Surface Effects in Crystal Plasticity

John Keats, writing to Fanny Brawne [1], said "I long to believe in immortality ••••• I wish to believe in immortality - I wish to live with you forever". So much of this talk will be concerned with the ductile behaviour of crystals, plasticity in its narrower sense. We shall consider a crystal which is deforming by slip, and shall expose a surface in this crystal. We first think of the sur­ face as a simple mathematical cut along a low-index plane. Then we allow for the relaxation of the newly-exposed atoms, and for surface irregularities, and we consider the effect of lattice va­ cancies which can enter at the surface. We consider the effect of dissolving off the surface layers, either intermittently or contin­ uously. Then the effects of adsorption or oxidation by normal con­ stituents of the atmosphere must be considered, the effects of sur­ face alloying, and finally those of special surface-active agents. But "All/Life death does end and each day dies with sleep" [2], and plasticity in its broader sense includes the fracture ~'1hich term­ inates flow. Here there is a bewildering array of effects. The medium in which the crystal flows may enhance its ductility enor­ mously, or it may cause it to break almost without plastic deform­ ation, or under a load which it has already supported.