Development of a Co-W Ferromagnetic High-Temperature Structural Alloy

Abstract

Rotors for space power generators require a compromise between high-temperature strength and magnetic properties. Rotors must withstand high centrifugal stresses at high temperatures and develop high induction for small exciting fields. Alloys considered for rotors include maraging steels, H-11 steel, and a Co-base alloy, Nivco. This paper discusses work to develop alloys with improved high-temperature strength and good high-temperature magnetic properties. Because Co has the highest Curie temperature of any metal and is a good base for high-temperature alloys, a NASA superalloy Co-25W-1Ti-12Zr-12C was chosen for further study. Modifications of this alloy were screened for magnetic induction (H = 100 Oe) over a range of temperatures. For increasing W, Curie temperature decreased for the base Co-XW-1Ti-12Zr-12C, and Bmax above 1000°F decreased. Below 1000°F, low W alloys show Bmax vs T inflections due to instability of the fcc phase. Stability of the fcc increased with increasing W and Fe. A preferred alloy, Co-712W-212Fe-1Ti-12Zr-12C, was evaluated for magnetic and mechanical properties in cast form and for mechanical properties as sheet. After aging at 1700°F for 72 h, this alloy had a coercive force of 10.5 Oe at 75°F, and a Bmax of 11.5 kG and 19.3 kG at 1200° and 1400°F, respectively. The stress rupture life at 1200°–1400°F was greater than that of the strongest high-temperature commercially available magnetic-structural alloy.