Showing papers by "Bhagwati Prasad Kashyap published in 2001"
TL;DR: In this article, a tensile test of AA 8090 Al-Li Alloy was performed at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers.
Abstract: Superplastic forming grade sheets of AA 8090 Al–Li alloy were observed to contain layers of different microstructure and microtexture across the thickness cross-section. Superplastic behaviour and its relationship to the concurrent microstructural and microtextural evolution of this sheet were studied at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123}〈634〉 type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}〈211〉 texture. The stress–strain rate (σ–e) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers were in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the...
17 citations
TL;DR: In this article, the effect of microstructural evolution on superplastic deformation parameters, such as the nature of σ-e plots, strain-rate sensitivity parameter, and activation energy, were studied for unstable and thermally stable microstructures of a Zr-2.5 wt pct Nb pressure-tube alloy.
Abstract: The effect of microstructural evolution on superplastic deformation parameters, such as the nature of σ-e plots, strain-rate sensitivity parameter, and activation energy, were studied for unstable and thermally stable microstructures of a Zr-2.5 wt pct Nb pressure-tube alloy. Two types of differential strain-rate tests (increasing temperature (IT) and decreasing temperature (DT), in the temperature range of 610 °C to 810 °C at 20 °C intervals) were conducted within a strain-rate range of 10−5 to 10−3 s−1. Single specimens were used to obtain the σ-e plots for all the test temperatures in the aforementioned temperature range. The effect of orientation (with respect to the axial direction of the tube) on the superplastic deformation parameters was also studied. The microstructural evolution was studied along the three orthogonal planes of the tube by water quenching underformed samples in the beginning of differential strain-rate tests at each test temperature. The observed apparent activation-energy values associated with deformation were in the two distinct ranges of 287 to 326 and 151 to 211 kJ/mole. In the temperature range of 730 °C to 810 °C, the apparent activation-energy value depended on the direction of approach of the test temperature. The mechanisms of superplastic deformation in this alloy were found to be dislocation climb—controlled creep in region III and grain-boundary sliding accommodated by grain-boundary diffusion or lattice diffusion in the α or β phases in region II. Based on the observed microstructural features, a model to explain the σ-e plots and apparent activation energy has been proposed.
13 citations
TL;DR: In this article, the results of a microtextural evolution study on AA 8090 Al-Li alloy were presented, showing that the surface layer was dominated by S {011}〈634〉 type texture, whereas the mid-thickness layer dominated by brass {011]〈211-type texture.
Abstract: A brief review of orientation imaging microscopy and an overview of the results of investigations related to textural evolution during superplastic deformation of a recent series of superplastic grade aluminium alloys, and specifically the results of the authors' study on microtextural evolution in AA 8090 Al–Li alloy, are presented. It was observed that the surface layer of the AA 8090 alloy sheet was dominated by S {011}〈634〉 type texture, whereas the midthickness layer was dominated by brass {011}〈211〉 type texture. In the course of superplastic deformation, textural weakening occurred, the rate and extent of which were different in the surface and centre regions, depending upon whether the two layers were deformed separately or as a composite in the full thickness tensile specimen. Textural weakening was generally more extensive at higher test temperatures and lower strain rates. Based on textural evolution and the presence of a large proportion of low angle boundaries, and their near eliminat...
12 citations
TL;DR: In this article, a simple instability condition based on the Ziegler's continuum principles is extended for delineating the regions of unstable metal flow during hot deformation of Nimonic AP-1 superalloy.
Abstract: A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of Nimonic AP-1 superalloy. The optimum hot working conditions for this material are suggested.
3 citations
Journal Article•
TL;DR: In this article, the authors examined the flow localization concepts as well as the simplified metallurgical stability criterion for delineating the regions of flow instabilities in the processing maps and suggested the optimum hot working conditions for the material.
Abstract: Using the test data of extra low interstitial (ELI) grade titanium alloy Ti-6Al-4V (wt.%) with microstructural observations, studies are made to examine the flow localization concepts as well as the simplified metallurgical stability criterion for delineating the regions of flow instabilities in the processing maps. The optimum hot working conditions for the material are suggested.
1 citations