Papers by Author: G.K. Dey

Abstract: Hot workability of Zr-2.5Nb-0.5Cu alloy has been investigated by means of hot compression test using Gleeble-3800^®, in the temperature and strain rate range of 700 to 925°C and 0.01-10s^-1, respectively. Deformation behavior was characterized in terms of flow instability using peak stress with the help of Lyapunov Function. The true stress-strain curves shows that softening occurs at all lower temperature and for entire strain rates of deformation. The instable flow was suggested by negative m value at deformation condition of 700°C (5 and 10 s^-1), while s value at 925°C (10 s^-1). The combined result of rate of change of m and s with respect to log strain rate suggest that the deformation condition ranges from 725-780°C (10^-2- 10^-1 s^-1) and 700°C (1-10 s^-1) representing safe domain for stable flow.

Abstract: The Indian PHWR uses Zr-2.5% Nb pressure tubes and its in-reactor performance mainly irradiation creep and growth depends strongly on its microstructure. A detailed microstructural examination was carried out on unirradiated pressure tubes off-cuts and an irradiated pressure tube S-07 of KAPS-2 (operated for 8 effective full power years (EFPYs)), Microstructural characterization was carried out using transmission electron microscopy. Microstructual observation of un-irradiated off-cuts shows the lamellar morphology of the -Zr along with the -phase present as stingers between two alpha laths as well as fine and coarse beta globules. The size of -Zr lamellae was found to be in the range from 0.17 to 0.2 m, 1.8 to 2.4 m and 1.7 to 2.8 m in the radial, circumferential and axial direction respectively (aspect ratio of 1:7:8). TEM-EDS analysis showed composition of the  phase tin the range of 15-50 wt%Nb. The irradiated pressure tube samples obtained from 13 locations were showing average alpha grain width, grain length and aspect ratio in the range of 0.17-0.27 micron, 1.7-2.3 micron and 7.1-8.5 respectively. Extensive modification in beta morphology could be seen at the high flux and high temperature regions. The  phase was observed to have globulised completely in many regions. They were present at the interface of -Zr laths as well as within the lath. The Nb concentration of the  phase appeared to have increased as the volume fraction had reduced. The microstructure details of irradiated and un-irradiated pressure tubes obtained in this study is expected to help in modeling the dimensional change occurring during irradiation in reactor.

Abstract: Thermal aging embrittlement of type 304L stainless steel weld is investigated on the basis of changes in microstructure, microhardness and electrochemical behavior after aging up to 20,000 h at 335, 365 and 400 °C. Spinodal decomposition and G-phase precipitation in the ferrite was observed after thermal aging. Aging led to increase in the hardness of ferrite phase while there was no change in the hardness of austenite. The changes in electrochemical behavior due to aging were studied using double loop electrochemical potentiokinetic reactivation (DL-EPR) test. Aging led to increase in the DL-EPR value which is attributed to Cr depletion in the ferrite phase.

Abstract: In this study a systematic characterization of the microstructural and textural evolution in each thermo-mechanical processing step of Zircaloy-4 rod fabrication has been carried out. The possible micro-mechanisms leading to the observed microstructural evolutions have been discussed. The thermo-mechanical steps followed resulted in a completely recrystallized microstructure and retention of the hot-extruded texture in the finished product.

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.

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.

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.

Abstract: Recent studies indicate that the existing criteria for amorphous phase formation are not precise. This study emphasizes that the extent of deviation from the linear relationship of the chemical contribution to enthalpy of the solid solution with the enthalpy difference between the amorphous phase and the constituent elements is a measure of tendency to bypass amorphous phase formation. Mechanical alloying experiments on a number of compositions in Al-Ni-Ti system showed that the compositions, not following the aforesaid relation, yield partial or no amorphization.

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'.

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.