Severe Plastic Deformation Driven Nanostructure and Phase Evolution in a Dual-phase Al0.5 CoCrFeMnNi High Entropy Alloy

Tummala, Sharath Reddy and Bhattacharjee, Pinaki Prasad (2017) Severe Plastic Deformation Driven Nanostructure and Phase Evolution in a Dual-phase Al0.5 CoCrFeMnNi High Entropy Alloy. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

The effect of severe plastic deformation on phase stability and microstructural evolution in a dual - phase Al 0.5 CoCrFeMnNi high entropy alloy ( HEA ) was investigated. For this purpose, the alloy was initially subjected to a series of thermo - mechanial processing steps consisting of warm - rolling and annealing to get the desired starting microstructure for further processing . The HEA discs (diameter = 10 mm; thickness = 1mm) were deformed by h igh Pressure t orsion (HPT) technique up to 5 complete rotations under an applied pressure of 6 GPa. The microstructures of the deformed and annealed samples were investigated using electron backsc atter diffraction (EBSD) and transmission electron microscopy (TEM) techniques, while the mechanical properties were evaluated using hardness and tensile testing. TEM analysis of the starting HEA showed the dual phase microstructure consisting of a mixtu re of FCC and B2 phases. The chemical composition analysis showed that the B2 phase was enriched in Ni and Al while the converse was true for the FCC phase . The HPT processed samples revealed the formation of nano - sized FCC grains and dispersion of the B2 phase inside the FCC matrix. The B2 phase was found to maintain the ordered structure even af ter 5 rotations . Hardness of the HPT deformed discs increased with increasing number of torsional rotations. The hardness homogeneity measured along the diameter o f the discs increased with increasing deformation. Careful TEM analysis of the samples annealed at 800 ° C for 1 hour (h) showed the formation of a recrystallized microduplex structure and precipitation of the σ phase enriched in Cr and Fe. Hardness results of the 800 ° C/1 h annealed sample indicated marked increase in hardness inhomogeneity along the diameter of the sample due to the formation of the σ phase. The 5 rotation HPT sample showed enhanced strength but limited ductility. Thus, σ phase formation adversely impacted the tensile ductility of the HEA. As the annealing temperature increased, the volume fraction of the σ phase was reduced. Thus, h igher ductility at the expe nse of strength was observed due to decreased σ phase fraction and increased annealed grain size

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