In-situ Observation Technology and Application of Fatigue Crack Initiation and Propagation for Cast Magnesium Alloys

The micro-fracture mechanism of cast magnesium alloys was investigated based on in-situ observation with scanning electron microscope (SEM).The effects of the α-Mg grain size and shape as well as the β phases (Mg17Al12) distribution on the fatigue crack initiation and propagation,and effect of the addition of Ca to magnesium alloy on the mechanical properties were examined.The fatigue cracks mainly initiated and propagated at the α-Mg boundaries.These cracks stably propagated in a scale of micrometers and did not absolutely propagate perpendicularly to the maximal principal stress direction.The fatigue crack propagating mechanism was dependant on the coupling of mode I and mode Ⅱ.Therefore,the stress intensity factor ΔK was not suitable for estimating the fatigue crack growth rate in microscope.