Papers by Author: Dinesh Srivastava

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Authors: Nachiket Keskar, Vikrant Raizada, Vaibhaw Kumar, Sanjay Kumar Jha, Dinesh Srivastava, Gautam Kumar Dey, Nudurupati Saibaba
Abstract: The hot extrusion of Zr-2.5Nb alloy has been simulated using an FEM based software and validated by the experimental measurements during real extrusion process. The flow stress data for Zr-2.5Nb alloy at different temperatures, strain and strain rates were generated by hot compression testing on Gleeble Machine. The flow stress data, obtained both below β transus temperature in two phase α+β region as well as above β transus temperature in β phase region, were directly used in the FEM simulation of extrusion process, eliminating the need for any assumptions regarding material plastic flow law. Two important extrusion parameters, viz. included die angle and reduction ratio, were varied and for each set of conditions, temperature, strain and strain rate at different nodal points of the tube at different stages of extrusion were calculated. The extrusion parameters were optimized to obtain minimum variation in the force, temperature, strain and strain rate in the extruded product. A fair agreement has been found between the measured values of the applied ram force and the temperature profile at the die landing area in a 3780 ton horizontal extrusion press and those obtained through simulations. The effect of the friction and heat transfer coefficient between the die chamber and work piece was also investigated in this study.
Authors: G. Jyoti, R. Tewari, K.D. Joshi, Dinesh Srivastava, G.K. Dey, S.C. Gupta, S.K. Sikka, S. Banerjee
Abstract: In the present study, specimens of Zr were subjected to shock compression of 11.6 GPa. TEM examination of the recovered samples revealed that during shock compression the a phase has transformed into the w phase. The orientation relationships (ORs) between the a and w phase have been determined using both the stereographic projection method and the correspondence matrix method. Our ORs have been found to belong to the Variant I OR given by Usikov and Zilbershtein (UZ) for statically compressed Zr samples. Our ORs are the same as the one reported by Song and Gray (SG) on dynamically compressed samples. In the present paper it has been shown that the OR of SG is a subset of the OR of UZ and is not apart from it. The mechanism of a®w transition with respect to occurrence of an intermediate b (bcc) structure, during the transition has also been explored. We also show in this study that the amount of the transformed w phase decreases with increasing oxygen content in the samples that were shock loaded to the same peak pressure, as is revealed by both the TEM and XRD results.
Authors: A.P. Srivastava, Dinesh Srivastava, K.G. Suresh, G.K. Dey
Abstract: Microstructure and magnetic properties of a nanocrystalline soft magnetic material having composition Co64.5 Fe3.5 Si16.5 B13 Ni1.5 Cu1 has been studied. Amorphous ribbon could be produced by melt spinning unit. DSC analysis showed four distinct crystallization events. Heat treated samples were characterized using XRD and TEM techniques. Co2B, and CoB phases were found to crystallize before magnetic phase a−Co. Addition of copper was proved to have adverse effect on soft magnetic properties.
Authors: Arijit Lodh, Indradev Samajdar, Raghvendra Tewari, Dinesh Srivastava, Gautam Kumar Dey, Prita Pant
Abstract: The present study deals with nanoindentation behavior of commercial Zircaloy 2 and high purity (5N purity) crystal bar Zirconium. The effect of crystal orientation was studied through high resolution electron diffraction, while a finite element model was developed to extract yield strength and strain hardening exponent from nanoindentation data. The study brings in clear signatures of orientation dependence of mechanical properties in hexagonal Zirconium.
Authors: Karri V. Mani Krishna, Sudipto Mandal, Ankur Agrawal, Vijay Hiwarkar, Dinesh Srivastava, Indradev Samajdar, Gautam Kumar Dey
Abstract: Grain boundary texture evolution in case of two of the Zr based alloys (Zircaloy-4 and Zr-2.5\%Nb) was studied. In case of Zircaloy-4, grain boundary texture evolution during $\beta$ $\to$ $\alpha$ phase transformation was monitored. Direct evidence of variant selection during this transformation is presented. In case of Zr-2.5\%Nb alloy, considerable increase in $\alpha/\beta$ interfaces following Burger's orientation relationship was noticed with increasing annealing time at 700 \textdegree{}C.
Authors: Supratik Roychowdhury, Vivekanand Kain, M. Gupta, S. Neogy, Dinesh Srivastava, G.K. Dey, R.C. Prasad
Abstract: Intergranular Stress Corrosion Cracking (IGSCC) of austenitic Stainless Steels (SS) in Boiling Water Reactor (BWR) environment is generic in nature in both the sensitised and the non-sensitized conditions. IGSCC in non-sensitized austenitic SS in the strain hardened condition has been reported without any grain boundary chromium depletion or impurity segregation. The present study ascertains the reason for IGSCC in BWR environment in non-sensitized condition and investigates the effect of nitrogen content in SS on the susceptibility to IGSCC. Two heats of type 304LN stainless steel with 0.08 and 0.16 wt. % nitrogen were used. Strain hardening was done by cross rolling at 200 °C to 20 % thickness reduction (warm rolling) to simulate the weld induced strain in constrained welds. Subsequently, Transmission Electron Microscopic (TEM) examination was carried out on the rolled SS. The deformation mode observed due to warm rolling was predominantly elongated twin and shear band (SB) formation in both the SS, terminating at the grain boundary regions. This resulted in higher stresses and strains making grain boundary susceptible to IGSCC. Presence of more dislocations at grain boundaries is a key feature for such enhancement in the susceptibility of non-sensitized SS to IGSCC. Formation of twins and SB occurred to a greater extent in the SS with higher nitrogen content indicating greater susceptibility to IGSCC in BWR environment. Crack growth studies done in simulated BWR environment at different Dissolved Oxygen (DO) levels showed higher crack growth rates in the SS with higher nitrogen content in the non-sensitised and strain hardened condition, confirming the higher susceptibility of SS with a higher level of nitrogen.
Authors: A.P. Srivastava, Dinesh Srivastava, K.G. Suresh, G.K. Dey
Abstract: Effect of copper addition in a Metallic glass 2714A on the nanocrystallization characteristics have been examined in this study. Amorphous ribbon of the alloy composition Co64.5 Fe3.5 Si16.5 B13.5 Ni1Cu1 were prepared by melt spinning technique. Nanocrystallization kinetics was studied using differential scanning calorimeter technique. The kinetic parameters such as activation energy and Avrami exponent were determined using two different non-isothermal analysis methods. The kinetic behavior of individual crystallization event has been rationalized on the basis of these results. The role of addition of copper on the crystallization behavior has been understood by comparing with Metallic glass 2714A. The isothermally annealed nanocrystallized microstructures were characterized by X-ray diffraction.
Authors: Santosh Kumar Sahoo, V. D. Hiwarkar, Prita Pant, Indradev Samajdar, Karri V. Mani Krishna, Gautam Kumar Dey, Dinesh Srivastava, R. Tewari, S. Banerjee
Abstract: The present study deals with deformation behaviour of textured Zircaloy 2 with two dominant orientations: basal and non-basal. During initial stages (20%), two distinct class of grains were observed – non-deforming/non-fragmenting grains and deforming/fragmenting grains. The so-called non- deforming/non-fragmenting grains remain equiaxed even after 50% of deformation. They also have insignificant in-grain misorientation developments and have more residual stresses. Dislocation dynamics simulation showed that the dislocation interactions/mobility is insignificant in basal orientations at room temperature deformations.
Authors: A.P. Srivastava, M. Srinivas, S. Sharma, Dinesh Srivastava, B. Majumdar, P.K. Pujari, G.K. Dey, K.G. Suresh
Abstract: Amorphous ribbons of composition Fe68.5Cu1Nb3Si18.5B9 were produced by melt spun unit. Positron annihilation technique along with DSC and XRD studies has been employed to characterize the nanocrystallization process. XRD results confirmed presence of Fe3Si and Fe2B phases. Two life time components could be fitted to life time spectra of amorphous and heat treated samples. Life time of positron in amorphous matrix was found to be 163.3 ps. Small life time components in nanocrystallized samples could be ascribed to positron annihilation within amorphous and nanocrystalline particles. Larger life time component could be attributed to positron annihilation in interfaces associated with primary and secondary phase particles.
Authors: Dinesh Srivastava, G.K. Dey, S. Banerjee
Abstract: The morphology and substructure of martensite is considered to arise from the lattice invariant shear (LIS) associated with the transformation and this may be slip, twinning or both. Out of the several possible slip shears and twin modes only a few satisfy invariant plane strain criteria of the phenomenological theory of martensite (PTMC). On the basis of crystallographic and energetic criteria, a simple model has been proposed for determining the factors which influence the selection of the preferred LIS mode. In the present work, it is found that for b ® a' martensitic transformation in Zr-2.5 wt%Nb alloy, the preferred slip system is {1101}a'<2113>a' and the preferred twin system is {1101}a'<415 3>a'.
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