Question

An alloy of Pb/Sn is much harder than either of the two metals it was made from. In the alloy, the sizes of the atoms of Pb and Sn are of different size. Propose an explanation as to why it is much more difficult to deform an alloy along planes of mixed atoms as compared to a pure metal where the atoms are all the same size.

Answer 1

This is the result of the sizes of the atoms in the alloy, because larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Thus the alloy becomes a source of production of dislocations. Plastic deformation creates a large number of dislocations. Higher the dislocation density in the material, higher is the resistance to movement of dislocations and hence higher forces are required for plastic deformation. This explains the increase in strength during the deformation.