Rubber-like behaviour and superelasticity of [001]-oriented FeNiCoAlNb single crystals containing γ′- and β-phase particles

The paper reports the effect of one-step and two-step ageing of the [0011-oriented Fe-28Ni-17Co-11.5Al-2.5Nb (FeNiCoAlNb) (at%) single crystals on the precipitate of gamma'- and beta-phase particles, M-s temperature and reversible deformation under tensile strain, within a wide temperature range from 77 to 373 K. It was shown that during one-step ageing (973 K for 5 h) in FeNiCoAlNb single crystals, gamma'-phase particles with L1(2)-type ordering are precipitated. Superelasticity (SE) of 4-5%, associated with the development of gamma-alpha' martensitic transformation (MT) under stress, is realized within the temperature range from 77 to 263 K. Two-step ageing (973K for 5 h and then 873 K for 2 h) results in a structure containing gamma'- and beta- (with B2-type ordering) phase particles. The precipitation of beta-phase particles increases M-s temperature up to 190 K, however, M-s temperature remains below 77 K, when only the gamma'-phase is precipitated. In the [001]-oriented crystals with a structure containing gamma'- and beta-phase particles, a rubber-like behaviour (RLB) of 1.4% at 77 K was found, which was associated with a change in the fine structure of the cooling martensite in the load-unloading cycle. For the first time, at tension in the [001]-oriented FeNiCoAlNb crystals with a structure containing two types of particles in the temperature range M-s < T< A(f) and from A(f) temperature up to 348 K, SE of 3-3.8% was found, due to the development of gamma-alpha' MT under stress. RLB and a wide SE range from M-s=190-348 K are related to the achievement of a high level of elastic energy Delta G(el) during gamma-alpha' MT and low values of the dissipated energy Delta G(diss) in the [001]-oriented FeNiCoAlNb crystals, with a structure containing gamma'- and beta- phase particles. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

The research demonstrates that one-step ageing leads to the formation of gamma'-phase particles in FeNiCoAlNb single crystals, while two-step ageing results in a structure containing both gamma'- and beta-phase particles. Different ageing methods affect the M-s temperature and reversible deformation, thus influencing the mechanical properties of the material.