Shape memory effect of [111]-oriented Cr20Mn20Fe20Co35Ni5 high-entropy alloy single crystals

Shape memory effect (SME) governed by the γ–ε martensitic transformation (MT) was studied in the [111]-oriented Cr20Mn20Fe20Co35Ni5 high-entropy alloy crystals under tension. The dependence of SME on the conditions of isothermal and isobaric deformation was established. It was shown that the maximum reversible strain of 7.4% and SME of 6.8% were observed under conditions of isothermal deformation

Investigation of the orientation dependence of marforming on superelasticity and shape memory effect in equiatomic TiNi single crystals under compression

The shape memory effect (SME) and superelasticity (SE) after marforming (deformation in the martensitic phase at 203 K, followed by annealing at 713 K, 0.5 h) were studied in the equiatomic TiNi crystals under compression. Marforming was carried out along [0 0 1]B2 and [0 1 1]B2 directions of B2-phase after strain of ε = 1.5ε0 (ε0 is the lattice deformation, which depends on the crystal orientation). SME and SE after marforming were studied along [0 0 1]B2 direction. It was shown that the maximum stress level of B2-phase σcr(B2) = 750 MPa, the lowest values of mechanical Δσ = 100 MPa and thermal ΔTh = 32 K hysteresis and the maximum temperature range of SE ΔTSE = 77 K were observed when marforming under compression was realized along the [0 0 1]B2 direction

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, ∆Gel, is generated, which significantly exceeds the energy dissipation, ∆Gdis. 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 ∆σ and thermal ∆Th hysteresis. The reasons for an increase in ∆Gel during the development of γ–α' 0 MT under stress are discussed

Orientation dependence and tension-compression asymmetry of shape memory effect in Ti50Ni40Cu10 single crystals

Transformation strain under stress in Ti50Ni40Cu10 alloy single crystals, with a two-step B2–B19–B19′martensitic transformation (MT), was investigated along three different crystallographic orientations, i.e., [117], [011] and [123], in order to reveal the orientation dependence and tension-compression asymmetry of the shape memory effect (SME) at different stress levels. It was shown that the SME depends on the crystal orientation and the deformation method (tension and compression). The maximum value of the SME εSME under tensile stresses turned out to be higher than the theoretical transformation strain resource ε0. In [117]-oriented crystals, εSME = 6.1% exceeded ε0 = 3.4% by 2.7%; in [011]-oriented crystals εSME = 9.1% exceeded ε0 = 7.14% by 1.96%; in [123]-oriented crystals εSME = 8.2% exceeded ε0 = 7.22% by 0.98%. Higher experimental values for εSME than the theoretical transformation strain resource ε0 at B2–B19–B19′MT was found to be associated with elastic mechanical twinning along the {110}В19 planes of B19-martensite. Under compression, in the studied orientations, εSME was less than ε0 for the B2–B19–B19′MT. This was due to lower stresses in the initial B2-phase and local plastic flow during development of an MT under stress at stresses of more than 200 MPa

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 [001]-oriented Fe-28Ni-17Co-11.5Al- 2.5Nb (FeNiCoAlNb) (at%) single crystals on the precipitate of γ′- and β-phase particles, Ms 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, γ′-phase particles with L12- type ordering are precipitated. Superelasticity (SE) of 4–5%, associated with the development of γ-α' mar-tensitic transformation (MT) under stress, is realized within the temperature range from 77 to 263 K. Two- step ageing (973К for 5 h and then 873 К for 2 h) results in a structure containing γ′- and β- (with B2-type ordering) phase particles. The precipitation of β-phase particles increases Ms temperature up to 190 K, however, Ms temperature remains below 77 K, when only the γ′-phase is precipitated. In the [001]-oriented crystals with a structure containing γ′- and β-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 Ms < T < Af and from Af temperature up to 348 K, SE of 3–3.8% was found, due to the development of γ-α' MT under stress. RLB and a wide SE range from Ms=190–348 K are related to the achievement of a high level of elastic energy ΔGel during γ-α′ MT and low values of the dissipated energy ΔGdiss in the [001]-oriented FeNiCoAlNb crystals, with a structure con-taining γ′- and β- phase particles

High-Strength Behavior of the Al0.3CoCrFeNi High-Entropy Alloy Single Crystals

The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (&sigma;0.1) at a test temperature above room temperature. This restricts their practical application at high test temperatures from 773 K to 973 K. In this study, we found that a high stress level was reached at the yield point &sigma;0.1 &asymp; G/100&ndash;G/160 (G is the shear modulus) of the [001]- and [1&macr;44]-oriented crystals of the Co23.36Cr23.29Fe23.80Ni21.88Al7.67 (Al0.3CoCrFeNi) high-entropy alloy (HEA) within a wide temperature range of 77&ndash;973 K under tension, due to the occurrence, of nanotwins, multipoles, dislocations under plastic deformation at 77 K and the subsequent precipitation of ordered L12 and B2 particles. It was shown that grain boundaries are not formed and the samples remain in a single-crystal state after low-temperature deformation and subsequent ageing at 893 K for 50 h. Achieving a high-strength state in the Al0.3CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (&tau;cr) at T &ge; 200 K (&tau;cr[1&macr;44] &gt; &tau;cr[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of &sigma;0.1 above 573 K, and reduces plasticity to 5&ndash;13% in the [1&macr;44] orientation and 15&ndash;20% in the [001] orientation

Effect of the Surface Oxide Layer on Shape Memory Effect and Superelasticity of [011]-Oriented Ti-50.1Ni Single Crystals

Effect of the surface oxide layer on the shape memory effect (SME) and superelasticity (SE) after marforming (deformation in the martensitic state, followed by annealing at 713 K for 0.5 h in an inert helium gas and in dry air) was investigated on Ti-50.1Ni (at.%) single crystals, oriented along [011]-direction, under compression. Quenched [011]-oriented crystals of the Ti-50.1Ni alloy experience a one-stage B2-B19′ martensitic transformation (MT) without SE under compression. Marforming leads to a two-stage B2-R-B19′ MT and creates conditions for SE. A thin TiO2 oxide layer of 170 nm thick was formed on the sample surface upon annealing at 713 K for 0.5 h in dry air. In [011]-oriented crystals without and with an oxide layer, maximum of the SE value reached 4%, and the SME was 2.4 and 2.6%, respectively. Appearance of an oxide layer upon annealing in dry air: (i) reduces the stresses of B2-phase by 50 MPa from Md to 473 K; (ii) decreases Θ = dσ/dε from 6.5 GPa in crystals without an oxide layer to 2.0 GPa with an oxide layer and (iii) does not affect the SME and SE values

Effect of marforming on superelasticity and shape memory effect of [001]-oriented Ni50.3Ti49.7 alloy single crystals under compression

The shape memory effect (SME) and superelasticity (SE) of Ni50.3Ti49.7 alloy single crystals were studied after quenching and marforming (deformation in the martensite phase, followed by annealing at 713 K for 0.5 h) along the [001] direction, under compression, in the high-temperature B2-phase. It was shown that marforming leads to the following: strengthening of the high-temperature B2-phase; an increase in the temperature range of SE; a decrease in the mechanical Δσ and thermal ΔТh hysteresis; and an increase in the transformation hardening coefficient Θ = dσ/dε, in comparison with the crystals after quenching. The reasons for improvements in the functional properties of [001]-oriented single crystals after marforming are discussed