Solid-solution hardening elements

Iron-niekcl base superalloys were developed primarily from the stainless steels. In the United Slates, these alloys included 19-9 DL. lb-25-6, and A-286, Luler, higher nickel contents were employed to take advantage of the superior oxidation resistance of nickel and the beneficial effects of y -forming elements. All iron-nickel base superalloys rely on solid solution hardening lo some extent. 

The extensive solubility of several of the alloying elements is the basis of solid solution hardening which scales roughly with the atomic-size difference of the solute and is, therefore, pronounced with W, Mo, Nb, Ta, and Al. Based on the face-centered cubic structure, Ni-based solid solutions show high ductility, fracture toughness, and formability. The basic corrosion resistance of Ni is strongly increased by alloying additions of Cr, Mo, and W. 

The solid solution phase a of Cu, dominating in most Cu materials, is hardened by the solute elements s through solid solution hardening which is proportional to the misfit parameter r/s. given by the relative difference in atomic radius to rcu r s = 2(rs - rcu)/( s+ Cu)- Since rc = 128,... 

Frequently, the solubility of the second element in the matrix phase cannot be neglected so that some of these atoms are dissolved in the matrix and increase the strength additionally by solid solution hardening. 

The superalloys are strengthened by addition of both solid-solution hardening and precipitate-forming elements. The Fe-Ni-base alloys can be strengthened the least and the Ni-base alloys the most (Fig. 5-8). The... 

The interstitial elements C, N, and B are potent solid-solution hardeners (Irvine et al., 1961), although too much of these elements can lead to embrittlement. Addition of approximately 0.003-0.05% boron is beneficial to the properties of superalloys. The boron segregates to grain boundaries where it inhibits grain-boundary tearing under creep-rupture loading. Small additions of boron also improve the hot workability of superalloys (Ramaswamy et al., 1972). Addition of up to 0.1 % zirconium has a beneficial effect similar to that of boron. 

Solid solution hardening of the y matrix resulting from the coherency strains or stiffening effect or larger atom elements such as chromium, molybdenum, and tungsten. 

When other elements dissolve in a metal to form a solid solution they make the metal harder. The solute atoms differ in size, stiffness and charge from the solvent atoms. Because of this the randomly distributed solute atoms interact with dislocations and make it harder for them to move. The theory of solution hardening is rather complicated, but it predicts the following result for the yield strength... 

Solution hardening is not confined to 5000 series aluminium alloys. The other alloy series all have elements dissolved in solid solution and they are all solution strengthened to some degree. But most aluminium alloys owe their strength to fine precipitates of intermetallic compounds, and solution strengthening is not dominant... 

There are several ways to harden alloys. A certain procedure to increase the resistance to the erosion corrosion is the hardening by solid solution. One adds an element to another to produce a solid solution that is resistant to the corrosion by hardening the metal. The thermal treatment is also a method to harden a metal or alloy, but it changes the microstructure and can induce a greater susceptibility to corrosion. Hardening by cold work is also an important procedure and it is the reason for using stainless steel to resist cavitation erosion. This material, initially hard, attains an even harder surface by cold work and becomes more resistant to attack and erosion. 

The addition of further alloying elements will always cause hardening, but not all elements have the same hardening effect. Hardening will also depend on whether the solute atoms are present in solid solution or as particles. Alloy hardening can be divided into... 

Hardening due to elements that are initially in solid solution and are precipitating as second phases (as is the case with age-hardenable alloys). 

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