Research On Long Period Stacking Ordered Mg-Zn-Y-Mn-(B) Alloys

Magnesium alloys with small density,high specific strength,good damping capacity,formability,as well as electromagnetic shielding properties have great application prospect on aerospace,automobile and military industry.However,the current magnesium alloys with low strength,poor plasticity and other shortcomings limit their engineering application.Recent years people have witnessed a great deal of attention being given to the Mg-Zn-Y alloys with long period stacking ordered(LPSO)phase due to their unique microstructures and excellent mechanical properties.Such magnesium alloys with high mechanical properties have been manufactured through a series of methods including alloying,heat treatment and hot deformation.However,a higher Y/Zn ratio is a prerequisite for the formation of LPSO phase,in which case the expensiveness of the rare earth Y entails limitations on its civil industrial application.Therefore,it is of vital importance to find an alloying element which can promote the formation of LPSO phase under low Y/Zn ratio.This paper selects Mg94Zn2.5Y2.5Mn1 alloy as the experimental subject and has conducted considerable research as follows.Mg-Zn-Y-Mn-(B)alloys were prepared by conventional casting method,prior to which the Mg-B master alloy prepared by sintering method of spark plasma(SPS)was added to Mg94Zn2.5Y2.5Mn1 alloy for the first time.After solid-solution treatment,the alloys were subjected to extrude treatment for better microstructure and mechanical properties.And the microscopic characterization and mechanical test performed on the as-cast,solid-solution treated and as-extruded samples were conduted to study the effects of minor B on their microstructures and mechanical properties.Additionally,study of the effects which B exerts over the evolution of W phase,LPSO phase and dynamic recrystallization behavior were investigated for the first time.Then,the effects of different processes on the microstructure evolution and mechanical properties of the alloys were studied.And systematical study on the mechanism of W phase spheroidizing,LPSO phase transition and dynamic recrystallization were conducted,with an aim to provide a new way of preparing the magnesium alloys with high strength and toughness.The experimental results are as follows:(1)The phase composition of the as-cast Mg-Zn-Y-Mn-B alloys mainly consists of ?-Mg matrix,18R-LPSO phase(Mg12YZn)and W-phase(Mg3Zn3Y2).And minor B can greatly refine the grain of the as-cast alloy,increase the volume fraction of LPSO phase and inhibit the precipitation of the W phase.With 0.003 wt.% B addition,the alloy achieved the optimal microstructures and mechanical properties with tensile strength of 252.5 MP and elongation of11.0%.(2)In the solid-solution treated Mg-Zn-Y-Mn-(B)alloys(furnace cooling),partial block 18 R LPSO phase along the grain boundary transformed into 14 H LPSO phase which homogeneously distributed in the grain interior.Additionally,the continuous network W phase at the grain boundary was spheroidized.But in the solid-solution treated Mg-Zn-Y-Mn-(B)alloys(water quenching),only the spheroidization of the W phase occurred.The transition of LPSO phase and W phase has a significant effect on the mechanical properties,especially on the plasticity of the alloy.(3)B can promote the spheroidization of the W phase,refine W particles and reduce its volume fraction in the solid-solution treated alloy and promote the formation of 14 H LPSO phase in the solid-solution treated alloy(furnace cooling).In addition,B can significantly strengthen the mechanical properties of solid-solution treated alloys.The tensile strength and elongation of the solid-solution treated B-containing alloy(water quenching)is 287.7 MP and21.1%,and of the solid-solution treated B-containing alloy(furnace cooling)is261.7 MPa and 20.5%.(4)The DRX of Mg-Zn-Y-Mn-(B)alloys occurred during the extrusion process,bringing out improved mechanical properties.And the DRX process can be delayed by the addition of B.The volume fraction of grains without DRX increased from 5.7% in the B-free alloy to 16.0% in the B-containing alloy.(5)The B addition can change the DRX behavior of the as-extruded Mg94Zn2.5Y2.5Mn1 alloy.And B-containing alloy exhibited both DDRX and CDRX behaviors while the B-free alloy mostly exhibited DDRX behavior.Additionally,minor B addition can strengthen the mechanical properties of extruded alloy,the tensile strength and elongation of which were up to 389.7MPa and 24.9%,respectively.