  "Micro-alloying", referring to the addition of small concentration of a
foreign metal to a given metallic glass, was used extensively in recent years
to attempt to improve the mechanical properties of the latter. The results are
haphazard and nonsystematic. In this paper we provide a microscopic theory of
the effect of micro-alloying, exposing the delicate consequences of this
procedure and the large parameter space which needs to be controlled. In
particular we consider two very similar models which exhibit opposite trends
for the change of the shear modulus, and explain the origins of the difference
as displayed in the different microscopic structure and properties.
