tailieunhanh - Ebook Welding metallurgy and weldability of nickel-base alloys: Part 2

(BQ) Part 2 book "Welding metallurgy and weldability of nickel-base alloys" has contents: Oxide dispersion strengthened alloys and nickel aluminides, repair welding of ni base alloys, dissimilar welding, dissimilar welding. | CHAPTER 5 Oxide Dispersion Strengthened Alloys and Nickel Aluminides The oxide dispersion strengthened alloys and nickel aluminides represent two specialty groups of Ni-base alloy that have been developed for demanding service environments, normally requiring good corrosion and creep resistance at elevated temperature. The oxide dispersion strengthened alloys are produced using a mechanical alloying process that uses a ball mill to mechanically mix the metal and oxide species. This mixture is then formed and heat-treated to develop the necessary mechanical properties. Nickel aluminides are based on either the NiAl the Ni3Al intermetallic system. Both these intermetallic alloys are quite strong and considerably lighter than conventional Ni-base superalloys because of the high volume fraction of aluminum, but they have negligible ductility and toughness. Because of the desirable strength-to-weight ratio, both the ODS and nickel aluminide alloys are of great interest to the aerospace turbine engine industry. Not surprisingly, these alloys have some unique weldability issues as described in this chapter. OXIDE DISPERSION STRENGTHENED ALLOYS Physical and Mechanical Metallurgy Ni-base oxide dispersion strengthened (ODS) alloys utilize a fine dispersion of insoluble oxide particles as their primary strengthening agent. Because the oxide particles are insoluble, these materials must be prepared with mechanical alloying techniques using a powder metallurgy route. The first step is to mix individual powders in a ball mill that consist of an oxide powder (typically yttria—Y2O3) along with powders of Ni, Cr, and a master alloy that contains additional elements. The oxide particles exhibit very fine particle diameters on the order of 25–50 nm (– μm). During mixing within the grinding balls, repeated deformation, fracture, and cold welding occurs to produce Welding Metallurgy and Weldability of Nickel-Base Alloys, by John N. DuPont, John C. Lippold, and .