I want to know if there is a way in which the void of the material inside the metal can be divided from micron size to nanometer size by splitting the voids into discrete voids of reduced size preferably without phase transformation .
I agree with Gertz - if they are micropores in castings then HIPping is the way to go [ref.1]. However the process/equipment is expensive. I assume that you mean by "splitting" the voids that in fact you mean to avoid forming large ones and form small ones only; this could be difficult - if voids nucleate at all they will probably grow due to thermodynamics (tiny voids have huge surface energy). Anyhow, for a better answer you need to provide more detail - e.g. alloy, process etc..
[ref . 1]
Kaiser, R., Williamson, K., O’Brien, C., Ramirez-Garcia, S., Browne, D.J., “Effects of hot isostatic pressing and heat treatment on cast cobalt alloy”, Materials Science and Technology, 31(11), 2015, pp. 1298-1304.