  Segregated impurities at grain boundaries can dramatically change the
mechanical behavior of metals, while the mechanism is still obscure in some
cases. Here, we suggest an unified approach to investigate segregation and its
effects on the mechanical properties of polycrystalline alloys using the
example of 3$sp$ impurities (Mg, Al, Si, P, or S) at a special type $\Sigma
5(310)[001]$ tilt grain boundary in Cu. We show that for these impurities
segregating to the grain boundary the strain contribution to the work of grain
boundary decohesion is small and that the chemical contribution correlates with
the electronegativity difference between Cu and the impurity. The strain
contribution to the work of dislocation emission is calculated to be negative,
while the chemical contribution to be always positive. Both the strain and
chemical contributions to the work of dislocation emission generally become
weaker with the increasing electronegativity from Mg to S. By combining these
contributions together we find, in agreement with experimental observations,
that a strong segregation of S can reduce the work of grain boundary separation
below the work of dislocation emission, thus embrittling Cu, while such an
embrittlement cannot be produced by a P segregation because it lowers the
energy barrier for dislocation emission relatively more than for work
separation.
