Hastelloy X alloy is commonly used to manufacture high-temperature components such as the combustion chambers of aero-engines by selective laser melting (SLM). In practice, even when using the same SLM process, heat treatment, and hot isostatic pressing process, there are differences in the microstructures and mechanical properties of different batches of Hastelloy X alloy parts. There is a strong correlation among these differences and differences in the compositions of the batches of Hastelloy X alloy powder raw material used. In particular, the influence of the Si element is the most significant. The existing research mainly focuses on the use of the Si element in the traditional casting or forging process for the preparation of nickel-based high temperature alloys, with little attention given to the Si element in the Hastelloy X alloy in SLM-related research. This study discusses the effect of Si on the microstructure and stress rupture properties of SLM formed Hastelloy X alloy parts, with a view to providing useful guidance for optimizing their quality.

Two batches of gas-aerosolized Hastelloy X powders with different Si compositions, A ( Si mass fraction of 0.071%) and B (Si mass fraction of 0.365%), are used in this experiment, with particle sizes ranging from 15 μm to 45 μm. The experiment is carried out using the SLM equipment. The SLM forming parameters of the two batches are the same, and the specimens are placed in the length direction parallel to the substrate (transverse specimens). After the SLM is finished, each sample is subjected to hot treatment at 1050 ℃ for 1 h to remove the thermal stress, and then hot isostatic pressing at 1150 ℃ for 2 h is performed. A scanning electron microscope (SEM) is used to observe the high magnification microstructure of the Hastelloy X alloy, and the compositions of the phases at the grain boundaries are analyzed using an SEM energy dispersive spectrometer (SEM-EDS). The type of needle-like precipitates is analyzed using a transmission electron microscope (TEM). The stress rupture properties are tested using an electronic creep and stress-rupture testing machine at 815 ℃ and 105 MPa. Finally, the stress rupture fracture is observed using an SEM.

Before the stress rupture test, the specimens in batch A contain precipitated Cr-rich and Mo-containing M₂₃C₆-type carbides with a chain-like distribution at the grain boundaries [Fig. 2(a)], while the specimens in batch B contain precipitated Mo-rich M₆C-type carbides distributed in a more continuous manner at the grain boundaries [Fig. 2(c)]. After the stress rupture test, there is no obvious change in the type of grain boundary precipitates (Fig. 3), but there are a large number of needle-like precipitates present in the grains of the specimens in batch B, with compositions that include Mo, Si, and W (Fig. 4). Diffraction patterns show that the precipitation phase has a tetragonal structure (Fig. 5), with the lattice constants of a=3.2, b=3.2, and c= 7.8, and atomic mass ratio of Mo, Si, and W is 1∶2.4∶0.3. The carbide morphology at the grain boundaries of the specimens in batch B promotes crack initiation and extension, affects the bonding force between grain boundaries, and leads to grain boundary embrittlement. A large number of needle-like MoSiW hard and brittle phases are precipitated within the grains during the stress rupture test, which reduces the stress rupture properties of the alloy.

Before the stress rupture test at 815 ℃ and 105 MPa, the carbides of the specimens in batch A are precipitated at both grain boundaries and inside the grains. The grain boundary precipitates have a chain-like distribution, and the carbide type is Cr-rich M₂₃C₆. In contrast, the carbides of the specimens in batch B are precipitated almost exclusively at the grain boundaries in a continuous pattern, and the carbide type is Mo-rich M₆C. After the durability test, no significant change is seen in the precipitates of the specimens in batch A, but the specimens in batch B show a needle-like precipitation phase (MoSiW) inside the grains. In addition, the durability properties of the specimens in batch A reach the standard of Hastelloy X alloy forgings, while the stress rupture properties of the specimens in batch B show low plasticity, and the average elongation is 6.4%, which is lower than that of the standard of Hastelloy X alloy forgings. The shape, distribution, and type of the Hastelloy X precipitation phases are closely related to the Si content. A high Si element content (mass fraction of 0.365%) leads to the significant precipitation of a hard brittle needle-like phase within the grains and continuous carbon outgrowth at the grain boundaries during the stress rupture test, which affects the bonding force between the grain boundaries and reduces the stress rupture properties of the alloy.