Yb On The Microstructure And Mechanical Properties Of Zk31 Magnesium Alloy At Room Temperature

Six Yb alloying ZK31-based magnesium alloys was studied in this paper. Thermal mechanical simulation, metallographic analysis, hardness test, room-temperature mechanical tension, x-Ray phase analysis and scan electron microscope observation was used to search the suitable plastic deformation parameters and heat treatment parameters. And the effect of Yb on the mechanical properties improvement of ZK31 magnesium alloy was also investigated.Yb and Zn distributed homogeneously in the grain, a small amount Yb-Zn intermetallic lied in the grain or along the grain boundary discontinuously, when the addition of Yb was lower than 1Wt.%, thus the alloy is deformable. If the addition of Yb is more than 2 Wt.%, both Yb and Zn segregated to the grain boundary to form friable ternary Mg-Zn-Yb intermetallic. Although the grain was refined obviously by the segregation of solute element, the alloy became undeformable due to the cracking introduced by fragmentary of net-like intermetallic which located on the grain boundary. While the addition of 0.5Wt.%Yb has minor affect on the improvement of the mechanical of the alloy, so the 1 Wt.% is the proper addition quantity.The improvement of the mechanical properties of ZK31 magnesium alloyed with Yb, mainly due to the grain refinement of as-extrusion ZK31 alloy(<5μm) by the inhibition of dynamic recrystallization(DRX) process which can benefit the properties after heat treatment. The mechanical properties of ZK31 can improved obviously by addition of lWt.% Yb, the best mechanical properties attained after T5 (423K/24h) treatment, σ_b=313MPa,σ_0.2 =293MPa, δ=19.1%.The result of thermal mechanical simulation shows that 663K/0.1^-1 is the optional deformation parameters, DRX can be activated easily under these conditions, while DRX could not be activated at 623K and 573K. The sample fractured before sufficient deformation, when the strain rate increased to 1s^-1. Twining occurred at the early stage of deformation, and introduced "polygonization morphology" into the grain, which transformed into low angle boundary(LAB) in the subsequent straining contributed to the fragment of coarsen cast grain.