A Study Of Fatigue Crack Growth In Extruded AZ31B Magnesium Alloy And Pure Magneisum

Magnesium (Mg) alloys have excellent mechanical properties such as low density, superior specific strength and stiffness, good physical machinability and recyclability. They are important future engineering metallic structure materials in electrons, transportation, aerospace and weapon industry. Because of their poor ability of plastic deformation, most magnesium alloys components are made by casting process. Due to pores and inclusions left from the casting process, cast Mg alloys do not sastify the engineering requirement for strength. Wrought Mg alloys exhibit superior physical properties and they become a subject of research. Cyclic load is usually applied on a real structure, and fatigue failure is the main failure mode for a structure in service. The fatigue properties of wrought Mg alloys are anisotropic due to initial texture and microstructure caused by extrusion and rolling processes. It is significant to study the fatigue crack growth of magnesium alloys with strong textureIn this thesis, the study focuses on the a detailed investigation on the fatigue crack growth (FCG) behavior and fracture mechanism of extruded pure Mg and AZ31B Mg alloy bar with strong texture. The microstructure, texture, crack growth path and fracture face were investigated by optical microscope (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The study was conducted using compact tension specimens oriented in three different directions with respect to the extrusion direction. The effect of texture on FCG of different direction specimen is discussed. The experimental results are used to evaluate two existing fatigue models (Walkers model and Wheelers model) for their capabilities to account for the R-ratio effect, the overload effect, and the high-low loading sequence effect. The following conclusions were reached:The extruded AZ31B Mg alloy consists of an inhomogeneous microstructure exhibiting equiaxed large grains and equiaxed small grain clusters. The average sizes of the large and small grains are approximately50μm and8μm, respectively. On the plane parallel to the extrusion direction, large grains are elongated due to the extrusion process and a lamellar microstructure composed of alternatively elongated large grain and small grain cluster is developed. On the other hand, the extruded pure Mg is mainly composed of equiaxed large grains with an average size of approximate150μm and a few small grains with a size of20μm. No elongated grains and lamellar microstructure were abserved in extruded pure Mg. A strong basal texture was observed in both extruded AZ31B Mg alloy and pure Mg. The basal planes of most grains are parallel to the extrusion direction and the c-axes of most grains are orientated along the radial direction. The texture factor of the extruded AZ31B Mg alloy and pure Mg are15.85and12.65, respectively.The specimen orientation with respect to the extrusion direction plays a critical role in FCG of extruded AZ31B Mg alloy. The threshold stress intensity factor ranges are similar in the T-L and L-T specimens with a value of0.95～1.34MP(?) and they are not significantly influenced by the load ratio. The threshold stress intensity range of the T-R specimens is1.04～2.43MP(?) and increases with the load ratio. Transgranular cracking is the major fatigue propagation mode in all the three specimen orientations. In the T-L and L-T specimens, two sets of2nd order <c+a> pyramidal slip induced cleavage dominates the transgranular cracking whereas both twin boundary cracking and <c+a> pyramidal slip induced cleavage exist in the T-R specimens. The cracks of the T-L specimens grow in typical mode I with the fastest FCG rate. The overal crack path of the T-R specimens follows Model1cracking, and local branching and cracking deviated from the horizontal direction are developed. A large angle of deviation from the horizontal line was observed in the crack path of L-T specimens. The crack tends to deviate or branch following the {1120} crystal planes. The FCG rate in the L-T specimens is the lowest among the three orientations. A nearly constant FCG rate was observed in the L-T specimens right after the threshold zone, and the da/dN-ΔKcurve in log-log scale exhibits three linear relationship with different slopes. The FCG rate increases with increasing stress ratio in all the specimens with three orientations. Walker"s model and Paris law can correlate well the constant amplitude FCG experiments with different R-ratios, and can be used to predict the FCG life and safety assessment for extruded AZ31B Mg alloy.For AZ31B Mg alloy, application of a single tensile overload during constant-amplitude loading results in an immediate decrease in the FCG rate to a minimum value in all the three specimen orientations. As the crack extends, the FCG rate recovers rapidly to the level expected during constant amplitude loading. The reduction in the FCG rate due to overloading is significant but the overload influencing zone size is small. No crack extension was detected during overloading in the experiment. In the high-low loading sequence experiment, the influence of the higher loading step on the crack growth of the subsequent lower loading step is dependent on the relative magnitude of the maximum loads in the consecutive loading steps. If the maximum loads in the two loading steps are identical, there is no loading sequence influence on the crack growth. With identical R-ratios or identical minimum loads in high-low two-step loading, the influence of the higher amplitude loading on the crack growth of the subsequent lower amplitude loading is similar to that of overloading in a constant amplitude loading. The modified Wheeler model can reasonably predict the influences of overloading and sequence loading on FCG.For extruded pure Mg, transgranular cracking is the major fatigue propagation mode in all the three specimen orientations. In the T-L and L-T specimens, two sets of2nd order<c+a> pyramidal slip (1122)<1123> and (1122)<1123> induced cleavage dominate the transgranular cracking whereas both{1012} twin boundary cracking and <a>,<c+a> pyramidal slip induced cleavage exist in the T-R specimens. Severe deviation from the pure Mode I cracking and many branchs were observed in the crack path of the L-T specimens. The FCG rate in the L-T specimens is the lowest in three orientations of specimens. The crack in the T-L specimen grows in typical Mode I direction with the fastest FCG rate among the three orienations for a given stress intensity factor range. The crack in the T-L specimen grows in Mode Ⅰ, and local deviation from Mode I cracking and a few branchs were observed in the crack growth path. The FCG rate discreases due to cracking dievation from Mode I, branching, and local microcracking. The da/dN-ΔKcurve under log-log scale exhibits two linear relationships with different slopes in all three orientations of specimens. The curves are divided to two parts at ΔK≈3.0MPa(?) for L-T and AK≡3.4MPa(?) for the T-L and T-R directions.For extruded pure Mg, application of a single tensile overload during constant-amplitude loading results in an immediate decrease in the FCG rate to a minimum value in L-T and T-R specimen orientations. The overload influencing zone size is less than0.1mm and0.2mm for L-T and T-R specimen orientations, respectively, which is much less than the predicted value by Wheeler’s model. No obvious overload influence was observed in the T-L specimen. Application of a compressure underload accelerates the FCG due to the residual tensile stress and twinning ahead of the crak tip.