Materials Science Forum Vols. 715-716

Abstract: Simulation of mobility-driven abnormal grain growth in the presence of particles in a 3D Potts Monte Carlo model has been investigated, and even though the driving force in this case is identical to normal grain growth, Zener pinning does not occur. Instead the particles seem merely to have a small inhibiting effect on the number of abnormal grains, and this effect only has a noticeable influence for volume fractions of particles above 5 vol%.

Abstract: Formation behavior of texture during high temperature compression of AZ80 is investigated on the specimens produced from the extruded bar and rolled plate. Special attention is paid on the formation of (0001) (compression plane) texture. Before deformation, (0001) is distributed frequently parallel to the compression plane in the specimen produced from the rolled plates, while (0001) is frequently normal to the compression plane in the specimen produced from the extruded bar. The deformation is performed at temperatures and strain rates ranging from 673K to 723K and 1.0×10^-4s^-1 to 5.0×10^-2s^-1, respectively. It is found that fiber textures are formed by the deformation and the main component of the texture varies depending on the deformation conditions. The sharpness of (0001) texture after deformation in the specimen produced from extruded bar is much weaker than that in the specimen produced from rolled plate. It is concluded that the growth of (0001) grains during deformation is the basic process for the (0001) texture evolution.

Abstract: The analysis of deformed and recrystallized zirconium, used in nuclear industry is presented. The main purpose of the present work is to describe and analyze changes in texture, microstructure and misorientation profile, which are observed during a complete thermomechanical treatment. Zirconium samples were channel compressed till various degrees of deformation. The samples were then annealed. In both deformed and recrystallized states, topological maps were measured using the well known EBSD (Electron Backscatter Diffraction) technique. The obtained data were necessary for further analysis which consisted of several approaches: quantification of textures, identification of principal components, volume fractions, misorientations, grain size and IQ histograms. For the misorientation analysis, several parameters have also been tested (ie. correlated misorientation profile, grain average misorientation, kernel, etc.). Based on this complete set of data a scenario is proposed to explain the observed microstructural evolutions.

Abstract: The effect of a magnetic field on texture and microstructure development in cold rolled (80%) commercially pure zirconium (Zr701) was investigated. X-ray diffraction and EBSD measurements were utilized for the texture and microstructure characterization. The results revealed that a magnetic field promotes grain growth in the investigated material. During annealings at 550°C this is particularly apparent from the faster development of specific (0/180, 35, 30) texture components and the bigger mean grain size after magnetic annealing. The magnetic annealing at 700°C resulted in an asymmetry of the two major texture components. This is due to a magnetic driving force for grain growth arising from the anisotropic magnetic susceptibility of zirconium. During annealing at 700°C the abnormal grain growth occurred. This behavior is attributed to the higher mobility of grain boundaries between grains misoriented by 30° around [000. The magnetic field essentially enhanced the observed abnormal grain growth.

Abstract: Cu/Ni composite electrodeposit was fabricated by electroplating nickel on the both sides of an electroplated copper sheet. In order to lower interfacial stresses between copper and nickel, the microstructure of nickel was controlled to consist of grains with a mean size of 15 nanometers. The different parts of the composite electrodeposit underwent different evolution of textures and microstructures during annealing. In the Cu electrodeposit, the as-deposited texture characterized by a relatively high <100>//ND and twin components transformed to be diffuse due to grain growth during annealing above 300°C. This is attributed to a large number of twins conducting the as-deposited microstructure. On the other hand, in the Ni electrodeposit, grain growth that takes place during annealing above 250°C corresponds to abnormal grain growth in terms of the scale change of the grain size. This grain growth also transformed the as-deposited texture of strong <100>//ND into a diffuse texture. In the interface between copper and nickel, the atomic diffusion was generated by excessive vacancies resulting from the grain growth during the annealing of nanostructured Ni electrodeposit. An 'interface texture' began to developed in the previous Cu region above 500°C, and the microtexture development was similar to the growth texture of the annealed Ni electrodeposit.

Abstract: Effect of carbide precipitation on pinning force and migration mechanism of boundaries of martensite laths was considered in a 3%Co modified P911. The dimensions of second phase precipitations, martensite laths and dislocation densities were measured by means of transmission electron microscopy. The pinning forces retarding the motion of the lath boundaries, that arise from M(C,N) nanoscale precipitations and M₂₃C₆ particles were evaluated by using different models. The pinning pressure evaluated by taking into account a non-uniform distribution of M₂₃C₆ particles was high enough to stabilize the lath martensite structure during tempering and long term ageing. On the other hand, significant coarsening of martensite laths occurred in neck portions of samples subjected to long-term creep tests. Additional effects from dislocation density and applied stress on the motion of lath boundaries are considered in some details.

Abstract: The present simulations have clearly demonstrated that the kinetics, as derived directly from the 3D Potts Monte Carlo simulations, deviate strongly from the classical JMAK theory. The Avrami plots exhibit a strong initial transient and the Avrami exponents are far from constant and generally much lower than predicted by the classical JMAK theory. However, by introducing a suitable time delay, t₀, due to a non-zero volume fraction of recrystallized grains at the start-up of the simulations, this initial transient can be removed and the Avrami plots are made close to linear at the same time as the Avrami exponent is in better agreement with theory.

Abstract: In this paper, modelling and plotting of recrystallization curves of copper-titanium powder materials with titanium content of 0.5%, porosity 5% and 10%. The mathematical model that describes an influence of temperature, degree of deformation, strain rate, initial grain size and porosity to grain size after deformation has developed. The interconnection of deforming parameters and structure has presented by function of several variables with analytical expression obtained by method of undetermined coefficients based on experimental data. Theoretical recrystallization curves for copper-titanium powder materials with different porosity have plotted. It has established that porosity decelerates the kinetics of structure formation during dynamical softening of porous powder materials.

Abstract: In recent papers Rios and Villa resorted to developments in stochastic geometry to revisit theclassical KJMA theory and generalize it for situations in which nuclei were located in space accordingto both homogeneous and inhomogeneous Poisson point processes as well as according to Materncluster process and surface and bulk nucleation in small specimens. Rigorous mathematical methodswere employed to ensure the reliability of the new expressions. These results are briefly described.Analytical expression for inhomogeneous Poisson point process nucleation gives very good agreementwith Cellular Automata simulations. Cellular Automata simulations complement the analyticalsolutions by showing the corresponding microstructural evolution. These new results considerablyexpand the range of situations for which analytical solutions are available.

Abstract: The relaxation-precipitation-controlling phase transformation (RPC) technique after deformation at non-recrystallization zone to refine the intermediate transformation microstructure has been simulated on a Gleeble-1500 thermo-simulator. The optical microscope, SEMTEMPTA(particle tracking autoradiography) technique to reveal the boron distribution were employed to study the variation of austenite grain size and subgrain size, the features of microstructure after RPC, precipitation and the evolution of dislocation configuration during the relaxation and the boron distribution. The results show that after relaxation at non-recrystallization zone, the subgrain formed inside an original austenite grain. With the relaxation time increasing, the size of the subgrains increased and the misorientation also increased. During the cooling after the relaxation the boron can also segregate at the boundaries of subgrains and the boron segregation can reveal the subgrains forming in deformed austenite before phase transformation. It has been found that during the relaxation strain induced precipitates occurs and these precipitates can pin the subgrain boundary and make it more stable. Comparing the subgrain size demonstrated by PTA with the optical microstructure a conclusion can be drawn that the packet of bainite generally cannot break through the boundaries of subgrains, so the subgrain appearing at the relaxation stage can confine the growth of the microstructure during the transformation in succeeding and the final bainite is refined.