominant planar fault is twins ($10 À3 ). The austenite grains were found to contain low to moderate density of stacking faults ($10 À4 -10 À3 ), which act as efficient nucleation sites of the "-martensites. Both X-ray and EBSD analyses estimated negligible twins in the austenite. Approximate average dislocation densities have been estimated and correlated with the grain structure. research papers J. Appl. Cryst. (2007). 40, 354-361 P. Sahu et al. Structure and microstructure evolution 357
A high-Mn Fe-26Mn-0.14C austenitic steel was hot deformed at 900 °C, 1000 °C, and 1100 °C with th... more A high-Mn Fe-26Mn-0.14C austenitic steel was hot deformed at 900 °C, 1000 °C, and 1100 °C with the strain rate of 5 s−1 followed by air cooling (AC) or water quenching (WQ). Deformation at 1100 °C results in the occurrence of dynamic recrystallization (DRX) that produces a fine grain size (∼10 μm), which was found to prevent the γ fcc → ε hcp transformation upon cooling. The transformation
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 -620 °C for different durations between 0.1 -600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α'-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the preexisting α'-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.
Physica B: Condensed Matter, 2013
A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr 2 ErNbO 6 (SEN)... more A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr 2 ErNbO 6 (SEN) is synthesized by the solid state reaction technique. Rietveld analysis of X-ray diffraction pattern reveals that SEN has monoclinic phase with space group P2 1 /n. Fourier transform infrared spectrum shows two phonon modes of the sample due to the antisymmetric NbO 6 stretching vibration. The compound shows significant frequency dispersion in its dielectric properties. The frequency dependent ac conductivity spectra follow the Jonscher's universal power law. The conductivity at 100 Hz varies from 1.38 Â 10 −6 Sm −1 to 2.34 Â 10 −4 Sm −1 with the increase of temperature from 303 K to 653 K respectively. The increase of conductivity can be explained from the Correlated barrier hopping (CBH) model. The relaxation mechanism of the sample in the framework of electric modulus formalism is modelled by Davidson-Cole equation. The activation energy of the sample, calculated from both conductivity and modulus spectra is found to be $ 0.8 eV which indicates that the conduction mechanism for SEN is due to ion hopping. The scaling behavior of imaginary electric modulus suggests that the relaxation describes the same mechanism at various temperatures.
Physica E: Low-dimensional Systems and Nanostructures, 2007
Nanocrystalline nickel ferrite (NiFe 2 O 4 ) is synthesized at room temperature by high-energy ba... more Nanocrystalline nickel ferrite (NiFe 2 O 4 ) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and a-Fe 2 O 3 powders. The structural and microstructural evolution of NiFe 2 O 4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and a-Fe 2 O 3 phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe 2 O 4 is formed within 1 h of ball milling. Particle sizes of all the phases reduce to $10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe 2 O 4 phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis. r
Philosophical Magazine Letters, 2008
Metallurgical and Materials Transactions A, 2014
personal use only and shall not be self-archived in electronic repositories. If you wish to selfa... more personal use only and shall not be self-archived in electronic repositories. If you wish to selfarchive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com".
Metallurgical and Materials Transactions A, 2012
A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation a... more A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), a¢-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to e-martensite. However, a small fraction (~4 pct) of a¢-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of e-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m 2 because of the increase in temperature during the cold rolling deformation.
Metallurgical and Materials Transactions A, 2007
The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C... more The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C has been studied using the Rietveld entire X-ray diffraction (XRD) pattern fitting technique. The decomposition of austenite was observed to be very sensitive to the cooling rate. A very high proportion (49 to 70 pct) of ε-martensites was found in the differently cooled specimens as well in the as-cast specimen. The density of various planar (stacking and twin) faults was also estimated in both ε-martensites and retained austenite. The significant variation in the extent of gamma fcc--> ε hcp martensitic transformation was interpreted in terms of athermal and isothermal martensites formed during cooling and grain size of austenite. The austenite grains were found to be comprised of a high to moderate density of stacking faults (˜10-3 to 10-2) and negligible twin fault probability (˜10-5). On the other hand, in the ε-martensites, the dominant planar fault was twins (˜10-3). Approximate values of the dislocation density within the retained austenite were also evaluated using the crystallite size and root-mean-square (rms) strain values obtained from the ``size-strain-shape'' analyses.
Metallurgical and Materials Transactions A, 2008
Materials Science and Engineering: A, 2002
... Permissions & Reprints. Microstructural characterization of FeMnC martensites a... more ... Permissions & Reprints. Microstructural characterization of FeMnC martensites athermally transformed at low temperature by Rietveld method. P. Sahu a , M. De Corresponding Author Contact Information , E-mail The Corresponding Author , a , S. and Kajiwara b. ...
Materials Science and Engineering: A, 2006
Materials Chemistry and Physics, 2011
a b s t r a c t X-ray diffraction studies employing the Rietveld analysis is reported on the infl... more a b s t r a c t X-ray diffraction studies employing the Rietveld analysis is reported on the influence of increasing Co substitution on the structural and microstructural evolution in AB 2 O 4 type spinel ferrites: Ni 1−x Co x Fe 2 O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The specimens were synthesized by the organic precursor method. Structure refinement reveals that the pure NiFe 2 O 4 is not an exact inverse spinel and about 11% of Ni 2+ ions occupy the tetrahedral (A) sites. Increasing Co concentrations had the effect of increasing the ratio of [Fe 3+ ] B /[Fe 3+ ] A and gradual expansion of the ferrite unit cell. The microstructure refinement estimates that the particle size values are in the order of ∼nm, ranging from 31 to 61 nm, which gradually increase with increasing Co doping accompanied by almost negligible lattice micro strains (∼10 −4 ). The corresponding particle size distribution for each specimen was obtained from the X-ray diffraction data from the basic assumption that the spherical nanoparticles follow the log-normal distribution. The size distribution for the pure NiFe 2 O 4 was also estimated from transmission electron microscope and agreed well with that obtained by the diffraction data analysis.
Materials Chemistry and Physics, 2003
Transformation reaction induced due to ball-milling of iron oxide, ␣-Fe 2 O 3 in both air and oxy... more Transformation reaction induced due to ball-milling of iron oxide, ␣-Fe 2 O 3 in both air and oxygen atmospheres under closed milling condition has been studied for detailed characterization of the microstructure of all the evoluted phases on milling up to 10 h. The methodology adopted for characterization involves Rietveld's whole X-ray profile fitting technique adopting the most recently developed software, material analysis using diffraction (MAUD), which incorporates Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s. strain). The analysis also considers lattice defect related features of the microstructure, viz. stacking, twin, compound fault density and dislocation density parameters. The study also undertakes quantitative estimation of volume fractions of the phases evoluted (Fe 3 O 4 : Fd-3m:1 and FeO: Fm-3m). The results reveal transformation of ␣-Fe 2 O 3 to Fe 3 O 4 and finally to FeO occurs in both air and oxygen atmospheres, and the reaction speed is slower in oxygen environment. The reaction is controlled by oxygen partial pressure, which decreases on continued milling. A critical oxygen partial pressure is reached at 3-4 h of milling, when Fe 3 O 4 phase attains maximum saturation with nano-order (7-8 nm) crystallite sizes, reduced r.m.s. strain and high dislocation density (∼10 12 cm −2 ). Prolonged milling (7-10 h) results in further reduction of oxygen partial pressure, resulting in complete transformation of ␣-Fe 2 O 3 and Fe 3 O 4 to FeO, having nano-order (6-7 nm) crystallite sizes, high r.m.s. strain (∼10 −2 ) and high dislocation density values (∼10 12 cm −2 ) in both the environments, except that the transformation reaction is slowed down in oxygen.
Vacuum, 2015
The present study investigates the microstructure and grain boundaries evolutions during recrysta... more The present study investigates the microstructure and grain boundaries evolutions during recrystallization of commercial FeCo-~7 wt.% V. The resulting microstructures were characterized using electron back scatter diffraction after annealing at different temperatures. At low temperatures (~400 C), where only recovery occurs during annealing, the fraction of low angle boundaries was increased. But, at high temperatures (~850 C), where recrystallization occurs during annealing, the fraction of high angle boundaries and low-S coincidence site lattice (CSL) boundaries (especially S3) were increased.
Physical Review Letters, Feb 1, 1997
High-pressure x-ray diffraction studies of the high temperature structural intermetallic, Ti3Al (... more High-pressure x-ray diffraction studies of the high temperature structural intermetallic, Ti3Al ( DO19 structure at NTP), were carried out with the motivation of finding clues for identifying the alloying elements for stabilizing the cubic L12 structure, which is expected to have better ductility. The present investigation revealed a structural transition from DO19 to DO24 structure in the pressure range 10-15 GPa. The geometrical relationship between these two structures and the possible mechanism of the transition have been elucidated. From this investigation follow two important results that may have significant bearing in the search for the cubic Ti3Al.
Microstructure and Texture in Steels, 2009
Page 1. 361 A. Haldar, S. Suwas, and D. Bhattacharjee (eds.), Microstructure and Texture in Steel... more Page 1. 361 A. Haldar, S. Suwas, and D. Bhattacharjee (eds.), Microstructure and Texture in Steels, © Springer 2009 Chapter 21 Evolution of Recrystallization Texture in AISI300 Series Austenitic Stainless Steels After Cold Rolling to Large Strain ...
A novel, low temperature synthesis technique is developed for fabrication of nanocrystalline CaCu... more A novel, low temperature synthesis technique is developed for fabrication of nanocrystalline CaCu 3 Ti 4 O 12 ceramic powders, using inexpensive and easily available reagents. Structural and microstructural characterization was undertaken by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The confirmation regarding the phase formation was done using the Reitveld analysis. The compound belongs to the cubic system with the lattice parameter a = 7.3985 Å, which agrees well with the values reported in the literature. The particles formed were spherical in shape, with the average size of 70 nm.
Controlled martensitic reversion annealing was applied to a heavily cold-worked metastable austen... more Controlled martensitic reversion annealing was applied to a heavily cold-worked metastable austenitic low-Ni Cr-Mn austenitic stainless steel (Type 201) to obtain different ultrafine austenite grain sizes to enhance the mechanical properties, which were then compared with the conventional coarse-grained steel. Characterization of the deformed and reversion annealed microstructures was performed by electron back scattered diffraction (EBSD), X-ray diffraction (XRD) and light and transmission electron microscopy (TEM). The steel with a reverted grain size $ 1.5 μm due to annealing at 800 1C for 10 s showed significant improvements in the mechanical properties with yield stress $ 800 MPa and tensile strength $ 1100 MPa, while the corresponding properties of its coarse grained counterpart were $ 450 MPa and $ 900 MPa, respectively. However, the fracture elongation of the reversion annealed steel was $ 50% as compared to $ 70% in the coarse grained steel. A further advantage is that the anisotropy of mechanical properties present in work-hardened steels also disappears during reversion annealing.
A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low str... more A deformation-dependent stacking fault energy (SFE) viewpoint is invoked to interpret the low strain rate tensile deformation of a Fe-27Mn-2.5Si-3.5Al austenitic steel at room temperature by using X-ray diffraction analyses and transmission electron microscopy (TEM) observations. The effective SFE of austenite increased with strain from 18to40mJmAˋ2,anditwasreasonedthatthiswastheresultoftheincreaseinstrainenergyofthestackingfaults(SFs)perunitareaduetoitsdependenceonthedislocationcharacteranddensity.Twinningwasobservedat218 to 40 mJ m À2 , and it was reasoned that this was the result of the increase in strain energy of the stacking faults (SFs) per unit area due to its dependence on the dislocation character and density. Twinning was observed at 2% strain and confirmed to occur by a glide mechanism of a 6 h1 2 1i Shockley partial dislocations, leading to the formation of overlapping intrinsic-extrinsic SF pairs representing a three-layer twin embryo, revealing periodic dislocation contrast in TEM. The early onset of twinning is attributed to the unusually low critical twinning stress of the steel, 18to40mJmAˋ2,anditwasreasonedthatthiswastheresultoftheincreaseinstrainenergyofthestackingfaults(SFs)perunitareaduetoitsdependenceonthedislocationcharacteranddensity.Twinningwasobservedat2200 MPa. In spite of twinning, the stacking fault probability (P sf ) of twinned austenite was remarkably low ($10 À4 ) at low strains, but increased moderately (to 10Aˋ3)uptofailurestrain.Attheemergenceoftwinning,thecorrespondingperfectdislocationdensitywaslow(10 À3 ) up to failure strain. At the emergence of twinning, the corresponding perfect dislocation density was low (10Aˋ3)uptofailurestrain.Attheemergenceoftwinning,thecorrespondingperfectdislocationdensitywaslow(10 14 m À2 ) but was well above the critical dislocation density required for twinning occurrence. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character from edge to screw type. The microstructural parameters of the steel estimated in direct or indirect relation to its SFE could explain its flow stress and strain hardening behavior.
Structure and microstructure evolution under various cooling rates of a wrought austenitic steel,... more Structure and microstructure evolution under various cooling rates of a wrought austenitic steel, Fe-26Mn-0.14C (composition in mass %), were studied by the Rietveld method of X-ray diffraction pattern fitting, grain boundary characterization by electron back-scattered diffraction (EBSD) and optical microscopy. Cooling rate, density of stacking faults, and austenite grain size and grain boundaries influence the observed fcc ! " hcp transformation and lead to significant anisotropic X-ray line broadening. Depending on the cooling conditions, the grain boundaries are misoriented at both lower and higher angles. In the "-martensites, the dominant planar fault is twins ($10 À3 ). The austenite grains were found to contain low to moderate density of stacking faults ($10 À4 -10 À3 ), which act as efficient nucleation sites of the "-martensites. Both X-ray and EBSD analyses estimated negligible twins in the austenite. Approximate average dislocation densities have been estimated and correlated with the grain structure. research papers J. Appl. Cryst. (2007). 40, 354-361 P. Sahu et al. Structure and microstructure evolution 357
A high-Mn Fe-26Mn-0.14C austenitic steel was hot deformed at 900 °C, 1000 °C, and 1100 °C with th... more A high-Mn Fe-26Mn-0.14C austenitic steel was hot deformed at 900 °C, 1000 °C, and 1100 °C with the strain rate of 5 s−1 followed by air cooling (AC) or water quenching (WQ). Deformation at 1100 °C results in the occurrence of dynamic recrystallization (DRX) that produces a fine grain size (∼10 μm), which was found to prevent the γ fcc → ε hcp transformation upon cooling. The transformation
Materials Science Forum, 2013
The influences of the heating rate and annealing duration on martensite formation and its reversi... more The influences of the heating rate and annealing duration on martensite formation and its reversion to austenite have been investigated in a 60% cold-rolled metastable high-manganese austenitic Type 204Cu stainless steel. A Gleeble 3800 thermomechanical simulator was used for dilatometric measurements. Cold-rolled steel pieces were either heated up to 1000 °C at various heating rates between 5 °C/s and 150 °C/s followed by quenching to room temperature, or heated and held at temperatures in the range of 450 -620 °C for different durations between 0.1 -600 s. In heating experiments, dilatation curves revealed an expansion of a specimen starting around 550 °C followed by contraction, both processes depending on the heating rate. These dimensional changes could be correlated to the formation and reversion of a ferromagnetic phase, α'-martensite. Some martensite was also formed during isothermal holding in connection with tempering of the preexisting α'-martensite before the following reversion, as established by magnetic measurements. Tempering of martensite was revealed by microhardness behaviour, X-ray diffraction analysis and transmission electron microscopy.
Physica B: Condensed Matter, 2013
A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr 2 ErNbO 6 (SEN)... more A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr 2 ErNbO 6 (SEN) is synthesized by the solid state reaction technique. Rietveld analysis of X-ray diffraction pattern reveals that SEN has monoclinic phase with space group P2 1 /n. Fourier transform infrared spectrum shows two phonon modes of the sample due to the antisymmetric NbO 6 stretching vibration. The compound shows significant frequency dispersion in its dielectric properties. The frequency dependent ac conductivity spectra follow the Jonscher's universal power law. The conductivity at 100 Hz varies from 1.38 Â 10 −6 Sm −1 to 2.34 Â 10 −4 Sm −1 with the increase of temperature from 303 K to 653 K respectively. The increase of conductivity can be explained from the Correlated barrier hopping (CBH) model. The relaxation mechanism of the sample in the framework of electric modulus formalism is modelled by Davidson-Cole equation. The activation energy of the sample, calculated from both conductivity and modulus spectra is found to be $ 0.8 eV which indicates that the conduction mechanism for SEN is due to ion hopping. The scaling behavior of imaginary electric modulus suggests that the relaxation describes the same mechanism at various temperatures.
Physica E: Low-dimensional Systems and Nanostructures, 2007
Nanocrystalline nickel ferrite (NiFe 2 O 4 ) is synthesized at room temperature by high-energy ba... more Nanocrystalline nickel ferrite (NiFe 2 O 4 ) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and a-Fe 2 O 3 powders. The structural and microstructural evolution of NiFe 2 O 4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and a-Fe 2 O 3 phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe 2 O 4 is formed within 1 h of ball milling. Particle sizes of all the phases reduce to $10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe 2 O 4 phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis. r
Philosophical Magazine Letters, 2008
Metallurgical and Materials Transactions A, 2014
personal use only and shall not be self-archived in electronic repositories. If you wish to selfa... more personal use only and shall not be self-archived in electronic repositories. If you wish to selfarchive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com".
Metallurgical and Materials Transactions A, 2012
A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation a... more A high-Mn austenitic steel was deformed in cold rolling to study the martensitic transformation and microstructure using X-ray diffraction and electron backscatter diffraction. Despite heavy deformation of 70 pct reduction (1.2 true strain), a¢-martensite could not be induced in this alloy, but about 90 pct of the austenite transformed to e-martensite. However, a small fraction (~4 pct) of a¢-martensite could be observed when the same alloy was subjected to low strain compression tests in a Gleeble simulator. The stability of e-martensite was attributed to the increase in stacking fault energy of the steel, expected to be more than 20 mJ/m 2 because of the increase in temperature during the cold rolling deformation.
Metallurgical and Materials Transactions A, 2007
The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C... more The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C has been studied using the Rietveld entire X-ray diffraction (XRD) pattern fitting technique. The decomposition of austenite was observed to be very sensitive to the cooling rate. A very high proportion (49 to 70 pct) of ε-martensites was found in the differently cooled specimens as well in the as-cast specimen. The density of various planar (stacking and twin) faults was also estimated in both ε-martensites and retained austenite. The significant variation in the extent of gamma fcc--> ε hcp martensitic transformation was interpreted in terms of athermal and isothermal martensites formed during cooling and grain size of austenite. The austenite grains were found to be comprised of a high to moderate density of stacking faults (˜10-3 to 10-2) and negligible twin fault probability (˜10-5). On the other hand, in the ε-martensites, the dominant planar fault was twins (˜10-3). Approximate values of the dislocation density within the retained austenite were also evaluated using the crystallite size and root-mean-square (rms) strain values obtained from the ``size-strain-shape'' analyses.
Metallurgical and Materials Transactions A, 2008
Materials Science and Engineering: A, 2002
... Permissions & Reprints. Microstructural characterization of FeMnC martensites a... more ... Permissions & Reprints. Microstructural characterization of FeMnC martensites athermally transformed at low temperature by Rietveld method. P. Sahu a , M. De Corresponding Author Contact Information , E-mail The Corresponding Author , a , S. and Kajiwara b. ...
Materials Science and Engineering: A, 2006
Materials Chemistry and Physics, 2011
a b s t r a c t X-ray diffraction studies employing the Rietveld analysis is reported on the infl... more a b s t r a c t X-ray diffraction studies employing the Rietveld analysis is reported on the influence of increasing Co substitution on the structural and microstructural evolution in AB 2 O 4 type spinel ferrites: Ni 1−x Co x Fe 2 O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0). The specimens were synthesized by the organic precursor method. Structure refinement reveals that the pure NiFe 2 O 4 is not an exact inverse spinel and about 11% of Ni 2+ ions occupy the tetrahedral (A) sites. Increasing Co concentrations had the effect of increasing the ratio of [Fe 3+ ] B /[Fe 3+ ] A and gradual expansion of the ferrite unit cell. The microstructure refinement estimates that the particle size values are in the order of ∼nm, ranging from 31 to 61 nm, which gradually increase with increasing Co doping accompanied by almost negligible lattice micro strains (∼10 −4 ). The corresponding particle size distribution for each specimen was obtained from the X-ray diffraction data from the basic assumption that the spherical nanoparticles follow the log-normal distribution. The size distribution for the pure NiFe 2 O 4 was also estimated from transmission electron microscope and agreed well with that obtained by the diffraction data analysis.
Materials Chemistry and Physics, 2003
Transformation reaction induced due to ball-milling of iron oxide, ␣-Fe 2 O 3 in both air and oxy... more Transformation reaction induced due to ball-milling of iron oxide, ␣-Fe 2 O 3 in both air and oxygen atmospheres under closed milling condition has been studied for detailed characterization of the microstructure of all the evoluted phases on milling up to 10 h. The methodology adopted for characterization involves Rietveld's whole X-ray profile fitting technique adopting the most recently developed software, material analysis using diffraction (MAUD), which incorporates Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s. strain). The analysis also considers lattice defect related features of the microstructure, viz. stacking, twin, compound fault density and dislocation density parameters. The study also undertakes quantitative estimation of volume fractions of the phases evoluted (Fe 3 O 4 : Fd-3m:1 and FeO: Fm-3m). The results reveal transformation of ␣-Fe 2 O 3 to Fe 3 O 4 and finally to FeO occurs in both air and oxygen atmospheres, and the reaction speed is slower in oxygen environment. The reaction is controlled by oxygen partial pressure, which decreases on continued milling. A critical oxygen partial pressure is reached at 3-4 h of milling, when Fe 3 O 4 phase attains maximum saturation with nano-order (7-8 nm) crystallite sizes, reduced r.m.s. strain and high dislocation density (∼10 12 cm −2 ). Prolonged milling (7-10 h) results in further reduction of oxygen partial pressure, resulting in complete transformation of ␣-Fe 2 O 3 and Fe 3 O 4 to FeO, having nano-order (6-7 nm) crystallite sizes, high r.m.s. strain (∼10 −2 ) and high dislocation density values (∼10 12 cm −2 ) in both the environments, except that the transformation reaction is slowed down in oxygen.