Influence of heat treatment on the microstructure evolution and elevated temperature mechanical properties of Hastelloy-X processed by laser directed energy deposition

Abstract

The microstructure and elevated temperature mechanical properties of Hastelloy-X (Hast-X) bulk structures built using Laser Directed Energy Deposition (LDED) based Additive Manufacturing is investigated in as-built (AB) and heat-treated (HT) conditions. Microscopic analysis show the presence of fine cellular and dendritic growth in AB samples and recrystallized and coarsened equiaxed grains in HT samples. The AB samples revealed random grain orientation with slightly preferred texture along the< 100 >plane, unlike in the HT samples. Further, the AB samples revealed the elemental segregation of Mo, Si and C and precipitation of Mo-rich carbides, which are absent in the HT samples. Micro tensile test results at ambient temperature showed excellent agreement with the mechanical properties obtained from automated ball indentation (ABI) tests of LDED built Hast-X in AB condition. Subsequently, ABI tests are used to evaluate the mechanical properties in AB and HT conditions from ambient temperature to 873 K. It is observed that the strength and ABI hardness decreased with increase in test temperature, while the strain hardening exponent and uniform ductility showed an opposite trend. The yield strength (YS) of AB sample is higher than conventional sample, while YS of HT sample is similar to the YS of conventionally processed wrought Hast-X taken from literature at all test temperatures. Further, the indentation size and material pile up around the indentation also increased with heat-treatment and increase in the test temperature. This study paves a way towards the successful deployment of LDED built Hast-X for various applications in hostile conditions.