I think yes.

thanks sir 

so we can say that the two element Si and Mg are immiscible according to their phase diagram

I think yes, they are mixed only above the liquidus line.

thanks sir, for your answers

best regards

 when you have intermetallic compound between two material in diagram phase, that mean it's a miscible system, in immicible systemme ther is no intermetallic and no solide solution between two material in ordinary temperature . eg: (Fe/ Mg) or (Fe/Cu).