The Investigation Of Microstructure,Mechanical Properties And Deformation Mechanism Of Extruded Mg-Y-Zn(Ni) Alloys

In recent years,magnesium alloys have attracted much attention because of their advantages such as low density,high specific strength,good casting and electromagnetic shielding properties.However,low strength and poor room temperature processing properties limit the application and development of magnesium alloys.There is a long period stacking order(LPSO)phase in Mg-RE(rare earth element)-TM(transition element)alloy,which has a unique atomic stacking order,which can improve the mechanical properties of Mg alloy.However,the type,morphology and distribution of LPSO phase affect the properties of magnesium alloys.Therefore,the evolution of LPSO phase in magnesium alloys and the strengthening mechanism for magnesium alloys have become the focus of research.Meanwhile,the deformation mechanism and strain hardening behavior of magnesium alloys containing LPSO phase under the condition of variable temperature and velocity are still unclear and need to be further studied and discussed.The solution of these problems will provide some theoretical basis for the design and development of high strength magnesium alloy.In this paper,Mg-2Y-1Zn(MYZ),Mg-2Y-0.5Zn-0.5Ni(MYZN)and Mg-2Y-1Ni(MYN)(at%)alloys are mainly studied.The microstructure evolution,strengthening mechanism and deformation mechanism of the alloys are studied by gravity casting,heat treatment and hot extrusion.The three as-cast alloys are mainly composed of?-Mg matrix,Mg₂₄Y₅ phase,Mg₂Y phase,Y-rich phase and coarse lamellar 18R LPSO phase precipitated at the grain boundary of?-Mg dendrite.The dendrite arm spacing of the as-cast alloy is gradually refined with the increase of Ni content,and the volume fraction of LPSO phase also increases.After homogenization,the LPSO phase changes,part of 18R LPSO phase changes to 14H LPSO phase and stacking fault(SFs),and a small amount of Mg₂Ni phase precipitates in MYN alloy.After extrusion,dynamic recrystallization(DRX)occurred in all three alloys,the grain size of the alloys is refined obviously,and three kinds of LPSO phases are precipitated:lamellar 18R LPSO phase,blocky 18R LPSO phase,and the thin strip 14H LPSO phase distributed in the grain.The tensile tests at room temperature show that the maximum tensile properties of the extruded MYZN alloy,which?_0.2??_UTS and?_f are 336,389 MPa and 12.6%,respectively.The high mechanical properties of the alloy are mainly related to LPSO phase strengthening,grain refinement,texture strengthening and stacking fault strengthening.The results show that the tensile strength of the alloy decreases with the increase of temperature and the decrease of strain rate,and the elongation increases gradually.The elongation of the extruded MYZN alloy is up to 130.3%at 300?and?=4×10^-4 s^-1,which giving expression to quasi superplasticity.By fitting the strain rate sensitivity coefficient(m)values at different temperatures,the m values at RT,100?,200?and300?are 0.03,0.04,0.05 and 0.17,respectively.Combined with the surface observation and fracture analysis of the tensile specimen after fracture,it is found that the slip band on the surface of the tensile specimen of extruded MYZN alloy at RT?200?,which proves that the main deformation mechanism of the alloy is dislocation slip.At 300?,the activation energy(Q)and fracture morphology of extruded MYZN alloy indicate that the deformation mechanism of the alloy is grain boundary sliding and dislocation slip.At the same time,with the increase of temperature,the fracture mode of the alloy also changes from intergranular fracture to transgranular fracture.During uniform plastic deformation at room temperature,the strain hardening exponent(n)of MYZ,MYZN and MYN alloy are 0.15,0.11 and 0.08,respectively,and strain hardening capacity(H_C)and 0.46,0.28 and 0.24,respectively.The results indicate that the strain hardening ability of extrusion MYZ alloy is higher,which is mainly due to the high Schmidt factor(0.31)of base plane,which decreases the?_non-basal/?_basal ratio,which is more conducive to the opening of non-base plane slip system.The activation of non-basal slip makes the dislocation aggregation more easily,increases the dislocation density,promotes the dislocation interaction,increases the dislocation storage,and improves the strain hardening ability of the alloy.The strain hardening ability of the alloy decreases with the increase of temperature.The results show that the strain rate of 1×10^-2 s^-1 has the best strain hardening ability at room temperature,which is mainly due to the fact that the alloy is conducive to a large number of dislocation activation at high strain rate and room temperature,which increases the hardening exponent and hardening rate of the alloy.