Crack Propagation in As-Extruded and Heat-Treated Mg-Dy-Nd-Zn-Zr Alloy Explained by the Effect of LPSO Structures and Their Micro- and Nanohardness.

Petra Maier,Norbert Hort,Benjamin Clausius,Benjamin Bittner,Asta Richter,Roman Menze

doi:10.3390/ma14133686

Abstract

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.

Highlights

The fine-grained microstructure consists of lamellar long-period stacking-ordered (LPSO) structures within the matrix and in the coarser grains there are fewer lamellae, but blocky LPSO phases, whose amount decreases with heat treatment time, especially the inner grain blocky phases
The amount and location of LPSO phases were evaluated by optical microscopy
Crack initiation and propagation were demonstrated with the focus on the interaction of the microstructural features and the crack growth

Summary

Introduction

Research on Mg-RE alloys covers many aspects: strength and ductility [1], creep [2], fatigue [3], corrosion [4,5], and their biological performance as biodegradable implants [6,7].there are only a few studies on the fracture mechanics, toughness and the influence of the microstructure on the crack propagation, especially in Mg-RE alloys containing Zn and consisting of long-period stacking-ordered (LPSO) structures.Resoloy® , an Mg-Dy-Nd-Zn-Zr alloy high in Dy, was developed for absorbable implants by Meko Laser Materials Processing in Sarstedt and the HelmholtzZentrum Geesthacht in Germany, is internationally patented [8] and shows excellent strength and ductility. Research on Mg-RE alloys covers many aspects: strength and ductility [1], creep [2], fatigue [3], corrosion [4,5], and their biological performance as biodegradable implants [6,7]. There are only a few studies on the fracture mechanics, toughness and the influence of the microstructure on the crack propagation, especially in Mg-RE alloys containing Zn and consisting of long-period stacking-ordered (LPSO) structures. Resoloy® , an Mg-Dy-Nd-Zn-Zr alloy high in Dy, was developed for absorbable implants by Meko Laser Materials Processing in Sarstedt and the HelmholtzZentrum Geesthacht in Germany, is internationally patented [8] and shows excellent strength and ductility. Mg-RE-Zn alloys, where RE are elements such as Y, Gd, Tb, Dy, Ho, Er, and Tm, form novel LPSO structures and show improved strength, fracture toughness, 4.0/)

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Results

Discussion

Conclusion