8-16 NICKEL AND NICKEL ALLOYS
8-16 NICKEL AND NICKEL ALLOYS
Nickel and high-nickel alloys have been used widely because of their superior corrosion resistance and high-temperature strength. Commercially pure nickel is almost as hard as
a low-carbon steel, but it cannot be hardened by heat treatment. The addition of small amounts of alloying elements can make the metal heat treatable by the age-hardening method. Nickel work-hardens rapidly when cold-worked.
Nickel—copper alloys contain nickel in an amount greater than 50% and, like copper— nickel alloys, are of the solid solution type, with FCC structure. The most important alloy in this group is Monel, which has a composition of 63% to 70% nickel and up to 30% copper. Another very important group of nickel-base alloys called superalloys comprises various nickel—chromium—iron alloys such as Inconels, René-s, Uldimets, and others (see Table 2A) and may be produced in cast, wrought, and powder forms. They are solid solution alloys having an austenitic matrix FCC containing solid solution strengtheners such as V, Cr, Mo, W, Co, and others. Most of these superalloys can be strengthened by the γ ’ precipitate coherent with the austenitic matrix. The precipitate is
based on the ordered Ni 3 Al structure in which Ni atoms are located at face centers and Al atoms or atoms of other elements such as Ti, Nb, and Ta at cube corners. Alloys containing 2—6% Nb are strengthened by γ ” precipitate of Ni 3 Nb having a body- centered tetragonal structure. The γ ” precipitate is much finer than the γ ’, and it may convert to an ordered Ni 3 Nb phase having an orthorhombic crystal structure. The presence of precipitate and the resultant coherency strain make it difficult for dislocations to penetrate the precipitate particles, thereby increasing the critical resolved shear strength. The γ ’ phase is quite stable at elevated temperatures as high as 900°C (1650°F), maintaining a virtually constant yield strength. The shape and the structural stability of the γ ’ precipitate depend on the misfit parameter δ as defined by Equation 4-
5. When the misfit parameter is small or the particle size is small, the γ ’ particles tend to assume spherical shape. With a large misfit and large precipitate continued aging results in a precipitate of cuboidal shape. Nb and Ti tend to increase the misfit whereas Fe and Mo tend to decrease it. For misfit nearly zero all γ ’ precipitate is very small and spherical.
(a) (b)
FIGURE 8-28 Optical micrographs of(a) stellite 6B (1.1% C, 30% Cr, 4.5% W, 1.5% Mo, 3% max Ni, 3% max Fe, balance Co). Round large spheres are M 7 C3; small spheres at grain boundaries are M 23 C 6 . FCC matrix. Hot-rolled sheet, X 413. (b) Hastelloy G (44.5% Ni. 0.03% C, 1.5% Mn. 22.3% Cr. 6.5% Mo. 20% Fe, 0.1% cb=Ta. FCC matrix. Precipitate is M 6 C. Hot-rolled sheet, X 150.
Most nickel-base superalloys contain carbides of types MC, M 6 C, and M 23 C 6 dispersed in the matrix or located in the grain boundaries (Fig. 8-28). Carbides of the MC type such as TaC, NbC, TiC, and VC have FCC structure and have a tendency to decompose
with increasing temperature. Carbides M 6 C and M 23 C 6 have a complex cubic structure and have a tendency to form along grain boundaries; with high Cr content they are stable in the range 870—980°C (1600—1800°F). The grain boundaries are rich in elements
such as boron, zirconium, and magnesium. Boride (M 3 B 2 ) particles form a hard, refractory precipitate that delays the onset of grain boundary tearing during creep.
A number of alloys known as Hastelloys and Chlorimets, particularly those containing chromium and molybdenum, are also solid solution types showing superior corrosion resistance to many corrosive environments as well as high-temperature strength and oxidation resistance. Hastelloys B and B-2 are nickel—molybdenum—iron alloys, Hastelloy C-4 and Illium alloys are nickel—chromium—molybdenum − iron alloys, and Hastelloy D is a nickel—silicon alloy.