Effect of interface morphology on intermetallics formation upon annealing of Al–Ni multilayer

doi:10.1016/j.jallcom.2013.04.140

Journal of Alloys and Compounds

Volume 576, 5 November 2013, Pages 257-261

https://doi.org/10.1016/j.jallcom.2013.04.140 Get rights and content

Highlights

•
Formation of aluminides at the interfaces in Al–Ni multilayer film.
•
Interfaces show different roughness and asymmetric composition.
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First phase formation depends on interface roughness.

Abstract

Following Benès rule, the initial phase formed at interface of Al–Ni multilayer on annealing is Al₃Ni. On further annealing it should give rise to intermetallics Al₃Ni₂, AlNi and AlNi₃, gradually increasing in Ni content. Using X-ray and polarized neutron reflectivity (PNR) we studied the depth dependent structure and magnetization of as-deposited and annealed Al–Ni multilayer, with thickness ratio of Al:Ni equal to 1:2 giving an overall atomic stoichiometry of 1:3 for Al:Ni. We identified asymmetric initial alloy phase formation at two interfaces, i.e. Ni on Al (Ni/Al) and Al on Ni (Al/Ni), of Al–Ni multilayer on annealing. The phases are identified as Al₃Ni and Al₃Ni₂ at Al/Ni and Ni/Al interfaces respectively. Our results indicate that the asymmetric alloy formation at interfaces is directly correlated with the interface morphology of as-deposited multilayer. Higher roughness at Ni/Al interface changes the local concentration of elements at interface, which results into change in effective heat of formation of the alloys and thus makes Al₃Ni₂ more favourable at Ni/Al interfaces. PNR results also suggested that alloy layers are magnetically dead.

Introduction

Alloys of nickel and aluminium are important in industrial applications and research fields because of their attractive properties like light weight, high strength, high melting point, good oxidation resistance and high mechanical strength [1], [2], [3]. Intermetallics can be produced by several methods like cold rolling [4], [5], ball milling or mechanical alloying [6], [7], shock compaction and self sustained reactions [8]. But special care has to be taken during new phase evolution in the system. Thermal annealing in vacuum is one of the suitable methods for inducing solid state reactions between pure Ni and Al layers to produce nickel aluminides [9], [10] in a multilayer thin film with alternating Ni and Al layers.

Ni and Al both are iso-structural (fcc) at room temperature with cell parameters 3.52 Å and 4.05 Å, respectively. Al atomic size is 13% larger compared to Ni, which also reflects in their respective unit cell parameters. They form several ordered intermetallics viz. Al₃Ni, Al₃Ni₂, AlNi, and AlNi₃ in order of increasing Ni concentration. Interface diffusion in thin films and multilayer systems is mediated through several external parameters like temperature [9] and reaction time [11]. In case of multilayer stacks, we find that thickness ratio of the respective components is also an important parameter [12]. Considering the number density (no. of scatterers per unit volume) of Ni as 9.1 × 10²² cm⁻³ and of Al as 6.02 × 10²² cm⁻³, in bulk, to get an alloy with 1:1 atomic ratio in a multilayer stack we should have a thickness ratio d(Al)/d(Ni) of 1.5:1 [9]. Similarly to get a stoichiometry of 1:3 in Al:Ni we should have a thickness ratio d(Al)/d(Ni) of 1:2. Thickness of the present system is based on these calculations. Between Al and Ni, Al with a lower melting point is the more mobile species. Usually during alloy formation at the interfaces Al₃Ni is the first phase that forms at lower temperatures of annealing typically (about 250–300 °C) [13]. Thickness of the component layers also might dictate the first phase formed at the interfaces, which further decides the kinetics of phase formation at the interfaces [12].

Earlier we had studied the structure and magnetism of a Al–Ni multilayer film and the effect of annealing on the system with thickness ratio, d(Al)/d(Ni) of 1:1 [11]. Using X-ray reflectivity (XRR) [14] and polarized neutron reflectivity (PNR) [15], we demonstrated that it is possible to quantify the composition of binary alloy formed at the interface of Al–Ni multilayer on annealing [11]. XRR and PNR are two non-destructive techniques that provide quantitative measures of the chemical and magnetic depth profiles of films with nanometer resolution averaged over the lateral dimensions of the entire sample (typically ∼100 mm²). Here we study the structural and magnetic properties of a Al–Ni multilayer and effect of annealing on the multilayer with a thickness ratio, d(Al)/d(Ni), of 1:2, which gives an atomic stoichiometry of Al:Ni equal to 1:3, making it rich in Ni. Also the as-deposited sample showed asymmetric roughness at interfaces as measured by reflectometry data. Ni on Al (Ni/Al) interfaces showed much higher roughness compared to Al on Ni (Al/Ni) interfaces. We annealed the sample at 160 °C from 1 h to 8 h and looked for the alloy composition at the interfaces. Our analysis shows that upon annealing the sample at 160 °C, Al₃Ni alloy formed at the Al/Ni (Al on Ni) whereas Al₃Ni₂ alloy layer has been found at the Ni/Al interface (Ni on Al) interface. Effective heat of formation rule predicts that the first interface alloy layer should be Al₃Ni [16]. According to Colgan et al. [13] formation of interface alloy will also depend on kinetics as well as interface composition. Our work specifically highlights this issue. According to the PNR analysis these interface alloy layers carry zero magnetic moment and hence are considered to be magnetically dead layers.

Section snippets

Experimental details

A multilayer sample Si/[Al(25 Å)/Ni(50 Å)] × 10 was prepared by ion beam sputtering at a base pressure of 2 × 10⁻⁹ Torr [11]. The deposition rate for both the elements was 0.1 Å/sec. During deposition the thickness of the layers were calibrated using a water cooled quartz crystal monitor. The samples were annealed at 160 °C for time intervals of 1 h, 4 h and 8 h. XRR and PNR data were collected after successive annealing. The XRR data were taken in a Bruker’s D8 advanced laboratory source and the PNR data

Results and discussion

Fig. 1 shows the PNR (R⁺ and R⁻) data from as-deposited and annealed Al-Ni multilayer. Closed (red)¹ and open (blue) circles in Fig. 1 depict the experimental spin dependent reflectivities R⁺ and R⁻, respectively. Fig. 1a shows the PNR profile for as-deposited multilayer sample. PNR profiles for sample annealed at 160 °C for 1 h, 4 h and 8 h are shown in Fig. 1b–d, respectively. Reflectivity plots were

Summary

We carried out depth dependent structure and magnetic properties of a Al–Ni multilayer (with thickness ratio of d(Al)/d(Ni) = 1:2) in as-deposited and annealed conditions (at 160 °C for 1–8 h) using XRR and PNR. The as-deposited multilayer showed asymmetric roughness at the interfaces. On annealing the sample at 160 °C for 1h we observed asymmetric alloy formation at interfaces, which might have resulted from asymmetric roughness in the as-deposited sample. Detailed analysis of XRR and PNR suggested

Acknowledgements

SB acknowledges the help received from Chuck Majkrzak of NCNR, NIST during the PNR experiment.

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Effect of interface morphology on intermetallics formation upon annealing of Al–Ni multilayer

Highlights

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Summary

Acknowledgements

Mater. Sci. Rep.

Acta Mater.

Acta Mater.

Intermetallics

Prog. Mater. Sci.

Physica B

Mater. Lett.

Acta Mater.

Scripta Mater.

J. Magn. Magn. Mater.

J. Vac. Sci. Technol. A

Int. J. Eng. Technol. IJET–IJENS