Mechanistic understanding of speciated oxide growth in high entropy alloys

Complex multi-element alloys are gaining prominence for structural applications, supplementing steels, and superalloys. Understanding the impact of each element on alloy surfaces due to oxidation is vital in maintaining material integrity. This study investigates oxidation mechanisms in these alloys using a model five-element equiatomic CoCrFeNiMn alloy, in a controlled oxygen environment. The oxidation-induced surface changes correlate with each element's interactive tendencies with the environment, guided by thermodynamics. Initial oxidation stages follow atomic size and redox potential, with the latter becoming dominant over time, causing composition inversion. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to elucidate the oxidation process and surface oxide structure evolution. Our findings deconvolute the mechanism for compositional and structural changes in the oxide film and will pave the way for a predictive design of complex alloys with improved resistance to oxidation under extreme conditions

Texture transition in friction stir processed Al powder compact

Green Al powder compacts of commercial purity with random orientation were subjected to single pass frictionstir processing (FSP) with different tool rotational and traverse speeds. The evolution of crystallographic textureobtained from large area electron back scattered diffraction were compared with the bulk texture of the nuggetzone characterized using synchrotron diffraction. Evolution of different deformation and recrystallization texturecomponents were discussed. While the grain size distributions were found to be independent of processparameters, the texture components and their strength of the FSPed samples were strongly influenced by theprocess parameters. Continuous dynamic recrystallization (CDRx) was found to be the primary restorationmechanism for most of the processing conditions leading to a bi-modal misorientation distribution. The possiblerelations between different texture components and the appearance or suppression of bimodal misorientationdistributions were discussed. Restoration mechanism changed to discontinuous dynamic recrystallization(DDRx) with the evolution of cube component at the stir zone along with random misorientation distribution.Dominance of a particular restoration mechanism depends on strain, strain rate and temperature attained duringthe processing

High cycle fatigue performance, crack growth and failure mechanisms of an ultrafine-grained Nb+Ti stabilized, low-C microalloyed steel processed by multiphase controlled rolling and forging

Abstract An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been correlated with microstructural features characterized by light optical (LOM), transmission electron (TEM) and scanning electron microscopy (SEM), aided with electron backscatter diffraction (EBSD). TEM analysis near the fracture zones of the fatigue tested samples and corresponding fractographic analysis corroborated well in explaining the improved fatigue life of the SG steel. The fatigue strength was found to have a linear relationship with the tensile strength in both types of processed samples. The fatigue strength of the forged specimens was estimated to be nearly twice than that of the untreated annealed steel, demonstrating significantly different fracture characteristics. Intergranular fracture is found to be dominant in the rolled/forged specimens, in comparison to the transgranular fracture observed in the as-received steel. Such variances in fatigue strength and fracture characteristics have been endorsed to their microstructural constituents. Superior combinations of yield strength (YS), tensile strength (UTS), elongation (% El.) and high cycle fatigue strength (σf) (YS = 1027 MPa, %El. = 8.3%, σf = 355 MPa) were obtained in multiphase-controlled 15-cycle multiaxially forged (MAFed) specimens (processed in intercritical α+γ phase regime). An enhancement of the fatigue strength can be ascribed to the formation of evenly dispersed nano-sized fragmented cementite (Fe₃C) particles (~35 nm size) present in the SG ferritic matrix (average ~280 nm size). The fine dislocation substructures/cells together with the nano-sized Fe₃C particles could efficiently block the initiation and propagation of cracks thereby enhancing the fatigue endurance limit of the steel. Superior mechanical properties together with high fatigue resistance in the SG material render the present steels highly beneficial for high-strength structural applications