Hot deformation and dynamic recrystallization of Mg-Gd-Y-Zn-Zr alloy containing high volume fraction 18R-LPSO phase

The deformation behavior including flow behavior, hot workability and dynamic recrystallization (DRX) mechanism of as-cast Mg-9Gd-3Y-3.75Zn-0.6Zr alloy containing 26.5% volume fraction 18R long-period stacking ordered (LPSO) phase distributed in continuous network morphology were studied by hot compression experiments at deformation condition of 350 degrees C-500 degrees C, 0.01-1 s(-1) strain rate and 0.75 true strain. The influence of deformation conditions on the microstructural evolution was analyzed by using optical microscopy (OM), scanning electron microscope (SEM), electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). Results display that stress exponent and deformation activation energy are 5.58 and 241.39 kJ.mol(-1) correspondingly, which suggested that dislocation climb is the principal deformation mechanism. The Arrhenius type constitutive equation is established as epsilon (center dot) = 4.09 x 10(16)[sinh(9.922 x 10(-3)sigma)](5.58)exp(-241394/RT). Microstructure evolution can be characterized by Zener-Holomon parameter (Z) which reflects the combining effect of temperature and strain rate on microstructure. At low lnZ (lnZ<36), the 18R-LPSO phase dissolves from continuous network to blocks and high DRX volume (>50 %) is realized by particle-stimulated nucleation and discontinuous dynamic recrystallization. At moderate lnZ (36 <= lnZ<40), a fine and coarse grains bimodal structure is formed at the original grain boundary by continuous dynamic recrystallization resulting in a DRX volume between 10 % and 50 %. At high lnZ (lnZ >= 40), layered 14H-LPSO phase precipitates and inhibits DRX (DRX volume<10 %) by absorbing dislocations. With combination of dissipation coefficient, instability factor and microstructure analysis, 400 degrees C/0.01-0.05 s(-1) and 450 degrees C/0.01 s(-1) are confirmed as optimal hot working domains for the as-cast alloy, under which conditions bimodal microstructure and equiaxed grains microstructure (average grain size 10.3 mu m) can be formed correspondingly.