Structure-property relations of cyclic damage in a wrought magnesium alloy

Abstract

The fatigue properties of an extruded Mg–3Al–0Mn magnesium alloy component were evaluated experimentally. Fully reversed, strain control fatigue tests were conducted on specimens extracted from regions with a varying grain size and texture. Scanning electron microscopy was employed to establish structure–property relations between microstructure and cyclic damage. Relations were drawn between microstructural features such as particle size, grain size, initial Taylor factor and the number of cycles to failure.

Introduction

Figure. 1. The initial average grain size and initial Taylor factor in the extruded magnesium AM30 alloy. The fatigue specimens were machined from sections A, B, C, E, G, and I as indicated in the cross-sectional view of the extruded alloy.

General Motors Research and Development Center (GM R&D) recently created a new extruded magnesium alloy, designated AM30 [17]. In terms of microstructural influences on fatigue damage, research has previously been conducted on similar magnesium alloys with regards to microstructural effects on the strain rate sensitivity [21] and grain size refinement effects on fatigue properties [22,23] . Magnesium alloys with a finer grain size exhibited a reduction in crack growth rate when compared to the same alloy with larger grains. Material orientation with respect to the extrusion direction and manganese content on fatigue life were shown to have a profound impact on both short and long crack development [24] . Furthermore, the authors suggested that the crack closure behavior was largely responsible for variations in the fatigue life of different specimen orientations. Further studies into the texture effects were conducted on a magnesium alloy, AZ31 [25] . These findings indicated that the fracture toughness of the material was significantly affected by the orientation of the specimen with respect to the extrusion direction. The objective of this research is to further quantify the cyclic behavior of AM30 and to obtain relations be- tween the microstructure of the material and cyclic damage. As such, structure–property relationships that correlate material texture, grain size and critical flaw size to fatigue life were determined.

Results

Figure. 4. Number of cycles to failure vs. the square root of the particle area (a), average grain size (b), and average Taylor factor (c) for the magnesium AM30 alloy. The locations from the rail cross- section of the specimens are also shown.

Figure. 2. EBSD mapping showing the texture characteristics of section A in the extruded direction (ED).

The specimens were machined from an extruded AM30 magnesium alloy crash rail parallel to the extrusion direction. These cylindrical dog-bone-shaped specimens had a gage length of 30 mm and a diameter of 6 mm. Since different regions of the rail exhibited varying average grain sizes and texture, multiple specimens were machined from six predetermined sections shown in Figure 1.

The average initial grain size and initial Taylor factors based on electron backscatter diffraction (EBSD) results of the rail cross-section are shown in Figure. 1. Figure. 2 shows the specific EBSD results from section A. The initial Taylor factors were only used to provide a comparative analysis of the distribution of the material strength in the rail component.

The objective of this study was to correlate structure–property relationships in order to predict the scatter that is typically observed in fatigue studies. The square root of the particle area vs. the number of cycles to failure is shown in Figure. 4a. The area of each particle was calculated using an image analyzing software program. The trend shows that specimens with smaller particles had greater fatigue resistance, and therefore better fatigue life, when compared to specimens with larger particles.

To further characterize the microstructure of the AM30 alloy in regards to the fatigue behavior, the average grain size and initial Taylor factors are plotted against the number of cycles to failure. Again, the spec- imens were machined from sections that contained various grain sizes and calculated initial Taylor factors. Figure. 4b displays the number of cycles vs. average grain size. While this plot displays significant scatter, the trend suggests that the alloy exhibits greater fatigue resistance for smaller grains compared to specimens with larger grains. This observation is somewhat unexpected considering larger grains typically provide better fatigue resistance in steel and aluminum alloys by providing grain boundary blocking that slows the rate of crack propagation. However, in this present study, smaller grains provided better fatigue resistance, which is consistent with a similar extruded Mg alloy [22].

References

Citation: Structure–property relations of cyclic damage in a wrought magnesium alloy, Bernard, J.D., Jordon, J.B., Horstemeyer, M.F., El Kadiri, H., Baird, J., Lamb, David, and Luo, Alan A., Scripta Materialia, v 63, n 7, p 751-756, 2010.