Strengthening point defects

An example of the type of data associated with solution hardening it is the mission of our models to explain was shown in fig. 8.2(a). For our present purposes, there are questions to be posed of both a qualitative and quantitative character. On the qualitative side, we would like to know how the presence of foreign atoms dissolved in the matrix can have the effect of strengthening a material. In particular, how can we reconcile what we know about point defects in solids with the elastic model of dislocation-obstacle interaction presented in section 11.6.2. From a more quantitative perspective, we are particularly interested in the question of to what extent the experimental data permit a scaling description of the hardening effect (i.e. r oc c") and in addition, to what extent statistical superposition of the presumed elastic interactions between dislocations and impurities provides for such scaling laws. 

Although a point defect may be considered to be a volume defect of atomic dimensions, in this section the common term point defect will be used. Impurity atoms are often also considered to be point defects. Perhaps a solute atom at a substitutional or interstitial site should also be considered as an imperfection, namely as a point defect, since there is a deviation from the original periodicity of the pure crystal. However, an impurity atom is present in a crystal unintentionally, whereas solute atoms are purposely added to a pure material. Generally, atoms are added to pure materials to enhance certain properties, mechanical or physical. Regarding mechanical properties, bear in mind that some impurities may strengthen a material, but others may be detrimental to its mechanical properties. Clearly, the common interest is to enhance the mechanical properties of a given material. 

Point Defects in Amorphous Ceramics and Their Strengthening (Effect)... 

Another type of point defect is the incorporation of an impurity atom. Impurity atoms may replace host atoms in the regular crystal structure, in which case they are called substitutional defects, or they may occupy an interstitial site as interstitial impurities. Impurities are often purposely introduced in a lattice to strengthen it (solid solution hardening) or to otherwise alter its properties, e.g., doping a semiconductor to tailor the number and sign of charge carriers. However, as seen later, it is virtually impossible to completely eliminate unwanted impurity atoms. 

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