STRUCTURAL ( SUPER ) PLASTICITY OF MAGNESIUM

Low density of magnesium alloys and relatively high specific strength are the main advantages of these materials. Magnesium alloys and composites are used in different industrial applications. As magnesium is recyclable, its extensive usage can minimize the negative impact on the environment due to non-degradable plastic wastes. Magnesium materials have low ductility at ambient temperature. This is a consequence of the hexagonal close-packed (hcp) crystal structure with a limited number of operative slip systems. The plasticity of magnesium alloys and composites increases at elevated temperatures where additional slip systems become active. However, low ductility of magnesium alloys prevents their application hence; the enhanced ductility of magnesium materials is of a special interest [1–4]. It is known that the fine grain size (d 10 μm) stable at higher temperatures is a main structural requirement for the occurrence of superplasticity. All characteristics of superplasticity (relative elongation to failure εf , flow stress σ, strain rate sensitivity of flow stress m, optimum strain rate for superplasticity and optimum deformation temperature for superplasticity) depend on the grain size. Fine grain structure with the grain size less than 10 μm may be obtained by various methods such as phase transformation, recrystallisation, forming of the microduplex structure or severe plastic deformation (equal channel angular pressing, reciprocating extrusion) [5–10]. Structural superplasticity was estimated in several magnesium alloys especially in alloys of the AZ series [11–13] or Mg-Li alloys [14–15]. The superplasticity phenomenon was also observed in ZK60 alloy [16], WE54 alloy [17] and QE22 alloy [18]. In papers [18, 19] the influence of preliminary thermomechanical treatment (hardening, overageing and hot extrusion) on the grain size was investigated. This prior thermomechanical treatment influences the number of recrystallisation nuclei and so the resulting grain size.