Physics of Metals and Metallography (v.110, #3)

In terms of the concept of the control wave process, it is proposed to consider the formation of internally twinned martensite crystals as a result of a consistent action of quasi-longitudinal waves, which determine the orientation of the habit plane, and of shorter longitudinal waves, which make the major contribution to the control of the growth of the main component of the twin structure. A simple set of equations corresponding to the propagation of a threshold deformation that leads to the formation of platelike regions is given. It is shown that the removal of the degeneracy with respect to the orientations of the expected twin boundaries is due to the existence of feedbacks upon the initiation of the Bain-deformation mechanism. It is noted also that the twin structure can be formed through the excitation of short-wavelength cells in the inter-phase region at the stage of the martensite-crystal growth.
Keywords: martensitic transformations; control wave process

Magnetic properties and structure of nonstoichiometric rare-earth transition-metal intermetallic compounds TbNi2Mn x (0 ≤ x ≤ 1.5) by N. V. Mushnikov; V. S. Gaviko; E. G. Gerasimov; P. B. Terent’ev; I. A. Tkach (210-217).
Crystal structure, magnetization, coercive force, magnetic susceptibility, and anisotropic magnetostriction of nonstoichiometric rare-earth transition-metal intermetallic compounds TbNi2Mn x (0 ≤ x ≤ 1.5) have been studied. The samples with x ≤ 1 have an fcc structure, whereas TbNi2Mn1.25 has a rhombohedral structure of the PuNi3 type. It has been found that the magnetic ordering temperature increases sharply when manganese is added. As the Mn concentration grows, the magnetization and the magnetostriction decrease monotonically, while the coercive force increases. The experimental data obtained have been interpreted on the assumption that a partial substitution of manganese for terbium in TbNi2Mn x leads to local distortions of the crystal field acting on Tb ions, to the appearance of a local uniaxial random anisotropy, and to the formation of a noncollinear magnetic structure in the terbium sublattice.
Keywords: rare-earth intermetallic compound; crystal structure; magnetization; magnetostriction

Structural and phase states in thin films of a Cu-12.5 Al-4.0 Mn alloy (wt %) that were obtained by thermal evaporation onto substrates of different type have been investigated by a variety of methods. As a result of additional heat treatments, there arise various states that determine the shape-memory effect. The possibility of obtaining a disordered state in the initial phase of such an alloy has been established.
Keywords: alloy; evaporation; temperature; films; structure; phases

Emission properties of thin-film alloys of immiscible components by F. F. Dotsenko; V. F. Bashev; S. I. Ryabtsev; A. S. Korchak (223-228).
Results of the investigation of emissive properties of Ba47W53, Ba59Ni41, and Ba33Ni67 (at %) alloys obtained by ion-plasma sputtering of composite targets are presented. It has been found that after vaporquenching, the elemental pairs Ba and Ni and Ba and W that are immiscible in the liquid state form alloys with a mosaic structure consisting of alternate microregions of different elements. Such a structure with alloy components W or Ni (having a high electron density) and Ba (with a low work function φ) produces regions of variable electron density and work function on its surface. This makes it possible to obtain high values of the density of the electron-emission current using these alloys.
Keywords: emission; vapor-quenching; mosaic structure; nanostructure

Data are generalized on the results of irradiation of nanostructured materials (hereafter, nanomaterials) based on metals, steels, and some compounds (such as oxides, nitrides, carbides, and intermetallic compounds) by metallic and gaseous ions in accelerators and by neutrons under reactor conditions. The effects of the fluence, energy of ions (neutrons), and irradiation temperature on the phase composition, amorphization, nanostructural parameters, impurity segregation, blistering and swelling, electrical and optical properties, hardness, strength, plasticity, fracture toughness, and other characteristics have been considered. Attention is given to the fact that the consolidated (bulk) nanomaterials and isolated nanoparticles behave differently under irradiation. The results of theoretical studies and simulation (by the moleculardynamics method) of the effect of irradiation on the properties of nanomaterials are analyzed.
Keywords: irradiation; nanomaterials; radiation defects; nanostructure

Effect of mechanical milling on the mobility of hydrogen in the ZrTi2-H system stabilized by hydrogen: NMR data by A. V. Soloninin; A. V. Skripov; O. A. Babanova; E. Yu. Medvedev; B. A. Aleksashin; M. A. Uimin; V. S. Gaviko (241-249).
Methods of X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) were used to investigate the effect of milling on the properties of the hydrogen-stabilized compound ZrTi2H3.9 with the structure of the metallic sublattice of the C15 type. It has been found that the milling leads to a phase separation in the initial sample: there appears an additional phase with a structure of the TiH2 type and an average grain size of approximately 10 nm. The mobility of hydrogen in the phase of the TiH2 type is considerably lower than in the phase of the C15 type. An analysis of the results of measurements of rate the spin-lattice relaxation of 1H nuclei in wide ranges of temperatures (20–420 K) and resonance frequencies (14.6–90 MHz) allowed us to estimate the parameters of the hopping motion of hydrogen atoms in the phase of the C15 type. It has been established that an increase in the time of milling leads to a decrease in the mobility of hydrogen in this phase.
Keywords: metal-hydrogen systems; nanostructured systems; diffusion; nuclear spin relaxation

Effect of external force fields on the domain structure of equiatomic CuAu alloy by A. Yu. Volkov; B. D. Antonov; A. M. Patselov (250-259).
Peculiarities of the ordered-structure formation in the equiatomic CuAu alloy, which are caused by changes in the sample dimensions and by an applied external force field, have been studied. The rate of the microstructure transformation in the ordered alloy has been found to increase with increasing thickness of the sample. It has been shown that tensile stresses applied during heat treatment can affect the growth direction of c domains and, thus, makes it possible to avoid the low-temperature cracking of the material.
Keywords: microstructure; phase transformations; gold-copper alloy

Influence of equal-channel angular pressing on the structure and mechanical properties of low-carbon steel 10G2FT by E. G. Astafurova; G. G. Zakharova; E. V. Naydenkin; S. V. Dobatkin; G. I. Raab (260-268).
Results are presented of the investigation of mechanical properties, microstructure, and phase composition of low-carbon steel 10G2FT (Fe-1.12Mn-0.08V-0.07Ti-0.1C) before and after equal-channel angular pressing (ECAP). It has been established that the ECAP of steel 10G2FT at T = 200°C in the case of the ferritic-pearlitic state and at T = 400°C in the case of the martensitic state leads to the formation of a predominantly submicrocrystalline structure with an average size of structural elements of approximately 0.3 μm, causes an increase in the strength properties, a decrease in the plasticity, and the localization of plastic flow. It has been experimentally shown that the initially martensitic structure after ECAP causes higher strength properties in comparison with the ferritic-pearlitic structure.
Keywords: equal-channel angular pressing; low-carbon steels; ferrite; pearlite; martensite; submicrocrystalline structure

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys by N. S. Surikova; A. A. Klopotov; E. A. Korznikova (269-278).
Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [ $$ ar 1 $$ 11] and [ $$ ar 1 $$ 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.
Keywords: plastic deformation; twinning; martensitic transformation; fragmentation; texture; nanostructure; microstructure; amorphous state

Structural and phase transformations in a titanium alloy of the transition class under the effect of deformation by A. G. Illarionov; I. V. Narygina; M. S. Karabanalov; S. L. Demakov; A. A. Popov; O. A. Elkina (279-288).
Structural and phase transformations in the VT22 (α + β) titanium alloy of the transitional class that occur during cold plastic deformation (CPD) by upsetting under the conditions of limited friction have been studied and the evaluation of the nonuniformity of deformation along the height of the deformed samples has been performed. It has been found that a CPD of the alloy quenched from the β field with degrees of reduction higher than 6% results in the formation of a martensite-like τ phase (bct) with an axial ratio c/a less than 1; upon upsetting with reductions exceeding 16%, α″ martensite appears in the structure. It is shown that the structural transformations in the alloy with the above degrees of deformation are realized in the following sequence: easy slip (ɛ = 1.5%); multiple slip (ɛ = 6%); the formation of a cellular structure (ɛ = 16%); the formation of deformation twins (ɛ = 23% and higher). The morphology of phases and structural constituents at all stages of the alloy treatment has been established and the distribution of the degree of deformation over the height of the deformed samples has been calculated.
Keywords: high-strength titanium alloys; cold plastic deformation; upsetting; phase and structural transformations; the morphology of phases; the morphology of structural constituents; the nonuniformity of deformation

X-ray diffraction, electron microscopy, microhardness measurements, and differential scanning calorimetry have been used to investigate the formation of the dislocation substructure and nanocrystalline and amorphous structures in Ti-Ni shape-memory alloys depending on the degree of cold deformation by rolling and post-deformation annealing. The moderate deformation (e = 0.25) leads to the formation of a developed dislocation substructure; with an increase in the deformation to e = 2, the dislocation substructure is gradually substituted by a mixed nanocrystalline and amorphous structures. The residual martensite completely disappears as the deformation increases in the interval of e = 2−3 or upon annealing in the interval of 200–300°C. Annealing at 400°C after a moderate deformation leads to the formation of a polygonized (“nanosubgrain”) dislocation substructure in austenite. As the initial deformation increases to e = 2, this structure is gradually substituted by a nanocrystalline structure of austenite. Annealing after deformation to intermediate degrees (e = 0.75−1.0) results in the formation of a mixture of nanocrystalline and submicrocrystalline polygonized structures.
Keywords: shape-memory alloys; thermomechanical treatment; nanostructure; substructure