Search

In the present study the superelasticity (SE) under tension/compression was investigated in the oligocrystals of Fe42.5Mn34Al15Ni7.5Ti1 (1% Ti), Fe42Mn34Al15Ni7.5Ti1.5 (1.5% Ti) and Fe41.5Mn34Al15Ni7.5Ti2 (2% Ti) (at %) alloys undergoing α (bcc)–γ′ (fcc) martensitic transformation. It was optically established that, first, with a growth in the Ti concentration from 1 to 2% the average and maximum grain sizes increase. Second, the optimal Ti concentration necessary to suppress the precipitation of the γ-phase particles during quenching is no more than 1.5%. SE strain studied at room temperature depends on the Ti concentration, heat treatment and stress state—tension/compression. In the quenched state the SE with value of 6% was observed only in 1% Ti oligocrystal under compression. Quenched oligocrystals was brittle and broke at the yield stress under tension. Aging at 473 K for 3 h leads to an increase in the SE value to 6.8% in 1% Ti oligo-crystal under compression, observation of the SE of 3.1% in 1.5% Ti oligocrystal and suppression of brittle fracture at the yield stress under tension. The SE strain in aged 1% Ti and 1.5% Ti oligocrystals under tension was eSE(1% Ti)=1.2% and eSE(1.5% Ti)=2%, respectively. SE was not found in a 2% Ti oligocrystal under tension/compression.

The effect of γ′-phase particles on the mechanical behaviour was studied in single crystals of the fcc CoCrFeNiAl0.3 (at %) high-entropy alloy (HEA) under tensile strain. It was shown that ageing of the CoCrFeNiAl0.3 HEA single crystals at a temperature of 823 K for 150 hours and at 893 K for 50 hours, results in the precipitation of γ′-phase particles of size d < 5 nm, with L12-type ordering. In the [1¯11]-oriented crystals, precipitation of γ′-phase particles leads to an increase in critical shear stresses τcr by 25–30 MPa in the temperature range T = 77–873 K, a decrease in the strain hardening coefficient Θ = dσ/de and an increase in plasticity of 11–14% relative to the initial crystals without particles. The decrease in Θ = dσ/de and the increase in plasticity are associated with the influence of the γ′-phase particles on the localization of the deformation, predominantly in one system.

For [001]-oriented single crystals of Fe-28Ni-17Co-11.5Al-2.5Ti (at %) alloy in tension, it was shown that particles of the γ’-phase with a size of 4–6 nm lead to the appearance of superelasticity, with a strain of eSE=4.5% and a shape memory effect of eSME=5.9%. Increasing the particle size to 10–12 nm reduces eSE to 3.6% and eSME to 3.5%. The paper discusses the causes of the influence of particle size on the reversible strain.