Chapter 9 - Creep of Intermetallics

Abstract

The term ‘‘intermetallics'’ is used to designate the intermetallic phases and compounds that result from the combination of various metals. Intermetallics often have high melting temperatures (usually higher than 1000°C), partly because of the strong bonding among unlike atoms, which, in general, is a mixture between metallic, ionic, and covalent to different extents. The presence of these strong bonds also results in high creep resistance. Another factor that contributes to the superior strength of intermetallics at elevated temperature is the high degree of long-range order. The effect of order is, first, to slow diffusivity. The reason for this is that the number of atoms per unit cell is large in a material with long-range order. Therefore, in alloys in which dislocation climb is rate-controlling, a decrease in the diffusion rate would result in a drop in the creep rate and therefore, in an increase of the creep resistance. Second, the presence of a high degree of long-range order may retard the viscous motion of dislocations. This is because when a dislocation moves in a non-perfect lattice, the long-range order is damaged and this leads to an increase in energy or a dragging force. Thus, the presence of order results in a decrease of the creep rate in the intermetallic alloys in which the mechanism of viscous glide of dislocations is rate controlling.