Explain briefly ALL of the following observations (a) For some systems, decomposition of an unstable solid solution into two phases does not occur by the nucleation and growth mechanism. (b) The average precipitate size continues to increase during a heat treatment, even after the equilibrium volume fraction has been reached. (c) Rapid quenching of a system with a different high and low temperature equilibrium crystal structure can often lead to a martensitic phase transformation. (d) During annealing, the dislocation density of a deformed metal will often show an initial gradual reduction followed by a rapid drop as annealing continues