Showing papers by "Srinivasa R. Bakshi published in 2018"

Microstructure and mechanical properties of oxide dispersion strengthened 18Cr-ferritic steel consolidated by spark plasma sintering

Abstract: This paper presents the results of an experimental study on evolution of the nanocrystalline microstructure in a mechanically alloyed Oxide dispersion strengthened (ODS) 18Cr ferritic steel powder during densification by spark plasma sintering (SPS) in the temperature range of 1273 K (1000 °C) to 1423 K (1150 °C). Systematic Electron Back-Scatter Diffraction analysis has been carried out to study the grain size distribution and texture as a function of consolidation temperatures. Based on the kinetics of the densification process and resultant microstructure/microtexture, a sintering temperature slightly above 1323 K (1050 °C) within a range of 50 K was found to be optimum. The 18Cr-ferritic steel powder consolidated at 1323 K (1050 °C) was also studied to understand the role of dispersoids on microstructure. The dispersoids exerted a profound influence on the strength as well as toughness of the steel by restricting the grain growth at high temperatures. Further, a signature of (1 1 0) grain cluster is observed during consolidation and its preferential growth with increase in sintering temperature is noticed which lead to the alignment of the (1 1 0) plane in the direction of applied pressure. The minimum creep rate of the consolidated steel under a load of 300 MPa was found to be 5E-7 h−1 and 1E-4 h−1 at 873 and 973 K (600 and 700 °C) respectively. The apparent activation energy for creep deformation was estimated as ~ 402 kJ/mol, which is typical of lattice diffusion assisted general climb mechanism of dislocations over the barriers such as present dispersoids.

Spark Plasma Sintering Process as a Tool for Achieving Microstructural Integrity

Abstract: The fact that real materials are not perfect crystals, is critical to materials engineering as the presence of crystalline defects is the most important feature of the microstructure which influences the mechanical properties. Exposure of materials to high temperature for long duration would result in structural failure, when a sub-size crack grows into a critical level. This paper concentrates on the novel application of spark plasma sintering (SPS) technique in achieving microstructural integrity of materials by crack closure using the superior capability of SPS for annealing the defects in materials. Due to the presence of applied compressive stress, expansion is restricted and brings about the closure of cracks. The crack surfaces then come in contact with each other, and energization between the crack surfaces causes them to bond. The presentation would bring about the nature of bonding achieved through SPS when two model systems, diffusion bonding of stainless steel discs and stainless steel with ferroboron powder, were considered and highlighted its applicability through systematic optimization.

Microstructure and Mechanical Properties of NiTiCuFe Multi-component Alloy

Abstract: NiTiCuFe multi-component alloy was synthesized using vacuum induction melting under argon atmosphere. As-cast structure shows a mixture of phases with FCC as the major phase. Microstructure of the alloy exhibits coarser non-equi-axed dendritic grains with spike-shaped inter-dendritic phases. Scanning electron image with an energy-dispersive X-ray spectrograph line scan taken across dendrites shows Cu segregation in the inter-dendritic region and other elements in the dendritic region. The result shows the presence of Fe2Ti Laves phase. Also the major phase is observed to be a Cu–Ni FCC phase. XRD analysis also confirms the presence of Fe2Ti Laves phase and Cu–Ni FCC phase. The presence of phases is confirmed using a transmission electron microscope (FEI Tecnai T20) operating at 200 kV. The alloy also exhibits good mechanical properties.