Thank you Volker Sir.

First of all you need to choose the criterium of mechanical properties optimization and understand clearly the possible mechanism of reinforcement... Otherwise all this correspondence is useless!

Thank you Igor L Shabalin Sir.

The question is very complex, mechanical properties of your intersting composition would depend at least on 3 parameters: composition /weight fraction/, size distribution  of the ceramisc filler and volume distribution of the filler. 

Yes Gutzov sir, I agree with you. Thank you.

But what weight fraction should I choose ? I am also looking some reports on on metal matrix nano-composites. In most of the papers the authors fixed a range of nanoparticle concentration like 0.01 to 0.05, or 0.1 to 0.5 wt% and saw the distribution and their effect on properties to comment on the optimum weight fraction/properties. 

I would plot mechanical properties as a function of  wt% of the filler and investigate the composites using X-Ray diffratcion.

To Dr. S. Gutzov

An important correction: vol.%..., but not wt.%... Although, it is clear for me that to convert wt% to vol.% sometimes is not easy for a researcher involved in some technological proceedings. However, only vol.% will be informative in this case. For a materials science and engineering problem this is the main concentration unit as the purpose of materials design is "how to fill a volume"...

To

Igor L Shabalin: I agree with Igor L Shabalin Sir. It should be the volume %. 

To  Dr. S. Gutzov: 

Yes, that would be a good approach to check mechanical properties as a function of filler concentration. However, selection of the range/interval of filler concentration in the composite is very important. How will you decide that? That is my concern. May be I am choosing the wrong concentration and I need a wider range like 1 to 5 wt% or volume % in which my optimum content may lie.

Thanks

To Igor Shabalin:

Dear Igor, you are right. but it is not difficult to change between wt % and vol %, if interactions are strong you can use optical microscopy for checking volume %. The range of filling, however, is a difficult question...

For aluminum matrix and ceramic particulates it is recommended to limit the ceramic reinforcement volume fraction to 15-25% for ductility and toughness issues. (Structural Composite materials, F.C. Campbell Chapter 20).

I don't know whether this helps, based on my personal experience on working with commercial nanoparticles, the bulk density of the nanoparticles are much lower than their micron sized counterparts. This affects the parameter of wt% and vol% calculations. This would be the first point to consider.

The second point, composite enhancement (if conducted uniformly) usually results in a J curvature (sometimes with inverted J). There is a either a maximum or minimum inflexion point. As to your case where there is no enhancement detected,  I suspect that the distribution of the materials is at fault. Did you examined the surfaces of the test pieces conducted under the mechanical tests? (I assume that tensile or flexural loading tests were performed).

Maybe you could refer to Westman and Hugil as a starting reference for bimodal powder mixing? 

May I know the size difference between the matrix and the nanoparticles?

Volume fraction or concentration of nano particles should be decided based on the specific properties expected from the composite. For example, tensile strength will improve to a critical volume fraction and decrease afterwards. However, wear rate will decrease with increase in volume fraction. 

It is very difficult to disperse nano particles in a molten metal. Some investigators used ultrasonic waves (generated using ultrasonic probe) to disperse nano particles. Some others used electromagnetic stirring of molten metal to disperse nano particles.  Both the methods may improve dispersion but complete absence of clusters is not guaranteed.

Thank you Sir, Guven, Alex, and Dinaharan.