Hi Pillai,

Vickers Hardness of an alloy depends on the micro constituents present in the alloy. The phases formed by addition of two elements can have very different  properties (including hardness) than the elements themselves. You need to know the phases before commenting its properties. As I can see from Bi-Sn phase diagram is a simple eutectic type, with rhomboidal Bi and BCT Sn with no intermetallic or other compounds. Therefore you can expect  to increase in vickers hardness with Bi addition. 

Check the following papers for additional info:

 https://link.springer.com/article/10.1007%2FBF02666734

http://www.gotrawama.eu/fonderia/The%20experimental%20study%20of%20the%20Bi%E2%80%93Sn,%20Bi%E2%80%93Zn%20and%20Bi%E2%80%93Sn%E2%80%93Zn%20systems.pdf

Hope this helps. Good luck.

Thank you Roisul.