Effect Of Alloying Elements On Indentation Creep Properties Of Heat Resistant Magnesium Alloys

Major in Material Processing Engineering Postgraduate:ZHANG Yao-cheng Supervisor:Prof.ZENG MingIn the project,the AE,AEC,AECJ and AECS magnesium alloys were prepared by adding proper amount RE,Ca,Sr and Si elements into as-cast Mg-Al matrix alloy. The elevated temperature indentation creep resistant properties of as-cast,solid solution state,and aging state alloys were investigated on a special apparatus.The microstructure and composition of the precipitated phases before and after creep test were performed.The results indicate:(1)The as-cast AE42 alloy was consist ofα-Mg matrix,acicular Al₁₁La₃ and granular Al₂La.Al₁₁La₃ and Al₂La,which have good thermal steady under 200℃, can guarantee the creep resistance of AE42 alloy by hindering gain boundary sliding and dislocation movement.The creep resistance properties of AE42 alloy were deteriorated because of the high solid solubility of Al element in theα-Mg matrix, which is beneficial to discontinuous precipitation ofβ-Mg₁₇Al₁₂ along the grain boundary.(2)The precipitation phase morphologies of AEC4108 containing La-Pr-Ce mischmetal(AEC4108L) were acicular and bone,which appeared along grain boundary,and granular which appeared in grain.The grain had been fined and there was fish-bone phase distributing along the grain boundary after adding a small quantity of Sr element into AEC4108L alloy.And if(0.2～0.8) wt.%Si was added into AEC4108L alloy,the grain had fined a little,and there was Chinese script type phase appearing in the grain.The precipitation phase morphologies of AECS alloys were rod,granular,bone and Chinese script,and the amount of rod and granular phases were increasing with the contents of Si.There was no new phase appeared in the grain and the contents of bone phase deceased in the AEC4108L alloy and AECJ410802L alloy,but the contents of short rod and granular phases increased in the AECS alloys after creep test.The compositions of precipitation phases keep invariant.The acicular,bone, granular,fish-bone and Chinese script phases were Al₁₁La₃,Al₂Ca,Al₂La,Al₄Sr and Mg₂Si,respectively.But the compositions of precipitation phases of AECS alloys are complicated.The phases were multivariate phase or the mixture of binary phases.(3)The alloys containing Nd-Pr misch rare earth have more significant grain boundary than the alloys containing La-Pr-Ce mischmetal,but have worse grain refinement.The grain of AEC4108 alloys added into a small quantity of Sr got more uniform,and the precipitation phases morphologies become more signification.The main phases morphologies of AEC4108 alloys are acicular,bone,granular and some lamellar,and there is fish-bone phase distributing in the grain after 0.2wt.%Sr adding into the AEC4108 alloys.The alloys containing Nd-Pr misch rare earth have better elevated creep resistance than the alloys containing La-Pr-Ce mischmetal,and Sr can improve the creep resistance of magnesium alloys.(4)The creep resistance of as-cast magnesium alloy is the best,solid-solution is secondly and aging is the worst.The precipitation phase morphologies of alloys had not changed much after heat treatment.There was some small lamellar and granularβ-Mg₁₇Al₁₂ precipitating along the grain boundary.The grain was coarsened during the indentation creep.Almost all theβ-Mg₁₇Al₁₂ dissolved in theα-Mg matrix after solid solution,but it precipitated along grain boundary again during aging.The high solid solubility of Al element in theα-Mg matrix is the predominant cause of the poor creep resistance of magnesium.(5) The creep data for these alloys could be correlated using an empirical equation(?)=Aσⁿ exp(-Q_c/RT) under the creep temperature 125℃,150℃,175℃,200℃and creep stress 55MPa,75MPa,95MPa,where n is stress exponent,σis stress,Q_c is creep activation energy.The stress exponent and activation energy had not changed too much under different stresses and temperatures.The stress exponents for AE42,AEC4108L,AECJ410802L and AECS410808 alloys are 3.06, 3.172,3.12 and 3.039,respectively.And the creep activation energies for AE42, AEC4108L,AECJ410802L and AECS410808 alloys are 72.4kJ/mol,33.985kJ/mol,50.487kJ/mol and 30.064kJ/mol,respectively.(6) The indentation steady creep rate of AE42 alloy is controlled by the grain boundary sliding led by dislocation viscosity slipping.And the predominant creep mechanism for AEC4108L,AECJ410802L and AECS410808 alloys is the dislocation viscosity slipping led by grain boundary sliding.