| dc.description.abstract | Additive manufacturing (AM), a burgeoning manufacturing method for producing metallic materials, induces extraordinary impacts on the resulting performance. It is of great importance to investigate the microstructure and nanomechanical properties of AM-ed metallic materials and the further understanding of process-structure-property relations in an effort to ensure that they can meet the demanding performance requirements. Herein, different additively manufactured (AM-ed) materials, including high entropy alloy (HEA), metallic glass (MG), Al alloy, and superduplex stainless steel (SDSS), have been studied.
For the AM-ed HEAs, the nanomechanical performance and deformation mechanisms in accordance with the microstructural properties remain unclear. In this work, the microstructure and nanomechanical properties of an AM-ed (CrCoNiFe)94Ti2Al4 HEA were investigated. The local mechanical properties including hardness, elastic modulus, and nanoscale creep deformation, were explored by nanoindentation-based measurement. Simultaneously, the crystallographic orientation dependence on the mechanical behavior of AM-ed HEA was carried out by combining with electron backscattered diffraction (EBSD). It is found that the {101}-grain has the highest hardness and elastic modulus, whereas the creep resistance of {111}-grain is the greatest, with the indicators of the creep mechanism showing lattice diffusion is the dominant mechanism. Two different states of HEA, as-printed and heat-treated, were utilized to explore the effect of heat treatment. Heat treatment in the current study can increase the hardness and elastic modulus but decrease the creep resistance slightly. This work elucidates the underlying mechanisms of grain orientation dependence on nanomechanical properties and the effects of heat treatment. Moreover, it also sheds light on the particular creep behavior at the nanoscale and creep mechanism of the AM-ed (CrCoNiFe)94Ti2Al4 HEA.
Nanoscale creep behaviors of MGs have also gathered considerable interests in recent years, owing to the distinct atomistic mechanisms of plasticity. However, the nanoscale creep properties and creep mechanism of the AM-ed MGs remain unexplored. In this work, the nanoscale creep behavior and creep size effect of a selective laser melted (SLM-ed) Zr-based MG were investigated by using nanoindentation. In an effort to gain more insight into the creep behaviors, the creep compliance and creep retardation spectra were extracted. The creep stress exponent (n) and shear transformation zone (STZ) volume, as the indicators of creep mechanism, were estimated. The creep resistance of the MG was found to decrease with the increasing applied peak loads. A potential mechanism for this creep size effect was revealed: the smaller of the STZ volume, as well as the greater ratio of plastic flow under the higher maximum load, are responsible for the decreasing tendency of creep resistance. This work provides an in-depth understanding of the atomistic mechanisms in the AM-ed MG during the nanoscale creep deformation process.
Heat input is one of the most important thermal-relative process parameters during the AM process. It is of great value to investigate the effect of heat input on microstructure, nanomechanical properties and its underlying mechanism. Wire-arc additive manufactured (WAAM-ed) Al 4047 alloys under different heat inputs were used in this work. The Al 4047 alloys showed hypoeutectic microstructure that consisted of primary Al (α-Al) dendrite and Al-Si eutectic. The effect of heat input on nanohardness and strain rate sensitivity (SRS) were investigated respectively through nanoindentation, on both local and global scale. The hardness of both individual phase and integral Al alloy decreased with the increasing heat input, in accordance with the trend of yield strength and microhardness in the previous studies. Different from the microscale mechanism that explained by the grain growth model and Hall-Petch relationship, nanoscale mechanism regarding the effect of heat input on hardness was revealed: the solute supersaturation of Si element in Al phase and the solid solution strengthening were enhanced owing to the higher cooling rate under lower heat input. The heat input had little effect on the SRS and activation volume in both the α-Al phase and Al-Si eutectic phase. Compared with the α-Al phase, higher SRS and lower activation volume were found in the eutectic phase, which is largely attributed to the great fraction of interfaces in the Al-Si eutectic phase where dislocation annihilation happened. It is hoped that this study leads to new insights on the understanding of the influence of heat input, and further benefits to optimization of the process parameters and thus improve the mechanical properties of the AM-ed materials.
The AM process often results in non-uniform microstructure and different mechanical properties in sequential layers, impacting the overall performance of the AM-ed component. However, it is extremely challenging to evaluate the local stress-strain behavior of each individual layer, owing to the limited size of the AM-ed layered structure. To this end, a framework for characterizing and predicting the mechanical evolution of AM-ed multiphase alloys by combing nanoindentation and microstructure-based finite element method (FEM) was proposed. The sample used in this study was SDSS manufactured by WAAM, and the microstructure varied from layer to layer. Firstly, the mechanical properties of the two constituent phases in each layer, including elastic modulus and hardness, were obtained by nanoindentation, and the indentation size effect (ISE) was also evaluated. The yield strength and hardening exponent of each phase were subsequently estimated by reverse analysis method, and therefore the constitutive behaviors of the individual phase, which served as input parameters for FEM, were acquired. By aid of real microstructure-based FEM under uniaxial tension, the overall stress-strain behaviors of each layer and the distributions of the stress and strain during the deformation process were investigated. This work provides a new avenue for the characterization of the multiphase alloys in AM industry, beneficial to the understanding of the mechanical evolution in AM-ed materials.
In summary, this dissertation investigated the nanomechanical properties of the AM-ed materials, as well as the effect of heat treatment and process parameters. Moreover, a framework for predicting the mechanical evolution of AM-ed multiphase alloys was proposed considering the non-uniform characteristics in subsequent layers. This work contributes to a better understanding of the deformation mechanisms, and serves as a reference for improving the performance and further engineering applications of AM-ed metallic materials. | en_US |
