scholarly journals Механизм упрочнения ультрамелкозернистого алюминия после отжига

2019 ◽  
Vol 61 (10) ◽  
pp. 1836
Author(s):  
М.Ю. Гуткин ◽  
Т.А. Латынина ◽  
Т.С. Орлова ◽  
Н.В. Скиба
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Quantitative Agreement ◽  
Triple Junctions ◽  
Energy Characteristics ◽  
Low Temperature Annealing ◽  
Critical Stresses ◽  
Ultrafine Grained ◽  
Sequential Transformation

A theoretical model is proposed that describes the mechanism of hardening of ultrafine-grained aluminum, obtained by severe plastic torsion deformation, after low-temperature annealing. In the framework of the model, hardening is realized due to the sequential transformation of the grain-boundary dislocation structure. In particular, plastic deformation occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the bulk of the grain, and the formation of walls of grain-boundary dislocations climbing along opposite grain boundaries. The energy characteristics and critical stresses for the emission of lattice dislocations are calculated. The theoretical dependences of the flow stress on the plastic deformation are plotted, which show good qualitative and quantitative agreement with experimental data.

Effect of Grain Boundary State on Diffusion and Diffusion-Controlled Processes in Ultrafine-Grained Materials Processed by Severe Plastic Deformation

2015 ◽  
Vol 5 ◽  
pp. 111-126
Author(s):  
Evgeny V. Naydenkin ◽  
Galina P. Grabovetskaya ◽  
I.P. Mishin
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Severe Plastic Deformation ◽  
Grain Boundaries ◽  
Experimental Studies ◽  
Boundary State ◽  
Diffusion Controlled ◽  
Ultrafine Grained ◽  
Ultrafine Grained Materials ◽  
And Diffusion

Experimental studies on the grain boundary diffusion and processes controlled by it in the ultrafine-grained metallic materials produced by various methods of severe plastic deformation are reviewed. Correlation between the increased diffusion permeability of grain boundaries and features of recrystallization and deformation development in these materials possessing the non-equilibrium state of grain boundaries formed during severe plastic deformation in the temperature range of T < 0.35Tm is demonstrated and analyzed.

Grain Boundary Design of Bulk Nanomaterials for Advanced Properties

2015 ◽  
Vol 5 ◽  
pp. 43-54 ◽  
Author(s):  
Ruslan Z. Valiev
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Severe Plastic Deformation ◽  
Grain Boundaries ◽  
Metals And Alloys ◽  
New Approach ◽  
Petch Relation ◽  
Ultrafine Grained ◽  
Grain Boundary Segregations ◽  
Bulk Nanomaterials

Nanostructuring of metals and alloys by severe plastic deformation techniques is an effective way of enhancing their mechanical and functional properties. The features of the nanostructured materials produced by severe plastic deformation (SPD) are stipulated by forming of ultrafine-sized grains as well as by the state of grain boundaries. The concept of grain boundary (GB) design of ultrafine-grained metals and alloys is developed for enhancement of their properties by tailoring grain boundaries of different types (low-angle and high-angle ones, special and random, equilibrium and nonequilibrium) and formation of grain boundary segregations and precipitations by SPD processing. The paper presents experimental data demonstrating the super-strength and “positive” slope of the Hall-Petch relation when passing from micro-to nanostructured state in a number of metallic materials subjected to severe plastic deformation. The nature of the superior strength is associated with new strengthening mechanisms and the difficulty of generation of dislocations from grain boundaries with segregations. This new approach is used for achieving the enhanced strength in several commercial Al and Ti alloys as well as steels subjected to SPD processing.

Atomistic Modeling of Grain Boundaries and Dislocation Processes in Metallic Polycrystalline Materials

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Douglas E. Spearot ◽  
David L. McDowell
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Behavior ◽  
Complex Stress ◽  
Boundary Structure ◽  
Polycrystalline Materials ◽  
Atomistic Modeling ◽  
Triple Junctions

The objective of this review article is to provide a concise discussion of atomistic modeling efforts aimed at understanding the nanoscale behavior and the role of grain boundaries in plasticity of metallic polycrystalline materials. Atomistic simulations of grain boundary behavior during plastic deformation have focused mainly on three distinct configurations: (i) bicrystal models, (ii) columnar nanocrystalline models, and (iii) 3D nanocrystalline models. Bicrystal models facilitate the isolation of specific mechanisms that occur at the grain boundary during plastic deformation, whereas columnar and 3D nanocrystalline models allow for an evaluation of triple junctions and complex stress states characteristic of polycrystalline microstructures. Ultimately, both sets of calculations have merits and are necessary to determine the role of grain boundary structure on material properties. Future directions in grain boundary modeling are discussed, including studies focused on the role of grain boundary impurities and issues related to linking grain boundary mechanisms observed via atomistic simulation with continuum models of grain boundary plasticity.

Fracture Strengths of Individual Grain Boundaries in Ni3Ai Using a Miniaturized Disk Bend Test

1991 ◽  
Vol 238 ◽  
Author(s):  
Douglas E. Meyers ◽  
Alan J. Ardell
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Coincident Site Lattice ◽  
Bend Test ◽  
High Angle ◽  
Load Drop ◽  
Site Lattice ◽  
Special Boundaries

ABSTRACTThe results of our initial efforts at measuring the fracture strengths of grain boundaries In Ni3Al using a miniaturized disk-bend test are presented. The samples tested were 3 mm in diameter and between 150 and 300 μm thick. An Ingot of directlonally-solidlfled, boron-free Ni3Al containing 24% Al was annealed between 1300 and 1350 °C to induce grain growth, producing many grain boundaries In excess of 1.5 mm in length. Specimens were cut from these In such a way that one long grain boundary was located near a diameter of the specimen. The relative orientations of the grains on either side of the boundary were determined from electron channeling patterns. Low-angle boundaries are so strong they do not fracture; Instead the samples deform In a completely ductile manner. High-angle boundaries always fracture, but only after considerable plastic deformation of the two grains flanking them. Fracture is Indicated by a load drop in the load vs. displacement curves. A method involving extrapolation of the elastic portion of these curves to the displacement at fracture is used to estimate the fracture stresses. This procedure yields consistent values of the fracture strengths of high-angle boundaries. The measured stresses are large (∼2 to 3 GPa), but considerably smaller than those required for the fracture of special boundaries, as predicted by computer simulations. No correlation was found between the fracture stresses or loads and the geometry of the high-angle boundaries, many of which are close to, but deviate from, coincident site lattice orientations.

Molecular Dynamics Analysis of the Acceleration of Intergranular Cracking of Ni-Base Superalloy Caused by Accumulation of Vacancies and Dislocations Around Grain Boundaries

2020 ◽  
Author(s):  
Ryo Kikuchi ◽  
Shujiro Suzuki ◽  
Ken Suzuki
Keyword(s):  
Molecular Dynamics ◽  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fatigue Loading ◽  
Transgranular Fracture ◽  
Dynamics Analysis ◽  
Intergranular Cracking ◽  
Creep Fatigue ◽  
Schmid Factor

Abstract Ni-based superalloys with excellent high temperature strength have been used in advanced thermal power plants. It was found that grain boundary cracking is caused in the alloy under creep-fatigue loading due to the degradation of the crystallinity of grain boundaries and the grain boundary cracking degrades the lifetime of the alloy drastically. In order to clarify the mechanism of intergranular cracking, in this research, static and dynamic strains were applied to a bicrystal structure of the alloy perpendicularly to the grain boundary using molecular dynamics analysis. In addition, the effect of the accumulation of vacancies in the area with high-density of dislocations on the strength of the bicrystal structure was analysed. It was found that the fracture mode of the bicrystal structure changed from ductile transgranular fracture to brittle intergranular one as strong functions of the combination of Schmid factor of the two grains and the density of defects around the grain boundary. The local heavy plastic deformation occurred around the grain boundary with large difference in Schmid factor between nearby grains and the diffusion of the newly grown dislocations and vacancies was suppressed by the large strain field due to the large mismatch of the crystallographic orientation between the grains. The accumulation of vacancies accelerated the local plastic deformation around the grain boundary. Therefore, the mechanism of the acceleration of intergranular cracking under creep-fatigue loading was successfully clarified by MD analysis.

scholarly journals Molecular Dynamics Study on the Impact of Cu Clusters at the BCC-Fe Grain Boundary on the Tensile Properties of Crystal

Metals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1533
Author(s):  
Haichao Zhang ◽  
Xufeng Wang ◽  
Huirong Li ◽  
Changqing Li ◽  
Yungang Li
Keyword(s):  
Molecular Dynamics ◽  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Tensile Properties ◽  
Structural Phase ◽  
Structural Phase Transformation ◽  
Tensile Process ◽  
Deformation Ability ◽  
The Impact

The molecular dynamics (MD) method was used to simulate and calculate the segregation energy and cohesive energy of Cu atoms at the Σ3{111}(110) and Σ3{112}(110) grain boundaries, and the tensile properties of the BCC-Fe crystal, with the grain boundaries containing coherent Cu clusters of different sizes (a diameter of 10 Å, 15 Å and 20 Å). The results showed that Cu atoms will spontaneously segregate towards the grain boundaries and tend to exist in the form of large-sized, low-density Cu clusters at the grain boundaries. When Cu cluster exists at the Σ3{111}(110) grain boundary, the increase in the size of the Cu cluster leads to an increase in the probability of vacancy formation inside the Cu cluster during the tensile process, weakening the breaking strength of the crystal. When the Cu cluster exists at the Σ3{112}(110) grain boundary, the Cu cluster with a diameter of 10 Å will reduce the strain hardening strength of the crystal, but the plastic deformation ability of the crystal will not be affected, and the existence of Cu clusters with a diameter of 15 Å and 20 Å will suppress the structural phase transformation of the crystal, and significantly decrease the plastic deformation ability of the crystal, thereby resulting in embrittlement of the crystal.

Mechanical Behaviour of Nanocrystalline Copper Related to Grain-boundary Structure

2002 ◽  
Vol 727 ◽  
Author(s):  
Y. Champion ◽  
P. Langlois ◽  
S. Guérin-Mailly ◽  
C. Langlois ◽  
M. J. Hÿtch
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Structural Analysis ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Mechanical Behaviour ◽  
Tensile Testing ◽  
Metallic Nanostructures ◽  
Nanocrystalline Powders ◽  
Clear Understanding

AbstractUnderstanding the mechanical behaviour of metallic nanostructures is a key issue for their development. On the one hand, knowledge of the plastic behaviour at various temperatures is essential to control the synthesis, forming, and machining of such materials. Equally, a clear understanding of atomic and mesoscopic mechanisms, involving defects and their interactions, is essential for the control of ageing and functional properties. Regarding plastic deformation at room temperature, there is now evidence for unusual behaviour in nanostructured metals. In addition to high resistance and ductility, tensile testing reveals peculiar elasto-plastic deformation. Such behaviour was initially attributed to grain-boundary sliding. However, intergranular areas (including triple junctions) may possess special properties compared to their microcrystalline counterparts. For example, low activation energies have been measured for grain-boundary diffusion and it has been observed that grain-boundaries may act as dislocation sources and nucleation sites for deformation twinning.In this paper, we report on analysis on bulk copper nanostructures. Grain-boundaries are studied, by cross-correlating information from mechanical tensile testing and structural analysis, including X-ray diffraction (XRD) and transmission electron microscopy (TEM). Macroscopic bulk specimens (with grain size of about 80 nm) are prepared by powder metallurgy techniques, modified to fit to the special properties of nanocrystalline powders. Processing includes coldisostatic pressing, sintering and differential extrusion. The powders used (grain size of 40 nm) are synthesised by evaporation and cryo-condensation of a metallic vapour within liquid nitrogen. Results on mechanical testing and structural analysis will be reported. Emphasis will be placed on the structure of grain-boundaries (type of grain-boundary, grain-boundary thickness) studied by TEM and high resolution TEM image analysed using the geometric phase technique. The nanostructure was revealed to be consist in agglomerate of nano-size grains separated by low angle grain-boundaries. Agglomerates are themselves separerated by general high angle boundaries. These observations will then be related to the unusual mechanical true stress-true strain curves of the metallic nanostructures.

Mechanism of Stress-Driven Grain Boundary Migration in Nanotwinned Materials

2018 ◽  
Vol 55 (1) ◽  
pp. 21-25 ◽  
Author(s):  
N.V. Skiba
Keyword(s):  
Theoretical Model ◽  
Grain Boundary ◽  
Twin Boundary ◽  
Boundary Migration ◽  
Grain Boundary Migration ◽  
High Angle ◽  
Twin Boundaries ◽  
Critical Stresses ◽  
Ultrafine Grained ◽  
Ultrafine Grained Materials

Abstract Stress-driven grain boundary (GB) migration in ultrafine-grained materials with nanotwinned structure is theoretically described. In the framework of the theoretical model, the stress-driven high-angle GB migration is accompanied by migration of twin boundaries which adjoin this GB. Energetic characteristics and critical stresses of the GB migration accompanied by the twin boundary migration are calculated.

Evaluation of Plastic Work Density, Strain Energy and Slip Multiplication Intensity at Some Typical Grain Boundary Triple Junctions

2010 ◽  
Vol 654-656 ◽  
pp. 1283-1286 ◽  
Author(s):  
Tetsuya Ohashi ◽  
Michihiro Sato ◽  
Yuhki Shimazu
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Crystal Plasticity ◽  
Strain Energy ◽  
Plastic Work ◽  
Slip Systems ◽  
Triple Junctions ◽  
Plastic Slip ◽  
Boundary Triple

Plastic slip deformations of tricrystals with simplified geometries are numerically analyzed by a FEA-based crystal plasticity code. Accumulation of geometrically necessary (GN) dislocations, distributions of the total slip, plastic work density and GN dislocations on slip systems, as well as some indices for the intensity of slip multiplication are evaluated. Results show that coexistence of GN dislocations on different slip systems is prominent at triple junctions of grain boundaries.

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