Apu Sarkar

Microstructure and Texture Evolution in Interstitial-Free (IF) Steel Processed by Multi-Axial Forging

In this study, severe plastic deformation (SPD) of Ti-bearing interstitial-free steel was carried out by multi-axial forging (MAF) technique. The grain refinement achieved was comparable to that by other SPD techniques. A considerable heterogeneity was observed in the microstructure and texture. Texture of multi-axially forged steels has been evaluated and reported for the first time. The material exhibited a six-fold increase in the yield strength after four cycles of MAF.

Hot deformation behaviour of niobium in temperature range 700–1500°C

Abstract Niobium was hot deformed in vacuum in uniaxial compression to a true strain of 0·6 in the temperature range of 700–1500°C and the strain rate range of 10−3–10 s−1. Strain rate sensitivity was calculated from the compression tests data and mapped out in contour plots with the aim to optimise the hot workability of niobium. The domain of hot workability was identified in the temperature range of 1200–1500°C and strain rate range of 10−2–1 s−1. In this domain the strain rate sensitivity was ∼0·15, the stress exponent 7·5 and the activation energy 246 kJ mol−1. Microstructure of the deformed samples showed features of dynamic recrystallisation within the high strain rate sensitivity domain and features of flow instability in the regime of low strain rate sensitivity. Compared to a previous study on Nb–1Zr–0·1C alloy, Nb showed a lower flow stress and an optimum hot working domain at lower temperatures.

Activation Volume and Density of Mobile Dislocations in Plastically Deforming Zr-1pctSn-1pctNb-0.1pctFe Alloy

Stress relaxation and strain rate change tests were carried out in annealed Zr-1pctSn-1pctNb-0.1pctFe alloy at room temperature. The apparent and true activation volumes were determined and the results obtained from different techniques were compared. The stress dependence of activation volume was studied in a wide range of stresses, and an analysis was made in the light of the thermal activation theory. The results suggest that surmounting of the Peierls barrier is the rate-controlling factor in the room-temperature plasticity of the alloy. Dislocation exhaustion rate during initial stress relaxation is higher in the alloy.

Microstructure Evolution and Mechanical Behaviour of Severe Plastically Deformed Cu

The present study reports the microstructure evolution and mechanical behaviour of severe plastically deformed pure Cu under cryogenic conditions. The samples were severely deformed by cryo rolling upto 50%, 75% and 95% deformation. Microstructure evolution of cryo rolled samples has been characterized by using optical, TEM and EBSD technique. The rolled samples were heat treated at various temperatures so as to control the recrystallization in the severely deformed samples. The effect of recrystallization on the mechanical behaviour was investigated in detail by tensile testing. The EBSD analysis performed on 95% rolled + heat treated samples showed that partial recystallized microstructure demonstrate an optimum combination of strength and ductility in cryo rolled Cu.

Microstructural characterization of the Portevin-Le Chatelier band in an Al-Mg alloy by X-ray diffraction line profile analysis

Al–2.5% Mg alloy exhibits the Portevin–Le Chatelier (PLC) effect at room temperature for a wide range of strain rates. Tensile test has been carried out on a flat Al–2.5% Mg alloy sample at a strain rate of 3.7×10 −6 s −1 . The strain rate was chosen so that the type C PLC band appears in the sample. X-ray diffraction profile has been recorded from the gauge length portion of the deformed sample to investigate the microstructure of the PLC band. Analysis revealed that the dislocation density is much higher within the band compared to the undeformed sample even at small strain. The PLC band in this alloy possesses an equal fraction of screw and edge dislocations.

High Temperature Uniaxial Compression and Stress–Relaxation Behavior of India-Specific RAFM Steel

India-specific reduced activity ferritic martensitic steel (INRAFM), a modified 9Cr-1Mo grade, has been developed by India as its own structural material for fabrication of the Indian Test Blanket Module (TBM) to be installed in the International Thermonuclear Energy Reactor (ITER). The extensive study on mechanical and physical properties of this material has been currently going on for appraisal of this material before being put to use in the ITER. High temperature compression, stress–relaxation, and strain-rate change behavior of the INRAFM steel have been investigated. The optical microscopic and scanning electron microscopic characterizations were carried out to observe the microstructural changes that occur during uniaxial compressive deformation test. Comparable true plastic stress values at 300 °C and 500 °C and a high drop in true plastic stress at 600 °C were observed during the compression test. Stress–relaxation behaviors were investigated at 500 °C, 550 °C, and 600 °C at a strain rate of 10−3 s−1. The creep properties of the steel at different temperatures were predicted from the stress–relaxation test. The Norton’s stress exponent (n) was found to decrease with the increasing temperature. Using Bird–Mukherjee–Dorn relationship, the temperature-compensated normalized strain rate vs stress was plotted. The stress exponent (n) value of 10.05 was obtained from the normalized plot. The increasing nature of the strain rate sensitivity (m) with the test temperature was found from strain-rate change test. The low plastic stability with m ~ 0.06 was observed at 600 °C. The activation volume (V*) values were obtained in the range of 100 to 300 b3. By comparing the experimental values with the literature, the rate-controlling mechanisms at the thermally activated region of high temperature were found to be the nonconservative movement of jogged screw dislocations and thermal breaking of attractive junctions.

Texture evolution during hot deformation of Moly-TZM

Moly-TZM was deformed at constant strain rate of 1.0 s(-1) to investigate the high strain rate deformation behaviour by microstructural and stress response change within a temperature range of 1400-1700 degrees C. To correlate the deformation behaviour with orientational change, recrystallization and recovery of the material, the microstructural investigation was undertaken using scanning electron microscopy (SEM) of electron back scattered diffraction (EBSD). Depending on the grain size and orientation spread recrystallized grains were identified and texture was calculated. Change in grain boundary characteristics with increasing temperature was determined by the misorientation angle distribution for the deformed and recrystallized grains. Subgrain coalescence and increase in subgrain size with increasing temperature was observed, indicating recrystallization not only occurred from the nucleation of the dislocation free grains in grain boundaries but also from the subgrain rotation and merging of the subgrains by annihilation of the low angle grain boundaries. Detailed studies on the evolution of texture of recrystallized grains showed continuous increase in fiber texture in recrystallised grains, in contrast to a mixed fiber + for the deformed grains.

Effect of symmetrical and asymmetrical tilt grain boundaries on the tensile deformation of zirconium bicrystals: a MD-based study

The aim of this article was to study the effect of symmetrical as well as asymmetrical tilt grain boundaries on the tensile deformation of irradiated bicrystalline zirconium. Molecular statics-based simulations were performed to generate both types of tilt grain boundaries with [0001] and

Study on Growth and Instability of Sesame in North-Eastern Hill Region of India

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