Nanoscale structure formation in nickel-aluminum alloys synthesized far from equilibrium

Abstract: The present study reports on the structure formation in thin epitaxial nickel-aluminum films (Ni1-xAlx; 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<0.11, the films exhibit the face-centered cubic random solid-solution Ni1-xAlx structure ({\gamma}). Whereas in the range x=0.11-0.24 the phase coexists with the ordered L12 structure ({\gamma}' phase). The two phases are homogenously intermixed forming a coherent and strained nano-solution, 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 nano-solution 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 nano-solution to dissolve into distinct {\gamma} and {\gamma}' phase domains and the hardness to decrease down to values of 4 GPa. These findings confirm the metastable nature of the as-deposited thin Ni1-xAlx alloy films and underpin the effectiveness of high supersaturation/undercooling for creating non-equilibrium phases and self-organized nanostructures upon synthesis of multicomponent materials.