No abstract

The influences of Ca addition (<1.2wt %) on structure and tensile fracture behavior of AZ91D alloy were investigated in this article. These results indicated that Ca addition can both form Al2Ca phase on interdendritic boundary and refine grain size. Ca addition evidently increases the yield strength but decreases the elongation of AZ91D alloy. The crack initiations of the AZ91D alloy and Ca-contained alloy both occur at the edges between the interdendritic eutectics and the α-Mg matrix. Failure of AZ91D alloy is brittle quasi-cleavage fracture, while Ca addition makes the failure tend to be cleavage.

The tensile properties and fracture mechanisms of fully lamellar TiAl based alloy was studied by room and high temperature (400°C) tensile test. The results indicated that: the crack initiated between weak lamellar interfaces．With tension increased, the cracks extended along lamellar direction. Crack propagation resistance was enhanced at colony boundary. Ligaments broke while load increased, and then the main crack and microcrack linked. The fracture process was that the main crack occurred firstly, microcrack appeared and then propagated, the main crack and microcrack were linked，and the alloy fractured finally; The ductility of the alloy at 400°C was superior to that at room temperature; The fracture mechanism of the fully lamellar TiAl based alloy is brittle cleavage fracture.

The compressive Properties and the effect of anti-damage were studied by means of compression and pre-compression damage tests. The study indicated that：The fracture mechanisms of compression tests were quasi-cleavage fracture attributed to the normal stress and shear stress. Fracture strength, the maximum strain and the fracture energy were reduced in varying degrees, and the damage was intensified while increasing preload value. The mechanical property of the TiAl alloy degrades because of the present of the cracks, and large amount of micro-cracks appear on the surface firstly.

The changes of the tensile properties of the full lamellar TiAl alloy under pre-compression were studied by means of pre-compression damage tests and tensile tests. The study indicated that：The changes of the tensile property of the damaged alloy at room temperature were different from that of high temperature. The mechanical property of the TiAl alloy degrades because of the present of the cracks, which decrease the load area of the alloy.