Research On Long Period Stacking Ordered Mg-Gd-Ni Alloys

As the lightest structural metal material in the world, magnesium alloyswith high specific strength and specific stiffness have attracted considerableinterest in automotive and aerospace application. However, the inferior plasticityand relative low strength strongly restrict their potential applications. In recentyears, many researchers have demonstrated that long period stacking ordered(LPSO) phase is excellent reinforcement phase of magnesium alloys because ofits high microhardness and elasticity modulus, thereby enhancing themechanical properties of magnesium alloys.In the present work, long period stacking ordered Mg96Gd3Zn1-xNix(x=0,0.1,0.3,0.5,1) and Mg95.7Gd3Ni1Al0.3Sry(y=0,0.1,0.2,0.3) alloys wereprepared by conventional permanent mold casting method and heat treatment.The effects of Ni on the formation of LPSO phase and the mechanical propertiesof Mg-Gd-Zn alloy were explored. The influences of alloying elements Al andSr on the microstructure and mechanical properties of Mg96Gd3Ni1alloy were investigated. The results are as follow:The addition of Ni not only refined the grains and induced the formation ofabundant LPSO phase in as-cast Mg-Gd-Zn alloy, but improved the strengthespecially the plasticity of the as-cast alloy. The tensile strength and elongationof the as-cast Mg96Gd3Ni1alloy were203MPa and8.8%, respectively, with anincrease of19.7%and63.6%respectively compared with Mg96Gd3Zn1alloy.After solid solution treatment, there were abundant LPSO phase in bothMg-Gd-Zn and Mg-Gd-Zn-Ni alloys, and the ratios of Gd and the total of Znand Ni were almost the same. The LPSO phase of Mg-Gd-Ni alloy had higherthermal stability, having no obvious change during the solid solution treatment.After aging heat treatment, the tensile strength and elongation of all theMg-Gd-Zn, Mg-Gd-Zn-Ni and Mg-Gd-Ni alloys increased obviously. Ni hadless effect on the increase of the tensile strength of the aging-treated alloys thanZn, but Ni can enhance the plasticity of both the as-cast and the aging-treatedalloys significantly compared with Zn.The addition of0.3at.%Al to Mg96Gd3Ni1alloy improved the volumefraction of LPSO phase obviously and refined the dendrites and grains.Furthermore, new quaternary Mg-Gd-Ni-Al phases with peculiar rod-like andpetal-like morphologies were uniformly distributed in the alloy and theirchemical compositions were Mg73.9Gd9.3Ni8.5Al8.3and Mg44.4Gd16.9Ni16.3Al22.4,respectively. The angels between petals of the petal-like phase were all about60degrees. The new phases had high thermal stability and microhardness. The addition of Sr refined the microstructure of Mg95.7Gd3Ni1Al0.3alloyfurther. The refining effect was fine with0.1at.%Sr, but excessive Sr additioninduced the formation of new ternary Mg-Gd-Sr phase. Additionally, Sr did notparticipate in the formation of rod-and petal-like phases, but promoted theirformation.With addition of0.3at.%Al and0.1at.%Sr, the Brinell hardness of theMg96Gd3Ni1alloy increased significantly owing to the refined microstructure,larger volume fraction of LPSO phase and the dispersion strengthening of thepetal-and rod-like phases.