Abstract:The hot tearing susceptibility of MgZn_2.5Y_xZr_0.5 (x=0.5, 1, 2, 4, 6) alloys was evaluated by thermodynamic calculations based on Clyne-Davies model. The microstructure and morphology of hot tearing regions of the alloys were observed by X-ray diffraction and scanning electron microscopy. The solidification temperature and shrinkage stress during the solidification of MgZn_2.5Y_xZr_0.5 alloys in the “T” type hot tearing permanent-mold were acquired with the attached computer. The effect factors of hot tearing susceptibility of MgZn_2.5Y_xZr_0.5 alloys, such as the solidification temperature interval, the variation of solid fraction in vulnerable region, the residual liquid fraction in the final stage, the type of the second phase of the alloys were discussed based on the above calculation and observation. The results demonstrated that the hot tearing susceptibility in the investigated alloys was found as follows：MgZn_2.5Y₂Zr_0.5>MgZn_2.5Y_0.5Zr_0.5>MgZn_2.5Y₄Zr_0.5>MgZn_2.5Y₆Zr_0.5>MgZn_2.5Y₁Zr_0.5. The highest hot tearing susceptibility of MgZn_2.5Y₂Zr_0.5 alloy related to the following reasons: the largest freezing range, the biggest changing of the variation of solid fraction in vulnerable region, the least liquid film in the final stage of solidification, the formation of the second phase which worsens the liquid flow and interdendritic feeding after dendrite coherency.

Key words: Mg-Zn-Y-Zr alloys; solidification curve; shrinkage stress; hot tearing susceptibility