(502f) Microscopic Characterization of Discontinuous Precipitation in a Nickel Aluminide Intermetallic Material

Engineering alloys containing intermetallic compounds such as Ni3Al have long been known to provide beneficial high-temperature properties including tensile strength, enhanced plasticity and superior resistance to oxidation and creep deformation. Ni3Al base alloys may be beneficial as a structural material for high-temperature heat exchangers for nuclear hydrogen generation due its high strength and superior corrosion resistance at elevated temperatures. Heavily cold-worked Ni3Al can undergo discontinuous precipitation, when subjected to recrystallization annealing. A systematic study of discontinuous precipitation behavior is essential as this particular phenomenon is expected to cause deterioration in mechanical properties of this otherwise high strength material. In order to study the discontinuous precipitation behavior in Ni3Al-base alloys, a detailed investigation was pursued on a 70 percent cold-rolled and annealed Ni3Al(B,Zr) alloy. Well developed cellular precipitates were observed on Transmission Electron Microscopy (TEM) examination of the alloy annealed at 700°C for 2h, 5h and 10h. The initial stage of discontinuous precipitation was accompanied by thickening of grain boundary along with the formation of semi-circular ring like patterns of these precipitates along the grain boundary. The shapes of these precipitates were found to change to globular or plate-like at higher annealing temperatures. In addition, dissolution (or reversion) of precipitates at high temperatures of annealing was also seen.