an indirect hardening effect by increasing the density of dislocations
due to hindering their annihilation. It has been shown [8] that
1 and 3 at.% Mg in severely deformed Al yielded a two and twelve
fold rise in dislocation density, respectively. In the latter case, the
shear stress increment due to the rise of the dislocation density
was one order of magnitude larger than the direct hardening effect
of Mg solute atoms. When the alloying elements form precipitates,
the annihilation of dislocations is also hindered, but this effect
strongly depends on the size and the distribution of precipitates
in the matrix, as well as on the nature of interface between the
matrix and the secondary particles. If the spacing between the precipitates
is larger than the distance between dislocations with
opposite signs and/or the interfaces are coherent or semicoherent
(i.e. the precipitates are shearable), the secondary particles only
weakly hinder the dislocation annihilation