Improvement in creep life of a nickel-based single-crystal superalloy via composition homogeneity on the multiscales by magnetic-field-assisted directional solidification

The improvement of the creep properties of single-crystal superalloys is always strongly motivated by the vast growing demand from the aviation, aerospace, and gas engine. In this study, a static magnetic-field-assisted solidification process significantly improves the creep life of single-crystal superalloys. The mechanism originates from an increase in the composition homogeneity on the multiscales, which further decreases the lattice misfit of γ/γ′ phases and affects the phase precipitation. The phase-precipitation change is reflected as the decrease in the γ′ size and the contents of carbides and γ/γ′ eutectic, which can be further verified by the variation of the cracks number and raft thickness near the fracture surface. The variation of element partition decreases the dislocation quantity within the γ/γ′ phases of the samples during the crept deformation. Though the magnetic field in the study destroys the single-crystal integrity, it does not offset the benefits from the compositional homogeneity. The proposed means shows a great potential application in industry owing to its easy implement. The uncovered mechanism provides a guideline for controlling microstructures and mechanical properties of alloys with multiple components and multiple phases using a magnetic field.

The following assumptions were made: 1) Only the solid and liquid phases are present, i.e., no pores are formed.
2) The liquid is Newtonian and incompressible, and the flow is laminar.
3) The solid and liquid phases have the same thermal properties and densities.
4) There is no diffusion of solutes in the solid phase. 7 5) The thermal properties are constant, which allows the use of the Boussinesq approximation. Hence, the density is constant except in the body-force term of the momentum equation.
6) The solid is stationary. The liquid and solid concentrations at the interface are in the local equilibrium.
With these assumptions, the governing equations for the transport processes in the melt during crystal growth can be described by the conservation laws for the mass, momentum, energy, solute, and phase field as follows: The x-axis component: The y-axis component: In the above equations, ρ is the melt density, V is the velocity vector, μ is the x-axis 8 velocity, ν is the y-axis velocity, t is the time, μl is the viscosity, K is the permeability, p is the pressure, σB is the electrical conductivity, B is the magnetic field intensity, βT is the thermal-expansion coefficient, βc is the solutal expansion coefficient, Tref is the reference temperature, cref is the reference concentration, λ is the thermal conductivity, cp is the specific heat, L is the latent heat, ɸ is the volume fraction of the solid, C is the average mixture concentration, kp is the equilibrium partition coefficient, D is a mixture diffusivity, Dl is the liquid's diffusivity, Ds is the solid's diffusivity, Mɸ is the motilities related to the interface kinetic coefficient, Ɛɸ is the gradient entropy coefficients related to the phase field, ɸ, mL is the liquid's slope, Cl is the liquid's concentration, TM is the pure component melting point, and WA is an energy hump, which is a constant.
The temperature-concentration relation is: where mL is the liquid's slope, and Cl is the liquid's concentration.
The boundary and initial conditions of the calculated domain are described in Fig. 8.
The given value of the parameters of Ni-40.38 wt.%.Cu is in Table 1.  Fig. 9(a). The MHD effect merges the two cell into one. Figure 9(b) is in the condition of being merged. Figure 9(c) shows the complete-mergement condition. With increasing the magnetic-field intensity, the cell 9 width becomes narrower as in Fig. 9(e). The convection velocity is also decreased with the MHD effect in Fig. 10. For example, the velocity in the melt is decreased by 30% at the middle time with the 2T magnetic field. The effect of MHD on the temperature field in the Ni-40.38 wt.% Cu alloy is not so obvious (Fig. 11). However, it make the temperature gradient in the liquid increase (Fig.   12). As we know, MHD could suppress the melt flow, which would increase the temperature gradient. On the contrary the thermoelectromagnetic convection (TEMC) induces a flow in the melt and further decreases the temperature. Therefore, the MHD control the convection on the macro scale.