Experimental Investigation On Fatigue And Failure Behavior Of Cast Magnesium Aluminum Alloys

The increasing use of magnesium-aluminum (Mg-Al) components undercyclic loading has triggered renewed interest on the fatigue properties of Mgalloys. The purpose of this thesis is to observe clearly the micro-crack growthbehavior especially at high temperature and evaluate quantitatively thecrack growth rate and fatigue life of cast Mg-Al alloys.qIn this thesis, some fatigue experiments for the smooth and flawedspecimens of cast AM50, AM60B and AZ91 alloys were carried out using theservo-hydraulic fatigue testing machine with scanning electronic microscopy(SEM). It was found that the microstructure had a great effect on themechanical properties of cast Mg-Al alloys. The smaller size and morehomogeneous distribution of α -Mg grains produced the higher strength ofMg alloys within average range of 5-50 μm. The higher volume fraction andnetwork structure of β -Mg₁₇Al₁₂ phase promoted the strength and brittlenessof Mg-Al alloys at the same time.From the in-situ observation with SEM of fatigue crack evolution andfacture analysis, the crack was found to propagate mainly along the grainboundary at room temperature and propagate through the α -Mggrains atelevated temperature. Microstructure played an important role on theinitiation and early propagation of fatigue small crack, while the appliedstress determined the propagation path of small cracks. The fracture analysismainly dealt with cleavage and pore accumulation. And at high temperaturethe fracture mode tended to change from cleavage to quasi-cleavage. At thesame time, this thesis reports the fracture analysis and FEA simulation.One of the most important efforts in this thesis was to develop a formulafor the evaluation of fatigue small crack growth rate and fatigue life of castMg-Al alloys based on the observation of micro-crack propagation at elevatedtemperature. The experimental results showed that the micro-crack growthrate dl /dN was a function of σ maxn and crack length l at specifictemperature, where there was a good linear relation between the index n andtemperature.