Size effects and the deformation behaviour of a dilute Mg-Zn-Ca alloy subjected to cyclic four-point bending

Abstract

In biodegradable stents made from dilute Mg-Zn-Ca alloys a small number of grains constitute the thickness of a sample limiting the mechanical constrain typically seen in polycrystalline alloys. At the same time such stents are exposed to complex loading conditions, which could potentially lead to premature failure particularly in the presence of limited slip mode activation. In order to improve our understanding of the prevailing micromechanics for such conditions hot-rolled, thin strip samples were produced with thickness to grain size ratios (t/D) of 100 and 17. Samples were extracted from such thin strips and subjected to interrupted cyclic four-point bending in order to study the in-grain misorientation axis (IGMA) evolution of selected grains during cyclic loading using quasi-in-situ electron back-scattered diffraction (EBSD). Grains were selected at the tensile and compressive faces of samples, and in the vicinity of the neutral band. Analysis of the evolution of IGMA and misorientation profiles during the bending showed that extension twinning was the dominant deformation mode in the compressive region for both t/D ratios. In addition, non-basal <a>-type slip systems were active as a result of work hardening. In the tensile region, the activity of <a>-type non-basal slip systems was observed for the t/D of 100, but basal/double basal slip was the dominant deformation mode for the t/D of 17. No signs of failure were observed for the sample with a t/D of 17 at the tensile region, implying that these basal slip modes can accommodate deformation largely without the assistance of other deformation modes. The neutral band evolution was mapped using grain orientation spread (GOS) analysis. In contrast to the continuum mechanics solution, a neutral band was observed instead of an axis. The position of the neutral band was dependant on the local orientation organization of the grains and the extent of work-hardening.