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.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 4–20, September, 2004.     

Magnetic memory effect in ZnO single crystals

Hardening of ZnO single crystals is found to occur after their treatment in a constant magnetic field (B = 1–2 T). A maximum increase in the microhardness of the crystals is observed within 3–4 h after magnetic treatment. Then, the effect gradually decreases and the microhardness regains its initial value in 2–3 days. It is revealed that the sensitivity of the microhardness to a variation in the magnetic field has a threshold character: the effect appears at a magnetic induction higher than a critical value, rapidly increases in a narrow magnetic field range ΔB (～0.3 T), and then reaches saturation. It is shown that the magnitude of the effect depends on the orientation of the magnetic field with respect to the polar axis of symmetry of the crystal and is independent of the crystallographic orientation of the measurement plane. The maximum increase in the microhardness (～20%) is observed for all the (0001), (11\(\bar 2\)0), and (10\(\bar 1\)0) faces studied in the magnetic field B ∥ [10\(\bar 1\)0]. No change in the microhardness is found to occur in the magnetic field direction B ∥ [0001]. A physical model related to the spin-dependent variations in the impurity subsystem of the crystal in the magnetic field is proposed.     

Crystal quality boosts responsiveness of magnetic shape memory single crystals

Magnetic shape memory alloys are promising materials to replace giant magnetostrictive materials and piezoelectrical ceramics in actuating devices due to the large magnetically induced strains. Ni-Mn-Ga is the most intense studied system due to its relatively high operational temperatures and the huge magnetically induced strains reported. Up to now the application of these materials is still limited by the operational temperature range. Additionally twin boundary mobility suffers from structural defects increasing the magnetic fields needed for significant and reproducible strains. The sample quality is affected by crystal inhomogeneity, porosity and impurities. Here new results are reported for the Ni-Mn-Ga class based on a set of single crystals grown by the SLARE method, recently developed by Mecklenburg et al. Single crystalline samples of Ni_49.7Mn_29.3Ga₂₁ of tetragonal martensitic structure exhibit a magnetic field induced strain of more than 4% below 170 mT and 6.5% at only 340 mT. Furthermore the operational temperature regimen could be expanded up to 65 °C.     

Mechanism for the bidirectional shape memory effect in titanium nickelide crystals

A mechanism for the bidirectional and all-round shape memory effects observed in titanium nickelide crystals is discussed quantitatively by using the theory of diffuse martensitic transformations (DMTs). These effects are associated with an anisotropic distribution of Ti₃Ni₄ particles, which arises in bent crystals subjected to annealing followed by relaxation of coherent microstresses produced by the particles. Using the DMT theory, the influence of the stepwise B2 → R → B19′ phase transition on the magnitude and sign of the radius of curvature of a thin strip of titanium nick-elide is calculated and the conditions are determined under which the bidirectional and all-round shape memory effects occur depending on structural factors and the geometrical parameters of the strip.     

Shape memory effect and superelasticity in a strain glass alloy

The shape memory effect and superelasticity are usually found in alloys exhibiting spontaneous martensitic transformation. Thus it is hard to imagine that such interesting effects can appear in a system without a martensitic transformation. In this Letter we show shape memory and the superelasticity effect in a nonmartensitic Ti48.5Ni51.5 alloy, which has no martensitic transformation but undergoes a "strain glass" transition. In situ x-ray diffraction experiment showed that the shape memory and superelasticity in strain glass stem from a stress-induced strain glass to martensite transformation and its reverse transformation. The new shape memory and superelasticity in strain glass extends the regime of the shape memory effect and superelasticity and may lead to novel applications.     

Shape memory effect and linear actuators based on Cu-Al-Ni single crystals

Recovery stress generation under thermal cycling has been experimentally studied in clamped shape memory Cu-Al-Ni single crystals up to 9% reversible strain. It is shown that such crystals are capable of repeated force generation upon heating up to 600 K and single actuation when heated to 700 K with a maximal stress of 350 MPa. The main principles of designing cyclic linear actuators are considered and a technique for calculating their force characteristics is proposed. The calculation is based on the mathematical model of linear actuator.     

Influence of incomplete shape memory deformation on the generation of reactive stresses in single crystals of the Cu-Al-Ni alloy

The influence of preliminary incomplete shape memory (SM) deformation on the generation of reactive stresses in single crystals of the Cu-13.6 wt % Al-4.0 wt % Ni alloy under conditions of constrained SM deformation has been studied. It has been found that, with an increase in the deformation, the value of reactive stresses increases linearly with increasing stress of preliminary deformation of the alloy.     

Synergetics of martensitic structures in shape memory crystals

The kinetic mechanism of the formation of spatially inhomogeneous martensitic structures in shape memory alloys is discussed theoretically. In terms of synergetics, the formation of these structures is the result of the self-organization of elementary transforming volumes associated with the motion of transformation dislocations along interphase boundaries. In contrast to the pure thermodynamic approach based on the Ginzburg-Landau theory of phase transformations, the kinetic method allows one to determine the proper physical scale of the phenomenon and to elucidate the effect of structural factors on the transformation parameters.     

Influence of reactive stresses on thermodynamics and kinetics of martensitic transitions in single crystals of the shape memory Cu-Al-Ni alloy

The heat of the β′₁-β₁-martensitic transition in single crystals of the alloy Cu-13.5 wt % Al-4.0 wt % Ni was studied under conditions of a constrained shape memory deformation and emergence of reactive stresses. The experiments were performed with samples bended in the form of a clamp. The sample was put into a continuous stainless steel ring, and this construction was placed in the capsule of a differential calorimeter. It was found that, with an increase in the magnitude of preliminary bend deformation, the maximum in the heat release (or heat absorption) curves decreases noticeably and its position shifts to higher temperatures. It was revealed that the latent heat of the transition decreases by a factor of 2 and 3 during heating and cooling, respectively. It was assumed that the observed effects are related to the influence of reactive stresses on the parameters of martensitic transitions. A quantitative analysis of the data obtained was performed in terms of the theory of diffuse martensitic transitions taking into account both the thermodynamic and kinetic factors.     

Mechanism of homogenization of the martensite state of crystals with a shape memory effect

The mechanism of homogenization of the martensite state of crystals with a shape memory effect under mechanical stresses applied to a crystal is theoretically analyzed in the framework of the thermodynamic approach and the theory of smeared martensite transitions. The homogenization of the martensite state of the crystal is considered for two variants of martensite that differ in all parameters (the temperature and the heat of transformation, spontaneous strains, etc.) and for many variants of martensite that differ from one another in the orientation of the habit planes.     

Ni54Fe19Ga27单晶的形状记忆和超弹性

本文研究了单晶Ni₅₄Fe₁₉Ga₂₇不同方向的形状记忆效应、超弹性和磁性. 研究发现,单晶样品具有良好的双向形状记忆效应.不同晶体学方向的相变应变随着热循环次数的变化而改变. 在外应力作用下,通过应力诱发马氏体相变,样品在[001],[110],[111]方向分别产生了3.3%, 2% 和3%的可回复应变平台.磁性测量结果表明马氏体的磁晶各向异性能约为4.8× 10⁵ erg/cm³,远远小于变体孪生所需机械应力能,因此磁场的作用是使磁矩发生转动而不是使孪晶界移动, 成功揭示了不能在NiFeGa中获得大磁感生应变的物理根源.     

Characteristic X-ray intensities: Dependence on orientation in BI-atomic single crystals

The ratio of characteristic X-ray intensities excited by electron Bloch waves in bi-atomic crystals has been investigated as function of the crystal orientation. The result indicates the possibility to use the electron probe for locating the site of impurity atoms in single crystals.     

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采用简化阴极的一维边界层模型,将同轴磁旋转电弧等离子体发生器的阴极与弧柱耦合求解,使用FLUENT软件,数值模拟了不同锥角阴极的形状对磁分散电弧等离子体、阴极弧根和阳极弧根位形的影响.结果表明:阴极弧根具有扩散特征,其电流密度为107A·m-2量级;阴极形状的改变引起阴极弧根位形和电流密度分布变化,从而影响等离子体参数分布;随着阴极锥角的增大,阴极弧根从阴极前端移动到阴极侧面,等离子体区域向下游偏移,等离子体轴向厚度减小.     

Diffusive shape memory effect in the V-H system

The effect of cold plastic deformation on the occurrence of the diffusive shape memory effect in the V-H system due to redistribution of the -phase arising from the Gorskii effect is investigated by means of three-point bending tests. The slight plastic deformation produced by the bending in the region of the -solid solution gives rise to a diffusive shape memory effect when there is no load, whereas large deformations completely suppress the diffusive shape memory effect. It is suggested that the reason for the occurrence and disappearances of the diffusive shape memory effect is internal residual stresses of a different scale level which occur after cold plastic deformation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 90–95, December, 1994.     

A molecular dynamics investigation of the deformation mechanism and shape memory effect of epoxy shape memory polymers

Following deformation, thermally induced shape memory polymers(SMPs) have the ability to recover their original shape with a change in temperature. In this work, the thermomechanical properties and shape memory behaviors of three types of epoxy SMPs with varying curing agent contents were investigated using a molecular dynamics(MD) method. The mechanical properties under uniaxial tension at different temperatures were obtained, and the simulation results compared reasonably with experimental data. In addition, in a thermomechanical cycle, ideal shape memory effects for the three types of SMPs were revealed through the shape frozen and shape recovery responses at low and high temperatures, respectively, indicating that the recovery time is strongly influenced by the ratio of E-51 to 4,4'-Methylenedianiline.