Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Abstract

Laser powder deposition was performed on a substrate of Inconel 738 using blended powders of Mar M247 and Amdry DF3 with a ratio of 4:1 for repairing purposes. In the as-deposited condition, continuous secondary phases composed of γ-Ni₃B eutectics and discrete (Cr, W)B borides were observed in inter-dendritic regions, and time-dependent nucleation simulation results confirmed that (Cr, W)B was the primary secondary phase formed during rapid solidification. Supersaturated solid solution of B was detected in the γ solid solution dendritic cores. The Kurz–Giovanola–Trivedi model was performed to predict the interfacial morphology and correlate the solidification front velocity (SFV) with dendrite tip radius. It was observed from high-resolution scanning electron microscopy that the dendrite tip radius of the upper region was in the range of 15 to 30 nm, which yielded a SFV of approx 30 cm/s. The continuous growth model for solute trapping behavior developed by Aziz and Kaplan was used to determine that the effective partition coefficient of B was approximately 0.025. Finally, the feasibility of the modeling results were rationalized with the Clyne–Kurz segregation simulation of B, where Clyne–Kurz prediction using a partition coefficient of 0.025 was in good agreement with the electron probe microanalysis results.

Appendix A. Calculation of dendrite tip radius

The equation to estimate dendrite tip radius, as developed by Kurz and Fisher,[40] was calculated using Eq. [A.1]

$$ R = \frac{{\lambda^{2} G}}{{4\Delta T^{\prime}}}, $$

(A.1)

where R is dendrite tip radius, λ is dendrite arm spacing, G is temperature gradient, and ΔT′ is the temperature difference between dendrite tip and non-equilibrium solidus, and

$$ \Delta T^{\prime} = T_{\text{L}} - T_{\text{S}} - \Delta T^{*}, $$

(A.2)

where T _L and T _S are liquidus and solidus temperature, respectively, ΔT* is tip undercooling and expressed in equation (A.3) at RSP,

$$ \Delta T^{*} = mC_{0} \left( {1 - \frac{1}{1 - P\pi \sqrt A }} \right), $$

(A.3)

where m is the slope of the liquidus line in the phase diagram and can be described as Eq. [11] for RSP, C0 is the nominal solute concentration, p is the complementary partition coefficient and displayed as 1 − k, k is effective partition coefficient, and

$$ A = \frac{V\varGamma }{{D\Delta T_{0} k}}, $$

(A.4)

where V is SFV, γ is the Gibbs–Thompson coefficient, D is diffusion coefficient, and ΔT ₀ = T _L − T _S.