A new characterization method of the microstructure using the macroscopic composition gradient in alloys
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I.
INTRODUCTION
A comprehensive description of the phase transformations should be realized in the form of the three-dimensional diagram having the temperature (T), time (t), and composition (c) axes.t1.2] The section parallel to the T and c axes is well known as the phase diagram, and the section parallel to the T and t axes is the TTT diagram. A section parallel to the c and t axes is also important, as well as the phase and TTT diagrams; nevertheless, it has not been noticed yet. In the T-t-c diagram, the experiments whose variables are the T-axis and t-axis are known as the thermal analysis and the isothermal aging, respectively. Since the c-axis is an important element of the T-t-c diagram, the experiment whose variable is composition c should be conducted. In the present work, we propose a new experimental method to determine the phase boundary and phase equilibria on the basis of the microstructural observation of the alloys having the continuous composition gradient. II.
EXPERIMENTAL PROCEDURES
There are many preparation methods to create the macroscopic composition gradient in the specimen, i.e., diffusion coupling, imperfect arc melting of sandwiched metals, imperfect homogenization of the coarse discontinuous precipitates, and so on. In the present work, the macroscopic composition gradient was realized in a Cu-Ti alloy by means of the imperfect homogenization of the coarse discontinuous precipitates. A Cu-4 at. pct Ti alloy was first prepared in the vacuum induction furnace, and then forged and rolled to a thin plate of about 1 mm in thickness and solution treated at 1173 K for suitable durations. After the homogenization at 1173 K, the specimens were aged at 973 K for a long duration to make a discontinuous Cu3Ti precipitate whose interlamellar distance was over 200 /xm. These specimens were again heated at 1173 K for a short
TORU MIYAZAKI, Professor, TOSHIYUKI KOYAMA, Research Fellow, and SENGO KOBAYASHI, Graduate Student, are with the Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466, Japan. Manuscript submitted December 14, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
duration so as to be imperfectly homogenized and then directly quenched into aging temperatures. By this heat treatment, the macroscopic gradient of the solute composition was realized in the Cu-Ti supersaturated solid solution. These specimens were aged at various temperatures for suitable durations and then quenched in iced brine. The specimens were prepared to thin foils by electropolishing in an electrolyte of HNO:CH3OH '= 1:3 at 240 K. The microstructural observation was performed with an analytical transmission electron microscope, and the solute composition analysis by energy dispersive X-ray spectroscopy (EDS) was concurrently performed at several locations in the same thin foil. The electron microscope, JEM 2000FX, was operated at 200 kV. The LaB6 flament was used at an accelerating voltage of 200 kV and a beryllium mesh was used. The K factor defined by the Cliff-Lorimer met
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