Recrystallization : the Actual Topics with Particular Focus on the Investigation of Transformation Kinetics

An attempt will be made to give an overview of the main areas of actual interest with respect to recent progress and to future prospects in the respective fields. In this connection the following areas are treated: The kind of material and the initial thermodynamical unstable state; the basic processes during heating up of a sample: recovery, nucleation, grain boundary migration; the end product, which is the result of an interaction of the basic processes. From these different areas attention is turned to the topic of "kinetics", focussing particularly on what we can learn about recrystalliiationand recovery-kinetics in metals as a result of modern calorimetric investigation. INTRODUCTION When the organisers of the colloquium asked me whether I would be willing to give a survey lecture on the actual state of the art of recrystallization problems, I was honoured by the trust placed in me and, after some thought, rather bewildered, because the desired topic represents a complex conglomerate of interwoven information, accepted views and controversial opinions. One way to unravel the mystery and give it structure is to look out for topical subjects. That is how the title of my talk originated. The paper is organized in the following way: To begin with an overview is given on the main areas of relevant interest with respect to recent progress and to fiture prospects in the respective fields. From these different areas the topic "kinetics" is treated in more detail, particularly the point: what can we learn about recrystallization and recovery kinetics in metals as a result of modern calorimetric investigation? This field has been chosen for three reasons: We are dealing with a central area of recrystallization; the field reflects rather well the ambivalence of agreement and scepticism so characteristic of many aspects of recrystallization; finally, the author can speak from experience. The search for topical subjects is not difficult, perhaps a little tedious. One has to look through the literature and the pertinent conference proceedings 11-81, It is a great deal more difficult however, to testifj. where notable progress has been made in recent times. Yet more delicate is the assessment of future potential. The reasons for this are manifold. Among other things, the valuation of a field depends on whether: (i) primarily scientific or practical criteria are involved (A typical example in this respect are grain growth phenomena in thin films. These processes are of great importance in the computer industry), (ii) young higMyers or old sceptics are offering their verdict (typical here is the valution of computer simulation and modelling). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1995301 C3-4 JOURNAL DE PHYSIQUE I V In order to obtain an objective valuation of the topical subjects from a scientific viewpoint, the author asked about 30 experts worldwide for their opinions. Therefore, what will be said about the topics in the first section is the author's personal view, influenced by many colleagues. To keep to the thread throughout, one can attempt to accommodate the whole recrystallization field in a systematic scheme. This is more or less inevitable, considering the life of a sample from its beginning to its final state. First of all we need information about the starting point: The sort of material we are dealing with and the initial thermodynamical unstable state. After that, the sample will be heated up. In a simplified sense, three basic processes can occur in the material: recovery, nucleation, grain boundary migration. Finally, we have to consider the end product which is the result of an interaction of these basic processes. After this curriculum vitae of a sample, we will take a look at those areas which have attracted the most attention in recent years and how the activity should be assessed. THE ACTUAL TOPICS Type of Material Most activities have been devoted to metallic materials either singleor multiphase in constitution. But within this group there are some fields which have been studied rather less, despite their practical relevance: ordered alloys, intermetallics, composites or multiphase systems, in which interaction of recrystallization with other reactions can occur [see for instance 19-1 1, 4711. The same holds for nonmetallic materials such as ceramics. Thermodynamical Unstable State An accurate microstructural desciption of the initial state is, from a fundamental point of view, an essential prerequisite for understanding the processes which take place in recrystallization. This also holds for practical applications.The whole complex may be regarded from two standpoints: The generation of instability and the characterization of the unstable state. Both areas have excited much interest, in the first instance the points listed in Fig. 1.') Remarkable progress, recorded by square dots (+), has been made in all of these fields besides in "grain morphology" and "arrangement of lattice defects". The future potential is probably particularly high in the areas characterized by an exclamation mark (!). This valuation of future potential is based on the availability of new or improved experimental techniques. Local values for instance can be determined nowadays rather precisely and fast in the electron microscope by nanobeam technique and by automated image processing 16, 12/. In the field of thermodynamic quantities powerfil calorimeric devices are available [see for instance 113,1411. A selection of recent contributions which are relevant to the matter handled in this subdivision is collated in Fig. 1 (and Figs 2-4) behind the respective topics. Recovery and Nucleation The joint treatment of recovery and nucleation is a consequence of the fact that, upon annealing, these two processes are generally the first to take their course in a plastically deformed metal. Theoretically, recovery and nucleation may be clearly differentiated in a well-defined way. In practice, this can prove difficult since both processes may overlap more or less strongly, according to the material. As a result, controversial opinions abound in this area. In addition there is a basic difficulty inherent in the term "nucleation": measurements register "perceptible nucleation", which may be related to a larger or smaller extent to growth processes. l ) There exist, of course, many more possibilities to produce instability than recorded in Fig. 1, for instance by quenching, particle bombardment, thennomechanical treatment etc. of a sample. The same holds for the subdivision of characterization, for instance by quantities like texture, strain gradient, magnetization, degree of order etc. These items which have not attracted too much attention in recent years have been indicated in Fig. 1 by dotted lines resp. by X.Y. The same selection criterion holds for the following Figs. 2-4. Fig. 1 : Initial unstable state Generation of Characterization of the Instability Unstable State / Plastic Deformation + i . ~ . Global /\ Local 110,15-171 Values Values The complex can be headed: energetic versus structural aspects, structural and chemical features, mechanisms and models. Compared with the last, the first two areas have received much attention: namely the points mentioned in Fig. 2 in particular. In all these areas remarkable progress (+) is evident. The future potential (!), too, may be judged noteworthy. This assessment of future development is based as before on new and improved measuring probes and preparatory techniques. Consequently, considerable enhancement of our knowledge in the future is anticipated. Free Energy + ! /25-271