The present study reports on the structure formation in thin epitaxial nickel-aluminum films (Ni<inf>1-x</inf>Al<inf>x</inf>; Al atomic fraction x up to x = 0.24 ) grown on MgO ( 001 ) substrates by magnetron sputtering. Experimental and computational data demonstrate that for x &lt; 0.11 , the films exhibit the face-centered cubic random solid-solution Ni<inf>1-x</inf>Al<inf>x</inf> structure ( γ phase). Whereas in the range x = 0.11–0.24 the γ phase coexists with the ordered L 1 2 structure ( γ ′ phase). The two phases are homogenously intermixed forming a strained coherent nanocomposite , which exhibits a single lattice parameter that expands as the Al content increases. Isothermal annealing of films containing x = 0.14 of Al, coupled with structural and nano-mechanical characterization, reveal that the coherent nanocomposite retains its overall integrity for temperatures up to 673 K , while the film hardness increases from 5.5 GPa (as deposited films) to 6 GPa . Further increase of the annealing temperature to 873 K and 1073 K causes the coherent nanocomposite to dissolve into distinct γ and γ ′ phase domains and the hardness to decrease down to values of 4 GPa . These findings confirm the metastable nature of the as-deposited thin Ni<inf>1-x</inf>Al<inf>x</inf> alloy films and underpin the effectiveness of high supersaturation/undercooling for creating non-equilibrium phases and self-organized nanostructures upon synthesis of multicomponent materials.