Материаловедение: свойства металлов. Матросова Т.А - 32 стр.

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Fig. 11.1. Alloy hardening in copper-nickel alloy

Several solid-state reactions produce gains in mechanical properties that are

important to engineers. Typical phase diagrams indicate the combination of phases

that is needed before a solid-state reaction can occur, but that alone does not mean

that a significant improvement will occur. A typical eutectoid reaction is a

prerequisite to eutectoid decomposition (fig. 11.2-a). However the iron-carbon

system is the only one of commercial value. When the solvus line which separates

one- and two-phase regions slopes to indicate decreasing solid solubility of B in A,

there is a chance that precipitation hardening may occur (fig. 11.2-b). Several

aluminum alloys have this type of phase diagram. Although many binary alloys have

similar solvus lines, aluminum alloys are the best example of precipitation hardening.

The requirements of the diffusion reaction are indicated in fig. 11.2-c. In this case the

diffusion of metal C into the alloy causes the composition of the hardenable alloy

(metal B in metal A) to shift from a single-phase region to a two-phase region. As

metal C diffuses into the solid solution, the overall composition gradually shifts into

the + region, so that the -phase (B

) begins to precipitate. The nitriding

process works according to this reaction because aluminum, chromium, and

vanadium form nitrides. Thus, when nitrogen gas diffuses into the steel surface that

contains those elements, their nitrides form within the surface of the metal to produce

an extremely hard surface. The unusual surface hardness is thought to be a result of

the fine dispersion of nitride particles rather than of the inherent hardness of the

nitrides alone.