Reorientation Of Crystal Lattice Research Articles

Features of Plastic Deformation and Fracture of a Dispersion-Strengthened V–4Ti–4Cr–(C, N, O) Alloy at Different Temperatures

4Using the methods of transmission and scanning electron microscopy, the defect substructure of a V–4Ti–4Cr– (C, N, O) alloy dispersion strengthened with nanosized oxicarbonitride particles and the features of its fracture after deformation via active tensile loading at the temperatures 20 and 800°C are investigated. It is shown that the most representative features of deformation at the elevated temperature is primarily activation of grainboundary mechanisms of deformation and fracture and secondly strain localization accompanied by crystallattice re-orientation. The reasons for these phenomena are discussed.

Nanodipoles of partial disclinations as carriers of non-crystallographic shear and crystal-lattice reorientation in nanocrystalline nickel and vanadium

Using electron microscopy, an investigation is performed of microstructure features of re-orientation microbands formed in nickel and V–4Ti–4Cr alloys under severe plastic deformation in the Bridgman anvils. It is shown that their formation could be described within the framework of a quasi-elastic mechanism of motion of partial-disclination dipoles, which is controlled by the flows of non-equilibrium point defects in the fields of local gradients of normal stress-tensor components. Operation of this mechanism offers an additional advantage of nanostructuring the defect substructure during plastic deformation of metallic materials, followed by formation of the structure states wherein the nanocrystallites measure a few nanometers.

Retention of the Twinning Σ3 Misorientation in the Process of Lattice Transformation during Cold Rolling of a Fe3 Pct Si Single Crystal

The article investigates the twinning evolution of two active systems during cold rolling of a (110)[001]-oriented Fe3 pct Si single crystal. An intriguing fact is revealed that for both twinning systems the Σ3 misorientation is retained, regardless of the fundamental difference in their transformation in the course of the matrix crystal lattice reorientation. One of the twinning systems retains the regular habit plane position, while the other acquires an irregular one. A model is proposed that describes the mechanism of such transformations.

Microstructure of Mo – 47% Re – 0.4% Zr alloys after rolling at room temperature. II. Peculiarities of mechanical twinning and formation of high-angle boundaries between microbands

It is shown that the Mo – 47% Re – 0.4% Zr alloy after rolling deformation (e ≈ 90%) at room temperature exhibits certain characteristic features, such as increased density of deformation defects with considerably faceted noncoherent boundaries and reorientation microbands with a specific range of high-angle disorientations around the direction of the 〈110〉 type. Using the concepts on direct-plus-reverse (along alternative systems) martensitic transformations as the mechanisms of plastic deformation and crystal-lattice reorientation, an analysis of possible mechanisms for formation of these states is made.

Recrystallization of plane strain compressed Al–1 wt.% Mn alloy single crystals of typical unstable orientations

A systematic study of crystal lattice reorientation in early stages of recrystallization has been carried out to correlate the orientations of recrystallization nuclei with the deformation microtexture and with slip systems. Microstructure and texture of Al-1 wt.% Mn single crystals of unstable initial orientations of {112}111, {100}001 and {001}110 have been examined by high-resolution field-emission gun scanning electron microscope local orientation measurements. All single crystals were channel-die deformed at room temperature and then annealed for a short time. It was shown that often observed presence of the 112 directions as rotation axes in the formation of new nuclei orientation directly suggested a close link with the deformation process.

Regularities and mechanisms of mechanical twinning in TiNi alloys

We generalize experimental findings and theoretical reasoning for a new mechanism of deformation and crystal lattice reorientation. This is the mechanism of direct plus reverse (B2 → B19(B19′) → B2) martensitic transformations as a mechanism of localized deformation band formation and mechanical twinning in the B2 phase of TiNi alloys. The paper gives atomic models of these transformations, analysis results for the distortion tensors and features of lattice reorientation in the transformation zones. The relation of mechanical twinning to yield stress asymmetry is analyzed for the case of tension and compression of TiNi alloy single crystals in directions of type . To explain this phenomenon, we introduce the notion of the factor of normal stresses reduced to the principal axes of the tensor of uniform transformation strain in the mechanical twinning zones. It is shown that this factor can be used for the quantitative estimation of yield stress asymmetry in [001] single crystals of TiNi alloys.

Crystal-lattice distortions upon the formation of localized-deformation bands via combined forward-plus-reverse martensitic transformations

The model of plastic flow as a combined forward-plus-reverse (on an alternative flow system) martensitic fcc → bcc → fcc transformation was used to theoretically investigate transformation-related distortions on the example of localized-deformation bands with a 60°〈110〉 crystal-lattice reorientation that are formed as a result of such a transformation upon rolling of austenitic steels. An analysis of the relative contributions of various modes of deformation and crystal reorientation to the total transformation distortion has been performed.

Mechanism of deformation and crystal lattice reorientation in strain localization bands and deformation twins of the B2 phase of titanium nickelide

Optical metallography and transmission electron microscopy were used to investigate the formation and microstructure of strain localization bands and deformation twins in single crystals of a TiNi alloy deformed by rolling. On the basis of these investigations, with the use of the theory of martensitic transformations based on the concept of cooperative thermal vibrations of coherent objects (close-packed planes) in crystals, a new mechanism of twinning in these alloys is suggested, namely, through the forward plus reverse (B2 → B19 → B2) martensitic transformations with the reverse transformation developing via an alternative transformation system. It is shown that using this mechanism one can describe on the same grounds the twinning in the crystal lattice of the B2 phase with twin planes of different ({1 1 2}, {1 1 3}, and {3 3 2}) Miller indices and the formation of strain localization bands with low-angle misorientations.

Evolution of defect substructure of metal alloys at microscopic and mesoscopic level under torsion

Transmission electron microscopy was used to investigate the reorientation of crystal lattice during the formation of ultrafine-grained (UFG) copper, nickel, and an alloy of Ni–18% Al–8% Cr–1% Zr–0.15% B (at.%) under severe plastic deformation by equal-channel angular (ECA) pressing and twisting at a high quasi-hydrostatic pressure. The crystal lattice was found to transform into a UFG state; it is fragmented at the nano-, micro-, and mesoscale levels. Possible mechanisms for the reorientation of the crystal lattice under deformation at the micro- and mesoscale level are discussed.

Structural changes without stable intermediate state in inelastic material. Part II. Applications to displacive and diffusional–displacive phase transformations, strain-induced chemical reactions and ductile fracture

Abstract A number of simple examples demonstrate the applicability of the general theory developed in Part I of this paper to various structural changes in solids, namely to displacive and generalized second-order phase transformations, twinning and reorientation of crystal lattice, ductile fracture, and strain-induced chemical reactions. The theory is extended to diffusional-displacive phase transitions. The following problems for elastic and elastoplastic materials are solved analytically: displacive and diffusional–displacive phase transitions in a spherical particle inside the space under external pressure, martensitic phase transition and twinning in ellipsoidal inclusion under applied shear stress, spherical void nucleation, crack propagation in a similar framework to the Dugdale model for a plane stress state. In most cases explicit expressions for the thermodynamic and kinetic conditions for structural changes and the geometric parameters of the nucleus are obtained and analyzed. The following typical cases in the determination of the geometric parameters of nucleus are found: solely from the principle of the minimum of transformation time and the kinetic equation without any constraints or with thermodynamic constraint; from the principle of the minimum of transformation mass and the thermodynamic criterion of structural changes (thermodynamically admissible nucleus); as an interatomic distance. For diffusional–displacive phase transformations, additional variants are related to the necessity to consider the diffusion equation (for the diffusion-controlled transformation) and constraints related to the maximum and minimum possible volume fraction of solute atoms. The nucleation kinetics for various nucleus geometries is compared.

Thermomechanics and kinetics of generalized second-order phase transitions in inelastic materials. Application to ductile fracture

A number of simple examples demonstrate the applicability of the general theory developed in Part I of this paper to various structural changes in solids, namely to displacive and generalized second-order phase transformations, twinning and reorientation of crystal lattice, ductile fracture, and strain-induced chemical reactions. The theory is extended to diffusional-displacive phase transitions. The following problems for elastic and elastoplastic materials are solved analytically: displacive and diffusional–displacive phase transitions in a spherical particle inside the space under external pressure, martensitic phase transition and twinning in ellipsoidal inclusion under applied shear stress, spherical void nucleation, crack propagation in a similar framework to the Dugdale model for a plane stress state. In most cases explicit expressions for the thermodynamic and kinetic conditions for structural changes and the geometric parameters of the nucleus are obtained and analyzed. The following typical cases in the determination of the geometric parameters of nucleus are found: solely from the principle of the minimum of transformation time and the kinetic equation without any constraints or with thermodynamic constraint; from the principle of the minimum of transformation mass and the thermodynamic criterion of structural changes (thermodynamically admissible nucleus); as an interatomic distance. For diffusional–displacive phase transformations, additional variants are related to the necessity to consider the diffusion equation (for the diffusion-controlled transformation) and constraints related to the maximum and minimum possible volume fraction of solute atoms. The nucleation kinetics for various nucleus geometries is compared.