Study On Fatigue Culmulative Damage Behaviors Of Cast Mg-Nd-Zn-Zr Alloys

Energy conservation and emissions reduction is a direction in the development road of modern automobile.Weight loss is the key way to solute the problem.It required that more and more light materials can apply in automobile instead of aluminum alloy,and magnesium alloy is a right material.NZ30K(Mg-3Nd-0.2 Zn-Zr,wt.%)magnesium alloy has good strength at room temperature or high temperature,and very is suitable for the preparation of engine cylinder body,automobile wheel hub and so on.Since these structural applications involve high stresses and cyclic loading,so it is important to understand the high cycle fatigue damage behaviors of cast magnesium alloys for their better structural applications.This paper studied the cumulative damage behaviors of NZ30 K alloy during high cycle fatigue test.In this paper,the process of fatigue cumulative damage and the evolution of surface morphology in 2-Dimension and 3-Dimension were observed,and the effect of stress amplitudes,fatigue cycles,grain orientation,grain size and heat treatment on the cumulative damage behaviors also were explored.The crack initiation behaviors also were studied.Results as follows: Two damage morphologies were observed in NZK alloy: the first one is Persistent slip markings(PSMs)existed in interior grains,and they are consist of lot of parallel straight lines.The second one is twin band(TB)which looks like a lens.Both deformation morphologies can increase the superficial area of parent grain.The increment from a TB is larger than the ones from a PSM.Generally,the increment rate of superficial area for each deformed grains is very high in the early stage of fatigue life time,and then decrease to a stable value.The influence of stress on fatigue damage mechanism of NZ30 K alloy is very significantly.Basal slip is a major damage mechanism under low stress amplitude which is equal to fatigue strength.In this condition,the twin band is very rarely.Twin damage mechanism became important under high stress amplitude which is larger than fatigue strength of 10 MPa.The influence of Schmid Factor on fatigue damage mechanism of NZ30 K alloy were studied.Results show that the greater the Basal Schmid Factor(BSF),the more probability the basal slip.When the BSF decrease,twin band can be formed very easily.When the BSF increase from 0 to 0.5,the percentage of slip bands increase,while the twin bands decrease.However,grain oriented cannot fully determine whether grain fatigue damage occurs,and the experimental results confirm that degree of fatigue damage is different among grains with a same orientation.So grain orientation is not the only factor to the fatigue damage mechanism.The effect of grain size on fatigue damage mechanism of NZ30 K alloy were studied.Results show that the fatigue strength of fine grain(~25?m)and coarse grain(~75?m)alloy are 69 MPa and 56 MPa,and about 13MPa(~23%)increment on fatigue strength from grain refinement.The percentage of deformed grain in fine grain and coarse grain are 68.8% and 32% under low stress amplitude.The percentage of deformed grain in fine grain and coarse grain are 77.9% and 56.4% under high stress amplitude.It indicates that the distribution of fatigue damage in fine grain alloy is more homogeneous than coarse grain alloy.All the results suggest that improving the distribution homogeneity of fatigue damage could be an effective way to increase the fatigue strength.The effect of heat treatment on fatigue strength and fatigue damage behaviors of NZ30 K alloy were studied.For as-cast alloy with coarse grain(grain size is 75?m,fatigue strength is 56MPa),the fatigue strength increase to 68MPa(~21% increment)after solid solution treatment(T4).The percentage of deformed grain is 54.1% under low stress amplitude,and the value increase to 83.6% under high stress amplitude.After aging treatment(T6),the fatigue strength can reach to 89MPa(~59% increment).The percentage of deformed grain is 3.8% under low stress amplitude,and the value increase to 54.7% under high stress amplitude.These results indicate that the fatigue damage in T4 alloy is more homogeneity than as-cast alloy due to the eutectic compounds in as-cast alloy were solute to the matrix completely.After aging treatment,the slip and twinning deformation become very difficult because the precipitates block the dislocation activity.The effect of stress amplitude,grain orientation,grain size and heat treatment on fatigue crack initiation behaviors in NZ30 K alloy were studied.This paper also statistic the proportion of slip crack and twin crack in different grain size and heat treatment alloy.In conclusion,almost all the fatigue cracks originate from intrusions inside PSMs in as-cast,T4-and T6-treated alloy under low stress amplitude.For the fatigue test under high stress amplitude,results show that the percentage of slip cracks and twin cracks in coarse-grained are 55.6% and 44.4%.For the fine-grained alloy,the percentage are 75% and 25%.The percentage in T4 alloy are 78.5% and 21.5%,while the percentage in T6 alloy are 71.4% and 28.6%.