V.Yu. Novikov

Simulation of grain growth in thin films with columnar microstructure

Grain growth in films on substrates was simulated under the suppositions that (1) all the grain boundaries have identical energy and mobility, (2) all the grains are columnar and differ in their diameter only, and (3) the growth process is inhibited by a constant drag force depending on the film thickness. The character of grain growth depends on the ratio of the most probable grain diameter in the initial microstructure to the film thickness. In the case when the ratio corresponds to that observed experimentally, the growth process evolves as abnormal grain growth (AG). It has been demonstrated for the first time that the AG process can develop in the absence of an additional driving force. The cessation of AG, when the fine-grained matrix is still present, was shown to result from the exhaustion of relatively small grains that could be consumed. The additional driving force resulting from a decreased surface energy of some grains leads to an increase of the volume fraction of abnormally large grains and to texture amplification. Texture development during grain growth can also take place in the absence of any additional driving force. Intensification of the AG results in texture enhancement.

Texture development during grain growth in polycrystals with strong preferred orientation

Abstract Texture alterations accompanying grain growth have been simulated by means of a new statistical model which for the first time takes into account the dependence of both the energy and mobility of grain boundaries on their disorientation. Model polycrystals with a single-component texture are assumed to consist of preferably oriented grains A (the main texture component) and randomly oriented grains B. Therefore, there are three types of grain boundaries in these polycrystals: high-angle A–B and B–B boundaries, and A–A boundaries with a smaller disorientation. A novel map showing the effect of both the scatter and intensity of the main texture component on the character of grain growth is obtained. Abnormal grain growth leads eventually to vanishing of the main texture component. However, the main component enhances at the incubation period of abnormal grain growth, especially in the case when the initial distribution of randomly oriented grains is displaced toward small sizes, as in materials with cube texture. The texture evolution during normal grain growth is strongly affected by the initial position of the size distribution of randomly oriented grains. If this distribution is displaced toward small sizes, the main texture component is enhanced. In contrast, normal grain growth results in vanishing of the main component if the initial size distributions of grains A and B are identical. Various texture evolutions observed in the simulations are shown to be a result of an interplay of the consumption and the growth of randomly oriented grains during the growth process.

Influence of heterogeneous nucleation on microstructure and kinetics of primary recrystallization

Abstract The influence of heterogeneous arrangement of nucleation sites and non-simultaneous appearance of nuclei during recrystallization on size distribution of recrystallized grains is studied by computer simulation. It is shown that the log-normal distribution in two-dimensional microstructure can be obtained at a certain combination of nucleation characteristics. A possible role of the other parameters of recrystallization process in microstructure development is discussed. Despite the nucleation heterogeneity, the kinetics of recrystallization is shown to obey the Avrami equation. It is supposed that the shape of the grain size distribution should be used as a characteristic which proves the correctness of any recrystallization model.

Computer simulation of microstructure evolution during secondary recrystallization

Abstract A brief description of a grain growth model and the results of its verification in the case of secondary recrystallization (SR) in 2D polycrystal are given. The variation coefficient of grain size distribution is shown to be a metric characteristic sensitive to the beginning of SR. The average grain size in topological class changes linearly with the class number, the coefficient of proportionality for secondary grains being six times greater than that for primary ones. The average topological class of grains adjacent to the growing one is inversely proportional to the topological class of the latter. These results lead to the conclusion that the average size of the former is smaller than the average size of matrix grains, which provides increased driving force for growth of large secondary grains.

Secondary recrystallization or normal grain growth

Abstract The grain growth process in the presence of pinning force was investigated by computer simulation. The pinning force was assumed to change its value from high to low, these changes being proceeded at different rates. The initial value of the pinning force was chosen in such a way that the changes of the average grain size were suppressed. It was shown that the mode of growth process during reduction of the pinning force is either normal grain growth or secondary recrystallization with the subsequent normal grain growth depending on both the rate of the pinning force reduction and the initial magnitude of the pinning force. An abrupt reduction of the pinning force leads to normal grain growth that can evolve more rapidly than in the presence of a low pinning force from the very beginning of the process. A gradual reduction of the pinning force provides the development of secondary recrystallization in case a noticeable grain size nonhomogeneity is evolved prior to the reduction of the pinning force.

Abnormal Grain Growth: Effect of Disperse Particles

Problems arising due to abnormal grain growth, the main of which concerns determination of the start and end of this process in materials with various initial microstructures and the behavior of disperse particles, are considered. Grain growth is studied by the method of numerical simulation. A pioneer diagram presenting the conditions of development and suppression of abnormal grain growth at a fixed rate of dissolution of particles is plotted in the “initial grain size – initial retarding force” coordinates. The causes of abnormal grain growth in nanocrystalline materials are analyzed, of which the principal one is supposed to be the presence of submicroscopic pores. The results obtained are used to develop novel methods for controlling the grain structure in various structural and functional materials.

Analog Studies of Thermomechanical Fatigue and Abrasive Wear of Cast and Forged Steels for “Autoforge” Dies

Processes of thermomechanical fatigue and abrasive wear of suspension-cast precipitation-hardening ferrite-carbide steel 30T6NTiC-1.5 and standard steel 4Kh5MFS are studied. The dominant kinds of fracture typical for dies for semisolid stamping are determined. The factors and parameters of cyclic temperature and force loading are shown to produce a selective action on the competing kinds of damage of the die steels. A comparative analysis of the properties of the steels is performed. Steel 30T6NTiC-1.5 is shown to have substantial advantages over steel 4Kh5FMS traditionally used for making “Autoforge” dies.

Initial magnetic permeability and microstructure of Fe-50% Ni alloy

Abstract It has been shown that in annealed commercial Fe—50% Ni alloy with random grain orientation, where coercivity is linearly dependent on the specific surface area of grain and twin boundaries, S, the initial permeability is strongly connected with S.

Recrystallization of heterophase alloys

Conclusions1.A dispersed second phase changes the recrystallization process in alloys, the distance between the particles being the controlling factor.2.Large particles with relatively large distances between them accelerate the formation of recrystallization nuclei, causing an uneven distribution of dislocations in the deformed material and creating dislocation sinks during annealing.3.Dispersed precipitates with small (<1–2 μ) distances between particles slow down recrystallization and permit retention of a considerably larger portion of cold hardening during heating to high temperatures.4.Fine particles of second phase existing in the metal before deformation slow down the formation of a cellular structure during deformation, broadening the cell walls, and leading to cells with small misorientation. This inhibits the formation of subgrains and their growth during annealing.5.Particles of second phase existing in the subgrain boundaries slow down their growth and retard the formation of recrystallization nuclei.6.Coalescence or solution of particles of second phase can induce accelerated growth of subgrains, formation of recrystallization nuclei and subsequent softening, with slow recovery in the deformed matrix.7.Coherent precipitates of second phase slow down recrystallization, inhibiting the rearrangement of dislocations and the movement of the boundaries of growing grains.8.Segregates in the boundaries of deformed grains and subgrains formed during annealing of deformed supersaturated solid solutions also slow down recrystallization. In this case the recrystallization texture also changes as the result of the difference in the holding of subgrain boundaries with different orientations.