Identification of a kinetic length scale which dictates alloy phase composition in Ni-Al interfaces on annealing at low temperatures

Abstract

Ni-aluminides are an important class of intermetallics from technological point of view. Ni-Al phase diagram has been studied in detail experimentally as well as theoretically. It is known that if annealed at low temperature, the first alloy phase is usually NiAl3 according to Bené's rule. It is also understood that heat of formation may get modified by local densities of the constituents forming the alloy. In this regard, it is important to identify a kinetic length scale for defining “local density” in a system. We have deposited ultrathin multilayers of Ni and Al of layer thickness in tens of nanometres with Ni:Al stoichiometric ratio as 3:1 and 1:3, respectively. Considering these stoichiometry, Ni3Al and NiAl3 are the thermodynamically favoured alloy phases in these samples. We used x-ray reflectivity, polarized neutron reflectivity, x-ray diffraction, and secondary ion mass spectroscopy to follow the alloy formation after annealing and identified the alloy phases at interfaces with nanometre resolution. Diffusion length of Ni and Al was obtained using Darken's law. Our results predict that ‘diffusion length’ is the unique length scale that connects kinetics to local density. In another interesting observation, using “virtual Kirkendall markers” at the interfaces, we showed asymmetry in consumption of Al for alloy formation, at Al on Ni (Al/Ni) and Ni on Al (Ni/Al) interfaces by comparing as-deposited and annealed states with respect to the markers.

Stoichiometry dependent inter diffusion and structural evolution in Al–Ni multilayer

“…In case of sample S1, we could get sizable fraction of Ni even after the final anneal but in case of S2 the fraction of Ni became nearly zero even after the first anneal at 150°C. This is also supported by XRR and PNR data [16]. In case of S1, alloy grain size increased from $50 Å to 170 Å and in S2 the grain size increased from $60 Å to 88 Å on annealing.…”

“…We also observed that Al being the more mobile species of the two, it has a higher diffusion length (140 Å) compared to Ni (33 Å) as estimated in Ref. [16]. This leads to higher ''local density'' for Al and formation of Al 3 Ni intermetallic phase at the interfaces in both S1 and S2 in line with Bené's rule.…”

“…Earlier we demonstrated that a kinetic length scale dictates the formation of intermetallic alloy on annealing of two Al-Ni systems with exactly opposite stoichiometry in Ni and Al [16]. Our results predict that 'diffusion length' is the unique length scale that connects kinetics to local density and results into asymmetry in consumption of Al for alloy formation, at Al on Ni (Al/Ni) and Ni on Al (Ni/Al) interfaces.…”

“…XRD data clearly indicates that the growth kinetics of the alloy phases are strongly dependent on the stoichiometry. While we are able to conclude that the alloy phases are crystalline, from XRD data, the exact composition of the alloy phases was identified from XRR and PNR and found to be Al 3 Ni in both the samples [16].…”