Regularities of Martensitic Transformations in New Medium-Entropy Single Crystals of CoNiAlFe Alloys

Abstract

This paper deals with the martensitic transformation and functional properties in the quenched single crystals of the Co35Ni35Al28Fe2 medium-entropy alloy, oriented along the [001]B2-direction. The microstructure and chemical composition of the single crystals have been studied in detail using transmission and scanning electron microscopy. The {111}L10 martensite twins up to 10-20 nm width and γ/γ′-phase precipitations larger than 100 μm are detected. The thermoelastic B2-L10 martensitic transformation upon stress-free cooling/heating in single crystals of Co35Ni35Al28Fe2 alloy is characterized by the accumulation of elastic energy, which is the driving force of the reverse martensitic transformation, and the low dissipation energy. The reverse transformation starts at lower temperatures than the forward transformation Ms>As. The regularities of the stress-induced B2-L10 martensitic transformation change due to an increase in the contribution of the dissipated energy and Msσ<Asσ. There is shape memory effect with the reversible strain (3.2±0.3)% and high temperature superelasticity with the reversible strain (3.3±0.3)% in the temperature range from 323 K to ≥548 K in the [001]B2-oriented single crystals. These crystals withstand stress up to 1200 MPa in compression without destruction.