Study on the High-temperature Creep Property of AM60 Magnesium Alloy with Composite Modification

Author:Jin Wei

Supervisor:liu yao hui

Database:Doctor

Degree Year:2019

Download:18

Pages:149

Size:6233K

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AM60 magnesium alloy is mainly used in seat frame,dashboard base,engine bonnet and other important parts.However,due to its poor high-temperature creep performance,AM60 magnesium alloy can only be used below 150oC.High-temperature creep performance of AM60 magnesium alloy has become a bottleneck in its development.How to improve the high-temperature creep performance of AM60 magnesium alloy is a key scientific problem to be solved.According to the creep behavior of magnesium alloy,the main creep mechanisms of magnesium alloy are atom diffusion,dislocation motion and grain boundary slip at high temperature.In addition,eutectic phase with low melting point distributed on the grain boundary is one of the key factors affecting creep property of magnesium alloy.In this paper,the creep mechanism of AM60 magnesium alloy was studied.On this basis,the creep resistance of AM60 magnesium alloy was improved by composite modification.At 150oC and 175oC,AM60 cast magnesium alloy mainly exhibits grain boundary slip creep,and the eutectic phase with poor thermal stability is favorable for grain boundary slip,which is the main reason for the poor high-temperature creep performance of AM60 alloy.The high temperature creep property of AM60 magnesium alloy was improved by Ce modification.Based on the principle of solid solution strengthening,the mechanical properties of AM60 magnesium alloy were significantly improved by adding 1.0 wt.%rare-earth Ce.Rare earth Ce strongly inhibit the nucleation and growth ofβ-Mg17Al12 phase with poor thermal stability in the alloy,and fine acicular Al11Ce3 phase with high thermal stability block and slow down the grain boundary sliding in the creep,which reduces the steady-state creep rate of AM60-1Ce alloy by an average of about 42.5%as compared with AM60 alloy and significantly improves the performance of high temperature creep.However,rare earth Ce cannot effectively inhibit grain boundary migration during the creep of AM60 magnesium alloy.High temperature creep properties of AM60 magnesium alloy were further improved by the composite modification using alkaline earth element Ca and rare earth Ce.When Ca element is added to AM60 magnesium alloy,it will preferentially react with Al to form a hard phase of Al2Ca,which can reduce eutectic phase and even gradually eliminate theβ-Mg17Al12 phase by controlling the content of Ca element.On the grain boundary,Al2Ca phase is a bone-shape high-temperature stable phase.This further eliminates the adverse effects of relative creep ofβ-Mg17Al12.In the creep process,Al2Ca phase firmly nails the grain boundary and prevents grain boundary slip,so that the steady-state creep rate of AM60-1Ce-1.5Ca alloy is 33.1%lower than the average creep rate of AM60-1Ce alloy,which further improves the high-temperature creep performance of AM60 magnesium alloy.The high temperature creep mechanism of AM60 magnesium alloy was changed by composite modification.The creep mechanism of magnesium alloy cast by AM60-1Ce-1.5Ca shows that the stress index n of magnesium alloy cast by AM60-1Ce-1.5Ca at 150oC is about 1.82,and the creep mechanism is mainly grain boundary slip creep.The stress index n of AM60-1Ce-1.5Ca cast magnesium alloy at175oC is about 3.07,and the creep mechanism is mainly dislocation slip creep.The Al2Ca phase formed in AM60-1Ce-1.5Ca casting magnesium alloy shows high thermodynamic stability in high-temperature creep process,which is the main factor preventing grain boundary slip and dislocation movement in creep process and the main reason for its good creep resistance.Heat treatment significantly affects the mechanical properties of AM60-1Ce-1.5Ca magnesium alloy at room temperature,and the creep properties of magnesium alloy can be improved by heat treatment optimizaiotn.The solid solution treatment(T4)is beneficial to the solid solution of alloy elements into the matrix of AM60,AM60-1Ce and AM60-1Ce-1.5Ca magnesium alloy,but has little influence on the high-temperature stable phase in the alloy.After T6 heat treatment,the secondary β’-Mg17Al12 phase in AM60 magnesium alloy was precipitated.Ce and Ca can effectively inhibit the precipitation of secondaryβ’-Mg17Al12 phase.After T6 heat treatment,the steady creep rate of AM60-1Ce-1.5Ca magnesium alloy decreased by about 4.7%.After T4 heat treatment,the steady-state creep rate of AM60-1Ce-1.5Ca magnesium alloy decreased by about 5.2%compared with that of T6 heat treatment.This indicates that heat treatment is also conducive to improving the high-temperature creep resistance of rare-earth Ce and alkaline earth Ca composite modified AM60magnesium alloy.The results show that atom diffusion,dislocation motion and grain boundary slipare the main creep mechanisms of AM60 magnesium alloy at high temperature.Therefore,composite modification with various elements is an effective method to improve the creep property of AM60 magnesium alloy at high temperature.However,different alloying elements should be responsible for inhibiting dislocation movement,hindering grain boundary slip and eliminating eutectic zone,respectively.In this paper,rare earth Ce can inhibit high-temperature atom diffusion and dislocation movement,but alkaline earth element Ca can eliminate the adverse influence of eutectic phase with low melting point,which can comprehensively improve the high-temperature creep resistance performance of magnesium alloy.