Papers by Author: Rajesh Prasad

Abstract: Development of bulk ultra-fine grained (UFG) materials by severe plastic deformation to attain improved mechanical properties is becoming more attractive and extensively studied nowadays. Equal channel angular extrusion (ECAE) is one of technique used effectively for obtaining bulk UFG materials. Novelty of this technique is one can build up significant amount of plastic strain by increasing the number of passes without much dimensional change. In present investigation dynamic recrystallization at deformation zones around the non-shearable second phase particles in Al-Li based alloy processed by ECAE is reported. Transmission electron microscopy technique involving imaging the regions of such deformation zones with different specimen tilt conditions is used. It is shown that the dynamic recrystallisation occurring in the proximities of second phase particles during the deformation at room temperature, leads to very fine grained microstructure. Observation of multiple active nucleation sites around even sub-micrometer sized non-deformable particles in the as-processed material indicates that the system exhibits efficiency >1 based on the concept of particle stimulated nucleation (PSN). Crystallites of ultra-fine/nanocrystalline size ranges are formed in the deformation zones around the non-deformable particles during deformation itself. Effect of short term post deformation annealing to understand the recovery and recrystallization was undertaken. Based on these results effect of optimal post deformation heat treatment conditions on the thermal stability of the microstructures is emphasized. It is suggested that with significant fraction of non-shearable particles it might be possible to get grain size in the nanocrystalline or ultra-fine range with relatively low effective strain levels using ECAE.

Abstract: In present study microstructural evolution during swaging on aluminium alloy based metal matrix composite (MMC) reinforced with 15% volume fraction silicon carbide (SiC) particles is presented. Samples were swaged at room temperature in steps with reducing die dimensions using rotary swaging technique. SEM and TEM are used to study the microstructural characteristics of swaged samples. SEM observations were made to understand the flow and deformation characteristics of deforming aluminium matrix in the presence of second phase and reinforced SiC particles during swaging. TEM observations on swaged samples confirmed the formation of ultra-fine grains in Al-15%SiC MMC. It is shown that the dynamic recrystallisation occurring in the proximities of second phase particles during the deformation at room temperature, leads to very fine grained microstructure.

Abstract: Equal channel angular extrusion (ECAE), involving intense plastic straining under high applied pressure is generally recognized and extensively studied top down approach for producing bulk ultra-fine grained (UFG) metallic materials, and even going down in size to the nanometer range. In this research efforts are made to identify conditional under which grains with size less than 100 nm form after ECAE. Evolution of microstructure of Al-Li based alloy processed by ECAE is analyzed using transmission electron microscopy (TEM). Observations on the effect of precipitates/second phase particles in the sample on the deformation characteristics and their role on the increased degree of grain fragmentation process is highlighted. Samples of Al-Li based alloy are solutionized, quenched and aged at different temperatures to obtain well formed precipitate laths/plates before subjecting to ECAE. During the deformation process these precipitates disintegrate into fragments and get dispersed into the Al matrix. The fragments of a few nanometers size bring about drastic changes in the flow as well as the recovery characteristics of processed samples. Evidence for dynamic recrystallisation taking place during the ECAE processing is presented. It was observed that optimal thermal treatment leads to more effective grain refinement and consequently an ultra-fine grained microstructure could be achieved even after single pass in Al-Li based alloy containing precipitates and second phase particles.