| Magnesium alloys are of interest in the transportation industries because of their potential for light weighting, yet adoption of these alloys has been limited due to poor mechanical properties. Precipitation hardening magnesium-rare earth-zinc-zirconium (Mg-RE-Zn-Zr, or RZK) alloys are considered having good potential for improved mechanical properties especially in elevated temperature (>125°C) applications, but the lack of in-depth understanding of structure-property relationship of RZK alloys hinders further development. In this thesis, two RZK alloys, i.e., Mg-Sm-Zn-Zr (SZK) and Mg-Nd-Zn-Zr (NZK), are investigated using a combination of experiment and CALPHAD-based (CALculation of PHAse Diagram) simulation. The purpose of this study is to obtain in-depth understanding of structure and property of the SZK and NZK alloys for potential industrial applications. The outcomes enrich the engineering community's knowledge of these two alloy systems, prove the application of computational simulation in novel alloy development, and provide instructions for further optimization of multicomponent RZK alloys.;For the recently-developed SZK alloy system, ternary equilibrium phase and phase equilibria are identified experimentally. A thermodynamic description is developed for Mg-Sm-Zn system with CALPHAD method based on experimental data from literature and this study. Furthermore, structure-property relationship of SZK alloys is illustrated experimentally by analysis on microstructural features (grain size, intermetallics, precipitates) and mechanical properties (compression properties, Vickers hardness).;Precipitation evolution and age-hardening response of the well-characterized NZK alloys are simulated using combination of CALPHAD and classical nucleation and growth model. Critical parameters, such as atomic mobility, thermodynamic property and interfacial energy of the key precipitate, are obtained according to literature data of NZ30K alloy (Mg-3Nd-0.2Zn-0.4Zr), which exhibits the optimum mechanical properties in NZK alloys. Mechanical properties of NZ30K and a series of NZK alloys are then modeled using simulated microstructure evolution data as inputs. Good agreement between simulations and experimental results from wide ranges of compositions and heat treatment reported by separate studies validates the accuracy and predictive capabilities of current simulation. Furthermore, database of NZK system is extended to SZK system with minor modifications. The calculation results show good agreement with experimental data from both this study and literature reports. |
