Nickel-aluminum-molybdenum phase equilibria
- PDF / 1,492,975 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 79 Downloads / 235 Views
I.
II.
INTRODUCTION
IN the mid-1970's,
a program was begun to develop a new family of turbine-blade materials with the increased temperature capabilities necessary for advanced highperformance engines. The program was conducted by the Government Products Division of Pratt and Whitney Aircraft, under the sponsorship of the Defense Advanced Research Projects Agency and the AFWAL Materials Laboratory. The Ni-A1-Mo system was chosen as a base, and an extensive investigation of alloys based on this system was undertaken. During a parallel research program to study the long-term stability of the Ni-A1-Mo base alloys an unexpected needle-like phase was discovered in some of the alloys.1 This phase was observed to form at temperatures as low as 870 ~ (1600 ~ to precipitate from and be in equilibrium with a 3'- ~/ matrix, and was later determined to be ~-NiMo. Phase diagrams found in the literature predicted the ~-NiMo phase to be in equilibrium with 3/-Ni and c~-Mo at this temperature and did not predict the occurrence of &NiMo for the compositional range of alloys in the long term stability studyY '4 It was postulated that the occurrence of a class II four-phase reaction, specifically 3' + c~ cooling// heating 3/' + 8, would explain these observed discrepancies. Hence, the task was undertaken to produce an experimentally validated ternary equilibrium phase diagram for the Ni-rich corner of the Ni-A1-Mo system in the temperature range from 1260 to 927 ~ and to investigate the postulated occurrence of the class II four-phase reaction.
EXPERIMENTAL PROCEDURE
The alloys used for this work were produced by both conventional arc-casting and powder metallurgy techniques. Melt stock for both powder and arc-cast alloys contained less than 1 pct total impurities, and the charge compositions are given in Table I. The powder was produced by the Rapid Solidification Rate (RSR) process at the Government Products Division, Pratt and Whitney Aircraft. The powder was sealed in stainless steel cans, hot outgassed at 371 ~ (700 ~ and extruded at 1260 ~ (2300 ~ with a reduction ratio of 16: 1. The extrusion can was removed on a lathe, and the extruded alloy was cut into slices - 2 mm thiCk and 10 mm in diameter. The remaining alloys were produced in a conventional multi-hearth arc-casting furnace using a nonconsumable tungsten electrode and a low partial pressure of argon. The buttons ranged from 50 to 100 gms each, and were flipped at least once and remelted to ensure more complete homogeneity. Specimens were cut from the arccast buttons measuring roughly 8 mm by 8 mm by 10 mm in height. To break up the coarse microstructure, cast alloys were forged at 1250 ~ in a vacuum of -0.0133 Pa (1 x 10 -4 torr) to a reduction in height greater than 50 pct. All the alloys were then placed individually in evacuated quartz tubes, back filled with high purity argon, and sealed for further heat treatment. After encapsulation, all alloys were given a short (--4 hours) heat treatment at 1290 ~ (2350 ~ to solution the 3", and were then equilibrated at
Data Loading...
