WHY SILICON IS HARD

Compared with pure metals and ionic salts, covalent solids such as sil icon are hard and brittle because dislocations do not move in them exc ept at high temperatures. A satisfactory explanation for this behavior has been lacking in spite of its great importance for the mechanics o f materials and structures. It is shown here that the critical atomic process leading to the observed brittleness is analogous to a chemical substitution reaction. Analysis of this analogy with the aid of a cor relation diagram yields the observed high resistive stress and high ac tivation energy. When a kink on a dislocation line moves, it breaks th e atomic bonding symmetry, a forbidden process.