A modern high strength ternary alloy containing 15Ni-8Sn-Cu has been developed and delivers new combinations of high strength and corrosion resistance. This alloy is in commercial use in aggressive tribological environments and has exhibited excellent performance characteristics. Prior work demonstrated excellent seawater corrosion resistance and reasonable corrosion resistance in NACE Levels I, III and V solutions. The alloy also exhibited SCC resistance in similar solutions. This paper documents the basic SCC and corrosion rate behavior of a new 150 ksi yield strength TS temper in Levels I, II, IV, V and VII environments. Microstructural observations, corrosion rate and resistance to SCC in full immersion proof ring static tensile tests are reported.
Copper spinodal alloys are a relatively new class of materials which are strengthened by an unusual phase transformation. Spinodal alloys are formed when an alloy with a homogeneous microstructure separates (or decomposes) into a phase comprised of nano-scale zones having two distinct compositions without interfaces.1 Although spinodal decomposition was predicted by the mathematics of thermodynamics near the turn of the twentieth century, it was observed only in ceramic systems for a number of years. Not until the 1960s was spinodal decomposition seen in the copper-nickel-tin system, where it was found to greatly increase strength.
In the 1990s, cast, solution annealed, and spinodally hardened Cu-15Ni-8Sn tempers designated CX (UNS C96900) became available in continuously cast advanced shapes for mechanical applications. Then hot worked, solution annealed, and spinodally hardened wrought products designated TX (UNS C72900) were developed as rods, bars, and tubes. The early 2000s have brought new hot worked, solution annealed, cold worked and spinodally hardened TS tempers (UNS C72900) with even higher strength.
Today's high strength copper-nickel-tin spinodal alloys, commonly known as ToughMet ® (1) , are performance materials that can stably operate under combinations of severe load and speed in bearing or bushing systems. The outstanding characteristics of spinodal copper alloys have led to applications in large jet aircraft, highly demanding off-road equipment, heavy-duty mechanical systems and in oilfield equipment, which requires the additional benefit of corrosion resistance in exploration and production environments.
This paper reports basic corrosion characteristics of the new high-strength TS temper and makes comparison of SCC and corrosion rate information to the CX and TX tempers, previously unreported in the literature but available elsewhere2.
Specimens of TS temper were machined from the mill products listed in Table I with the nominal composition indicated. The mechanical properties after spinodal hardening are given in Table 2. Additional information relating to the basics of the processing of each temper is also shown. Fundamentally, TS temper is a hot worked, solution annealed and cold worked Cu-15Ni-8Sn alloy which has been spinodally hardened to 160ksi (1100Mpa) minimum ultimate tensile strength.
Environmental cracking samples are threaded tensile samples with a gage diameter of 6.4 mm (0.250 in.) fabricated from the nominal mid-radius position of production rods.
