Study On Tension-compression Low Cycle Fatigue Behavior Of Casting Mg–10Gd–3Y–0.5Zr Magnesium Alloy

Mg–10Gd–3Y–0.5Zr alloy is regarded as a promising structural material to be used on aircrafts and spaceships due to its?'precipitates and high thermal stability.Since the structural components will experience high temperature and fluctuating load,the high temperature mechanical properties and fatigue properties of the alloy are important technical indicators.In this thesis,the main object of study is the low-pressure sand casting and gravity die casting Mg–10Gd–3Y–0.5Zr alloy.The high temperature tensile and compressive properties of the alloy has been researched,as well as the low cycle fatigue life,the low cycle fatigue behavior and the low cycle fatigue damage behavior at different numbers of cycles.The high temperature tensile and compressive tests show that at room temperature,the tensile properties of sand cast T6 is better than metal cast T6 alloy,while at high temperature(250 ~oC),the metal cast T6 alloy is better than sand cast T6 alloy.This is due to the different microstructure of sand cast and metal cast alloy and the different effect of grain size and defects at different temperatures.However,the compressive properties are not affected by different casting process.Both sand cast and metal cast alloys show anomalous phenomenon in high temperature tensile tests.The sand cast alloy shows best ultimate tensile strength at 150 ~oC while the metal cast alloy at 200 ~oC.The compressive properties of both alloy decreases monotonously with temperature growth.Both alloys show tensile-compressive yield asymmetry,the compressive yield strength and tensile yield strength ratios at room temperature are 1.13(sand cast)and 1.26(metal cast),and temperature has no obvious effect on tensile-compressive yield asymmetry.The low cycle fatigue tests results show that at lower strain amplitude,the difference of fatigue life between different alloys is obvious.At strain amplitude=0.25%,the sand T6alloy has the longest fatigue life,and then followed by Metal T6,Sand cast and Sand T4alloy.The relation of strain amplitude and fatigue life can be fit by Manson-Coffin formula.The cyclic hardening and softening can be observed during fatigue tests,which is more obvious at higher strain amplitude.Sand T6 alloy has the longest softening stage(after 40%fatigue life),followed by sand T4 alloy(after 60%fatigue life)and metal T6alloy(after 70%fatigue life).There is almost no softening stage for sand cast alloy.The fatigue damage behavior tests show that,the cyclic hardening is due to the impediment of dislocation slip by the slip lines and twin boundaries,and the cyclic softening is due to the propagation and merge of fatigue cracks.PSB and the surface of the specimen are preferential areas for fatigue cracks initiation and propagation.At strain amplitude=0.6%,cracks initiates at about 10%-30%of the fatigue life,while at strain amplitude=0.3%,cracks initiates at about 50%-70%of the fatigue life.?'can impede crack propagation and merge at higher strain amplitude,and postpone crack initiation at lower strain amplitude,which indicates that aging treatment is an effective way of preventing fatigue crack propagation and improving fatigue properties.