Shape Memory Effect and Superelasticity of [001]-Oriented FeNiCoAlNb Single Crystals Aged under and without Stress

Abstract

The two-step ageing of Fe-28Ni-17Co-11.5Al-2.5Nb (at. %) single crystals under and without stress, leads to the precipitation of the γ′- and β-phase particles. Research has shown that γ–α′ thermoelastic martensitic transformation (MT), with shape memory effect (SME) and superelasticity (SE), develops in the [001]-oriented crystals under tension. SE was observed within the range from the temperature of the start of MT upon cooling Ms, to the temperature of the end of the reverse MT upon heating Af, and at temperatures from Af to 323–373 K. It was found that at γ–α′ MT in the [001]-oriented crystals, with γ′- and β-phase particles, a high level of elastic energy, ΔGel, is generated, which significantly exceeds the energy dissipation, ΔGdis. As a result, the temperature of the start of the reverse MT, while heating As, became lower than the temperature Ms. The development of γ–α′ MT under stress occurs with high values of the transformation hardening coefficient, Θ = dσ/dε from 2 to 8 GPa and low values of mechanical Δσ and thermal ΔTh hysteresis. The reasons for an increase in ΔGel during the development of γ–α′ MT under stress are discussed.

Microstructure and Superelastic Properties of FeNiCoAlTi Single Crystals with the <100> Orientation under Tension

“…Recently, FeNiCoAlXB (X: Ta, Nb, Ti) alloys have been reported to have greater than 4% superelastic strain [1-3] and 1.6% shape memory strain [4], and single crystals have exhibited at least a 5% recoverable strain [5][6][7][8]. The martensitic transformation (MT) of the FeNiCoAlXB (X: Ta, Nb, Ti) alloy system is face-centered cubic (fcc) (austenite) to bodycentered tetragonal (bct) (martensite).…”

“…Previous aging heat-treatment studies of FeNiCoAlXB (X: Ta, Nb, Ti) alloys have reported that the optical aging temperatures of these alloys, to obtain superelastic properties, ranged from 600 to 700 • C [1][2][3][4][5][6][7][8]. Tanaka et al [1] found that Fe 40.95 Ni 28 Co 17 Al 11.5 Ta 2.5 B 0.05 (at.%) alloys aged at 600 • C for 72 h show a 13.5% superelastic strain at room temperature, and the diameter of the precipitate was 3 nm.…”

“…FeNiCoAlTi single crystals with <100> orientation aged at 600 • C for 4 h possessed 6% superelastic strain at −80 • C. The precipitate size was around 5 nm. Chumlyakov et al [7] used a two-stage aging heat treatment (600 • C for 4 h and 600 • C for 2 h) and achieved a 3% superelastic strain at −100 • C. In previous studies, good superelastic properties could only be obtained at low temperatures. When the test temperature was at room temperature, the superelastic strain was deteriorated due to very low transformation temperatures [5,6,9].…”

Shape Memory Properties and Microstructure of New Iron-Based FeNiCoAlTiNb Shape Memory Alloys

“…Tseng et al [23] found that using Ti to replace Ta could reduce the aging heat treatment times. A FeNiCoAlTi single crystal with <100> orientation aged at 600 • C for 4 h possessed 6% superelastic strain when the test temperature was at −80 • C. Chumlyakov et al [36][37][38] showed that adding Nb to this alloy could increase the hardness of the austenite matrix and improve their superelastic properties. FeNiCoAlNb single crystals with <100> orientation in two-step aging (700 • C-0.5 h + 700 • C-3 h) demonstrated 7% superelastic strain in tension and 13.5% in compression.…”

“…FeNiCoAlNb single crystals with <100> orientation in two-step aging (700 • C-0.5 h + 700 • C-3 h) demonstrated 7% superelastic strain in tension and 13.5% in compression. The precipitates size ranged from 5 to 8 nm [38].…”