Nickel-based superalloys, an alloy system bases on nickel as the matrix element with the addition of up to 10 more alloying elements including chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. Through the development and improvement of nickel-based superalloys in the past century, they are well proved to show excellent performance at the elevated service temperature. Owing to the combination of extraordinary high-temperature mechanical properties, such as monotonic and cyclic deformation resistance, fatigue crack propagation resistance; and high-temperature chemical properties, such as corrosion and oxidation resistance, phase stability, nickel-based superalloys are widely used in the critical hot-section components in aerospace and energy generation industries.

The success of nickel-based superalloy systems attributes to both the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed manufacturing processes. The microstructure of the modern nickel-based superalloys consists of a two-phase configuration: the intermetallic precipitates (Ni,Co)3(Al,Ti,Ta) known as γ′ phase dispersed into the austenite γ matrix, which is firstly introduced in the 1940s.  The recently developed additive manufacturing (AM) techniques, acting as the disruptive manufacturing process, offers a new avenue for producing the nickel-based superalloy components with complicated geometries. However, γ′ strengthened nickel-based superalloys always suffer from the micro-cracking during the AM process, which is barely eliminated by the process optimization.

On this basis, the new compositions of γ′ strengthened nickel-based superalloy adapted to the AM process are of great interest and significance. This study sought to design novel γ′ strengthened nickel-based superalloys readily for AM process with limited cracking susceptibility, based on the understanding of the cracking mechanisms. A two-parameter model is developed to predict the additive manufacturability for any given composition of a nickel-based superalloy. One materials index is derived from the comparison of the deformation-resistant capacity between dendritic and interdendritic regions, while another index is derived from the difference of heat resistant capacity of these two spaces. By plotting the additive manufacturability diagram, the superalloys family can be categorized into the easy-to-weld, fairly-weldable, and non-weldable regime with the good agreement of the existed knowledge. To design a novel superalloy, a Cr-Co-Mo-W-Al-Ti-Ta-Nb-Fe-Ni alloy family is proposed containing 921,600 composition recipes in total. Through the examination of additive manufacturability, undesired phase formation propensity, and the precipitation fraction, one composition of superalloy, MAD542, out of the 921,600 candidates is selected.

Validation of additive manufacturability of MAD542 is carried out by laser powder bed fusion (LPBF). By optimizing the LPBF process parameters, the crack-free MAD542 part is achieved. In addition, the MAD542 superalloy shows great resistance to the post-processing treatment-induced cracking. During the post-processing treatment, extensive annealing twins are promoted to achieve the recrystallization microstructure, ensuring the rapid reduction of stored energy. After ageing treatment, up to 60-65% volume fraction of γ′ precipitates are developed, indicating the huge potential of γ′ formation. Examined by the high-temperature slow strain rate tensile and constant loading creep testing, the MAD542 superalloy shows superior strength than the LPBF processed and hot isostatic pressed plus heat-treated IN738LC superalloy. While the low ductility of MAD542 is existed, which is expected to be improved by modifying the post-processing treatment scenarios and by the adjusting building direction in the following stages of the Ph.D. research.

MAD542 superalloy so far shows both good additive manufacturability and mechanical potentials. Additionally, the results in this study will contribute to a novel paradigm for alloy design and encourage more γ′-strengthened nickel-based superalloys tailored for AM processes in the future.

Additive manufacturing (AM), e.g., laser powder bed fusion (LPBF) technique, has become a powerful manufacturing process for producing metallic components with the advantages of design freedom, net-shape-forming flexibility, product customization, and reduced lead time to market. Nickel-based superalloys is one of the most significant alloy families used at elevated temperatures. Nickel-based superalloys commonly contain up to 10 more alloying elements like chromium, aluminum, cobalt, tungsten, molybdenum, titanium, and so on. The great capacity of the nickel-based superalloys for high-temperature operation is ensured by the well-tailored microstructures with the assistance of carefully doped alloying elements, and the intently developed corresponding manufacturing processes. However, high-performance nickel-based superalloys generally suffer from structural integrity issues during AM process, i.e., this class of superalloys is highly susceptible to crack. Therefore, new nickel-based superalloys adapted to AM process with tailored chemical composition are under the urgent call. Meanwhile, high-temperature performance is another prioritized target for the new superalloys.The first topic is the chemical composition-dependent cracking mechanisms. The interdendritic region formed at the last-stage solidification has been found as the cracked spaces. Owing to the suppression of precipitate formation, the cracking mechanism is generalized as (1) the large mismatch of the solidification steps accounting for the crack initiation, and (2) the large mismatch of load-bearing capacity accounting for the crack propagation, between the dendritic and interdendritic regions. To quantitatively formulate the additive manufacturability of nickel-based superalloys, herein a two-parameter-based, heat resistance, and deformation resistance (HR-DR) model, has been successfully proposed to predict the printability on accounting for the relation between chemical composition (both major and minor elements) and cracking susceptibility. The concept of this model is formulated as that if the interdendritic region obtains both higher heat and deformation resistances than the rest dendritic region, this alloy is expected to be crack resistant. Validated by the experimental results and hitherto reported literature data, the HR-DR model provides an excellent sound prediction on the crack susceptibility of nickel-based superalloy during AM process. By considering the combination of additive manufacturability and high-temperature performance, a novel high-strength nickel-based superalloy, MAD542 has been developed based on the materials selection procedure from 921,600 candidate compositions. In addition, another precipitation-strengthened nickel-based superalloy, Alloy738+ has been developed based on the modification of the composition of heritage superalloy IN738LC, aiming for improving the additive manufacturability, creep, and oxidation resistance.

The second topic is the post-processing treatments related to microstructural evolution and mechanical properties. Owing to the thermal history during the LPBF process, the as-built microstructure commonly consists of columnar grains nearly parallel to the building direction with strong crystallographic texture. Subjected to the post-processing treatments, the solution treatment is the key to controlling the grain evolution. It has been shown for both LPBF MAD542 and heritage LPBF CM247 superalloys, the high crystallographic texture is maintained at the sub-γ′-solvus temperatures because of the grain boundary pinning effect from grain boundary precipitates. Whilst the crystal anisotropy is highly reduced by the treatment at super-γ′-solvus temperatures driven by the means of recrystallization. However, fully recrystallized microstructure with low texture largely reduced the mechanical properties by the embrittlement manner at elevated temperatures accordingly.

The third topic is the examination of creep and oxidation performance of various LPBF superalloys. A strong building direction-dependent creep performance is found for an LPBF IN738LC superalloy fabricated by the vertical and horizontal build. Vertically built samples show 7-40 times longer rupture life and approximately 2 times longer elongation at fracture than the horizontally produced samples, for the creep at 150-300 MPa at 850 °C. To evaluate the short-term creep performance, constant displacement rate-controlled slow strain rate tensile (SSRT) testing was carried out. The constant load-controlled creep and SSRT are correlated by deformation rate-based power-law type analysis. The new superalloy LPBF MAD542 generally displays a 5 times slower deformation rate than the LPBF IN738LC superalloy at 850 °C. The new superalloy Alloy738+ shows a comparable creep performance to LPBF IN738LC. Oxidation tests were conducted at 850/950/1050 °C. The new superalloy Alloy738+ presents an excellent oxidation resistance at 850 and 950 °C. By comparison, for example, Alloy738+ has 3 times slower oxidation kinetics than IN738LC at 950 °C.

The several investigations associated with the composition/processing/property in multiple precipitation-strengthened nickel-based superalloys fabricated by AM in this thesis have proven that the materials development requires comprehensive in-depth considerations. The presented results can contribute to the fundamental understanding and/or serve as the reference data for other superalloys by AM from the properties perspective.