Influence of extrusion parameters on microstructure, texture and mechanical properties of a low Mn and high-Ca containing Mg-2.9Zn-1.1Ca-0.5 Mn magnesium alloy

Abstract

A newly developed low Mn and high-Ca containing magnesium alloy, Mg-2.9Zn-1.1Ca-0.5 Mn (wt.%) based alloy was processed by low-temperature extrusion. The average recrystallized grain size decreased from 1.64 to 0.58 μm with a decrease in the extrusion speed from 1 to 0.01 mm s−1, but increased to 2.11 μm with increasing extrusion temperature from 270 to 350 ℃. The finest and uniform recrystallized grain structures with substantial nano-sized MgZn2 precipitates and α-Mn particles were obtained in the 270 ℃/0.01 mm s−1 sample. The texture intensity including basal (0002), prismatic type I (10−10) and prismatic type II (11–20) decreased accompanied by increasing the deformation temperature or extrusion speed. The weakest texture intensity for the 270 ℃/1 mm s−1 sample was derived from high degree of recrystallization, inhomogeneous recrystallized grain structure and the precipitates. Superior uniaxial tensile strength (yield strength of ∼352.5 MPa, ultimate tensile strength of ∼382.3 MPa and elongation of ∼7.1%) were achieved in the 270 ℃/0.01 mm s−1 sample. The improved yield strength was mainly ascribed to fine grain strengthening and precipitates strengthening relative to dislocation strengthening and back stress strengthening. A positive slope of work-hardening rate (dθ⁄dσ > 0) extruded at 270 ℃/0.01 mm s−1 was related to the sharp basal texture and insufficient slip. High θ value at stage IV after extrusion at 270 ℃/1 mm s−1 with abundant dimples and weak texture contributed to high ductility. Massive micro-cracks formed along the twins in the coarse deformed grains led to the failure during tension.