Microstructural Analysis of Rapid Solidification and Undercooling in the Al-Ge System
- PDF / 2,683,460 Bytes
- 8 Pages / 417.6 x 639 pts Page_size
- 115 Downloads / 217 Views
MICROSTRUCTURAL ANALYSIS OF RAPID SOLIDIFICATION AND UNDERCOOLING IN THE Al-Ge SYSTEM M.J. KAUFMAN AND H.L. FRASER Department of Metallurgy and the Materials Research Laboratory University of Illinois Urbana, IL 61801 ABSTRACT Submicron powders, amorphous films and melt spun ribbons of various Al-Ge alloys have been analyzed to determine the relative roles of undercooling and cooling rate in the production of non-equilibrium structures. All analyses were performed in transmission electron microscopes equipped with energy dispersive x-ray spectrometers. The submicron powders, produced by electro-hydrodynamic atomization, were analyzed in their as-received condition and then annealed and/or melted using the electron beam as a local heating source. Once molten, the liquid droplets were undercooled at different cooling rates by varying the rate of beam obstruction. In this manner, a number of different microstructures were produced. These included metastable crystalline phases and mixed amorphous/crystalline structures. By combining this technique with a microscope heating stage, it was possible to carry out controlled dynamic undercooling experiments and determine phase selection as a function of undercooling and composition. The amorphous films were rapidly heated with the electron beam in the microscope and metastable as well as stable phases were produced. The results of these complementary analyses will be compared and discussed with reference to current models and theories of rapid solidification. INTRODUCTION Recently, it has been realized that liquid metal undercooling is an extremely important parameter controlling microstructural evolution during rapid solidification [e.g. 1-5]. For example, it is becoming common to discuss this parameter when describing such non-equilibrium phenomena as metastable intermediate crystalline phase formation, partitionless solidification (solute trapping) and metallic glass formation. Although the importance of this term has been recognized and discussed, only a limited number of experiments have been performed which attempt to relate undercooling to microstructural development. The purpose of the present paper is to describe microstructural evolution in Al-Ge alloys with an emphasis on the importance of undercooling and, to a lesser extent, cooling rate. This alloy system was chosen in part because a wide range of metastable phenomena has been reported in the literature and considerable controversy exists over the correct identification of metastable crystalline phases and solute trapping (Ge in a-Al) levels [e.g. 6-8].
Mat.
Res. Soc.
Symp.
Proc.
Vol.
28 (1984)
Published by Elsevier Science Publishing Co.,
Inc.
336
EXPERIMENTAL PROCEDURE Al-20, 30, 36, and 42 Ge* alloy ingots were produced by induction melting high purity elements under Ar and chill casting in a copper mold. Ingot sections were then used to make rapidly solidified melt spun ribbon and submicron powders. The latter were produced by the electrohydrodynamic (EHD) atomization process.** Also, an Al-5OGe
Data Loading...
