Microstructural evolution, high temperature tensile deformation behavior, and deformation mechanism in an Mg-Zn-Y-Ca-Zr alloy processed by multidirectional forging and hot rolling

To explore high temperature ductility, a new Mg-2.70Zn-1.34Y-0.37Ca-0.02Zr (wt.%) alloy has been fabricated by novel multidirectional forging (MDF) and hot rolling. The microstructure and mechanical properties were investigated. The average grain size of MDF + hot rolled alloy is 10.89 +/- 0.90 mu m refined from the as-cast average grain size of 60.03 +/- 0.72 mu m. The ultimate tensile strength of 260.51 +/- 1.03 MPa, yield strength of 192.29 +/- 1.21 MPa, and elongation of 17.83 +/- 0.72 % were obtained at room temperature. For high temperature tensile behavior, microstructural examination revealed that continuous dynamic recrystallization and discontinuous dynamic recrystallization are the main softening mechanism in the temperature range of 573-673 K, while dynamic grain growth with bimodal grains and twins is discovered at 723 K in this alloy. X-ray diffraction and scanning electron microscopy -energy dispersive spectroscopy examinations revealed that the constituent phases are composed of alpha-Mg solid solution and intermetallic compounds of Ca2Mg6Zn3, Mg3YZn6 (I -phase), and Mg3Zn3Y2 (W-phase). The microstructural evolution, such as dynamic recrystallization and dynamic grain growth at desired tensile temperatures, is related to the thermal stability of constituent phases. The elongation to failure of 215.4 % was demonstrated at 673 K and 1.67 x 10-2 s-1, exhibiting high strain rate quasi-superplasticity. A power-law constitutive equation was established. The deformation activation energy of 177.948 kJ/mol and stress exponent of 4.494 revealed that the dominant deformation mechanism of this alloy at elevated temperatures of 573-723 K is dislocation climb controlled by lattice diffusion.