Deformation Mechanism in the Crack-Tip Region of Fine-Grained Magnesium Alloy
- PDF / 629,122 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 32 Downloads / 225 Views
TRODUCTION
MAGNESIUM alloys have a high potential for application as structural materials since they are the lightest among all the structural alloys in use. For use in structural applications, their mechanical properties must satisfy both reliability and safety requirements, although the fracture toughness in magnesium alloys generally has been reported to be lower than that in aluminum alloys.[1] One of the reasons for their low fracture toughness is the formation of {10-12} type deformation twins that occurs easily at the beginning of plastic deformation.[2,3] The deformation twins are known to compensate for the lack of slip systems by rotating the crystals in magnesium; however, the interface between these twins and the matrix becomes the crack propagation route in the fracture toughness test due to the limitation in geometry.[4] On the other hand, the grain size was reported to affect the formation of deformation twins or its related deformation mechanism. The formation of deformation twins can be reduced by grain refinement.[5–9] The dominant plastic deformation mechanism transforms from the twins to the dislocation slip by the refinement of the grain structures.[10–12] The fracture mechanism also changes from a brittle fracture on account of the deformation twins to a ductile fracture associated with void formation. In addition, the HIDETOSHI SOMEKAWA, TADANOBU INOUE, and ALOK SINGH, Senior Researchers, and TOSHIJI MUKAI, Group Leader, are with the National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan. Contact e-mail: SOMEKAWA. [email protected] Manuscript submitted November 9, 2010. Article published online February 24, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
fracture toughness tends to increase with grain refinement in magnesium[13] and magnesium alloys.[7] The occurrence of a ductile fracture and high fracture toughness in fine-grained magnesium alloys supposedly relates to the blunt crack tip that appears during the fracture toughness test; however, the detailed microstructural evolution and fracture mechanism at the beginning of the fracture toughness test have not been clarified yet. In this study, the deformed microstructure of the fine-grained magnesium alloy, which was stopped at the beginning of the fracture toughness test, was investigated using transmission electron microscopy (TEM) observation combined with the focused ion beam (FIB) technique that allows for the preparation of sitespecific microsamples for microstructural observation. In addition, the finite element analysis (FEA), which is considered to be a tool for realizing quantitative evaluation during deformation, was used to evaluate the strain rate and temperature quantitatively at the crack tip during the fracture toughness test in the fine-grained magnesium alloy. The effect of deformation speed on the deformation behavior was also discussed from the deformed microstructural observations and FEA results.
II.
EXPERIMENTAL AND NUMERICAL PROCEDURES
A fine-grained Mg-2.4 at. pct Zn binary alloy was us
Data Loading...
