High Cycle Fatigue of Cast Mg-3Nd-0.2Zn Magnesium Alloys
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CAST magnesium alloys are increasingly used in automotive, aerospace, and other transportation industries due to their low density, high strength to weight ratio, and good castability compared with other structural materials.[1,2] As many structural applications such as automotive engine blocks and engine cradles involve cyclic loading, it is very important to understand the fatigue behavior of these alloys. Fatigue properties of cast components strongly depend on casting flaws and microstructural characteristics.[3] The castings generally demonstrate easy fatigue crack initiation due to the presence of casting flaws such as porosity and oxides at or below the casting surface.[4,5] In addition to casting defects, the fatigue and fracture properties of cast alloys also strongly depend on the microstructure including grain size and second-phase particles and precipitates, which are determined by their composition, casting, and heat treatment conditions.[4–7] An extensive understanding of the role of microstructural constituents on fatigue properties of Mg castings is lacking and is extremely important particularly when magnesium alloys are expected to be increasingly used in structural components. NZ30K (Mg-3Nd-0.2Zn with minor additions of Zr as grain refiner) is a recently developed magnesium alloy[7] which offers high strength and ductility as well as good creep resistance; thus, it is very attractive to ZHENMING LI, Ph.D. Student, PENGHUAI FU and YINGXIN WANG, Assistant Researchers, LIMING PENG and GUOHUA WU, Professors, are with the National Engineering Research Center of Light Alloys Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China. Contact e-mail: [email protected] QIGUI WANG, Materials Technical Specialist-Process & Property Modeling and Optimization, and ALAN A. LUO, Technical Fellow, are with General Motors, Warren, MI 48090. Manuscript submitted January 3, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
automotive powertrain and structural applications. The NZ30K alloy has been used in low pressure sand cast (LPSC) V6 engine blocks. The microstructure of the as-cast NZ30K alloy comprises magnesium matrix and Mg12Nd eutectic compounds. It has been reported that the cast NZ30K alloy can achieve high yield and ultimate tensile strength at room temperature with proper heat treatment.[7–9] Different aging treatments were shown to lead to different precipitates in the microstructure.[7] At the peak-aged condition (T6-PA: 10 to 16 hours at 473 K (200 C)), fine b¢¢ particles with DO19 structure (a = 0.64 nm, c = 0.52 nm) are the dominant strengthening phase, while b¢ precipitates of fcc structure (a = 0.742 nm) are the dominant phase when the alloy is aged at 523 K (250 C) for 10 hours (T62-OA).[7,8] However, no comprehensive study of the effect of heat treatment on the fatigue properties of this alloy without defects is yet reported. Furthermore, the effectiveness of grain refinement using Zr during remelting of this allo
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