Strengthening Methods and Plasticity of Polycrystals

A crystal yields when the force xh (per unit length) exceeds /, the resistance (a force per unit length) opposing the motion of a dislocation. This defines the dislocation yield strength 

If an impurity (copper, say) is dissolved in a metal or ceramic (aluminium, for instance) at a high temperature, and the alloy is cooled to room temperature, the impurity may precipitate as small particles, much as sugar will crystallise from a saturated solution when it is cooled. An alloy of A1 containing 4% Cu ( Duralumin ), treated in this way, gives very small, closely spaced precipitates of the hard compound CUAI2. Most steels are strengthened by precipitates of carbides, obtained in this way.  

Small particles can be introduced into metals or ceramics in other ways. The most obvious is to mix a dispersoid (such as an oxide) into a powdered metal (aluminium and lead are both treated in this way), and then compact and sinter the mixed powders. 

The obstacles thus exert a resistance of/o = 2T/L. Obviously, the greatest hardening is produced by strong, closely spaced precipitates or dispersions (Fig. 10.2). 

When crystals yield, dislocations move through them. Most crystals have several slip planes the f.c.c. structure, which slips on 111) planes (Chapter 5), has four, for example. Dislocations on these intersecting planes interact, and obstruct each other, and accumulate in the material. 

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