|Title:||Additive manufacturing of high entropy alloys by laser engineered net shaping|
|Advisors:||Chan, K. C. (ISE)|
Lasers -- Industrial applications
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xxiv, 174 pages : color illustrations|
|Abstract:||Differing from conventional alloys with a single major element, high entropy alloys (HEAs) comprise multiple elements (typically ≥5) in significant atomic fractions (often 5 to 35 at.%), possibly with alloying elements, and have higher configurational entropies. Since their first emergence in 2004, HEAs have attracted considerable research interest because they define a new and near-infinite compositional space with diverse microstructures and improved properties. However, the vast majority of HEAs (e.g. CrMnFeCoNi) were processed by casting, possibly followed by cold deformation and subsequent annealing. For particular hard or brittle HEAs (e.g. AlCoCrFeNiTi0.5), certain post processing routes (e.g. cold deformation) are not applicable. Furthermore, these approaches are often used to generate simple geometry parts and require post tooling. It is also worth mentioning that, similar to conventional alloys, HEAs often face the dilemma of strength-plasticity trade-off. To address these issues, the laser engineered net shaping (LENSTM) process, a typical additive manufacturing (AM) process, was adopted to manufacture HEAs in this project. The LENSTM process allows for the manufacturing of 3D complex-shaped parts with fine microstructures and good mechanical properties in a single print. The LENSTM process also provides the flexibility of multi-material (i.e. multi-phase) design, and may be a feasible approach to achieve enhanced strength-plasticity synergy of HEAs. In this study, we first performed a systematic study on the microstructural evolution and cracking behavior of a simple ternary equi-atomic CrCoNi alloy during the AM process. The increased laser scan speed generates more heterogenous grain structures (i.e. columnar plus equiaxed). Furthermore, the increased laser scan speed achieves more pronounced quenching effects and hence finer microstructures, and a quantitative relationship between the cooling rate and celluar spacing is established. The increased laser scan speed can trigger the solidification cracking of the CrCoNi alloy due to larger thermal gradients and thermal stresses. Based on the above findings, the microstructures and mechanical behavior of various AM-ed HEAs were further investigated. The LENSTM-deposited FCC CrMnFeCoNi HEA exhibits a multi-scale microstructure, i.e. columnar grains, solidification substructures and dislocation substructures. The tensile deformation process is mainly accommodated by dislocation activities with the assistance of deformation twinning. The high tensile yield strength of the LENSTM-deposited CrMnFeCoNi HEA originates from the initial-dislocation strengthening. The decreased uniform tensile elongation of the LENSTM-deposited HEA CrMnFeCoNi is attributed to the increased dynamic dislocation recovery rate and hence the weakened work hardening capability.|
The AlCoCrFeNiTi0.5 HEA was also additively manufactured by the LENSTM process. The microstructure of the LENSTM-deposited AlCoCrFeNiTi0.5 HEA consists of proeutectic B2-structured dendrites delineated by lamellar or rod-like B2/A2 eutectic structures. Such microstructures are successfully described with the aid of Scheil's solidification simulation. Furthermore, proeutectic B2-structured grains are totally equiaxed and randomly textured. A high density of nucleation sites (a minimum of 3 × 109 /mm3) is estimated, which is attributed to the frequent dendrite fragmentation. The volume fraction Φ values of equiaxed crystals at the solidification front are estimated to be greater than 49% for the various G - V combinations established in this study, indicating that fully equiaxed grain microstructures can be formed and hence providing a theoretical basis for our experimental findings. In view of the strength-plasticity trade-off of monolithic AM-ed HEAs, we additively manufactured CrMnFeCoNi/AlCoCrFeNiTi0.5 laminated HEAs that exhibit an enhanced strength-plasticity synergy during compression (yield strength up to 990 MPa and no complete fracture until 80% strain), surpassing those of monolithic bulk HEAs. The enhanced strength-plasticity synergy originates from heterogenous microstructures of ultra-hard BCC equiaxed grains and soft FCC columnar grains periodically arranged in the AlCoCrFeNiTi0.5 and CrMnFeCoNi lamellae respectively. In this research, the microstructures and mechanical behavior of various HEAs manufactured by the LENSTM process are investigated. The findings demonstrate the capability of the LENSTM process for manufacturing HEAs with high performance and also provide guidelines for producing fully equiaxed alloys during the AM process. This work also demonstrates a feasible and flexible way to design HEAs with heterogenous microstructures and improved mechanical properties.
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