There is no WHY. It is dissolving because the  Chemnal Affinity going from B4C  to  B4 plus N  in solution is positive or Gibbs Free energy dissolution is negative. Then it is a natural process  to break up 'n wet medium/ But this doesn t mean that this happens every liquid medium.

Dear Hamed, here B4 C  as it stands,  it implies that it a stochiometric compound, which can't be mechanical alloying or mixture.  But you may talk about the mechanical mixture of  B4C and Alumina  Al2O3  particles, which are put together  by close contact forces,  but still each grain keeps  its chemical identity  only the surface structure  may show some distortions or deformations.

There can be one other interesting explanation. The particles of B4C may break up equally in both environments (wet and dry), but in Ar or N2 atmosphere, it is possible that particles get re-welded and that's why appear larger in the microscope. But in wet milling, using medium like ethanol etc., absorption of moisture or oxygen from the air somehow prevents particle welding which then appear comparatively smaller in size. Needs further input...

One may have another plausible interpretation for the fragmentation of  B4C particles when the milling is done in wet medium such as ethanol or toluene. The particles submerged into these medium immediately they get wet  because of the hydrophilic : bonding action of water and methyl groups., which established  homopolar cohesive bond between particle that the types of van der Waals Forces hold a crystal together.

If the surface specific  Helmholtz1 free energy of these grains  covered by these homolar liquids should show great decrease than the surface roughness generated by the relaxation of  strain energy stored during the pelleting stage may turn into cracks as recently demonstrated by Ogurtani group by computer simulations.   The growth and propagation of these micro-cracks, through the stress assisted surface drift diffusion  driven by the reduction in the  Global Helmholtz  free energy (bulk plus surface)   may cause fragmentation or the decimation of particles into smaller pieces. 

1. I used here Helmholtz rather than Gibbs Free energy is due to the fact that the over all system is under the  isochoric conditions, which means no external traction and body forces are acting.  

Dear Latif,

Ball milling of Aluminum-Boron Carbide powder mixture are  a Ductile- Brittle milling system. Which was explained in many paper. In ductile -brittle  systems,  ductile powder  are flattened  in initial stages and convert to  equiaxed particle by cold welding mechanisms in final stage of  milling. In this system brittle powder such as B4C  are embedded between ductile powder. By my own experience both in wet milling  and dry milling, I think that cold welding mechanism are postpone in wet milling because of  presence of liquid medium which result in more free B4C particle in milling chamber (without embedded in aluminum  particles).  This free B4C particles are prone to fracture more than B4C particles which are embedded in dry milled Al-B4C powder mixture.

For more information you  can see this paper:

https://www.researchgate.net/publication/222412071_Synthesis_and_structural_characterization_of_AlB4C_nano-composite_powders_by_mechanical_alloying

Article Synthesis and structural characterization of Al–B4C nano-com...

Thank you very much Mehrdad Khakbiz for giving us a new dimension to think over.

When you ball-mill, you want them to break up into finer particles, Don't you?

B4C is a ceramic material. It is not crystalline like Al. During mechanical attrition all materials tend to break (depending upon the individual mechanical strenghth and impact force during attrition).