Grain Refining Mechanism And Influncing Factors Of Mg-RE-Zr Alloys

Mg-RE-Zr alloy is widely used in cars, aerospace, microelectronics as a resultof its low density and excellent properties. The influence of original Zr particles ongrain refining, grain refining mechanism of Mg-RE-Zr alloys as well as themicrostructure evolution of Zr-rich core with various cooling rate were analyzed bymeans of optical microscope(OM), scanning electron microscopy(SEM), inductivelycoupled plasma-atomic emission specrometry (ICP-AES) and XRD facilities.The original Zr particles play an important role on grain refinement. The statesof Zr particles in Mg-Zr master alloys of the same composition are different. Zrcontained in Mg-33.3Zr master alloy was mainly in form of zirconium particlesdistributed on the matrix. The size of Zr particles in Mg-Zr Master-1 were in rangeof 0~13μm, while 0~10μm in Mg-Zr Master-2. The results show that whenseparately inserted 8% Mg-Zr Master-1 and Mg-Zr Master-2 into pure magnesium,with the size of Zr particle decreasing from 2.39μm to1.77μm, the size of Zr-richcore decreased from 24.89μm to 16.68μm and the density of Zr-rich core increasedfrom 313/mm2 to 459/mm2. The average grain size decreass from 55μm to 40μm.According to statistics calculation, the density of Zr-rich core was proportional tothe density of original Zr particles between 1~8μm, so the formation of Zr-rich coremainly depends on the original Zr particles.When RE and Zn were added into Mg-Zr alloys, the maximum soluble Zrincreased from 0.597% to 0.98%. Compared with Mg-Zr alloys, the average Zr-richcore size in WE43 decreased from to 10μm, the density of Zr-rich core increased to759/mm2, and the average grain size declined to 20μm. The increase of soluble Zron one hand promoted the precipitation ofα-Zr particles at the peritectictemperature thus promoted the heterogeneous nucleation, on the other handincreased the Grain Restriction Factor(GRF) thus enhanced the restriction effect ofgrain growth.As the cooling rate declined from 100℃/s to 0.67℃/s, the average grain size ofWE43 alloy increased from 9μm to 52μm. The morphology of Zr-rich core wasvaried with cooling rate. They in turn evoluted to multi-layer halo shape, thensingle-layer halo shape final to no halo shape. The layer halo Zr-rich cores areproducts of ribbon segregation during rapid solidification, and the no layer haloZr-rich cores are the products of peritectic reaction and diffusion in solid phase.With the cooling speed increased, the average size of undissolved Zr in the center ofZr-rich core decreased, which probably because the increased subcooling reducedthe critical nucleation radius.