Grain size refinement due to relaxation of disclination junction configurations in the course of plastic deformation of polycrystals

A model is proposed for the formation of the substructure in polycrystals during plastic deformation. According to this model, fragmentation of a grain occurs through the formation of a system of diagonal low-angle boundaries, which originate at the edges of a rectangular grain. Misorientation boundaries form through relaxation of a nonsymmetric junction quadrupole disclination configuration accumulated at the grain corners under severe deformation when the disclination strength reaches a certain critical value. The energetics of this process is analyzed. A general case is considered where the disclinations at the junctions of the chosen grain differ in strength. The energetic approach used makes it possible to determine the misorientation angle ω_x of the resulting boundaries corresponding to the maximum energy gain and to find the dependence of this angle on the degree of asymmetry of the quadrupole configuration of junction disclinations. According to the proposed model, the splitting of a grain with a short edge greater than 0.5 μm is energetically favorable and decreases the latent energy of the grain for any ratio between the junction disclination strengths if the grain length-to-width ratio is less than 30. It is shown that the minimum possible grain size in the proposed model does not exceed 0.1 μm.     

Plasticity limit of metals during hot plastic deformation

A model of plasticity limit has been derived in the condition of hot plastic deformation, where dynamic recrystallization takes place, through the ratio between the rate of grain boundary sliding and the overall deformation rate. If fracture occurs preferentially at the grain boundaries we can replace the grain boundary deformation through the energy needed to cause fracture and express the temperature influence on the deformation stress. The plasticity limit is then the function of Zener-Hollomon parameter and deformation stress, where the exponent of deformation stress has a value of –4·3.     

Effects of severe-strain-induced defects on the mechanical response of two kinds of high-angle grain boundaries

ABSTRACT

Grain boundaries of metallic materials subjected to severe plastic deformation exhibit significantly enhanced diffusivity and excess energy compared with their relaxed poly- or bi-crystalline counterparts even when the macroscopic degrees of freedom are the same in both types of grain boundaries. Boundaries of excess energy are/can be relaxed by annealing. As a first step in accounting for this experimentally observed high-energy state of general high-angle grain boundaries subjected to severe plastic deformation, a concept of localised basic shear units and the presence of localised extra free volume in these units situated in different locations in the grain boundaries, which was originally proposed to explain steady-state structural superplastic flow, is made use of. Using MD simulation, the mechanical response of these modified grain boundaries is compared with that of their relaxed state. The results are also compared with a case of a homogeneous distribution of extra free volume within the grain boundary. The localised shear units containing extra free volume introduced in the grain boundaries are found to alter their physical and mechanical features strongly, which, in turn, drastically affect, consistent with experimental results, the mechanical response of the heavily deformed material.     

Deformation-induced changes in single-phase Al-0.1Mg alloy

Al-0.1wt.%Mg has been chosen to explore the effects of deformation on the microstructure of nominally single-phase materials. For these materials, the rate of static grain growth is much higher compared to that of Zener pinned systems and dual-phase alloys, where growth is hindered due to the pinning force exerted by second-phase particles on grain boundaries. Therefore, deformation-induced microstructural changes in single-phase alloys occur without any restricting pinning pressure. This paper illustrates the complex effect of deformation on the microstructural changes mainly associated with dynamic recovery. The process affects the initial microstructure due to an intensive substructure development. Dynamic recrystallisation, associated with the formation of new grains, is considered to be a transient phenomenon that quantitatively influences the mean grain size formed during straining. This work also aims to estimate the stored energy of the deformed regions and texture components to explore their contribution to the migration of high-angle grain boundaries.     

The effect of antiphase domains on the energy spectra of special boundaries in alloys with the <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript> superstructure

The special grain boundaries in ordering alloys with the L1₂ superstructure have been investigated by optical metallography and transmission diffraction electron microscopy. The relative energy of the boundaries Σ3 and Σ9 in Ni₃Fe alloy with a short-range order is determined. The energy of these boundaries in an alloy with long-range order is estimated. The energy of twin grain boundaries increases at the phase transitions A1 → L1₂ due to the formation of antiphase grain boundaries in those boundaries. The spectra of special boundaries over Σ and their distributions, depending on the relative energy, change as well. The average relative energy of special grain boundaries in alloys with the L1₂ superstructure increases with increasing the energy of antiphase boundaries.     

Dislocation structures. Part II. Slip system dependence

Part I established, via extensive transmission electron microscopy investigations, that the type of dislocation structure formed in metals of medium-to-high stacking fault energy upon deformation in tension or rolling to moderate strain levels (≤0.8) depends strongly on crystallographic grain orientation. This paper analyzes the grain orientation-dependent structures in terms of the active slip systems, focusing on the crystallographic plane of extended planar boundaries (geometrically necessary boundaries). The analysis establishes slip systems as the factor controlling the dislocation structure. Five fundamental slip classes, consisting of one to three active slip systems, have been identified. Multiple activation of these slip classes is also considered. The slip classes give rise to different types of dislocation structure, of which all except one contains geometrically necessary planar boundaries aligning with unique crystallographic planes (not necessarily slip planes). A slip class leads to the same type of structure, irrespective of the macroscopic deformation mode, as also demonstrated by successful predictions for shear deformation.     

Microstructural evolution of AZ31 magnesium alloy subjected to sliding friction treatment

Microstructural evolution and grain refinement mechanism in AZ31 magnesium alloy subjected to sliding friction treatment were investigated by means of transmission electron microscopy. The process of grain refinement was found to involve the following stages: (I) coarse grains were divided into fine twin plates through mechanical twinning; then the twin plates were transformed to lamellae with the accumulation of residual dislocations at the twin boundaries; (II) the lamellae were separated into subgrains with increasing grain boundary misorientation and evolution of high angle boundaries into random boundaries by continuous dynamic recrystallisation (cDRX); (III) the formation of nanograins. The mechanisms for the final stage, the formation of nanograins, can be classified into three types: (i) cDRX; (ii) discontinuous dynamic recrystallisation (dDRX); (iii) a combined mechanism of prior shear-band and subsequent dDRX. Stored strain energy plays an important role in determining deformation mechanisms during plastic deformation.     

Channelling of Plastic Deformation by Interfaces in TiAl Intermetallics

Plastic deformation in the lamellar microstructure of the L1₀ tetragonal phase is strongly affected by special rotational lamellar interfaces on the {111} close packed planes and by general grain boundaries separating the lamellar colonies. The activity of possible deformation modes in Ti-rich TiAl alloys is explained considering in addition to generally known asymmetry of deformation twinning the asymmetry of superdislocation motion. The restrictions imposed on the direction of propagating deformation by the lamellar interfaces are analysed in detail. Even in the case when the transfer of plastic deformation across the interfaces does not occur, the presence of interfaces as strong obstacles to moving dislocations and deformation twins can lead to localisation of strain parallel to the lamellae, to so called channelling of deformation. General grain boundaries can also significantly influence plastic deformation by stress redistribution due to the compatibility stresses arising from the crystal elastic anisotropy and from the anisotropy of plastic deformation.     

Contribution of tilt boundaries to the total energy spectrum of grain boundaries in polycrystals

By measuring temperatures T _w for the transition from the incomplete to complete wetting of grain boundaries in poly- and bicrystals, the width of the spectrum of tilt grain boundaries and their contribution to the total energy spectrum of grain boundaries in polycrystals have been experimentally estimated. It has been shown that the tilt grain boundaries correspond to a rather narrow (only 5–10%) portion in the total energy spectrum of grain boundaries in polycrystals. In metals with a low stacking fault energy (copper, tin, zinc), the tilt grain boundaries belong to 10–20% of the grain boundaries with the highest transition temperatures T _w (hence, with low energies). In a metal with a high stacking fault energy (aluminum), the values of T _w for the tilt grain boundaries lie nearly in the middle between the minimum (T _w,min) and maximum (T _w,max) transition temperatures from the incomplete to complete wetting of grain boundaries. This means that grain boundaries with the structure corresponding to a lower energy than that of the symmetric twin boundaries (or stacking faults) can exist in aluminum.     

Grain Boundary Dynamics: A Novel Tool for Microstructure Control

The reaction of grain boundaries to a wide spectrum of forces is reviewed. Curvature, volume energy and mechanical forces are considered. The boundary mobility is strongly dependent on misorientation, which is attributed to both grain boundary structure and segregation. In magnetically anisotropic materials grain boundaries can be moved by magnetic forces. For the first time a directionality of boundary mobility is reported. Flat boundaries can also be moved by mechanical forces, which sheds new light on microstructure evolution during elevated temperature deformation. Curvature driven and mechanically moved boundaries can behave differently. A sharp transition between the small and large angle boundary regime is observed. It is shown that grain boundary triple junctions have a finite mobility and thus, may have a serious impact on grain growth in fine grained materials. The various dependencies can be utilized to influence grain boundary motion and thus, microstructure evolution during recrystallization and grain growth.     

Theoretical study of hydrogen absorption near symmetric tilt grain boundaries in Pd and TiFe

The results of calculations of the atomic and electron structure of Pd and TiFe with symmetrical Σ5 tilt grain boundaries obtained using the methods of electron density functional theory are reported. Hydrogen sorption at tilt grain boundaries and corresponding surfaces is considered. It is shown that the hydrogen absorption energy increases in magnitude by ∼0.2 eV at the Pd Σ5(210) grain boundary and by ∼0.5 eV in B2-TiFe with the Σ5(310) grain boundary. The binding energy of hydrogen in palladium, as well as in TiFe, in the most preferred positions at the surface is higher than near grain boundaries. It is found that, as in the case of a defect-free material, the following tendency is observed at a symmetrical tilt grain boundary: the strong bond of the impurity at the grain boundary in the metal or alloy matrix reduces the sorption energy of hydrogen.     

Emission of partial dislocations by grain boundaries in nanocrystalline metals

A theoretical model is proposed to describe the emission of partial dislocations by grain boundaries in nanocrystalline materials during plastic deformation. Partial dislocations are assumed to be emitted during the motion of grain-boundary disclinations, which are carriers of rotational plastic deformation. The ranges of the parameters of a defect structure in which the emission of partial dislocations by grain boundaries in nanocrystalline metals are energetically favorable are calculated. It is shown that, as the size of a grain decreases, the emission of partial dislocations by its boundary becomes more favorable as compared to the emission of perfect lattice dislocations.     

Effect of microstructure on fracture in age hardenable Al alloys

ABSTRACT

In the present study, the fracture behaviour of AA6016 alloy was investigated during bending deformation. Wrap-bend tests were conducted and the material was subjected to different bend angles to study crack propagation. The average grain size of the as-received material is approximately 45?μm. The aspect ratio of the grains was changed from 0.53 to 0.40 during bending. The presence of deformation bands was observed during bending in both tensile and compressive regions of the sample. No orientation correlation was observed between the deformation band and its corresponding parent grain. The Schmid factor inside the deformation bands was higher than that of the parent grain, which indicates that the deformation bands accommodate strain during bending. The crystallographic texture evolved significantly during bending deformation. The strength of cube texture component decreases with increasing bend angle and new texture components formed during bending. These new texture components favour either single slip or duplex slip. A mixture of intra-granular and inter-granular fracture occurs during bending. It is observed that inter-granular crack propagation is predominantly favoured along high-angle boundaries, and grain boundary de-cohesion occurs in regions where the misorientation angle is greater than 40°. The formation of deformation-induced coincidence lattice site (CSL) boundaries is also observed during bending and it is shown that the volume fraction of CSL boundaries of Σ3 type increases with increasing bend angle. The current study shows that the formation of deformation-induced CSL boundaries of Σ3 type in AA6016 alloy can improve its inherent resistance to crack propagation during bending.     

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Correlation among grain boundary character,carbide precipitation and deformation in Alloy 690

The correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715^oC for 15?h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.     

Changes in the diffusion properties of nonequilibrium grain boundaries upon recrystallization and superplastic deformation of submicrocrystalline metals and alloys

The effect of an increase in the coefficient of the grain-boundary diffusion upon recrystallization and superplastic deformation of submicrocrystalline (SMC) materials prepared by severe plastic deformation has been studied. It is shown that the coefficient of the grain-boundary diffusion of the SMC materials is dependent on the intensity of the lattice dislocation flow whose value is proportional to the rate of the grain boundary migration upon annealing of SMC metals or the rate of the intragrain deformation under conditions of superplastic deformation of SMC alloys. It is found that, at a high rate of grain boundary migrations and high rates of superplastic deformation, the intensity of the lattice dislocation flow bombarding grain boundaries of SMC materials is higher than the intensity of their diffusion accommodation, which leads to an increase in the coefficient of the grain-boundary diffusion and a decrease in the activation energy. The results of the numerical calculations agree well with the experimental data.     

Grain boundary sliding during ambient-temperature creep in hexagonal close-packed metals

Even at ambient temperature or less, below their 0.2% proof stresses all hexagonal close-packed metals and alloys show creep behaviour because they have dislocation arrays lying on a single slip system with no tangled dislocation inside each grain. In this case, lattice dislocations move without obstacles and pile-up in front of a grain boundary. Then these dislocations must be accommodated at the grain boundary to continue creep deformation. Atomic force microscopy revealed the occurrence of grain boundary sliding (GBS) in the ambient-temperature creep region. Lattice rotation of 5° was observed near grain boundaries by electron backscatter diffraction pattern analyses. Because of an extra low apparent activation energy of 20 kJ/mol, conventional diffusion processes are not activated. To accommodate these piled-up dislocations without diffusion processes, lattice dislocations must be absorbed by grain boundaries through a slip-induced GBS mechanism.