Study On The Low Cycle Fatigue Behavior Of Rare Earth Magnesium Alloys

In this paper,under the total strain amplitude controlled mode ,die casting magnesiurnalloys AE42,AZ91 and AM50 with the addition of RE element Nd were exerted upon tensile-press load,which was carried out in the condition of stress ratio R equal to -1 and strain amplitude ranged from 2 ×10-3 to 1.5 × 10-2 ,and the low cycle fatigue behaviors were investigated .In addition,by means of optical.SEM and TEM ,the microstructoues and low cycle fatigue fracture surfaces under different conditions were examined respectively and the phases were analyzed by XRD.The results showed that the addition of RE can significantly refine the grains and after solution treatment in 415℃, beta phases (Mg17Al12) in both AE42 and AZ91 almost totally dissolved in Mg matrix,while RE phases still distributed dispersively at grain boundaries of alpha - Mg.The addition of RE can ont only refine the grains but also make the toughness of die-casting magnesium alloy better than that of magnesium alloys without RE,however,after the solution treatment the situation was the contrary,resulting in that the transition-life of magnesium alloys with RE was longer than that of magnesium alloys without RE,while the trend changed after solution treatment.The dispersion-strengthening played a key role in AE42,so the influence of heat treatment on cyclic hardening of AE42 was not apparent ,while for AZ91, after solution treatment the dispersion-strengthening almost have no action .while because of the work-hardening the hardness ratios become smaller with the bigger strain amplitudes before the solution ,but after the heat treatment the trend is the contrary.All the three alloys have no apparent characteristic region and there are more than one fatigue cores on the fracture surfaces.Most of the cores are caused by the defects on surface of the specimen or in the sample and the surface of the fracture becomes neater with lower strain amplitude.Thefracture surface is composed of dimples and cleavages and the dimples are tougher with the bigger strain amplitude.