The microstructural changes and corrosion response of Resoloy Ò , a resorbable Mg-Dy-based alloy, are the focus of this study. Hardness, tensile and compressive, and bending tests are used to monitor the changes in the mechanical properties of this material. The corrosion behavior is investigated by stress corrosion of C-rings. Hot-extruded tubes are solution heat-treated at different temperatures and times. The as-extruded condition shows a homogeneous fine-grained microstructure with matrix long-period stacking-ordered (LPSO) structures. Heat treatment at low temperatures and for short times does not significantly change the microstructure but reduces the hardness. Solution heat treatment at relatively high annealing temperatures and long annealing times causes grain growth, resulting in reduced hardness. The microstructure becomes inhomogeneous, medium-sized grains grow, the matrix LPSO structures dissolve, and small bulk LPSO phases develop. The matrix LPSO structures have a positive effect on the corrosion behavior. In particular, the short-term annealing condition shows the most uniform corrosion morphology. Resoloy Ò is not free of pitting corrosion, but none of the samples fails by cracking.

This study focuses on the characterization of the failure behavior of an extruded Mg10Dy1Nd alloy during stress corrosion. The microstructure, hardness, strength and corrosion behavior of binary alloys Mg10Dy and Mg1Nd are compared to those of the ternary alloy system. The ternary alloy Mg-Dy-Nd that is not fully recrystallized has the highest hardness but lacks ductility. The investigated alloys twin during plastic deformation. Static C-ring tests in Ringer solution were used to evaluate the stress corrosion properties, and stress corrosion could not be found. None of the alloys failed by fracturing, but corrosion pits formed to various extents. These corrosion pits were elliptical in shape and located below the surface. Some of the pits reduced the remaining wall thickness significantly, but the stress increased by the notch effect did not lead to crack initiation. Furthermore, the C-ring specimen was subjected to compressive loading until fracture. Whereas the Mg1Nd alloy showed ductile behavior, the alloys containing Dy fractured on the tensile side. The crack initiation and growth were mainly influenced by the twin boundaries. The Mg10Dy1Nd alloy had an inhomogeneous microstructure and low ductility, which resulted in a lower fracture toughness than that of the Mg10Dy alloy. There were features that indicate hydrogen-assisted fracture. Although adding Nd decreased the fracture toughness, it reduced the grain size and had a positive influence on the corrosion rate during immersion testing.

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DieMag633, a cast Mg-Al-Ba-Ca alloy, was the focus of this study. Brittle interdendritic phases strongly influenced the crack initiation and propagation under quasi-static and fatigue loading. Especially under tensile loading, the material showed a low resistance to failure. Selected fatigue loading sequences were applied to investigate their influence on crack propagation. DieMag633 in this study contained shrinkage cavities and pores of significant size and irregular distribution. Even though pores played a role in initiating the crack, it was mainly influenced by the Ba- and Ca-rich phases, being and staying much harder under deformation than the Mg-matrix. Apart from the fatigue crack propagation region under fatigue loading, there was no transgranular cracking found within the dendritic α-Mg grains. Only under compression did the dendritic α-Mg grains bridge the crack from one brittle phase to another. Transgranular cracking within the compact Ba-rich phase was very pronounced, starting with many microcracks within this phase and then connecting to the macrocrack. The lamellar Ca-rich phase showed also mainly transgranular cracking, but being small lamellae, intergranular cracking was additionally found. The hardness increase under deformation depended on the loading condition; a compression load strain-hardened the material the most. µCT analysis was applied to characterize the amount and location of the shrinkage cavities and pores in the individual gauge length.

The investigation of the crack propagation in as-extruded and heat-treated Mg-Dy-Nd-Zn-Zr alloy with a focus on the interaction of long-period stacking-ordered (LPSO) structures is the aim of this study. Solution heat treatment on a hot extruded Mg-Dy-Nd-Zn-Zr (RESOLOY®) was done to change the initial fine-grained microstructure, consisting of grain boundary blocky LPSO and lamellar LPSO structures within the matrix, into coarser grains of less lamellar and blocky LPSO phases. C-ring compression tests in Ringer solution were used to cause a fracture. Crack initiation and propagation is influenced by twin boundaries and LPSO lamellae. The blocky LPSO phases also clearly hinder crack growth, by increasing the energy to pass either through the phase or along its interface. The microstructural features were characterized by micro- and nanohardness as well as the amount and location of LPSO phases in dependence on the heat treatment condition. By applying nanoindentation, blocky LPSO phases show a higher hardness than the grains with or without lamellar LPSO phases and their hardness decreases with heat treatment time. On the other hand, the matrix increases in hardness by solid solution strengthening. The microstructure consisting of a good balance of grain size, matrix and blocky LPSO phases and twins shows the highest fracture energy.