Nickel solid solution strengthening

Hafnium is an effective solid solution strengthener at higher temperatures for other alloys such as nickel aluminides (39,40). 

Mickel-llase Superalloys. The nickel-basc superalloys are Ihe most complex in composition and microslrueuirex and. in must respects, the most successful high temperature alloys. The earliest superalloys were wrought, ie fabricated to final size by a mechanical working operation. Later alloys have incorporated higher aluminum plus tilanium contents, as well as molybdenum lor solid-solution strengthening (Nimonics 115 and 1201. 

Solid-solution-strengthened iron, nickel, and cobalt alloys... 

The nickel base alloys are produced from a group of alloys which have chemical compositions generally over 50 % nickel and less than 10 % iron. They are mainly strengthened by intermetallic precipitation in an austenitic matrix. The cobalt base alloys have a high Co content (40 to 70 %X high Cr (over 20 %), high W (7 to 15 %) and they are strengthened by a combination of carbides and solid solution hardeners. 

Microstmcture of Rodent after casting is composed of large grains/den-drites of the austenite matrix, which is the solid solution of nickel strengthened chromium and molybdenum. The matrix is additionally strengthened by dispersion of molybdenum silicide precipitates. A small number of carbide and aluminum oxide particles were also observed. The presence of eutectics y-P rather weakens the structure of the alloy. 

Nickel-based alloys, which form the bulk of alloys produced, are basically nickel-chrome alloys with a face-centered cubic solid-solution matrix containing carbides and the coherent intermetallic precipitate y-NijlAfTi). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminum, titanium, niobium, and tantalum are made to combine with nickel in the y phase, and additions of molybdenum, tungsten, and chromium strengthen the solid solution matrix. 

The NiAl intermetallic phases are of particular importance because of their use in strengthening Ni-based alloys, especially the superalloys used in high-temperature gas turbine blades (the Ni-Al phase diagram is shovm in Figure 12.12). Nickel aluminide (Ni-Al) is the simplest of these compounds. It is described by the Strukturbericht symbol B2, which tells us it has the CsCl structure, and the Pearson symbol cP2 tells us we have a cubic lattice with two atoms per unit cell. Therefore NiAl has Ni atoms on the comers of a cube and an A1 atom in the center (or vice versa). (Note this is not a bcc structure because the atom in the center is not the same as those on the comers.) The width of this phase in the phase diagram tells us that some solid solution is possible within the phase. 

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