Materials Science Forum Vols. 495-497

Abstract: One very often observes that the texture inheritance in BCC to HCP phase transformation shows variant selections, even though no external stress field is applied. These variant selections are related to the metallurgical state, the microstructure and the texture of the parent phase. From our own investigations, we came to the conclusion that the variant selections we observed in some phase transformations of various materials were influenced at different degrees by the elastic behaviour of the parent phase. Considering the transformation strain of each variant and the elastic anisotropy of the parent, we have build variant selection models based on energy minimum of elastic strain and assuming different types of interactions. The simulation results of texture transformation of a zircalloy sample show that the elastic characteristics of the parent phases are key parameters involved in the variant selection.

Abstract: While electron backscatter diffraction (EBSD) has become an established technique within materials characterization labs around the world, the technique is still relatively young and new applications are continuing to emerge. Automated EBSD or Orientation Imaging Microscopy (OIM) systems are being used in combination with other equipment within the scanning electron microscope (SEM) to perform in-situ measurements. This includes tensile stages for observing changes in local orientation during deformation and heating stages for studying orientation changes arising during recrystallization and grain growth as well as phase transformations. In addition to these temporally three-dimensional studies, spatially three-dimensional studies can be performed by in-situ serial sectioning in microscopes equipped with both electron and focused ion beams. These in-situ techniques are briefly reviewed. The review is followed by a detailed analysis of in-situ heating experiments on copper. The movement of grain boundaries during recrystallization and subsequent grain growth are tracked. The effect of orientation relationships on grain boundary mobility and nucleation are explored. No special relationship with grain boundary mobility was observed. However, twins appear to play a significant role in the nucleation process.

Abstract: High magnetic fields were applied to the austenite to proeutectoid transformation and tempering process in a 42CrMo steel. The thermodynamic and kinetic effects of the high magnetic field on the austenite decomposition show that it can obviously increase the amount of the product ferrite and accelerate the transformation by enhancing the Gibbs free energy difference between the parent and product phases. Moreover, the magnetic field can considerably lower the amount of low angle misorientations of ferrite in pearlite colonies and obviously increase the frequency of S3-29 coincidence boundaries, especially S3 boundaries, of the ferrite. But it has no obvious effect on crystallographic orientation distribution. When the field is applied to the high temperature tempering process, it can effectively prevent the directional growth of cementite along martensite plate boundaries and twin boundaries by increasing both the cementite/ferrite interfacial energy and the magnetostrictive strain energy. Finally, particle-like cementite is obtained. The magnetic field also obviously retards the formation and growth of the ‘distortion-free’ regions of the matrix.

Abstract: The mechanisms by which textures can be inherited in transformed phases are discussed in the light of different transformation mechanisms. Possible origins of variant selection in the different cases are reviewed and classified. Evidence is presented for a hitherto unsuspected source of variant selection that arises from the stresses which are generated during transformation due to the presence of micro-segregation. Some model predictions show the potential effect of this phenomenon on textures in bainite or martensite.

Abstract: This paper gives an overview of our recent works on the effect of magnetic annealing, i.e. annealing in a magnetic field, on the evolution of texture and grain boundary microstructure in ultra-fine grained and nanocrystalline magnetic materials differently produced; rapidly solidified Fe-6.5mass%Si ribbons, electrodeposited nanocrystalline nickel, and nanocrystalline Fe78Si9B13 alloy ribbon crystallized from the amorphous state. It was found that the effect of magnetic annealing was powerful and useful for controlling grain growth resulting in the evolution of different types of texture and grain boundary microstructure, depending on the condition of magnetic annealing. In particular, the magnetic crystallization of amorphous Fe78Si9B13 alloy was found to be powerful for producing a nanocrystalline material with a sharp texture and a special grain boundary microstructure.

Abstract: The preferential initiation of dynamic recrystallization (DRX) at triple junctions (TJs) in stainless steel polycrystals was investigated in compression at 1123 K to 1323 K at a strain rate of 2 x 10-4 s-1. Nucleation appeared at TJs at strains as low as 0.1. This strain is only about 1/5 to 1/2 of the peak strain at which DRX is conventionally believed to occur extensively. Furthermore, DRX nucleation was not observed to take place at grain boundaries or in the matrix at this strain. The probability of DRX nucleation at TJs increased monotonically with strain and temperature. It also depended on the angle, y, between the compression axis and the sliding boundary. That is, when the angle, y, approaches 45 degrees, the probability of DRX nucleation at TJs is higher. These results reveal the important role of grain-boundary sliding (GBS) on DRX nucleation at TJs. It should also be noted that more than 90% of the grains nucleated at TJs were twins. Such dynamic twinning suggests that the essential DRX nucleation mechanism is twinning.

Abstract: Magnetic annealing at five different magnitudes of field was conducted to evaluate the effect of the field on the recrystallized microstructure of Fe-0.75%Si samples. At higher fields the retardation during recrystallization is compensated by the magnetic filed driving force that causes an increase in the grain boundary mobility of grains that have a certain relationship with the direction of the field

Abstract: A theory for abnormal grain growth (AGG) in polycrystalline materials is revisited and extended in order to predict AGG in textured polycrystals. The motivation for the work is to improve our understanding of the origins of the Goss texture component, {110}<001>, during annealing of Fe-Si sheet. Since the AGG phenomenon in grain-oriented electrical steels is known to be dependent on the presence of a dispersion of fine second phase particles, the grain boundary properties are treated as representative of the homogenized behavior of the material, and not necessarily the properties that would be measured directly. The predictions of AGG are presented in the form of maps in Euler space, showing which texture components are most likely to grow abnormally. For different models of grain boundary properties applied to a theoretically derived texture, different sets of texture components are predicted to grow; neither model, however, predicts growth of the Goss component.

Abstract: Orientation relationships between individual crystals can be readily represented in Rodrigues-Frank space because of the one-to-one correspondence between each misorientation and a vector in the fundamental zone of this space. This is done by integrating the rotation angle and axis into a three-component vector. In this study, the three classical orientation relationships describing the γ-to-α transformation, namely the Bain, Kurdjumov-Sachs and Nishiyama- Wassermann, are represented in Rodrigues-Frank space. Also considered are the somewhat less common Pitsch and Greninger-Troiano relationships. The misorientations between these types of transformation variants are displayed in R-F space based on alternative reference systems to highlight the differences. Examples of the various crystallographic relationships between fcc and bcc crystals during the γ-to-α transformation are given to demonstrate the advantages of the use of this space.

Abstract: A newly developed model based on vertex concept is presented in this paper. Contrary to its standard version, which is strictly deterministic, some concepts of Monte-Carlo type method were introduced. It makes the model more flexible and allows to introduce some parameters appearing in vertex movement equations, which are not easy to express in analytical form. Initial microstructure in the model is characterized by topology, crystallographic orientations and stored energy values of the grains. The boundary energies and mobilities are anisotropic in general. Nucleation mechanism of a given type is selected at the beginning of calculations. Deformation texture, stored energy distribution and initial microstructure are input parameters of the model. The aim of the calculations is to predict the texture and microstructure modifications during recrystallization. The model was also applied to the study of the kinetics of grain growth and recrystallization. The preliminary tests of the model are presented.