E. Yu. Panchenko

Effect of disperse Ti3N4 particles on the martensitic transformations in titanium nickelide single crystals

The effect of the size and volume fraction of Ti3N4 particles in Ti-(50.3–51.5) at % Ni single crystals on their martensitic transformation temperatures and temperature hysteresis is studied. Aging at T = 673−823 K leads to a nonmonotonic change in the martensitic transformation temperatures and temperature hysteresis, which is related to a change in the Ni concentration in the matrix, the hardening of the high-temperature phase, a change in the elastic and surface energies generated upon the martensitic transformations, and the internal stresses that appear because of the difference in the lattice parameters of the particles and the matrix. As a result of the high strength of the B2 phase and the high elastic and surface energies that are generated upon the martensitic transformations due to the precipitation of particles of size d < 40 nm at an interparticle distance λ< 50 nm, the martensitic transformation temperatures decrease down to the suppression of the R-B19′ transitions upon cooling to 77 K. A thermodynamic description for the martensitic transformations in heterophase crystals is proposed, and an analogy between the martensitic transformations in heterophase Ti-Ni single crystals with nanoparticles of size d = 20–100 nm and those in single-phase Ti-Ni polycrystals with a grain size of 50–200 nm is found.

Orientational dependence of shape memory effects and superelasticity in CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals

The dependence of deforming stresses, shape memory effect (SME), and superelasticity (SE) on the orientation of the single crystal axis, test temperature, and disperse particle size is examined for CoNiGa, NiMnGa, CoNiAl, FeNiCoTi, and TiNi single crystals. The orientational dependence of SME, SE, and temperature interval of the development of martensitic transformations (MT) under loading and SE is established. The influence of disperse particles on magnitudes of SME, SE, and mechanical hysteresis is discussed.

High-Temperature Superelasticity and the Shape-Memory Effect in (001) Co-Ni-Al Single Crystals

Development of thermoelastic B2-Ll0 martensitic transformations (MTs) has been investigated upon cooling/heating and under stress using single crystals of Co40Ni33Al27 and Co35Ni35Al30 (at %), oriented along the [001] direction. It is shown that the temperature intervals of the development of superelasticity TSE and the values of the mechanical (Δσ) and temperature (ΔT) hysteresis are determined by the chemical composition of crystals and the type of deformation (tension or compression) along the longitudinal axis [001]. The effect of the phase and dispersed particles on the development of martensitic transitions, the mechanical stabilization of the stress-induced martensite, and the temperature interval of superelasticity †TSE has been investigated. Conditions necessary for the development of high-temperature superelasticity (at T = 570–580 K) have been determined and a thermodynamic description of the temperature interval of superelasticity has been suggested.

Superelasticity in high-strength heterophase single crystals of Ni51.0Ti36.5Hf12.5 alloy

The effect of precipitated disperse H-phase particles on the thermoelastic B2–B19′ martensitic transformation (MT) under compressive load has been studied on [001]-, [236]-, and [223]-oriented single crystals of Ni51.0Ti36.5Hf12.5 (at %) alloy in the initial (as-grown) state. It is established that, in Ni51.0Ti36.5Hf12.5 single crystals containing disperse H-phase particles with dimensions within 125–150 nm at a volume fraction of ~30%, neither the critical stresses of martensite formation nor superelasticity strain depend on the orientation. Fully reversible B2–B19′ MTs in Ni51.0Ti36.5Hf12.5 single crystals have been observed in tests at external axial stresses up to 1700 MPa and temperatures up to Tt ~ 373 K.

High-temperature superelasticity during B2-L10 martensite transformations in Co40Ni33Al27 crystals

The temperature interval ΔTSE of superelasticity in [001]-oriented Co40Ni33Al27 (at. %) single crystals strained by tension and compression has been studied. It is established that ΔTSE in tension amounts to 220 K, and the reversible B2-L10 martensite transformations in loaded samples take place at 590 K. In the samples strained by compression, ΔTSE decreases to 105 K, and the superelasticity is observed up to 420 K.

Peculiarities of high-temperature superelasticity in Ni–Fe–Ga single crystals in compression

The high-temperature superelasticity and temperature dependence of the yield stress of 14M and L10 martensite in [001]-oriented single crystals of Ni54Fe19Ga27 (at %) in compression alloy have been studied. As the temperature increases, the sequence of stress-induced martensitic transformations (MTs) changes from L21–14M to L21–L10. The yield stress of L10 martensite weakly depends on the temperature and is 1.7 times lower than that of 14M martensite. The temperature interval of superelasticity in [001]-oriented single crystals of Ni54Fe19Ga27 under compression is determined by the growth of critical stresses with increasing temperature, the coefficient of strain-hardening, and the yield stress of L10 martensite.

Stress-Induced Thermoelastic Martensitic Transformations and Functional Properties in [011]-oriented NiTiHfPd Single Crystals

The stress-induced martensitic transformation in the [011]-oriented Ni45.3Ti29.7Hf20Pd5 (at. %) single crystals in as-grown, homogenized and aged states were investigated in compression. It is experimentally shown that heat treatments of single crystals result in increase in martensitic transformation temperatures, two-fold decrease in reversible strain and increase in strain-hardening coefficient. As-grown single crystals demonstrate large temperature range of superelasticity (up to 140 K), large reversible strain (up to 4.3%) and large work output in comparison with homogenized and aged crystals.

The effect of thermal cycling and stress-assistant ageing twoway shape memory effect in [1̄23]-oriented Co40Ni33Al27 single crystals

The effect of thermal cycling through an interval of B2-L10 martensitic transformation (MT) under action of external stress and tensile stress-assistant ageing on the two-way shape memory effect in [23]-oriented Co40Ni33Al27 (at.%) single crystals are investigated. For the first time it is experimentally established that tensile stress-assistant 100 MPa ageing at 573 K for 1 h along [23]-direction of Co40Ni33Al27 single crystals creates the necessary conditions for two-way shape memory effect (TWSME) with the reversible strain up to =2.4 (±0.3)% at cooling/heating. The TWSME in quenched [23]-oriented Co40Ni33Al27 single crystals can be induced by thermal cycling through an interval of B2-L10 MT under action of constant external stress 50 MPa with the reversible strain less than 1%.