Fracture Behavior Of Cast Mg–Gd–Y–Zr Magnesium Alloys

As recently developed high strength magnesium alloys, Mg–Gd–Y–Zr magnesium alloys are very attractive as structural materials in aircraft and space machinery. Attempts have been made to use Mg–Gd–Y–Zr alloys in complicated structural applications formed by low-pressure sand casting. Quenching cracks are always observed in these structures if cooled in water after solution or aging heat treatment. However, few literatures about sand-cast Mg–Gd–Y–Zr alloys are available, and no studies involved air-cooled heat treatment can be found. The mechanical properties and fracture mechanisms should be investigated due to safety and reliability requirements.In this study, the effects of heat treatment and Gd and Zr contents on mechanical properties and fracture behaviors of Mg–Gd–Y–Zr alloys produced by low-pressure sand casting were mainly investigated. The Mg–10Gd–3Y–0.5Zr magnesium alloy was also included for comparison in this study. The results shows:As-cast Mg–Gd–Y–Zr magnesium alloys mainly contained α-Mg and β phase(Mg24(Gd,Y)5) attributed along the grain boundaries. The increase of Gd content from 9 to 11 wt. % leads to an increase in the amount of eutectic phase and the addition of Zr in Mg–Gd–Y–Zr alloys refines the grains. After T6 heat treatment, Mg–Gd–Y–Zr magnesium alloys mainly contained α-Mg + γ + β′(Mg5(Gd, Y).As-cast Mg–10Gd–3Y–0.5Zr magnesium alloys produced by low-pressure sand casting and gravity permanent casting showed similar mechanical properties. The yield strength, ultimate strength and elongation of low-pressure sand cast alloy were 147 MPa, 215 MPa and 1.2 %; and the yield strength, ultimate strength and elongation of gravity metal cast alloy were 150 MPa, 226 MPa and 1.5%. After T6 heat treatment, the tensile properties were significantly improved. The yield strength, ultimate strength and elongation of low-pressure sand cast alloy were 240 MPa, 358 MPa and 3.5%, and the yield strength, ultimate strength and elongation of gravity metal cast alloy were 237 MPa, 334 MPa and 1.9%, respectively.The deformation mechanism of as-cast Mg–10Gd–3Y–0.5Zr magnesium alloys was mainly about dislocation slipping and twinning under quasi-static tensile load. The alloy exhibited quasi-cleavage fracture. After T6 heat treatment, deformation mechanism was mainly about dislocation slipping, and low-pressure sand cast alloy showed quasi-cleavage fracture while gravity permanent cast alloy showed a mixed fracture mode of quasi-cleavage fracture and intergranular fracture.The impact toughness of as-cast Mg–10Gd–3Y–0.5Zr magnesium alloys produced by low-pressure sand casting and gravity permanent casting were 16.3 J?cm-2 and 18.1 J?cm-2. After T6 heat treatment, the impact toughness were increased to 33.6 J?cm-2 and 26.4 J?cm-2, respectively.The fracture surfaces of as-cast Mg–10Gd–3Y–0.5Zr magnesium alloys under impact load all were radiation zone, and fiber area and tear lips area were not observed. The main deformation mechanism was about dislocations and twining, and the twinning was a very important deformation mechanism. The as-cast alloys exhibited quasi-cleavage fracture. The fracture surfaces of T6-treated Mg–10Gd–3Y–0.5Zr magnesium alloys were mainly radiation zone, but a small tear lips area was observed at the marginal zone of the fracture surfaces. Deformation twinning is an important deformation mechanism for T6-treated alloys. The low-pressure sand cast alloy showed quasi-cleavage fracture while gravity permanent cast alloy showed a mixed fracture mode of quasi-cleavage fracture and intergranular fracture under impact load.The S-N curves of as-cast Mg–10Gd–3Y–0.5Zr magnesium alloys produced by low-pressure sand casting and gravity permanent casting were similar, and the fatigue strength was about 90 MPa. After T6 heat treatment, the fatigue strength and fatigue life of alloys were improved. The fatigue strengths of sand casting and metal casting alloys increased to 110 MPa and 100 MPa, respectively.The fatigue cracks of low-pressure sand cast alloys mainly initiated at the free surface of the samples while the fatigue cracks of gravity permanent cast alloys mainly initiated at the shrinkage or inclusions in the vicinity of the sample surface. The fatigue crack initiation of both as-cast and T6-treated alloys was related to the rupture of slip bands, and these cracks propagated though the grains after initiation.Both Gd and Zr contents had influence on Mg–Gd–Y–Zr magnesium alloys. Gd content was not significantly affect the fatigue strength of Mg–10Gd–3Y–0.5Zr magnesium alloys, but it affected the fatigue life under relatively high stress amplitude load. The alloy with high Gd content showed long fatigue life under relatively high stress amplitude load. Zr content significantly affected the fatigue strength and fatigue life. The alloy with high Gd content showed high fatigue strength and long fatigue life.The different region of the cast engineering structures exhibited different fatigue strength from100 MPa to 115 MPa. The samples whose fatigue crack initiated from free surface showed higher fatigue strength and fatigue life.The different region of the cast engineering structures exhibited different fracture toughness. The fracture toughness of as-cast alloy from the tested region were 12.3 MPa?m1/2 and 12.1 MPa?m1/2, and after T6 heat treated, the fracture toughness were improved to 17.3 MPa?m1/2 and 16.3 MPa?m1/2, respectively. The fracture toughness of as-cast and T6-treated alloy produced by gravity permanent casting were 13.4 MPa?m1/2 and 15.7MPa?m1/2.