Research On High-strength Mg-Y-Ni-Zn Alloys With A Long-period Stacking Ordered Phase

Magnesium alloys have been applied in electronic, aerospace and atumobile industries as structural materials due to their high specific strength and specific stiffness. However, the poor mechanical properties and resistance to corrosion limit its further application. In recent years, a new alloy reinforced by long period stacking ordered phase(LPSO) structure with superior mechanical properties raise more and more attentions because of its superior mechanical properties at ambient and high temperatures.In this paper, researches had been done on forming and transformation mechanism of LPSO phase in high strength Mg-Y-Ni-Zn alloys prepared by conventional cast followed by hot rolling. The influence of alloying elements on microstructures and mechanical properties as well as the strengthen mechanism of LPSO phase were studied systematically. The results were as follow:(1) Microstructure of Mg96Y3NixZn1-x alloys were mainly composed of18R type LPSO phase and a-Mg matrix. The block shaped LPSO phase gradually changed to lamellar structure with phase transforming temperature reduced by several tens degrees along with the decrease of Ni addition.(2) After solution treatment at500℃for24h, the block shaped or lamellar LPSO phase had the tendency to transform to needle like14H LPSO phase in the interior of Mg grains gradually with the decrease of Ni content in Mg96Y3NixZn1-x alloys. When the Ni content decreased to0.2at.%, the block shaped or lamellar LPSO nearly transformed to needle like14H LPSO in the interior of Mg grains completely.(3) The a-Mg grains were remarkable refined when the Ti was added to Mg96Y3Nii alloy. The smallest grains of9um were gained when the Ti content reached0.2at.%Ti. The enrichment of Ti in the solid-liquid interface reduced the diffusion of elements and increased undercooling, consequently, grain refinement was obtained by inhibiting growth of grains and increasing the nucleation rate in the undercooling zones.(4) Kinking deformation of various degrees occurred in LPSO phases and a small part of LPSO phases broken into particles in Mg96Y3Ni1Zn1alloy at the same time after62%reduction by6passes hot rolling. In addition, dynamic recrystallization (DRX) grains and twin crystal were found in worked Mg matrix simultaneously, it was interesting to note that twin crystal was also found in DRX grains. The pinning effect of tiny rare earth rich phase and particle LPSO at grain boundaries restricted the growth of DRX grains.(5) Cracks were found on a majority of LPSO phases, partial LPSO phases were divorced from bulk LPSO phases into partials in Mg96Y3Ni1alloy. Kinking deformations on LPSO phases were less than in Mg96Y3Ni1Zn1alloy. Twin crystal and DRX grains were also occurred in Mg matrix.(6) The mechanical properties of two alloys after62%reduction by6passes of hot rolling improved remarkable. The tensile strength and elongation of Mg96Y3NiiZn1and Mg95.8Y3Ni1Ti0.2alloys were383MPa,16%, and358MPa,18.5%respectively.(7) Kinking deformation induced by rotating of crystal lattice improved the deformability of LPSO phases as well as the amount of interface. The extra interface would hinder the movement of dislocations and the expand of microcracks. Stress concentration was released by forming kinking, which reduced the properties of occurrence of crack.