WELDING RESEARCH Fig. 23 — Semiquantitative (standardless) XEDS line scan across dendrites in a single-pass This reaction sequence and temperature range is generally similar to that expected in the binary Ni-Gd system. Simple binary Ni-Gd alloys with less than about 13 wt-% Gd exhibit a similar two-step solidification sequence consisting of primary austenite formation followed by a terminal eutectic reaction involving the Ni17Gd2 intermetallic at 1275°C (Ref. 44). By comparison, the multicomponent Ni-Cr-Mo-Gd alloys examined here complete solidification at ~ 1258°C by a terminal eutectictype reaction involving the Ni5Gd intermetallic. Thus, although the secondary phase within the terminal eutectic constituent is different in each case, the terminal reaction temperatures are very similar. In fact, as shown in Fig. 26, a pseudo binary solidification diagram has recently been developed (Ref. 38) for this alloy that is similar to the phase diagram of a binary eutectic alloy. In this case, the “solvent” is represented by the Ni-Cr-Mo solid solution γ -austenite phase and Gd is treated as the solute element. Although the diagram does not account for the minor variation in matrix Mo concentration that occurs due to microsegregation, the similarity of this γ -Gd binary system to a binary eutectic system is readily evident in several ways, including the assolidified microstructure consists of primary γ dendrites surrounded by an interdendritic eutectic-type constituent in which the secondary phase in the eutectic is solute rich; the amount of eutectic-type constituent increases with increasing solute content; and the proportional amount of each phase within the eutectic constituent is relatively insensitive to nominal solute content (Ref. 38). Also note that the eutectic temperature is not strongly dependent on the nominal Gdconcentration. The pseudo binary diagram developed for these alloys is useful for interpreting the weldability results because it permits direct determination of the solidification temperature range and amount of terminal γ /Ni5Gd eutectic constituent as a function of Gd concentration. The fraction of terminal γ /Ni5Gd eutectic constituent (fe) that forms during solidification can be calculated with the Scheil equation (Ref. 45) via (1) where CGd Gd C C e Gd 1 1 e is the concentration of Gd in the liquid at the eutectic reaction (14.7 wt- % Gd, Fig. 26), CGd o is the nominal Gd concentration, and k is the distribution coefficient for Gd. Equation 1 is valid for conditions in which the diffusivity of solute (Gd) in the solvent (γ) is insignificant. Previous work (Ref. 37) has shown that essentially no Gd is dissolved in the γ matrix, which results in the equivalent condition of negligible solute diffusivity in the solvent. The lack of Gd solubility in austenite also indicates that k for Gd is 0, and Equation 3 reduces simply to (2) Gd C C e Gd = The solidification temperature range of fusion welds is best represented by the separation between the on-heating liquidus temperature (TL) and on-cooling eutectic temperature (Te) because solidification initiates epitaxially at the weld interface without the need for undercooling (Refs. 46, 47). As mentioned previously and discussed in more detail elsewhere (Ref. 38), the eutectic temperature (Te) does not vary significantly with Gd concentration and can be represented by an average value of Te = 1258°C. The liquidus line in Fig. 26 can be expressed by a simple linear equation of the form (3) = + Gd where To is the melting point of the Ni-Cr- Mo “solvent” and mL is the liquidus slope. Linear regression analysis of the phase diagram leads to values of To = 1422°C and mL = –11.2°C/wt-% Gd. Thus, the solidification temperature range (ΔT) can be directly determined as a function of Gd concentration via (4) Gd Δ = + − e The solidification temperature range is important from a weldability perspective because it controls the size of the solid + liquid “mushy” zone that trails the fully molten weld pool. Assuming a constant temperature gradient in the solid + liquid region (i.e., fixed welding parameters and sample size/geometry), the size of the mushy region is given simply by the ratio of the solidification temperature range to temperature gradient. Thus, alloys with narrow solidification temperature ranges are generally crack resistant because the mushy zone is relatively small. The influence of the amount of terminal eutectic liquid is a bit more complicated. At very low amounts of terminal liquid, the cracking resistance is generally not adversely affected because there is not enough liquid to cover the solidification grain boundaries and interdendritic regions. Thus, solid-solid boundaries can be easily f e o k = ⎡ ⎣ ⎢⎢ ⎤ ⎦ ⎥⎥ − f o e TL To mLCo T To mlCo T WELDING JOURNAL 41-s GTA weld of Alloy 740H after homogenization at 1100°C for 4 h. A — Light optical micrograph of region of interest; B — SEM micrograph of region of interest; C — concentration profile for major alloying elements; D — concentration profile for γ ′-forming elements. Fig. 24 — Varestraint results showing maximum crack length as a function of Gd concentration in Alloy C-4.
Welding Journal | February 2014
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