S. P. Nikanorov

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.

Temperature effect on the elastic properties of yttrium garnet ferrite Y3Fe5O12

The Young’s moduli along the [100] and [110] crystallographic directions and the shear modulus along the [100] direction in a high-purity yttrium garnet ferrite single crystal are measured in the temperature range from 20 to 600°C. All the independent elastic constants are calculated for this temperature range. The behavior of the elastic moduli and elastic anisotropy factor is analyzed in the vicinity of the critical temperature of the magnetic phase transition.

DETECTION OF SHOCK-WAVE-INDUCED INTERNAL STRESSES IN Cu-Al-Ni SHAPE MEMORY ALLOY BY MEANS OF ACOUSTIC TECHNIQUE

University of Antwerpen (RUCA), IMS, Middelheimlaan 1, B-2020, Antwerpen, Belgium(Received April 12, 2000)(Accepted in revised form June 21, 2000)Keywords: Copper alloy; Martensitic transformation; Impact shock-wave loading; Elastic properties;Internal frictionIntroductionA response of materials, exhibiting the thermoelastic martensitic transformation (TMT), on the appliedmechanical stress leads to the superelasticity, plasticity of the martensitic phase, shape memory effect,high damping capacity, depending on a variety of parameters such as alloy composition, temperature,magnitude of the applied stress, etc. [1,2]. A predictability of such response is of a great importancefrom the engineering standpoint. Since the TMT is a diffusionless phase transition, the determinationof a threshold time to induce the TMT is a challenging fundamental problem. Another aspect, relatedto both of the above mentioned issues is the performance of TMT materials under the ultimateconditions of high-energy impact loading by stress pulses with short duration. In the present work theacoustic technique has been used to detect the structural changes induced by shock-wave loading ofCu-Al-Ni crystals. The same acoustic technique provides us with an estimate of the upper time limit toinduce the TMT and plastic deformation of the martensitic phase.ExperimentalPlate-shaped samples with dimensions of about 1 3 3 3 30 mm

Structural and physicomechanical properties of directionally crystallized aluminum-silicon alloys

Aluminum-silicon alloys (from 8 to 25 wt % Si) have been prepared by directional crystallization of shaped samples by the Stepanov growth at a solidification rate of 103 μm s−1. The dependences of the microhardness, Young’s modulus, internal friction, yield stress, and ultimate tensile stress of the alloys on the silicon content have been studied. It has been shown that the ultimate tensile stress has a maximum, and the yield stress has a kink at 15 wt % Si; the composition corresponds to the eutectic composition at the solidification rate used. The silicon content in the eutectics increases with an increase in the solidification rate. The increase in the ultimate tensile stress is explained by an increase in the volume fraction of the more strength fine-crystalline structure of the eutectics as a result of the decrease in the volume fraction of more plastic dendrites of the primary crystals of the α-Al solid solution. The decrease in the ultimate tensile stress of the hypereutectic alloy is determined by the increase in the volume fraction of brittle primary silicon crystals of various shapes.

Generation and relaxation of reactive stresses in Cu-Al-Ni shape-memory alloy

Reactive stresses in Cu-Al-Ni shape-memory single crystals are experimentally determined on constrained samples heated at a constant rate in the temperature range 293–800 K. At temperatures up to 600 K, the stresses increase with temperature. At higher temperatures, they begin to decrease as a result of the decomposition of the β-phase and vanish at 800 K. The theory of diffuse martensitic transformation is used to calculate the reactive stresses, including the case when the volume fraction of the β-phase decreases, at temperatures above 600 K.

Recovery of Young’s modulus upon annealing of nanostructured niobium produced through severe plastic deformation

The effect of temperature (in the range 20–500°C) on the Young’s modulus of nanostructured niobium with Ta impurity content <0.5 wt % and that of O2 < 0.1 wt % and with a mean grain size of ≅200 nm is studied. The transformation of polycrystalline niobium into a nanostructured state is performed through severe plastic deformation by equal-channel angular pressing. The Young’s modulus is found to increase in two stages as the temperature of isothermal annealing is gradually increased. The mechanisms of recovery of the elastic modulus upon annealing of the nanostructured niobium are discussed in the context of the modern concepts of the defect structure of deformed metals.

Influence of temperature and strain on the amplitude-dependent internal friction of high-purity aluminum

The dislocation amplitude-dependent friction (ADIF) of high-purity (99.999%) polycrystalline aluminum is investigated in the temperature range 7–300K at vibrational strain amplitudes of 10−7–10−4 for samples in the annealed and deformed (by quasistatic, shock, and ultrasonic loading) states. The ADIF is a multistage effect in the indicated temperature and vibration amplitude ranges. Analysis of the amplitude-temperature spectra of the ADIF permits separation of components attributable to: interaction between dislocations, the interaction of dislocations with pinning points, and pure dislocation relaxation (the interaction of dislocations with the Peierls relief). ADIF is observed to depend nonmonotonically on the initial quasistatic strain determined by strain hardening and recovery processes.

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge Systems at Centrifugation

Crystallization in the Al-Si, Al-Ge, and Al-Si-Ge systems at centrifugation is studied. Of them, the Al-Si system is the least prone to sedimentation. In the others, sedimentation considerably changes the structure of the alloys at the bottom of the ingots compared with their top. At certain concentrations of the constituents, the number of crystallites in the lower part of the ingot is larger than in the upper part and the crystallites at the bottom are coarser than at the top. The Si: Ge atomic ratio in the Al-Si-Ge system changes by a factor of 2–12 against the initial ratio (1: 1) when the (Si + Ge) concentration changes as a result of centrifugation. Also, this ratio changes over the crystal surface (in the samples not subjected to centrifugation, this ratio remains unchanged over the surface). Crystallites in the Al-Si-Ge system are covered by Ge.

Influence of Gravity on the Habit of Single Crystals of Compounds Grown from Solutions

The effect of different gravity levels on the growth habit of single crystals of some compounds has been determined. Isometric crystals were obtained both at microgravity and at 10–11g. Accelerations of 2–6g promoted the formation of elongated and flattened crystals. The microhardness of KCl and KBr crystals increased with increasing acceleration at which they were obtained).

Structure, microhardness, and strength of a directionally crystallized Al-Ge alloy

The structure, microhardness, and strength of binary directionally crystallized aluminum alloys with 35, 43, 53, 57, and 64 wt % germanium have been investigated. It has been shown that the eutectic microhardness is constant in the composition region under study. The microstrength of primary crystals of the solid solution of germanium in aluminum with the dendrite structure increases with increasing germanium concentration. However, the difference in the microhardnesses of the eutectic and dendrites, which was determined for each of compositions on the same specimen, does not exceed the measurement error. It has been assumed that the change in the strength of the alloy having the composition in the hypoeutectic region is determined by the redistribution of the volume fractions of the eutectic (α-Al and eutectic germanium) and the domains of primary crystals of the solid solution. This dependence can be described by the mixture rule. Above the eutectic composition, the alloy decomposes in a brittle manner; its strength is likely dependent not only on the content of the components, but also on the form and orientation of primary germanium crystals.