Deformation Mechanism in the Crack-Tip Region of Fine-Grained Magnesium Alloy

Abstract

The deformation mechanism in the crack-tip region of a fine-grained Mg-2.4 at. pct Zn binary alloy was investigated by focused ion beam (FIB) and transmission electron microscopy (TEM) observation and finite element analysis (FEA) at the beginning of the fracture toughness test. The deformed microstructure observations showed the formation of subgrains instead of deformation twins in the fracture toughness tested sample, which was performed at a conventional crosshead speed of 1 mm/min. By preventing the formation of deformation twins at the beginning of the test, the crack tip of the fine-grained magnesium alloys became blunted, and thus, the alloys obtained high fracture toughness. Finite element results showed that the temperature increased 50 to 110 K, and the strain rate became two orders of magnitude higher; however, this temperature increment was not sufficient to form high-angle grain boundaries, i.e., a complete occurrence of dynamic recrystallization. On the other hand, the deformed microstructure observations in the sample, which was tested at a crosshead speed of 50 mm/min, showed the formation of nano-order {10-12} deformation twins and subgrains. The formation of deformation twins was caused, in part, by the severe strain from the operation of a high strain rate in the crack-tip region.

OS19-3-3 Development of high strength and toughness magnesium alloys by microstructural controls

“…Typical TEM imeges are shown in shows streaked or splite spots; the presence of subgrain boundaries. The formation of subgrains at the crack-tip is related to the low increment temperature for occuring dynamic recrystallization in magnesium [5]. In addition, the deformation twins, which are believed to become the crack propagation site, are not visible in this image.…”

High-strain-rate nanoindentation behavior of fine-grained magnesium alloys

“…For example, the strain-rate in the crack-tip region during fracture has been reported to be at least 100 times higher than the global rate. 11 The mechanical properties and deformation behaviors of magnesium and magnesium alloys are unique compared with those of more conventional metallic materials, because of their hexagonal close packed (hcp) structure. The major slip system in single crystalline and coarse-grained magnesium alloys at room temperature lies on the basal plane, and deformation twins form to compensate for the lack of slip systems during plastic deformation.…”