Gibbs phase rule for Gibbs free energy says that phases during a phase transition must be in mechanical, chemical, and thermal equilibrium, e.g. $T_1=T_2$,$G_1=G_2$, and $P_1=P_2$, where the subscripts represent different phases (phase 1 and phase 2), T is temperature, P is pressure, and G is Gibbs free energy.
My understanding of these rules is that the constant temperature and pressure come from the fact that the decrease of Gibbs Free energy only becomes synonymous to the second law of thermodynamics when pressure and temperature are constant.
For systems where pressure isn't constant, but where for example, volume is constant, and temperature remains constant, we use Helmholtz free energy. However I was wondering if there is an equivalent Gibbs phase rule for phase transitions in equilibrium, where $T_1=T_2$,$F_1=F_2$, and $V_1=V_2$, where F is Helmholtz free energy and V is volume. I was just wondering if this is right.