Effect of rare earth elements addition on thermal fatigue behaviors of AZ91 magnesium alloy

Influences of rare earth (RE) elements addition on thermal fatigue behaviors of AZ91 alloy were studied. Repeated heating and cooling cycles were applied on the samples at 170 and 210 degrees C to develop thermal fatigue cracks. Crack growth mechanisms and microstructural influences were investigated by optical and scanning electron microscopy (SEM) as well its energy dispersive X-ray spectroscope (EDS). Thermal fatigue behaviors were observed to improve successively by addition of the RE up to 2wt.%. This improvement was attributed to the consummation of aluminum in melt by precipitation of the needle shaped Al11RE3 phases. This process wits attributed to the reduction of Mg17Al12 phase volume fraction and consequent decrease of the brittle Mg/Mg17Al12 interface which was the main reason for weak thermal properties of the alloy at rather high temperatures. Further additions of RE. however, reduced file thermal shock resistance of the samples by increasing the mean length of the brittle needle shaped phases.