Continuous Ordering Reactions in NiAl/Ni 2 , AlTi Dual Phase Alloys
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CONTINUOUS ORDERING REACTIONS IN NiAl/Ni DUAL PHASE ALLOYS
,
AlTi
N.C. TSO AND J.M. SANCHEZ, Henry Krumb School of Mines, Columbia University, New York, NY 10027 ABSTRACT The Cluster Variation Method is used to study metastable phase equilibrium and possible ordering reactions in the Ni2 AlTi÷+NiAl dual phase alloy system. The Cluster Variation Method was also used to model the corresponding binary alloys and the results were extrapolated to the ternary system. The occurrence of a continuous ordering reaction in the Ni-Al-Ti system is proposed and possible decomposition reactions are discussed. INTRODUCTION The Ni-Al-Ti system, and in particular the two phase intermetallic Ni 2AlTi/NiAl, is widely regarded as a promising material for high-temperature applications. This system has shown, for example, creep strengths comparable to that of commercial nickel-base superalloys [1,2]. Despite the fact that these two-phase intermetallics have generated a great deal of interest there is, at present, a dearth of information concerning equilibrium and metastable phase transitions in the Ni-Al-Ti system. In this work, a thermodynamic model based on the Cluster Variation Method (CVM) is used in order to investigate equilibrium between bcc-based phases in the ternary Ni-Al-Ti system. The same model has recently been applied to the study of binary alloys [3,4], as well as Y-Y' phase equilibrium and site occupancy in the Y' phase of ternary nickel aluminides [5,6]. Both the Ni2 AlTi (L2,) and the NiAl (B2) phases are ordered structures based on the bec lattice (see Fig. la and ib). The L2,÷-B2 phase transition is observed in a number of systems including the Mg-Ag-Re (Re: rare earth element), the CuMn-Al, some binary magnetic alloys and the Ni-Al-X (X: either group IVB or VB element). In most cases, the L21,÷B2 transition is second order whereas in the Ni-Al-Ti, Ni-Al-Ta and Ni-Al-Hf systems the transition is first order. Several reasons may be invoked to account for a first order transition in these Ni-Al-X systems [7,8], such as the presence of a miscibility gap and/or large volume differences between these two phases. In this work, however, we are mostly concerned with the fact that the Ni 2AlTi++NiAl transition may take place metastably as a continuous order-disorder transition without initial compositional changes. In order to address the problem of modeling this metastable phase transition, we begin with the thermodynamic description of the corresponding binary systems. In the present case we apply the model to a full description of the Ni-Al and Ni-Ti phase diagrams, and a less complete description is offered for the Ti-Al binary system. Available thermodynamic information for the ternary system is also used in order to determine the parameters of the model. THERMODYNAMIC MODEL The computation of phase diagrams requires an accurate description of the free energy of formation of t e system. In our model, the free energy of formation of a solid phase *, AFf, is given by:
0
1 x F( + AH( - T(AS1+Sn)
(1)
i ii f vib conf w
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