Alloy Oxidation - Environmental Degradation of Metals - page 285

As an empirical measure of properties, half the melting temperature, i.e., 0.5
T m , has been used as a criterion (where T m is in C, a temperature at which most

metals still retain sufficient strength for engineering purposes). Thus, high-tem-
perature materials can be defined as those having melting points above 1300

°

C
where the performance temperature would be 0.5 T m , i.e., 923 K [1]. On this
basis, all of the metals of the first three groups of the periodic table, as well as
the rare earth metals, are unsuitable. Some of the rare earth metals have accept-
able melting points, but when alloyed, especially with other transition metals,
they form eutectics with very low melting points. Sn, Pb, and Bi, due to their
low melting points, may also be disregarded, and the actinides are unsuitable due
to their strong radioactivity as well as scarcity. Scarcity also eliminates the transi-
tion metals of the Pd and Pt groups. This leaves the possible metals as arranged
in Table 6.1, which are divided into two groups according to a system described
by Kofstad [2].
The usefulness of the metals in group A at high temperatures is doubtful be-
cause of their high affinity for interstitial elements like O, N, C, and H. These
elements are easily dissolved in such metals, forming ordered and sometimes
metastable martensitic phases that not only affect the physical properties of the
metals but also appear to have adverse effects on their oxidation resistance.
The group B metals have been studied extensively, and the basic mechanisms
of their strengthening are fairly well understood, particularly for Ni-based alloys.
The basic mechanisms involved include solid solution hardening; dispersion
hardening; precipitation of borides, nitrides, and carbides; and intermetallic pre-
cipitation and fiber reinforcement. Therefore, in high-temperature alloy develop-
ment it is necessary to establish the compatibility of the alloy additions made to
promote these mechanisms with those promoting oxidation resistance.
The optimum conditions for oxidation resistance occur when a compact, ad-
herent, protective oxide layer develops on the metal surface. The rate of further
degradation is then governed by the growth rate of the scale, which in turn is
controlled by the solid-state diffusion of cations or anions through it. Develop-

°

Table 6.1 Division of Possible High-Temperature Metals into Two Groups [2]

Group A m.p. (°C) Group B m.p. (°C)

Ti 1668 Ni 1453

Zr 1852 Co 1495

V 1900 Fe 1535

Hf 2222 Cr 1875

Nb 2468 Mo 2610

Ta 2996 W 3410