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The effect of two-stage ageing in a stress-free state and under tensile stress on the precipitation processes of dispersed particles, temperature range of γ–α′ martensitic transformations (MT), superelasticity and shape memory effect was investigated on [001]-oriented single crystals of the Fe–28% Ni–17% Co–11.5% Al–2.5% TiNb (at %) alloy. It was shown that two-stage ageing leads to simultaneous precipitation of γ′- and β-phase particles. β-phase particles were not found to inhibit the γ–α′ MT and led to the appearance of a tensile two-way shape memory effect (TWSME), which was 3.8% in stress-assisted aged crystals. The reasons for the appearance of a TWSME were discussed.

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.

Two-way shape memory effect (TWSME) and its stability under cooling/heating cycling through the martensitic transformation (MT) temperatures in stress-free condition were investigated in mono-phase [001]-oriented single crystals of the ferromagnetic Co49Ni21Ga30 (at %) alloy. TWSME is induced by thermomechanical training– cooling under different external compressive stress and subsequent stress-free heating. It is shown that the TWSME strain eTWSME depends on the applied stress in the training cycle. The maximum of the eTWSME equal to 4.2(±0.2)% is achieved after training under σext=125 MPa and then decreases with increase of σext. The value of eTWSME = 4.2(±0.2)% is equal to the theoretical transformation strain for [001]-orientation at B2-L10 MT in compression. In crystals after training under σext=125 MPa with maximum eTWSME, the stability of the TWSME strain and martensitic start temperature Ms is observed up to 150 cooling/heating cycling. From 150 to 300 thermocycle, TWSME strain decreases from 4.1(±0.2)% to 3.5(±0.2)%, and the Ms temperature decreases from 251 to 242 K.

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.

Multiferroic meta-magnetic shape memory alloys are well known for exhibiting large magnetic field induced actuation strains, giant magnetocaloric effects, magneto-resistance, and structural and magnetic glassy behaviors. Thus, they are candidates for improving modern day sensing, actuation, magneto-resistance, and solid-state refrigeration processes. Until now, however, experimental apparatuses have typically been able to probe a limited ferroic parameter space in these materials, i.e., only concurrent thermal and mechanical responses, or magnetic and thermal responses. To overcome this barrier and better understand the coupling of multiple fields on materials behavior, a magneto-thermo-mechanical characterization device has been designed and implemented. This device is capable of compressing a specimen at load levels up to 5300 N collinearly with applied fields up to 9 T between temperatures of −100 °C and 120 °C. Uniaxial stress, strain, temperature, magnetic field, and the volumetric average magnetization have been simultaneously measured under mixed loading conditions on a NiCoMnIn meta-magnetic shape memory alloy and a few selected results are presented here.