  Nickel-based superalloys and near-equiatomic high-entropy alloys containing
Molybdenum are known for higher temperature strength and corrosion resistance.
Yet, complex solid-solution alloys offer a huge design space to tune for
optimal properties at slightly reduced entropy. For refractory Mo-W-Ta-Ti-Zr,
we showcase KKR electronic-structure methods via the coherent-potential
approximation to identify alloys over 5-dimensional design space with improved
mechanical properties and necessary global (formation enthalpy) and local
(short-range order) stability. Deformation is modeled with classical molecular
dynamic simulations, validated from our first-principles data. We predict
complex solid-solution alloys of improved stability with greatly enhanced
modulus of elasticity ($3\times$ at 300 K) over near-equiatomic cases, as
validated experimentally, and with higher moduli above 500~K over commercial
alloys ($2.3\times$ at 2000 K). We also show that optimal complex
solid-solution alloys are not described well by classical potentials due to
critical electronic effects.
