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

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.

Атомное разупорядочение и ОЦК -> ГЦК превращение в сплаве Гейслера Ni-=SUB=-54-=/SUB=-Mn-=SUB=-20-=/SUB=-Fe-=SUB=-1-=/SUB=-Ga-=SUB=-25-=/SUB=-, подвергнутом мегапластической деформации кручением под высоким давлением

Heusler L21 alloy Ni54Mn20Fe1Ga25 subjected to megaplastic deformation by high pressure torsion was first systematically investigated by in situ phase x-ray diffraction, transmission and scanning electron microscopy. It is established that shear deformation by torsion at room temperature grinds the polycrystalline structure of the alloy to a nanocrystalline and partially amorphous state. It is found that as the pressure value (from 3 to 5 GPa) and the degree of deformation (from 2 to 5 revolutions) increases, total atomic disordering and stepwise structural-phase transformation according to the scheme B2(BCC)→A2(BCC)→A1(FCC) occur. It is shown that annealing at temperatures below 570 K entails devitrification of the amorphous phase, and at 620 K and above – the restoration of the L21 structure. The dimensional effect of suppressing thermoelastic martensitic transformation in a nanostructured austenitic L21 alloy with a grain size less than 80 nm while its cooling down to 120 K. The ability to thermoelastic martensitic transformation and shape memory effect in submicrocrystalline ultrafine alloy after recrystallization annealing at temperatures exceeding 600 K is restored.

Влияние режима старения в мартенситном состоянии под сжимающей нагрузкой на двусторонний эффект памяти формы в монокристаллах ферромагнитного сплава Ni-=SUB=-49-=/SUB=-Fe-=SUB=-18-=/SUB=-Ga-=SUB=-27-=/SUB=-Co-=SUB=-6-=/SUB=-

The effects of stress-induced martensite aging (SIM-aging) along the [110]B2-direction on the thermoelastic martensitic transformation in Ni49Fe18Ga27Co6 single crystals were investigated. It was experimentally established that the effective regime of SIM-aging (at T = 423 K, 1 h under a compressive stress 450 MPa) results in a stabilization of stress-induced martensite and inducing the tensile two-way shape memory effect with reversible strain of +9.0 (± 0.3) % along the [001]B2-direction which is the perpendicular to the SIM-aging axis. Maximum work output of Wmax = 0.14 J/g (1125 kJ/m3) that can be realized using the two-way shape memory effect was obtained.