Materials Week '97: Tuesday PM 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 Tuesday afternoon, September 16.

MICROSTRUCTURE EVOLUTION, CHARACTERIZATION AND MODELING: Session IV: Solid-Solid Systems I

Session Chair: G. Spanos, Naval Research Laboratory, Code 6324, 4555 Overlook Ave. SW, Washington, DC 20375-5000

EXPERIMENTALLY DETERMINED NUCLEATION AND GROWTH RATES DURING RECRYSTALLIZATION: B.R. Patterson, S. Grandhi, M.J. Papo, Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294-4461

The effect of the amount of cold work on nucleation and growth rates during recrystallization has been studied using Al-0.23% Cu. Nucleation rates were determined from estimation of the number of grains per unit volume as a function of time. Both a site saturated nucleation component and a component with a continuously increasing nucleation rate were observed, with the magnitudes of both components increasing with the amount of cold work. Growth rates, computed from the Cahn-Hagel equation, were found to remain constant throughout recrystallization, with higher rates at greater amounts of cold work.

2:30 pm

COMPUTER SIMULATION OF MICROSTRUCTURAL EVOLUTION IN Ni-RICH TiNi SHAPE MEMORY ALLOYS: D.Y. Li, L.Q. Chen, Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802

Microstructural evolution in a coherent two-phase mixture is driven by the minimization of the total free energy which includes the bulk chemical free energy, the interfacial energy and the elastic energy generated by coherency strains, as well as by external stresses or strains. A particular case, Ti₁₁Ni₁₄ precipitation in TiNi shape memory alloys, was investigated using computer simulations based on a continuum field model in which the orientational difference between precipitate variants is distinguished by structural field variables whereas the compositional difference between the precipitate and the matrix is described by a concentration field variable. The temporal variation of the field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations and the Cahn-Hilliard diffusion equation. Precipitate morphologies and interactions under strain-constraints were studied. Influences of a strain gradient on the morphology, migration, and the variant distribution was also analyzed. Work supported by the U.S. Office of Naval Research.

3:00 pm

COMPUTER SIMULATION OF MORPHOLOGICAL EVOLUTION AND STRESS-INDUCED RAFTING OF PHASE IN Ni-Al SUPERALLOYS: D.Y. Li, L.Q. Chen, Department of Materials Science & Engineering, The Pennsylvania State University, University Park, PA 16802

The morphological evolution and coarsening kinetics of phase particles in Ni-Al superalloys under applied external stresses were investigated. A computer simulation model based on a diffuse-interface kinetic field model was employed. In this model, a two-phase microstructure is described by a concentration field and a three-component long-range order parameter field. The temporal evolution of the field variables was determined by solving the non-linear Ginzburg-Landau and Cahn-Hilliard equations. The elastic inhomogeneity was taken into account using the effective medium approximation recently proposed by Khachaturyan. It is shown that an applied stress results in rafting structures whose orientations depend on the magnitude and direction of the applied stress. The effect of intrinsic coherent stress and the applied stress on the two-phase equilibria will be discussed. Work supported by the U.S. Office of Naval Research.

3:30 pm

MICROSTRUCTURE DEVELOPMENT IN Ni-BASE SUPERALLOY IN738LC: Ercan Balikci, A. Raman, R. Mirshams, Louisiana State University, Baton Rouge, LA 70820

IN738LC is one of the recently developed Ni-base superalloys, having some prominent features for high temperature applications. Like the other superalloys, IN738LC owes these exceptional features to its fcc Ni-rich matrix strengthened by gamma prime, L12 Ni3(Al, Ti), precipitate phase. The volume fraction of the precipitate phase in this alloy is about 40-43%. A standard heat treatment is generally applied to IN738LC, which is a solution treatment of 1120 C/2h/AC or AAC and a subsequent aging treatment at 850°C/24h/FC. In this present study it was observed that this solution treatment neither produces a single phase solid-solution matrix, nor do the aging treatments subsequent to this solution treatment change the microstructure appreciably. It is found in this study that 1225°C is the lowest solutionizing temperature with 4 hours holding time and cooling to room temperature by AAC (accelerated air cooling) or WQ (water quenching) to produce the single phase solid-solution. Aging treatments were performed subsequent to 1200 C/4h/AAC and 1250°C/24h/AAC solution treatments, and similar microstructures were obtained. A precipitate morphology change from spheroidals to cuboidals was observed at 1050°C. A very important result of this study is that no double aging is necessary to develop a unimodal-cuboidal precipitate microstructure as cited in the literature. A single aging treatment at 1050°C for 24 hours subsequent to the solution treatment at 1200°C/4h yields unimodal-cuboidal precipitates. IN738LC shows a unimodal grain size microstructure up to 1140°C beyond which a very distinct duplex precipitate grain size microstructure sets in. Below 1140°C, precipitate grain growth was through precipitate coalescence and solute absorption from the matrix. Above 1140°C, precipitate dissolution is more favorable. The activation energy calculations show that beyond 1140°C, the precipitates are in dissolution mode, not in growth.

4:00 pm

MICROSTRUCTURAL EVOLUTION OF DEFORMED Ni-BASE SINGLE CRYSTAL SUPERALLOYS PRIOR TO RECRYSTALLIZATION: A.M. Dalley, H. Dong, D. Zhao, W.L. Moore, J.J. Valencia, Concurrent Technologies Corporation, 1450 Scalp Avenue, Johnstown, PA 15904

Single crystal nickel-base superalloys CMSX-6 (1st generation) and CMSX-4 (2nd generation) have been examined for microstructural evolution of the gamma prime strengthening phase in the as-cast and thermo-mechanically processed (TMP) conditions. The goal of this work is to advance the ability to manufacture components from cast single crystals. After being solution heat treated, specimens were plastically deformed by compression, rolling, or bending. Combinations of processing temperatures, strain rates and total strain produced non-recrystallized and recrystallized microstructures, including some that exhibited a secondary transformation of the cellular precipitation type. Those that did not recrystallize were subsequently heat treated to evaluate the ability of the microstructure to resist recrystallization. Light optical, SEM and micro-orientation textural analyses were performed to document the gamma prime microstructural evolution. This work was conducted by the National Center for Excellence in Metalworking Technology, operated by Concurrent Technologies Corporation, under contract N00140-92-C-BC49 to the U.S. Navy as part of the U.S. Navy Manufacturing Science and Technology Program.

4:30 pm

STUDY OF THE PRECIPITATION KINETIC OF Al-Zn AND Al-Mg ALLOYS USING DILATOMETRIC TECHNIQUES: Ney José Luiggi, Mirna Betancourt1, Glenys Hernandez and Luis Acuña, GFM. Dpto. de Física. Escuela de Ciencias, Universidad de Oriente, Cumana 1I, UT Cumana, Apdo. Postal 299, Sucre, Venezuela

A dilatometric study was undertaken to characterize the precipitation process in Al-26\% wt. Zn and Al-12\% wt. Mg binary alloys. The samples were cut in 26x4x4 mm³ parallelepipeds and homogenized at 500°C for 15 hours. The measurements were taken in an automatic 402 E NETZSCH dilatometer with a sensibility of 0.2 m_m. A previous study at different heating rates permitted us to determine the interval of temperature where the dilatation process exhibits a slope change, which is associated with a structural transformation process. This temperature range lies between 142 and 310°C for the Al-Zn alloy and between 240 and 417°C for the Al-Mg alloy. Isothermal aging of the samples were carried out in those temperature ranges, three different behaviors being observed for the Al-Mg alloy: a) A moderate initial growth to a peak followed by a decrease of the volume of the samples in the temperature range between 242 and 280°C. b) A monotonous and steady growth in the neighborhood of 320°C and c) A violent initial growth and a subsequent waning in the volume of the samples. All these behaviors indicate different precipitation processes. The Al-Zn alloy manifest a violent volume growth, followed by a decrease associated with the dissolution of the phase previously developed. The thermal expansion curves, smoothed and normalized to the maximum of the volume change at a given temperature, were used to obtain the precipitated or dissolved fraction of the phases formed at each aging temperature. At the end of this aging process, these observances allowed us to determine the solubility curve of the different phases under study. A Jhonson-Melh-Avrami type model is proposed to analyze the kinetics and evaluate the typical kinetic parameters for each alloy. In the particular case of the Al-Zn alloy we detect two processes with different activation energy, the values of which are (50 ± 5) Kcal/mol for the lowest temperature and (30 ± 5) for temperatures between 270 and 310°C. A similar study was undertaken for the Al-Mg alloy, the results being in agreement with the experience. TTT curves are reported for each case.