Effect of precipitate shapes on fracture toughness in extruded Mg-Zn-Zr magnesium alloys
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commercial Mg–6 wt% Zn–0.5 wt% Zr (ZK60) alloy was used to investigate the effect of precipitate shapes on fracture toughness. The ZK60 alloy was extruded at a temperature of 653 K (extruded alloy). The extruded alloy was annealed at 633 K for 86.4 ks (annealed alloy), and then the annealed alloy was aged at 448 K for 100 ks (aged alloy). The average grain size in all the conditions was about the same, 13.5 ± 1.0 m. The extruded and aged alloys had different shaped precipitates: spherical and rod shaped precipitates, respectively. The plane-strain fracture toughness KIC of the extruded, annealed, and aged alloys were estimated to be 22.4, 20.2, and 21.0 MPam1/2, respectively, by the stretched zone analysis. Transmission electron microscopy (TEM) observations showed that the deformation during the fracture toughness test was dominated by a dislocations on the basal slip planes in all the conditions. Such dislocations are commonly activated in magnesium alloys during the tensile and compression tests. The spherical shaped precipitates were found to be more effective than the rod shaped precipitates for improving the fracture toughness in the magnesium alloy.
I. INTRODUCTION
Magnesium alloys have a high potential for use as structural materials, as they are the lightest among all the structural alloys in use. To be used in structural applications, the alloys’ mechanical properties must satisfy both reliability and safety requirements. A method of ensuring this is to investigate their fracture toughness. Several reports have been made on the fracture toughness in magnesium alloys.1–5 However, the values of the fracture toughness in magnesium alloys have been generally reported to be much lower than those in aluminum alloys.6–8 Thus, research for improvement of the fracture toughness of magnesium alloys is a major issue. It has been reported that the fracture toughness in an extruded magnesium alloy is affected by its texture, i.e., the basal plane distribution in which basal plane of the grains lie parallel to the extrusion direction.9 Extruded samples of magnesium alloys having a precrack normal to the basal plane distribution are found to have higher fracture toughness in comparison with the samples having a precrack parallel to the basal plane distribution due to the difference of surface energy in basal and non-basal planes. In addition, the fracture toughness in magne-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0112 J. Mater. Res., Vol. 22, No. 4, Apr 2007
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sium10 and magnesium alloys11,12 can be improved with grain refinement. From the microstructural observations of fracture toughness tested samples in Mg–Al–Zn alloys, it is observed that the grain refinement helps in (i) prevention of formation of deformation twins, which are the origin of micro-cracks, and (ii) activation of dislocations on basal as well as non-basal planes.13 Therefore, controlling the texture and/or the grain refinement a
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