High-temperature deformation of two phase structures

Abstract

Some of the ways in which two phase crystalline substances deform are reviewed. By the addition of a small amount of a finely dispersed second phase the resistance of a pure material to deformation can be spectacularly increased. The effect is quite disproportionate to the amount of second phase added. It stems from the fact that crystal dislocations have to increase in length to circumvent the particles and the energy needed to produce this effect must be supplied by the external machine causing deformation. If equal amounts of equally deformable phases are present (as in certain alloys of eutectic or eutectoid composition) then the material has a low resistance to deformation but a very high ductility: the phenomenon of superplasticity. Deformation now occurs by sliding at the interphase boundaries (i.p.bs). Superdislocations whose glide causes this sliding are effectively trapped in the boundary - a condition of thermodynamic equilibrium which (it is shown) gives rise to the observed deformation properties. At higher deformation rates dislocations appear in the grains (particles) and may form into cells. They are not, however, trapped in the cell boundaries and are observed to produce strain by traversing the cells. The aggregate now exhibits the same deformation as the cells of which it is composed. The cell size is that which minimizes the free energy for a given rate of deformation and the ductility is high, but not so high as that produced in the superplastic state. A new mechanism of deformation appears if transformation continues during straining. Large, self-cancelling internal stresses and strains occur and the external strain rate polarizes the latter, producing external strain. The external strain rate bears the same proportionality to the external stress as does the internal strain rate to the internal stress. An analogy is provided by irradiation creep where crystallization of self-interstitials created by bombardment occurs continuously from the supersaturated solid solution. Here the rate of transformation and the internal stress have actually been measured and the predicted creep characteristics correlate well with inpile creep data.