n value for hexagonal closed packed structure
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Answer:
The limited slip systems in Mg alloys (hexagonal close-packed (HCP) structure) make them susceptible to cleavage fracture. A discontinuous cleavage results in the TGSCC of Mg alloys (Winzer et al., 2005). Various researchers (Chakrapani & Pugh, 1975, 1976; Meletis & Hochman, 1984) have reported that TGSCC in Mg alloys is a result of alternating processes of electrochemical dissolution and discontinuous crack advancement due to crystallographic constraints of HCP system. TGSCC resulted in fracture surfaces consisting of flat and parallel facets separated by perpendicular steps, which is consistent with cleavage mechanism. The matching and interlocking opposite fracture surfaces confirmed the occurrence of cleavage, which was difficult to explain by a dissolution model (Chakrapani & Pugh, 1975, 1976).
Mg alloys are reported to evolve considerable amount of hydrogen even at open circuit potential (OCP) as well as undergo localized corrosion. Hence, combined effects of hydrogen-assisted stress corrosion cracking (HASCC) and localized dissolution have been suggested to play an important role in deterioration of mechanical properties of AZ91D in modified simulated body fluid (m-SBF) (Choudhary & Singh Raman, 2012). In the case of magnesium alloys, an anodic polarization (which would normally not facilitate hydrogen generation in other alloy systems) is found to accelerate the SCC because hydrogen is generated even at such potentials due to the negative difference effect which is exclusive to Mg alloys (Stampella et al., 1984; Uematsu, Kakiuchi, & Nakajima, 2012; Winzer et al., 2005).
The TGSCC observed in Mg alloys has been widely attributed to mechanism involving hydrogen (H). Uematsu et al. (2012) showed that SCC of wrought AZ31 magnesium alloy was dominated by hydrogen embrittlement (HE). They reported higher crack propagation rate with increasing magnitude of cathodic potential that facilitated generation of hydrogen. Meletis and Hochman (1984) suggested that the presence of cleavage features at fracture surfaces could also be attributed to HE.
4.2.3 The extrusion processing parameter
1.
Preheated temperature of magnesium alloy ingots: Magnesium has a close-packed hexagonal structure. At room temperature, magnesium has only three slip systems, and its plastic deformation depends on the coordination of slipping and twinning deformation. When the temperature is lower than 250 °C or so, the plastic deformation includes the basal plane slipping and twinning, so the plastic deformability is weak. When the temperature is higher than 250 °C, the slipping of prism plane orientation comes into play. At the higher temperature, the slipping could occur on the prism plane orientation. Figure 4.5 shows the slipping system of magnesium.
So, the plastic deformation of magnesium alloy occurs at high temperature. The appropriate deformation temperature range is 350–460 °C. The magnesium alloy ingots must be preheated before extrusion deformation. The preheated temperature is higher than the deformation temperature, that is, about 450–530 °C. To ensure the uniform temperature distribution of magnesium alloy ingots, one hour heat retaining is needed.
2.
Preheating of extrusion die: When the high-temperature magnesium alloy ingots come in contact with the cold extrusion die, cracks easily appear. At the same time, the temperature of ingots decreases quickly, and the extrusion process would become difficult. So, the extrusion die should be preheated together with the magnesium alloy ingots.
3.
Deformation rate: When the deformation rate of magnesium alloy ingots is high, the thermal effect of the deformation process will increase the temperature of the extrusion billet, which would decrease the flow stress, and the required extrusion pressure will drop. But when the deformation rate is too high, the required extrusion pressure actually increases. This is due to deformation hardening of the metal in the hot extrusion process. The deformation hardening could be softened by the recrystallization process. However, it needs time to recrystallize. If the deformation rate is too fast, the softening effect of recrystallization will not take place and then the deformation resistance increases.
4.
Lubricants: During the hot extrusion process of magnesium alloy welding wire, in order to reduce the deformation resistance, it is necessary to add the appropriate lubricant in the extrusion cylinder. As a result of the high temperature extrusion above 400 °C, graphite lubricant is selected. In addition, the presence of lubricants can also play a role in insulation and can improve the service life of the extrusion die.