March_2022_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 1 8 kinetically. For this study, Pandat (a CAL- PHAD-based thermodynamic software) was employed to understand the vari- ations in solidus and liquidus tempera- tures within the two terminal alloys. Along with this, important parameters like gamma prime solvus temperature, equilibrium phase fractions as a func- tion of temperature, and corresponding phase compositions were also deter- mined. Initial trials involved identifying the major equilibrium phases in a pure Ni to pure Nb graded structure (Fig. 3). For this calculation, 11 major phases, namely liquid (L), face cen- tered cubic (fcc), body centered cubic (bcc), ordered bcc (B2), gamma prime (L1 2 ), gamma double prime, delta, mu, sigma, Laves, and AlNbNi 2 , were select- ed. As shown in Fig. 3, the Ni-Nb bina- ry phase diagram exhibits an fcc crystal structure on the pure Ni side, and a bcc crystal structure on the pure Nb end. There are also intermediate intermetal- lic phases—delta and mu, with possible compositions of Ni 3 Nb and Ni 6 Nb 7 , re- spectively, coupled with associated eu- tectic reactions. Subsequently, in the modeling effort for commercial IN718 and C103 alloys, a six-zone linear composition- al gradation was chosen to predict the phase evolution of these 11 phases. Thus, if the compositional variations may be defined as (x*(Ni alloy) + y* (Nb alloy)), where x and y are weight fractions of the two terminal alloys and x + y = 1, then the gradient steps for six-zone linear compositional grada- tion were set up such that the unicom- positional zones are with x = 1, 0.8, 0.6, 0.4, 0.2, and 0. Thus, the terminal al- loy compositions were represented by x = 1 (IN718 with nominal composition of 54Ni-0.7Al-19Cr-17Fe-3Mo-5.3Nb-1Ti wt%) and x = 0 (C103 with nominal com- position of 89Nb-10Hf-1Ti wt%). CALPHAD-based thermodynam- ic predictions of this compositional- ly graded structure are compiled in Tables 1-3. Table 1 shows a tabula- tion of predicted solidus and liquidus Fig. 3 — Ni-Nb binary phase diagram as predicted by Pandat software. TABLE 1 — PREDICTED SOLIDUS, LIQUIDUS, AND FIRST PHASE TO SOLIDY FOR SIX-ZONE LINEAR COMPOSITIONALLY GRADED MODEL Fraction of IN718 alloy Fraction of C103 alloy Liquidus temp, °C Solidus temp, °C Freezing range, delta °C Primary solid phase to form in liquid 1 0 1330 1260 70 FCC 0.8 0.2 1279 939 340 Laves (C14) 0.6 0.4 1510 844 666 Laves (C14) 0.4 0.6 1533 1015 518 Laves (C14) 0.2 0.8 2019 1005 1014 BCC 0 1 2411 2372 39 BCC TABLE 2 — PHASE PREDICTION AND CORRESPONDING PHASE FRACTION AT 25°C FOR SIX-ZONE LINEAR COMPOSITIONALLY GRADED MODEL Compositional gradient Predicted phase and phase % at 25°C Fraction of IN718 alloy Fraction of C103 alloy FCC Gamma prime, L1 2 Delta Laves, C14 Mu AlNbNi 2 BCC Other minor phases 1 0 45 11.6 19.7 5.6 18.1 0.8 0.2 53.7 4.8 5.1 5.2 28 3.2 0.6 0.4 26.6 62.2 4.3 2.3 4.6 0.2 0.8 27.5 12.5 1.5 52.1 6.4 0 1 92.7 7.3