Investigators at the Johnson Space Center White Sands Test Facility (WSTF) have developed specifications of the amounts of alloying elements needed to increase the specific strengths of nickel alloys while consistently maintaining their burn resistance. The issue of burn resistance versus strength arises because pure nickel resists burning in pure oxygen at pressures up to >104 psia (>69 MPa), but does not have enough specific strength to satisfy the requirements for use in engineered structures.
The high burn resistances of nickel and of some nickel alloys make them attractive for some structural applications. These alloys can be used in such oxygen-enriched structures as hyperbaric chambers and spacecraft, where fires could have catastrophic results. Because the specific strength of pure nickel falls short of structural needs, elements with high specific strength and low burn resistance — such as aluminum and titanium — are alloyed with nickel. Unfortunately, prior to this investigation, large variations in burn resistance among batches of nominally identical commercial nickel alloys had been observed. For example, while one batch of a commercial nickel alloy was found to burn in pure oxygen at a pressure > 104 psia ( ≈ 69 MPa), another batch was found to burn at a pressure 3 psia ( ≈ 34 MPa). This inconsistency renders the use of commercial nickel alloys in oxygen-enriched structures problematic at best, disastrous at worst. The WSTF investigators sought to discover why this inconsistency occurs.
The investigators identified an inherent looseness in the composition ranges of commercially produced, high-temperature, high-strength nickel alloys, which makes the alloys more likely to burn in oxygen-rich atmospheres. Combustion occurs because the alloying elements that are not burn-resistant — aluminum, silicon, manganese, titanium, chromium, and iron — are used in varying quantities to form primary solid solutions with nickel. The investigators found that even small variations in alloying content can significantly affect burn resistance. For example, in preliminary tests on two batches of the nickel alloy Haynes 214, one batch burned at a pressure above >104 psia (>69 MPa) while the other batch burned at only 3 × 103 psia ( ≈ 21 MPa), yet both batches were within manufacturer's specifications. Further study led to the conclusion that the difference between the burn resistances of the two batches was attributable to looseness in the proportions of the alloying elements aluminum and chromium.
If the proportions of the alloying elements in nickel alloys were better regulated, the primary solid solution phases could retain both the burn resistance of pure nickel and the strength of the alloying elements. The investigators found that if elements are added in the quantities shown in the table, the specific strengths of nickel alloys can be increased without reducing resistance to burning. The findings of this investigation are expected to ensure that differences among the burn resistances of manufactured nickel alloys can be reduced, without adversely affecting the safety of individuals who live and work in oxygen-rich atmospheres.
This work was done by Joel M. Stoltzfus of White Sands Test Facility and Moti J. Tayal of Rockwell International for Johnson Space Center. MSC-22698