B2 Phases and their Defect Structures: Part I. Ab Initio Enthalpy of Formation and Enthalpy of Mixing in the Al-Ni-Pt-Ru
- PDF / 76,222 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 205 Views
S5.14.1
B2 Phases and their Defect Structures: Part I. Ab Initio Enthalpy of Formation and Enthalpy of Mixing in the Al-Ni-Pt-Ru System Sara Prins1,2, Raymundo Arroyave1, Chao Jiang1,3 and Zi-Kui Liu1 1 Department of Materials Science and Engineering, Pennsylvania State University, University Park PA-16802, USA 2 CSIR-NML, PO Box 395, Pretoria, 0001, South Africa 3(now at) Department of Materials Science and Engineering, Iowa State University, Ames, IA-50011, USA ABSTRACT The enthalpies of formation of the bcc phases in the Al-Ni-Pt-Ru system, particularly in the Al-Ru binary and Pt-Al-Ru ternary subsystems, were calculated by first principle methods. The enthalpies of formation for stoichiometric bcc-B2 phases have been calculated using both the GGA and LDA approximations, while the enthalpies of formation for B2 phases with large amounts of constitutional defects (both vacancies and anti-site atoms) were calculated using the Special Quasirandom Structures (SQS) approach. The enthalpies of mixing for the disordered bcc-A2 phases have also been calculated with SQS by mimicking the random bcc alloy with the local pair and multisite correlation functions. The calculated B2 lattice parameters for the different defect structures were compared with experimental results. These results are used as input values for the CALPHAD modified sublattice model to describe the A2/B2 phases with one Gibbs energy function. INTRODUCTION In the Al-Ni-Pt-Ru system, investigated for its importance in the jet turbine engine industry, the B2 phase stabilities (NiAl and RuAl are the two stable binary B2 structures in this system) are important from several perspectives. NiAl is used as protective bond coat to γ/γ’ Ni-based superalloy (NBSA) matrix as it is structurally and chemically more stable at high temperatures than the γ/γ’ matrix. Using a Pt-modified aluminide, (Ni,Pt)Al, the stability of the bond coat can be improved significantly [1]. RuAl exhibits the same good high-temperature properties as NiAl while showing exceptional room-temperature toughness and ductility [2]. Because of this, B2RuAl has also been proposed as an alternative to B2-NiAl as a bond coat material [3], as it has a higher melting temperature, and Ru is an important matrix strengthening alloying element in the 4th generation NBSAs . The B2 phases in the Al-Ni-Pt-Ru system can exist over a wide composition range due to the presence of anti-site (ASD), vacancy (Va) defects and even a combination of both. The defect structure in these phases affects their mechanical properties and thermodynamic stability [4-7]. Although the stable B2 phases in this quaternary system have been investigated for quite a long time, there is still uncertainty regarding the defect structure resulting from adding other constituents, such as Pt, to the structure. A modification of the defect structure can lead to dramatic changes in both the thermodynamic (thermal stability) and kinetic properties (interdiffusion, creep) of these phases. Although there have been several investigations focused
Data Loading...
