Lattice constants and compositions of the metastable Ni 3 Nb phase precipitated in a Ni-15Cr-8Fe-6Nb alloy
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I.
THE
INTRODUCTION
JIS NCF 3 (INCONEL* X-750) is a Ni-based,
*INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.
heat-resistant alloy hardened by precipitation of the g ' phase (Ni3 (Al, Ti): cubic (P), L12 type, metastable). The authors prepared the alloy (hereafter referred to as X750M), in which the precipitation-hardening phase was changed from the g ' phase into the g " phase (Ni3Nb: tetragonal (I), DO22 type, metastable) by substituting most of the Al and Ti in NCF 3 with Nb. The age-hardening process for this alloy, the formation and growth rate of the g " phase,[1] the shift from g " phase to d phase (Ni3Nb: orthorhombic (I), DOa type, stable),[2] the growth rate of the cellular d phase,[3] the morphological change in the g " phase,[4] the lattice constant and g "/g lattice mismatch,[5] the stacking fault in the g " phase,[6] etc., have been investigated and reported. It become clear that the g " phase precipitated in the X-750M alloy forms a thin plate and grows according to the Ostwald ripening mechanism. Moreover, it has been clarified that the lattice constant of a g matrix reaches the maximum immediately after the solution treatment, decreases gradually as the aging time elapses, and reaches a constant value after a fixed amount of time has passed. The lattice constants of the g " phase increase gradually during the aging process. The lattice constants of the g and g " phases showed a remarkable temperature and aging time dependence. It has been suggested that the composition of
KIYOSHI KUSABIRAKI, Lecturer, is with the Department of Intelligent Information Engineering, Toyama University, Toyama 9308555, Japan. HAJIME KOMATSU, Research Metallurgist, is with the Department of Structure Analysis, Nippon Steel Technoresearch Corporation, Kawasaki 211-0035, Japan. SHUUICHI IKEUCHI, formerly Graduate Student, Department of Metallurgical Engineering, Toyama University, is Metallurgical Engineer with Hitachi Metals Co. Ltd., Mooka 321-4367, Japan. Manuscript submitted January 25, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
both phases changes gradually with aging and is dependent on the aging temperature. The relationship between the lattice constants and chemical composition of the g " phase precipitated in the X750M alloy was clarified in this research by using an Xray diffractometer and an analytical electron microscope with an energy-dispersive X-ray (EDX) system.
II.
EXPERIMENTAL METHOD
The alloy used in this study was melted in vacuum, hot rolled at 1373 K in two stages, heated at 1298 K for 600 seconds (10 minutes), water cooled, and then cold rolled at 83 pct reduction to a thickness of 0.5 mm. The chemical composition of the alloy is shown in Table I. The coldrolled sheet of 0.5 mm in thickness was cut into squares 15 3 15 mm, solution treated in vacuum at 1373 K for 1.8 ks (0.5 hours), and then quenched with water. The specimens were then aged for up to 720 ks (200 hours) under vacuum at a temperature between 943 and 1073 K. The lattice constants of the aged spe
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