Atom Probe Tomography Characterization of Thin Film Multilayers
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Atom Probe Tomography Characterization of Thin Film Multilayers G.B. Thompson1,2, M.K. Miller3, R. Banerjee2and H.L. Fraser2 1
now at University of Alabama Department of Metallurgical and Materials Engineering, Tuscaloosa, AL 35487 2 The Ohio State University Department of Materials Science and Engineering, Columbus, OH 43210 3 Oak Ridge National Laboratory Metals and Ceramics Division, Oak Ridge, TN Abstract As the individual layer of Ti is reduced in thickness in a Ti/Nb multilayered thin film structure, the Ti layer can undergo a change in phase stability from hcp to bcc. Due to its high spatial resolution, atom probe tomography (APT) is ideally suited to characterize the compositional variations in such thin film structures. A series of hcp Ti / bcc Nb and bcc Ti / bcc Nb multilayers have been sputtered deposited and prepared as APT specimens using a Focus Ion Beam (FIB) milling procedure. The APT results have shown a substantial interdiffusion of Nb into the bcc Ti layers to a pseudo-equilibrium concentration of approximately Ti-20at%Nb while maintaining a compositionally modulated interface. In contrast, the hcp Ti layers indicated little Nb interdiffusion within the layers. Thermodynamic volumetric free energy modeling has indicated that this unexpected Nb interdiffusion facilitated the bcc phase stability. The coupling of APT results to the pseudomorphic bcc Ti phase demonstrates the capability APT has in quantifying the compositional characteristics in these types of multilayered nanocomposite systems. Introduction When individual layers in a multilayered thin film stack are reduced in thickness, a change in phase stability can result in one or more of the layers [1-6]. This has been referred to as pseudomorphism [7]. Recently, we have reported the change in hcp to bcc phase stability for Ti in a series of Ti/Nb multilayers [8]. These changes in phase stability have been rationalized using a classical thermodynamic model [9]. In this thermodynamic methodology [9], the stabilization of either hcp Ti or bcc Ti occurs because of the dominance of either the volumetric or interfacial free energy in stabilizing one of these two phases. This competitive free energy interaction can be described in terms of a Ti/Nb bilayer unit cell within the multilayer [8-9]. The Ti/Nb bilayer unit cell has a length scale, λ, given as (1) λ = hTi + hNb where hTi and hNb are the individual thicknesses of either the Ti or Nb thin films respectively. Additionally a volume fraction of Ti (fTi) and Nb (fNb) are associated with the unit cell structure. This unit cell structure is then repeated numerous times generating the Ti/Nb multilayered stack. The total free energy, ∆G, for stabilizing either the hcp or bcc polymorphs of Ti in the unit cell is given as (2) ∆G/A = [∆GTi(fTi)]λ + 2∆γ where A is the fixed surface area of the film, ∆GTi is the volumetric free energy difference between pseudmorphic bcc Ti and bulk equilibrium hcp Ti phases, fTi is the volume fraction of Ti
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in the unit bilayer, λ is the bilayer spacing
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