Study of hot tearing susceptibility and crack arrest mechanism of Mg-Y-Ni alloys containing LPSO structure

The hot tearing susceptibility (HTS) and crack arrest mechanism of long-period stacking ordered (LPSO) reinforced Mg-4Y-xNi (x = 0.5, 1, 2, 3, 4 wt%) alloys were investigated using a "T"-shaped constrained rod casting mold equipped with a stress acquisition unit and a thermal analysis system. Among the experimental alloys, all the alloys except WN44 alloy have high HTS. With increasing Ni content, the crack volumes at the hot spots of the castings were 0.033 cm3, +infinity (complete fracture), 0.062 cm3, 0.020 cm3 and 0.004 cm3, respectively. The influence of Ni content on the HTS was well reflected in the hot tearing initiation temperature (Thti), which was measured as 514 degrees C, 593 degrees C, 549 degrees C, and 436 degrees C, respectively. Notably, the Mg-4Y-4Ni (WN44) alloy exhibited no detectable Thti and demonstrated the lowest HTS. The Mg-4Y-1Ni (WN41) alloy with columnar grain morphology initiated hot tearing at a relatively low stress evolution rate of 4.3 N/s due to localized stress concentration during the early solidification stage, corresponding to a critical load of only 10.2 N. The presence of fine grains accelerated the stress evolution in the Mg-4Y-2Ni (WN42) alloy to 14.6 N/s. However, the increased liquid film thickness and intergranular bridging delayed the initiation of hot tearing. Furthermore, the increase in residual liquid content enhances the crack healing capacity. Although the healed cracks existed in the alloy as defects similar to macro-segregation, the crack propagation was still effectively hindered. The Mg-4Y-xNi alloys should be considered for both high strength and low HTS, and the findings will provide a technological pathway and a theoretical reference for the magnesium alloys to reduce the HTS.