Abstract
Alloys of Ti-7%A1-2%Nb-1%Ta (Ti-721) containing 0.26 to 1.56% Pd were tested in two different heat-treatment conditions. For one of these, termed "sensitized", the heat treatment was designed to favour the precipitation of OE2 (Ti 3A1), whereas for the other, termed "desensitized", the heat treatment was designed to suppress Ti 3A1 precipitation. In tests on unstressed specimens of Ti-721, it was found that an alloying addition of 0.26% Pd was sufficient to bring about passivity in strongly acid 3.5% NaC1 solutions. However, improvements in resistance to environmental cracking (EC) in 3.5% NaC1 solution were shown only by alloys containing 0.38 to 1.56% Pd. Ti-721 alloy containing a2 (Ti 3A1) is known to be highly sensitive to EC. Despite moderate improvements imparted by palladium, the sensitized Ti-721 Pd alloys remained unacceptable in so far as resistance to EC was concerned. Desensitized Ti-721 Pd also showed improved resistance to EC at 0.38 to 1.56% Pd, these alloys showing a more acceptable level of resistance. Probably because of a solid solution strengthening mechanism, 0.75 to 1.56% Pd brought about minor increases in yield and ultimate tensile strength and also in fracture toughness. An additional slight increase in yield strength was caused by the sensitizing heat treatment at all palladium levels.
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