Microstructure evolution and strengthening mechanisms in Ni36Co30Cr11Fe11Al12-Nb high entropy alloys

To reveal the microstructure evolution and the strengthening mechanism, the Ni36Co30Cr11Fe11Al12-xNbx (x = 2, 4, 6, 8, 10 at%) high entropy alloys (HEAs) were designed. Phase composition, phase prediction, tensile properties and strengthening mechanisms were researched in detail. Results show that the alloys consist of face-center cubic (FCC) phases and Laves phases. The regularity of several factors, such as electronegativity difference and d-orbital energy level, predicted that the volume fraction of Laves phase is increased with the increase of Nb/Al ratio. The yield strength and the strain of Ni36Co30Cr11Fe11Al8Nb4 HEAs are 670 MPa and 16.5%, respectively. The alloy shows the typical hypoeutectic structure. The effects of solid solution strengthening and the interfacial strengthening were analyzed. When the Nb content> 6 at%, the effect of the interfacial strengthening is stronger compared with solid solution strengthening The FCC-Laves interface shows the higher barrier strength. The micrographs of dislocation structures show that the extensive dislocation pileups appear at the incoherent interface. The incoherent interface may shear easily and attract gliding dislocations due to its low shear strength, which explains the sharply decrease of the ductility.