Shape Memory Effect and Superelasticity in [001] Single Crystals of Fe–Ni–Co–Al–Nb(B) Ferromagnetic Alloy

Abstract

Shape memory effect (SME) and superelasticity (SE) during thermoelastic martensitic transformation (MT) from the FCC high-temperature γ-phase to the BCT α'-martensite are investigated in Fe – 28% Ni – 17% Co – 11.5% Al – 2.5% Nb (Nb) and Fe – 28% Ni – 17% Co – 11.5% Al – 2.5% Nb – 0.05% B (NbB) (at.%) single crystals oriented for tension along the [001] direction after aging at 973 K for 10 h. Non-equiaxial (NiAl) β-phase particles with thickness d and length l equal to 60–80 and 340–500 nm, respectively, and volume fraction f ≥ 3–5% are precipitated in Nb crystals during aging simultaneously with the (FeNiCo)3(AlNb) γ´-phase with sizes d = 12.5–16.5 nm. It is shown that precipitation of the β-phase with f ≤ 3–5% in the crystal volume does not reduce the crystal plasticity, and SME of 4.2% and SE up to 6.5% under loading are observed during thermoelastic γ–α′ MT in single crystals in a wide range of temperatures from 77 to 293 K. The β-phase is not detected in NbB crystals during aging. It is established that boron in NbB crystals slows down the aging processes: the γ′-phase particles have sizes 6.5–8 nm. The SME of 4.2% and SE up to 4.0% are observed in NbB crystals at temperatures from 77 to 243 K.