Why Y b(x)Lu(1-x)Al(3) and Y b(x)Y (1-x)InCu(4) have quite opposite concentration dependence of the Vickers microhardness

We report on the results of the Vickers microhardness measurements of the Y b(x)Lu(1- x)Al(3) alloy system. It is shown that the Mott-Nabarro theory is not able to explain the concentration dependence of the measured microhardness, as is possible in some similar Kondo alloy systems containing Ce and U. Interestingly enough, while microhardness increases with the Yb content in Y b(x)Lu(1-x)Al(3), it decreases with Yb in the Y b(x)Y (1-x)InCu(4) alloy system. Therefore, we try to explain the experimental data, taking into account the difference of the electronic structure and the change of the electronic structure with alloying in both systems.