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

TL;DR

This study investigates the formation and stability of nanoscale structures in nickel-aluminum thin films synthesized far from equilibrium, revealing metastable phases and the effects of annealing on their microstructure and hardness.

Contribution

It demonstrates how high supersaturation enables the creation of non-equilibrium nanoscale phases and self-organized structures in Ni-Al alloys.

Findings

Nanoscale phase coexistence observed in films with x=0.11-0.24.

Nano-solution remains stable up to 673 K with increased hardness.

Higher annealing temperatures cause phase separation and hardness decrease.

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.