On the Creep-Rupture Strength of Molybdenum Composite Cast with Nimonic 90 Alloy

Abstract

As molybdenum which is a typical refractory metal has poor oxidation resistance, but it has high strength at elevated temperatures, it cannot be used at high temperatures in air. We have tried a new coating method to use molybdenum under the above-mentioned condition. According to this new method, commercially-pure molybdenum rods are covered with Nimonic 90 nickel-base alloy which is hot-workable and oxidation-resistant and forge-worked. We named this technique the composite cast-working method.
After the composite cast materials were hot-swaged, they were creep-rupture tested at 1000°C with a stress of 5 to 10 kg/mm² in air. The results are as follows:
(1) It is possible to work the hard and brittle diffusion zone formed in the interface between molybdenum and the covering alloy without cracking.
(2) It is possible to heat and work molybdenum freely in air, and to make the materials stable by the following working even if the discontinuities in bonding were formed when molybdenum was composite cast with Nimonic 90 alloy.
(3) The creep-rupture strength of the composite cast materials at 1000°C in air is improved to a great extent as the part of the molybdenum core is increased, while the effect on the strength is negligible when the molybdenum core is small.
(4) The creep-rupture strength of these materials depends on the area of both molybdenum and diffusion zones, and the strength for this area is comparable to the strength of molybdenum under the same condition in vacuum. From this result, it is indicated that the new coating method using Nimonic 90 alloy can protect molybdenum perfectly from the oxidation at elevated temperatures in air.