Cryogenic Toughness Through Microstructure Control in an Iron-Nickel-Titanium Alloy

The selection of composition and processing of an iron--nickel--titanium (Fe--Ni--Ti) alloy having a favorable combination of strength and toughness at cryogenic temperatures is discussed. Criteria on alloy composition lead to the choice of low interstitial Fe--/sup 12/Ni--0.25Ti. The alloy has a naturally favorable microstructure, but cryogenic toughness must be imparted through grain refinement. Several alternate procedures are identified; four of these are discussed in detail, along with the resulting tensile properties at 77/sup 0/K, Charpy impact energies at 77 and 5/sup 0/K, and behavior in fracture toughness tests at 77/sup 0/K. Alloys are obtained having yield strengths to 150 ksi with excellent impact toughness to 5/sup 0/K.