A Thermodynamic Approach in Tuning Phase Stability in Nanocomposite Multilayers
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L3.40.1
A Thermodynamic Approach in Tuning Phase Stability in Nanocomposite Multilayers G.B. Thompson1,2, R. Banerjee2, and 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
Abstract Changes in the crystallographic phase stability of individual layers in a multilayered thin film stack are expected to have a significant influence upon the functional properties of the structure. The ability to predict and tune these phase stability states is of relevant importance in order to maximize the functional properties of the multilayer. A classical thermodynamic methodology, based upon competitive volumetric and interfacial free energies, has been used in the prediction and subsequent confirmation of the hcp to bcc phase stability in a Ti/Nb multilayer. An outcome of this model is a new type of phase stability diagram that can be used to predict the hcp Ti and bcc Ti phase stability as a function of length scale and volume fraction. The Ti layers were subsequently alloyed with a bcc-stabilizing element. The alloyed sputtered deposited Ti layers were able to stabilize the bcc Ti phase to a larger layer thickness as compared to the unalloyed Ti/Nb multilayers. The percentage of alloying element added to the Ti layer in controlling the critical transition thickness between the two phase states had good agreement with the predictions proposed by the thermodynamic model. Introduction We have recently reported a series of hcp to bcc Ti phase stability changes in Ti/Nb multilayers [1]. These changes have been rationalized using a thermodynamic model originally presented by Dregia et al. [2]. In this model, a multilayer of A/B alternating stacking can be can be represented by a volume fraction of either fA or fB and a compositional modulation, λ, which is equivalent to the bilayer thickness of layer A plus layer B in the A/B unit stacking cell. The total free energy change between a pseudomorphic and bulk equilibrium state of either A or B in the unit cell can be described as (1) ∆G/A = ∆γ = [∆GA(fA) + ∆GB(fB)]λ + 2∆γ where ∆Gi is the volumetric free energy difference between the pseudomorphic and equilibrium states of component i (where i is A or B), A is the fixed surface area of the film, ∆γ is the interfacial free energy reduction between the pseudomorphic and equilibrium phases, and λ and fi are as defined above. Note that fA•λ or fB•λ is the respective layer thickness of either A or B. To maintain the simplicity of equation (1) we will assume that all terms that scale with volume, such as strain energy, and all terms that scale with area are contained in the volumetric, ∆Gi, and area, ∆γ, terms respectively. When necessary the strain energy can be treated as a separate energy contribution [3]. In the framework of equation (1), either the A and/or B layer can under go a change in phase stability to a pseudomorphic phase. This will result in a positive increase i
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