Investigation On Creep Behavior And Microstructural Deformation Mechanism Of Mg₉₇Y₂Zn₁ Alloy

Long Period Stacking Ordered Structure?LPSO?in Mg-RE-Zn alloys can significantly improve the high temperature properties,especially creep resistance,and broaden the application prospects of magnesium alloys in automotive,aerospace and other fields.However,the current research mainly focuses on the type,transformation and its influence on the strength of LPSO structure.The microscopic deformation mechanism of magnesium alloy containing LPSO structure and the type,quantity,morphology and distribution of LPSO structure on the creep behavior of magnesium alloy.The impact is still lack of systematic research.In this paper,Mg₉₇Y₂Zn₁ alloy was used as the research object.Mg₉₇Y₂Zn₁ alloys with different types,quantities and distributions of LPSO phases were obtained by different annealing processes.The in-situ tensile test method was used to explore the microscopic deformation mechanism of LPSO-containing magnesium alloys.From the perspective of microscopic analysis,the influence of the type,quantity and distribution of LPSO structure on the creep behavior of the alloy was systematically studied.The creep mechanism of magnesium alloy alloy containing LPSO structure was discussed by using classical theory and microscopic analysis.The as-cast microstructure of Mg₉₇Y₂Zn₁ alloy consists of?-Mg and bulk 18R-LPSO phase.After annealing at 500°C for 15h,the 18R-LPSO phase is partially dissolved in the matrix,and the14H-LPSO phase is precipitated and distributed in the crystal.Inside and near the 18R-LPSO phase.After annealing at 500°C for 30 h,the amount of the second phase in the alloy further increases,the number of bulk 18R-LPSO phases decreases,and more lamellar 14H-LPSO phases appear in the crystal.The in-situ tensile test results show that the LPSO structure acts as a strengthening phase in the magnesium alloy.As the applied stress increases,the?-Mg matrix transfers the load to the LPSO structure,and stress concentration occurs at the LPSO structure.crack.At the same time,different orientations of the slip zone are generated during the deformation process.As the stress increases,stress concentration occurs at the intersection of the slip bands in different directions,and the cracks sprout and expand here.In the Mg₉₇Y₂Zn₁ alloy containing LPSO phase structure,the bulk 18R-LPSO phase distributed along the crystal/dendritic crystal has high thermal stability,which can effectively pin the grain boundary and hinder the movement of the grain boundary;at the same time,the thermal stability generated in the crystal The 14H-LPSO can interact with dislocations to limit dislocation motion,thereby improving the creep resistance of the alloy.The combination of the two aspects gives the alloy a high creep resistance.Creep experiments were carried out on alloys with different heat treatment conditions under523K-623K and 20MPa-90MPa.The results show that the as-cast Mg₉₇Y₂Zn₁ alloy containing only18R-LPSO phase has the best creep resistance.According to the classical theory,the strain index n of the as-cast condition,the 500°C-15h annealed state and the 500°C-30h annealed Mg₉₇Y₂Zn₁alloy under the selected creep conditions are all in the range of 4-7,which is a dislocation climbing mechanism.The creep activation energy Q is closer to the grain boundary diffusion energy,which is the grain boundary slip mechanism,and the two are not uniform.No grain boundary slip was observed in the microscopic analysis of the alloy creep process.At the same time,18R-LPSO maintained its microstructure during the creep process,indicating that the 18R-LPSO structure has high thermal stability and at the same time The process of the transformation effectively limits the movement of the grain boundaries.Microscopic observations have found that the LPSO structure hinders the dislocation motion.On the one hand,the dislocations are hindered at the 18R-LPSO structure,stopping or forming a dislocation wall.At the same time,the 14H-LPSO structure including internal stacking faults also interacts with dislocations,hindering dislocation motion.The comprehensive analysis shows that the creep of Mg₉₇Y₂Zn₁ alloy is affected by interface slip and dislocation climbing.