Materials Week '97: Wednesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday morning, September 17.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION, AND MODELING: Session V: Solid-Solid Systems II

Session Chairperson: M. Rappaz, Ecole Polytechnique Fdrale de Lausanne, DMX-G, CH-1015 Lausanne, Switzerland

MODELING OF PHASE TRANSFORMATIONS IN HYPOEUTECTOID FE-C STEELS DURING HEATING: A. Jacot¹, M. Rappaz¹ and R.C. Reed², ¹Laboratoire de mtallurgie physique, Ecole Polytechnique Fdrale de Lausanne, DMX-G, CH-1015 Lausanne, Switzerland; ²Department of Materials Science & Metallurgy University of Cambridge Pembroke Street, Cambridge, CB2 3QZ

Reaustenitisation from ferrite/pearlite microstructures in Fe-C steels is observed to occur in two steps. The pearlite dissolution which takes place just above the eutectoid temperature is followed by the ferrite to austenite transformation at higher temperature. At moderate heating rate these reactions are mainly governed by the diffusion kinetics of carbon in austenite. Since the diffusion scales and the subsequent transformation kinetics are very different, two separate models have been developed for the pearlite to austenite and the ferrite to austenite transformations. The dissolution of pearlite is described with a two-dimensional finite element approach using a deforming mesh and a remeshing procedure. The diffusion equation is solved in austenite () for a typical domain representative of the periodic structure of ferrite () and cementite () lamellae. The / and / interfaces are allowed to move with respect to the local equilibrium condition including curvatures effects via the Gibbs-Thompson coefficient. The model is able to predict the concentration field and the shape of the interface at different stages of the pearlite dissolution. Maps representing the steady state dissolution rate as a function of the temperature and lamellae spacing will be shown for small values of overheating. The appearance of a non-steady state regime at higher temperature will be discussed. The transformation of ferrite into austenite is described with a two-dimensional explicit finite volume technique. The diffusion equations are solved for a domain representative of the - grain structures, using a hexagonal grid and periodic boundary conditions. The discrete a/g boundary is represented by special interfacial elements which separate -elements from -elements. An -element always undergoes a transition to an /-element before becoming . This procedure allows to handle the displacement of the interface while respecting the flux condition at the interface. Simulated microstructures showing the dissolution of ferrite regions in the austenite matrix are presented at different stages of the phase transformation. Specifically, the influence of the microstructure scale and of the heating rate on the transformation kinetics and homogenization time has been investigated. Reverse TTT-diagrams calculated with this 2-D model are compared with experimental results from the literature.

9:05 am

THE EFFECTS OF INTERFACIAL STRUCTURE ON BOUNDARY MIGRATION IN Ni-Cr AND Fe-C ALLOYS: G. Spanos, R.A. Masumura, Naval Research Laboratory, Washington, DC 20375-5000; G. Chen, W.T. Reynolds, Jr., Virginia Polytechnic Institute, Blacksburg, VA 24061

The effects of growth ledges on interface migration and sympathetic nucleation of precipitates at interphase boundaries is examined with finite difference-based numerical calculations using the ledge growth model of Enomoto. Two alloy systems are considered: (1) Ni-45wt%Cr, and (2) Fe-C alloys. For the Ni-45wt%Cr alloy, direct measurements of ledge spacings and heights as a function of isothermal growth time and temperature are used as input into the model, and excellent agreement is found between experimentally measured and calculated lath thickening rates. These results are compared to similar calculations published previously for plate lengthening in the Fe-C system. Finally, the model is used to examine ledge nucleation and the driving force for sympathetic nucleation in Fe-C alloys. These calculations indicate that the return of super saturating at local regions of ledged ferrite: austenite interfaces should allow for plenty of driving force for sympathetic nucleation of ferrite crystals atop preexisting ferrite crystals, and the preferred sites for such nucleation are discussed. Results will be analyzed with the aid of color maps of the calculated solute profiles associated with ledged growth interfaces in both Ni-45wt%Cr and Fe-C alloys.

9:40 am

THREE DIMENSIONAL RECONSTRUCTION AND CLASSIFICATION OF CEMENTITE PRECIPITATES: M.V. Kral, G. Spanos, Naval Research Laboratory, Code 6324, Physical Metallurgy Branch, Washington, DC 20375-5343

The three dimensional nature of cementite precipitates in an Fe-1.34%C-13.0% Mn alloy has been revealed by two experimental techniques. A serial sectioning/computer reconstruction technique enables the visualization of entire grains of material from any perspective, and resolution is limited only by the thickness of material removed for each section and the number of sections that are taken. Scanning Electron Microscopy (SEM) of deep-etched specimens also shows excellent resolution of precipitates, but only near a specific plane of polish. The combined use of these methods on isothermally transformed specimens provides detailed information about the true three dimensional morphology, connectivity and growth of precipitates. Three-dimensional projections of various morphological types of cementite precipitates found in these materials are presented along with data on volume fractions and size measurements of all three dimensions. The resulting observations have enabled a more precise classification of precipitates than was previously possible with two dimensional microscopy techniques.

10:15 am BREAK

10:30 am

THE DEPENDENCE OF THE DEVELOPMENT OF RECRYSTALLIZATION TEXTURE ON THE STORED DEFORMATION ENERGY: Baolute Ren, Manufacturing Technology Laboratory, Corporate Research & Development, Reynolds Metals Company, 3326 East Second Street Muscle Shoals, AL 35661-1258

Among the many metallurgical and processing parameters which influence the development of recrystallization texture, it is generally believed that stored energy is the most important parameter. It determines the driving force for both nucleation and growth of crystal grains during primary recrystallization. In the present work, the development of stored energy is simulated on the basis of the Taylor/Bishop-Hill model. The simulation results indicate that the stored energy is not homogeneous. Therefore, the driving force for the development of recrystallization texture is not homogeneous. The effect of the stored energy in the forms of the grain boundary area, sub-boundary, and deformation zone on the recrystallization texture development is discussed.

11:15 am

DEVELOPMENT OF CUBE RECRYSTALLIZATION TEXTURE IN WARM PLANE DEFORMED ALUMINUM: EXPERIMENTS AND MODELING: Roger D. Doherty, Indradev Samajdarand Le Chun Chen, Department of Materials Engineering, Drexel University, Philadelphia, PA 19106

Heavily plane strain deformed fcc metals aluminum and copper show, in certain cases, a remarkable texture change from a stand rolling texture, containing only a small amount of cube texture, to a very strong cube texture with 75% or more of the volume occupied by recrystallized grains having a near cube texture. Recent experiments have shown that the development of strong cube texture arises from two features of the microstructural evolution during deformation: (i) The quasi stability of cube oriented material and (ii) The significantly lower stored energies of the deformed cube bands that are elongated in the rolling direction. A simple model for the development of recrystallized cube texture has been developed and successfully tested. Current studies are investigating the role of starting grain size, starting cube texture and deformation conditions on the input parameters of the model, which are the deformed cube band spacing, its thickness, and the frequency of cube grain nucleation from these bands in competition with nucleation of grains of other orientations.

11:40 am

NONCONVENTIONAL X-RAY DIFFRACTION TECHNIQUES FOR COATINGS: O.B. Girin, Yu. O. Proshenko, V.I. Bekerav, Dept. Physics, State Metallurgical Academy of Ukraine, Prospekt Gagarina 4, Dnepropetrovsk 320635, Ukraine

An improved package of nonconventional Xray diffraction techniques for coating characterization is described. The package permits (I) characterization of texture with due regard for anisotropy of crystal defects, (ii) characterization of substructure in oriented coatings, and investigation of substructure anisotropy in the various components of texture, and (iii) in situ studies of coating structure formation during deposition. These techniques combined with TEM, FIM and DTA revealed some effects throwing new light on formation of crystalline substructures and textures in electrodeposits. A mechanism and a model of texture evolution in electrodeposits is discussed. A model of electrodeposit structure formation involving liquid metal is addressed.