Study On The Fatigue Crack Propagation Behavior And Elevated Temperature Tensile Properties Of Magnesium Alloys AM50 & AZ91

Based on investigating a large amount of documents,the fatigue crack propagation behavior of Mg alloys AM50 & AZ91 is studied in this paper,as well as the tensile mechanical properties at elevated temperature.Micro-mechanics of tensile deformation and the FCPR are also investigated by SEM & TEM.Studies on the fatigue crack propagation rate(FCPR) indicate that load ratio(r) has a great effect on the FCPR:the greater load ratio is,the higher the FCPR becomes, and the lower Kth shows.Compared with load ratio,the thickness of the specimen has a lower effect on the FCPR. There is a slight drop of the FCPR when the specimen s thickness increases.However, the thickness of the specimen exerts a great influence on Kth and fatigue life(N): the thicker specimen is,the higher fatigue life(N) becomes, and the greater Kth shows.Under control of the same conditions,the FCPR of AM50 is not so high as that of AZ91.There are some correlations between load ratio(r) and the fatigue surface: when load ratiobecomes great,small flat surfaces increase while dimples decrease. During the course of the fatigue crack propagation, there lies crack-closing effect in both two alloys and the main crack-closing mechanic is plasticity-induced one. More obvious crack-closing effect on AM50 is thought as the key reason for AM50's higher fatigue life(N),compared with AZ91. As AM50 has better plasticity, stronger capacity of deforamtion hardening , higher kop and lower Keff value, AM50 shows a better crack-closing effect than AZ91.It is found that temperature has a great effect on the elevated tensile mechanical properties of Mg alloys by Mg alloys' tensile tests. b & 0.2 decrease while 6 increases with the increasing ofthe temperature. There lies deforamtion-hardening phenomenon during the course of the tensile deforamtion; temperature has an obvious effect on deformation -hardening constant (d /d ),which decreases gradually when temperature rises.Contents of RE have an effect on the tensile mechanical properties at high temperature to some degree. Proper amount of RE can improve b & 0.2 apparently and 5 a little, because RE can form compounds of good thermal stability with Magnesium, which can be effectively strengthened by heat treatment. A better strengthening effect on AM50 by adding RE has a relation to the content of Zn in the matrix alloy. A large content of Zn in AZ91,which leads to the formation of ( MgZn) 12RE, is thought as the basic reason for a poor strengthening effect of AZ91 when RE is added, compared with AM50.