Static Grain Growth Research Articles

Mechanical Grain Growth in Nanocrystalline Copper

Nanograined materials have some unusual properties. To maintain the small size of the grains, grain growth should be avoided. But recently grain growth has been observed under an indenter at liquid-nitrogen temperatures. Such grain growth has never been reported before. How can this happen and how can it be prevented? These questions are answered here using a simple tilt boundary. It is found that high purity and nonequilibrium structure are necessary conditions for mechanical grain growth. The material must be pure enough so that free dislocations are available to move out of the boundary. But the boundary should not be in the lowest-energy state so that extra dislocations are available to be emitted by stress. Based on these conditions, methods can be devised to avoid low temperature grain growth.

High-pressure torsion-induced grain growth in electrodeposited nanocrystalline Ni

Deformation-induced grain growth has been reported in nanocrystalline (nc) materials under indentation and severe cyclic loading, but not under any other deformation mode. This raises an issue on critical conditions for grain growth in nc materials. This study investigates deformation-induced grain growth in electrodeposited nc Ni during high-pressure torsion (HPT). Our results indicate that high stress and severe plastic deformation are required for inducing grain growth, and the upper limit of grain size is determined by the deformation mode and parameters. Also, texture evolution suggests that grain-boundary-mediated mechanisms played a significant role in accommodating HPT strain.

Strain–anneal growth of Zr 701 large crystals

The present paper establishes the optimal conditions for grain growth in low alloyed Zr during strain–annealing. It is shown that it is possible to produce quite large crystals, whose maximum size (8–9 mm) is of the order of magnitude of the largest crystals normally produced in pure zirconium. Micro-tensile specimens could be produced containing a single grain in the effective gauge length. Strain-induced boundary migration is then proposed as the preferential growth mechanism, and a simple calculation allows one to conclude that both tilted {0001}〈10 1 0〉 and {0001}〈11 2 0〉 orientations have a growth advantage linked to a predicted minimum stored energy.

Fabrication of large single-domain Sm123 superconductors by OCMG method

Melt processing of Sm-Ba-Cu-O bulk superconductors was studied to find the optimum conditions for fabricating a large single-domain superconductor. First, the holding time at the maximum temperature for the partial melting is important for the fabrication of a uniform bulk, especially for a large diameter. When the holding time at the maximum temperature during partial melting was not enough, two separated regions were observed, which can be diminished by sufficient holding time. Next we studied the optimum conditions for grain growth, varying the temperature gradient and cooling rate. The processing condition with which we have succeeded in growing a single-domain SmBa 2Cu 3O y (Sm123 hereafter) with dimensions of 35 φ × 15 mm 2 is a temperature gradient of 7.5 °C cm −1 with a slow cooling rate of 0.33 °C h −1.

Abnormal Grain Growth: Computer Simulation

The data on nucleation and necessary conditions for abnormal grain growth obtained by computer modelling are presented. Microstructure in the model description of grain growth is characterized by the grain size distribution and the distribution of the grain boundary properties. In a polycrystal with random grain orientation a pinning force promoting a completed abnormal grain growth should decrease in two stages i.e., first a slow one associated with particle coarsening or particle dissolution and then a rapid one caused by particle dissolution. In a polycrystal with weak texture a slightly increased boundary mobility of some large grains can lead to their rapid growth which results in the beginning of abnormal grain growth as well as in the appearance of a strong texture provided the pinning force is sufficient for matrix stabilization. In a polycrystal with a strong single-component texture, abnormal grain growth can evolve at both an intermediate scatter and a high intensity of the main component. An increased mobility of the boundaries with a thin liquid layer cannot alone provide the development of abnormal grain growth in oxide ceramics.

Texture Development by Templated Grain Growth in Liquid‐Phase‐Sintered α‐Alumina

The distribution and orientation of platelet‐shaped particles of α‐alumina in a fine‐grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α‐Al2O3 grains after sintering at 1400°C. A CaO + SiO2 liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.

The theory of structural superplasticity—IV. Cavitation during superplastic deformation

The mechanism for nucleation of cavities at the grain triple junctions and on the second-phase disperse particles located in the grain boundaries was proposed. Nucleation of cavities at the grain triple junctions is connected with the mismatch in the plastic rotations caused at the grain boundaries by accumulation of sessile components of spreading dislocations. Cavitation on the disperse particles is due to the defect layer which is forming on the precipitate-matrix interphase and whose internal stress field and elastic energy increase during intergrain deformation. Peculiarities of realization of the mentioned mechanisms in the conditions of active grain growth were considered. Expressions were obtained which describe the dependence of the number of cavities per unit area and their volume fraction on the rate and extent of superplastic deformation as well as on the parameters of the material microstructure.

Influence of precipitate dispersion and texture differences through the sheet thickness on selective grain growth in silicon-iron

Two steels having slightly different chemical compositions were transformed by a conventional two-stage cold rolling process. Different dispersions of second phase particles (grain growth inhibitors) were obtained by controlling the Mn/Cu ratio in the steels, which ultimately came to have significantly different core losses as a consequence of a different magnetic permeability and the size of the secondary recrystallized grains. After primary recrystallization and just before secondary recrystallization, the steels were analyzed for grain size distribution, texture, and particle size evolution. The size of every single grain was classified in relation to its orientation in different positions through the sheet thickness. The evolution of grain size distribution for grains near (±10°) the {110}〈001〉 was found to differ from that for grains with the other orientations. This is in agreement with the theory of grain growth in the presence of texture and special orientation relationships between classes of grain having different orientation. Magnetic properties were related to the starting conditions (frequency and dispersion around the exact Goss orientation) of grain growth after primary recrystallization. The amount of inhibitor and the effect of process parameters (like temperature of intermediate annealing, partition of cold rolling degree, etc.) were found to play a decisive role in improving the intensity of {110}〈001〉 grains and in reducing their dispersion after primary recrystallization.

Two-step continuous sintering schedules for β″ -Al 2O 3 ceramics

A method was developed for preparing ion-conductive membranes of Li 2O-stabilized β″ -Al 2O 3 with a fine-grained microstructure, density above 98% of the theoretical one and resistivity of 5–8 Ω cm. without additional thermal treatment. To this effect powders which contained about 90% β″ -Al 2O 3 were synthesized. A two-step continuous sintering schedule was applied with temperatures of the first step above, and of the second step around the melting point of the β″-A1 2O 3 - NaAlO 2 eutectic respectively. Under the conditions of this firing schedule grain growth is inhibited and a high degree of densification is attained with a minimum transient liquation of the eutectic. The contribution of each of the firing steps to the main characteristics of the membranes is assessed. The relationship between the resistivity, the phase composition and the microstructure is discussed.

Grain growth and densification during liquid phase sintering of W-Ni

Mixtures of single crystalline spherical W particles of 200–250 μm dia., fine W powder of 10 μm size and fine Ni powder were sintered at 167°C. Depending on the amount of liquid phase present complete densification is obtained after different sintering times and this is always accompanied by pronounced growth of the large W spheres at the expense of the smaller grains. With limited amounts of liquid phase present, under the influence of the densification force arising from the pores, grain growth occurs preferentially away from the areas where the grains are in close contact. Thus grain shape accommodation occurs during growth and this enables complete densification. In order to achieve rapid densification during liquid phase sintering, conditions of rapid grain growth are therefore favourable.