Development of boron-enhanced inconel 718 with superior thermomechanical properties for high-temperature concentrated solar power applications

Abstract

This study presents the characteristics of a modified boron-enhanced Inconel 718 for elevated mechanical strength and excellent optical properties for the next generation of solar receiver tube applications. While the standard in industry to produce high absorptive surfaces is through utilizing coatings, it becomes more challenging to maintain for high-temperature applications (>800 °C) for a long duration. The present study intends to directly increase the absorptivity of the Inconel 718 and bypass the need for coatings via Additive Manufacturing (AM) using boron-enhanced Inconel 718 powders. The effects of post-heat treatment and thermal cycling on microstructure, mechanical, and optical properties were analyzed systematically. The laser powder bed fusion (LPBF) technique enabled the boron content in Inconel 718 to increase up to 5000 ppm without microstructural defects (i.e., process defects). Increased boron content induced a larger amount of eutectic $\gamma$ phase (involving Laves phases) development, leading to enhanced tensile strength and microhardness. Furthermore, it is observed that after heat treatment and thermal cycling, with high boron concentration the Laves phase morphology changed to a more interconnecting web-like structure. Thus, it is important to study the possible concentration of boron that can be added to the alloy using the LPBF process. A specially designed post-heat treatment was applied to remove the Laves phase with a long-striped shape and produce a smaller, granular-shaped Laves phase. Compared to pure Inconel 718, the boron-enhanced Inconel 718 showed that its microhardness increased to 36.6 % at the as-printed stage and up to 9.2 % after a proper heat treatment. Boron-doped Inconel 718 altered the optical properties by demonstrating that reflectance decreased by 10 %. This approach could lead to the development of more resilient and high-performance receiver tubes capable of withstanding extreme operating conditions, reducing maintenance costs, and extending the lifespan of CSP components. This study aims to remove the reliance on coatings with limited lifetimes by directly increasing the absorptivity of the utilized alloy. It is expected that limiting downtime that would otherwise be utilized for recoating solar absorber tubes could provide a more reasonable return on investment after considering operational expenses.