Thermodynamic modeling of multicomponent alloy systems for the simulation of bulk Co alloys and related Coatings

Description

Co-based alloys received much attention because of its outstanding physical and chemical properties; however industrial applications as a high temperature material are limited by insufficient strength. The addition of alloying elements offer the opportunity for a considerable improvement, which makes Co-based alloys possibly become promising candidates for the next generation of superalloys. To extend the temperature application range of high temperature materials metallic coatings are applied to protect the substrate, extend the working time and improve the working temperature. In the present work, thermodynamic databases describing C-Co-Cr-Ni-Ta-W bulk alloys and Al-Co-Cr-Ni-Y alloy coatings were developed, which can be used for materials design of bulk superalloys and bond coat, respectively. To this end appropriate phase models were selected for all the phases appearing in the systems and especially for the Laves, Sigma, Mu and R phases, with particular attention to the consistency between thermodynamics and crystal structure. Two important ternary subsystems Co-Cr-Ni and Co-Cr-Ta were completely assessed while other systems were partially taken from literature and possibly adapted to our models until the whole C-Co-Cr-Ni-Ta-W database was built. As a result, several multi-component commercial alloys involving C, Co, Cr, Ni, Ta and W can be simulated. As for the coatings database, all the binary and ternary subsystems of the Al-Co-Cr-Ni-Y system were critically reviewed on the basis of the available literature. Then thermodynamic descriptions of binary systems taken from the literature were reviewed and refined if necessary. For each phase the model selection procedure is discussed in detail. A single Gibbs energy equation is used to model the order/disorder relation between ordered L12 and its disordered structure A1, as well as between ordered B2 and its disordered structure A2. The energy of vacancies in the A2 phase is adjusted to enhance its rationality. The Al-Co-Cr, Al-Co-Ni, Al-Cr-Ni, Al-Co-Y, Al-Cr-Y, Al-Ni-Y and Co-Ni-Y systems are modeled using the CALPHAD approach. Phase equilibria in the Al-rich corner are also considered during the optimization. According to the comparison, very good agreement between calculated results and experimental data is obtained. Based on the previous work, some extrapolations and brief discussions about the Al-Co-Cr-Ni quaternary system are performed. Satisfactory agreement between the calculations and experimental results are obtained, unless the stability range of the ζ phase is slightly underestimated. A reliable and self-consistent thermodynamic database for the Al-Co-Cr-Ni-Y multicomponent system is firstly obtained. As a conclusion a short analysis of the influence of Y addition on the Al-Co-Cr-Ni phase relations is presented based on the results of our calculations. The thermodynamic descriptions of the C-Co-Cr-Ni-Ta-W and Al-Co-Cr-Ni-Y systems developed in this work can be used to support the design of new alloying compositions for bulk alloys and related coatings. Moreover they also play an important role in the kinetic simulation of the interactions occurring at the joint between coatings and bulk alloys when in service

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