Abstract :
Effectof one-step strain annealing on grain boundary charact er distribution and boundarynetwork topology in equiatomic CoCrFeMnNi high entropy alloy and its effect on aqueouscorrosion

It is well known that grain boundaries play a dominant role ininterfacial controlled properties such as creep, corrosion and oxidation.Approaches to improving the properties are through alloying or throughmicrostructural engineering. In recent times high entropy alloys are beingdeveloped on the premise that increasing the number of components will resultin sluggish diffusion kinetics. However, recent results on CoCrFeMnNi system donot seem to support the above premise and enhanced diffusivity has beenobserved in a multicomponent system. On the other hand, grain boundaryengineering (GBE) is an alternative approach and in this case the compositionneed not be altered. GBE has been shown to enhance the properties of themat erials. In the present work, one-step strain annealing has been used toachieve GBE microstructure in an equiatomic CoCrFeMnNi alloy withan fcc structure. Cast and homogenised samples are cold rolled to reductions of5%, 10% and 15% and annealed at 1173 K, 1223K, 1273 K, 1323 K and 1373 K for 1hto 6h duration. Electron Channeling Contrast Imaging (ECCI) is used to studydislocation substructure in the as-rolled condition. The microstructures followingprocessing were characterised using electron backscattered diffraction (EBSD)in scanning electron microscope. The EBSD data was analysed to evaluate grainsize, special boundary fraction, triple-junction distribution and twin relateddomain (TRD) statistics such as TRD size, length of longest chain (LLC) andnumber of grains in TRD. The optimum processing parameters were identifiedbased on the GBE parameters. Potentio-dynamicpolarization and electrical impedance spectroscopy were performed to study the effectof GBE microstructure on passivation behavior and reducing localized attack in0.1 M and 0.6 M NaCl solution. Following cold rolling to 5% reduction andannealing at 1223 K for 1h, GBE microstructure was obtained. In GBE microstructure, the coincident site lattice (CSL)boundaries fraction is enhanced from (53 ± 1 )% to (71 ± 1 )% with simultaneousincrease in triple triple junctions containing 2 or 3 CSL boundaries from (20 ± 1)% to(40 ± 2 )% . In potentiodynamic polarization, GBE sample exhibited relativelynoble potential with respect of AR sample annealed at the same temperature in0.1 M and 0.6 M saline solution. A small passive region of narrow range (57 ± 6mV ) exhibiting metastable pitting is observed in GBE sample polarized at 0.1 M saline solution