  Understanding the phase transformation pathways (PTPs) and microstructural
evolution in multi-phase HEAs will aid alloy and process designs to tailor the
microstructures for specific engineering applications. In this work, we study
microstructural evolution in two-phase HEAs where a disordered parent phase
separates into a mixture of two phases: an ordered phase ($\beta'$) + a
disordered phase ($\beta$) upon cooling following two different PTPs: (i)
congruent ordering followed by spinodal decomposition in the ordered phase and
then disordering of one of the ordered phases, i.e., $\beta \rightarrow \beta'
\rightarrow \beta_1' + \beta_2' \rightarrow \beta + \beta_2'$ and (ii) spinodal
decomposition in the disordered phase followed by ordering of one of the
disordered phases, i.e., $\beta \rightarrow \beta_1 + \beta_2 \rightarrow
\beta_1+\beta'$. We systematically investigate the effects of equilibrium
volume fractions of individual phases, free energy landscapes (in particular,
the location of the critical point of the miscibility gap relative to the
compositions of the final two equilibrium phases), and elastic modulus mismatch
between the two equilibrium phases on the microstructural evolution of these
HEAs. We focus on the following morphological characteristics: bi-continuous
microstructures vs. precipitates + matrix microstructures, ordered matrix +
disordered precipitates vs. disordered matrix + ordered precipitates, and the
discreteness of the precipitate phase. This parametric study may aid in
multi-phase HEA design for desired microstructures.
