This - Calphad calculated value of numerical derivative -  Q has some interesting applications in the field of metal solidification. In fact, Q reflects the phase diagram of the alloy system and its reciprocal has been found to have a relationship with grain size d on solidification, which empirically has been  found in some cases to be linear:
where a and b are constants, as illustrated with some examples from the literature for Mg and Al alloys. Before Calphad use,  Q values were calculated from the conventional relationship:
Q=m*c0(k−1)
where m is the slope of the liquidus, c0 is the solute concentration, and k is the equilibrium distribution coefficient.
More recently some other possible correlation of Q with grain size d have been found, for instance:
where B is a constant independent of alloy composition.