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.