What are the properties of alloys?
What seems to be a set of simple questions leads one into the field of metallurgy, computational thermodynamics, mechanical properties, and goodness knows what else. 

For example, steel has very different mechanical properties from pure iron.  There are about a bazillion different kinds of steel, each with different impurities mixed in to give specific mechanical properties, optimizing strength, fatigue life, corrosion resistance, etc. Much of the development has been through insight gained by experience.  More recently, the CALPHAD approach is giving more fundamental insight through thermodynamic modeling, but predicting the mechanical properties $\it a\ priori$ is still difficult to impossible.

As for mercury, there are a large number of binary phases of Hg with other elements.  These are readily found in binary alloy references, or online at the ASM Alloy Phase Diagram Database (assuming your library has access).  ASM has info on >60 Hg binary diagrams, and a number of ternary systems as well.