Pseudoelastic deformation and generation of reactive stresses in a Cu-Al-Ni shape-memory alloy in the temperature range 4.2–293 K

Pseudoelastic deformation and the magnitude of reactive stresses in Cu-14.2% Al-4.5% Ni shape-memory alloy single crystals were studied experimentally in the temperature range 4.2–293 K. It is established that pseudoelasticity and the shape-memory effect are observed in this alloy over the entire temperature range indicated above. It is found that, as the constrained samples are heated at a constant rate from liquid-helium temperature, the reactive stresses increase continuously at temperatures of up to 100 K and then remain constant. When the temperature of preliminary deformation is 77 K, the generation of reactive stresses with an increase in temperature occurs by two stages, which agrees with the multistage behavior of the pseudoelastic-deformation curves of this alloy above the liquid-nitrogen boiling temperature. Using the theory of diffuse martensitic transitions, a quantitative calculation is performed of pseudoelastic-deformation curves and reactive-stress curves over the temperature range 4.2–293 K under conditions of two-stage behavior of the martensitic transformation.     

Generation and relaxation of reactive stresses in a Cu-Al-Ni shape memory alloy upon cyclic temperature variation in the range 293–800 K

The generation and relaxation of reactive stresses in Cu-Al-Ni shape-memory alloy single crystals studied during a single cycle of temperature variation in the range 293–800 K under conditions of the β-phase decomposition (above 600 K) are found to depend on the degree of β-austenite decomposition at the stage of decreasing temperature. The higher this degree, the lower the stresses relaxed and generated upon decreasing temperature and the higher the critical temperatures of the reverse martensitic transitions. Moreover, loading the alloy by reactive stresses during a heating half-cycle causes not only a reversible martensitic shape memory deformation but also an additional austenitic shape memory deformation to occur when the temperature is decreased.     

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.     

Martensitic transformation in single-crystal Cu-Al-Ni shape-memory alloy under load: Visualization by photoacoustic microscopy

Photoacoustic microscopy is applied to observe the surface structure of Cu-Al-Ni shape-memory single crystals in both the loaded and unloaded state. Visualizing the early stages of the loading-induced martensitic transformation is demonstrated to be feasible. The photoacoustic images are distinguished to advantage from the corresponding CCD images by a higher contrast between different phases of the shape memory alloy.     

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.     

Martensitic structure of a Cu-Al-Ni alloy after a single cyclic change in the temperature and the action of reactive stresses

The martensitic transformations in a Cu-13.4 wt % Al-4.0 wt % Ni alloy subjected to a single cyclic change in the temperature in the range 293–680 K under conditions of constrained shape-memory deformation are studied by differential scanning calorimetry. These martensitic transformations are found to be closely related to the temperature dependences of the reactive stresses generated in constrained alloy samples during a single heating-cooling cycle. The substantial change in the behavior of these dependences during heating to a temperature above 600 K is caused by the strong effect of the decomposition of the β-phase solid solution on the parameters of the martensitic transformations in this alloy.     

Characterization of defects in a martensitic CuAlNi shape-memory alloy

A water-quenched martensitic CuAlNi shape-memory alloy was investigated by a combination of coincidence Doppler broadening and positron-lifetime spectroscopy, supported by positron-lifetime calculations. We find a high defect concentration in the as-quenched samples. The positron-lifetime calculations suggest that the defects are not only single vacancies but also vacancies associated with dislocations and stacking faults. Annealing in the martensitic phase has no significant influence on the vacancy concentration but results in a different chemical environment around the vacancies. After aging in the austenitic phase the vacancy concentration decreases significantly. PACS 61.72.Ji; 78.70.Bj     

Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn

AuZn undergoes a shape-memory transition at 67 K. The de Haas-van Alphen effect persists to 100 K enabling the observation of a change in the quantum oscillation spectrum indicative of a catastrophic Fermi surface reconstruction at the transition. The coexistence of both Fermi surfaces at low temperatures suggests an intrinsic phase separation in the bulk of the material. In addition, Dingle analysis reveals a sharp change in the scattering mechanism at a threshold cyclotron radius, attributable to the underlying microstructure driving the shape-memory effect.     

Observation of a continuous phase transition in a shape-memory alloy

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au0.52Zn0.48. Elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q =(1.33,0.67,0) at temperatures corresponding to each sample's transition temperature (TM = 64 and 45 K, respectively). Although the new Bragg peaks appear in a discontinuous manner in the Au0.52Zn0.48 sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near TM are in favorable accord with a continuous transition. A fit to the pressure dependence of TM suggests the presence of a critical end point in the AuZn phase diagram located at TM* = 2.7 K and p* = 3.1 GPa.     

Interfacial diffusion relaxation of internal stresses in solids

The relaxation of internal stresses due to interfacial diffusion in a two-phase solid is studied theoretically with the help of the onsageristic approach of irreversible thermodynamics. In this note we derive an expression for the rate at which internal stresses associated with misfit caused by bonding a flat surface of one material to a rough surface of another. The two phases are treated as isotropic clastic substances. It is assumed that the components of only of the solids are capable of leaving their positions of migrating along the interface. The driving force for this process is minimization of total energy-clastic plus interfacial energy. We show that the time constant for relaxing these stresses is proportional to the cube of the wavelength of the roughness.     

Influence of neutron irradiation on the martensitic transformations and shape-memory effect in a TiNi alloy

The paper reports measurements of the strains and electrical resistance of a TiNi shape-memory alloy under irradiation in the low-temperature helium circuit of a nuclear reactor. Irradiation of the alloy in martensitic state at 170 K revealed that the transition temperatures from cubic to rhombohedral and from rhombohedral to monoclinic phase decrease exponentially with increasing dose. No change in the shape-memory effects and transformation plasticity was observed up to a dose of 6.7×10²² n/m². Keeping the sample at 340 K without irradiation restores (increases) partially the transition temperatures. The relations observed can be assigned to a change in the degree of long-range order in the lattice caused by neutron irradiation. Fiz. Tverd. Tela (St. Petersburg) 40, 1705–1709 (September 1998)     

Photostimulated relaxation of internal mechanical stresses in epitaxial SOS structures

The nature of internal mechanical stresses in the thin silicon-on-sapphire (SOS) epitaxial films is studied, and their value is estimated. They are ～10¹⁹ Pa and have a compressive character. The effects of pulsed laser and lamp annealings on stress relaxation are analyzed, and stress relaxation is shown to reach 90% under certain annealing conditions. An electron mechanism, which is based on a change in the quantum state of the electron subsystem of a crystal during pulsed photon annealing, is proposed for annealing of structural defects.     

Structural relaxation of FeNiB amorphous alloy

Young's modulus was measured in rapidly quenched ribbons of Fe₄₄Ni₃₈B₁₈ annealed at a rate of 5 K/min up to different temperatures. An oscillating behavior was observed. Samples annealed up to temperatures that correspond to maxima and minima values of Young's modulus were studied with Mössbauer spectroscopy in order to correlate the macroscopic parameter with the hyperfine field distribution.     

Stress relaxation in cadmium-silver alloy single crystals

A series of cadmium — silver alloy single crystals containing up to 0·25 at % Ag were deformed in tension at a strain rate of 1×10^?4s^?1. The tensile tests were carried out at temperatures between 77 and 199 K. Stress relaxation experiments were performed to investigate the concentration dependence of the activation volume. Over the temperature range investigated, the activation volume at beginning of deformation decreases proportionally toc ^?2/3 wherec is the atomic concentration of silver as solute. The results are discussed on the basis of the interaction between dislocations and solute atoms.     

Size-dependent multilayer relaxation of nanowires and additional effect of surface stresses

A theoretical investigation was carried out on multilayer relaxation, surface energy and surface stress in [001] oriented rectangular f.c.c metal nanowires using the modified embedded atom method. Surprisingly, the multilayer relaxation behavior depends on the wire sizes in absolutely distinct trends for inward-relaxation metals Ag, Cu, and for outward-relaxation metals Ir and Ni. Specifically, the outmost interlayer relaxation increases with decreasing cross-section areas for Ag, Cu nanowires, while it decreases for Ir, Ni nanowires. This is due to the additional effect of surface stress, which, with the surface energy, increases with decreasing wire width monotonically.     

Observation of a strongly nested Fermi surface in the shape-memory alloy Ni0.62Al0.38

The Fermi surface topology of the shape-memory alloy Ni0.62Al0.38 has been determined using Compton scattering. A large area of this Fermi surface can be made to nest with other areas by translation through a vector of approximately 0.18[1,1,0](2pi/a), which corresponds to the wave vector associated with martensitic precursor phenomena such as phonon softening and diffuse streaking in electron diffraction patterns. This observation is compelling evidence that these phenomena are driven by the enhanced electron-lattice coupling due to the Fermi surface nesting.     

Calculation of shear stresses in Ni3Ge alloy crystals

We present calculations of yield stresses in uniformly deformed regions of Ni₃Ge crystals at temperatures from T=77 to 673 K, taking into account shape changes in the crystals. We evaluate the Schmid factor including deformations. The influence of sample shape changes on the bending of slip planes in homogeneously and inhomogeneously deformed crystal regions is discussed.State Academy of Architecture and Construction, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 49–53, June, 1995.