In this research, the effects of rare earth elements addition on thermal fatigue behaviors of AZ91 alloy were studied. Repeated heating and cooling cycles were applied on the samples at 170 deg C and 210 deg C to develop thermal fatigue cracks. Crack growth mechanisms and microstructural influences were investigated by optical and scanning electron microscopy (SEM) as well as energy dispersive X-ray spectroscopy (EDS). Thermal fatigue behaviors were improved successively by addition of up to 2-wt percent RE. This improvement is attributed to consummation of the aluminum of the melt by precipitation of the needle shape Al_(11)RE_3 phases. This process reduces the volume of Mg_(17)Al_(12) phases and consequently decreases the brittle Mg/Mg_(17)Al_(12) interfaces which are the main reason of weak thermal properties of the alloy at rather high temperatures. Further additions of RE, however, reduced the thermal shock resistance of samples by increasing in the mean length of the brittle needle shape phases.
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