Effects And Mechanisms Of The Mg₁₇Al₁₂ Phase On The Static Recrystallization Behavior Of Mg-Al-Zn Alloys

Magnesium?Mg?and its alloys are the lightest metallic structural materials,which have received extensive attention in the automotive,aerospace and other fields because of its low density and high specific strength.However,the applications of Mg alloys have been limited mainly because their hexagonal close-packed?HCP?crystal structure,which limiting the number of slip systems of magnesium at room temperatures.It is an effective way to improve the property of materials by refining grains.Recrystallization is an important method to refine the grains and obtain good mechanical properties of magnesium alloys.The features of the second phase have great influence on the recrystallization of magnesium alloy,and therefore the microstructure of recrystallization of Mg alloys can be controlled by regulating the second phases.In recent years,the relationship between the second-phase and recrystallization is still controversial,which is one of difficulties in magnesium alloys.Therefore,the dynamic precipitation behavior and aging precipitation behavior of Mg₁₇Al₁₂ phases in Mg–Al–Zn alloy have been studied by controlling the rolling process and heat treatment parameters.The effect of pre-existing Mg₁₇Al₁₂ phases on static recrystallization of Mg–Al–Zn alloy has also been explored.The main conclusions are as follows:?1?The effect of Al content on the dynamic precipitation behavior during hot rollingof Mg–Al–Zn alloys was investigated.With the increase of Al content,there weresignificant differences in the size,quantity and distribution of Mg₁₇Al₁₂ phases.InAZ31,Mg₁₇Al₁₂ phases were hardly observed after hot rolling.However,a largeamount of fine Mg₁₇Al₁₂ precipitates?¹00 nm?existed in AZ61.For AZ91,abimodal particle size distribution of Mg₁₇Al₁₂ phase was preserved?90nm–1.5?m?.?2?The role of dynamic Mg₁₇Al₁₂ precipitation content in the recrystallizationbehavior of hot-rolled Mg–Al–Zn alloys was investigated.The experimentalresults showed that the AZ31 exhibited the fastest recrystallization rate,while theAZ61 recrystallized slower than the AZ91 alloy.The effect of the features ofMg₁₇Al₁₂ phases on static recrystallization behavior was explored.It was foundthat the fine Mg₁₇Al₁₂ phases in AZ61 can hinder the migration of subgrainboundaries and grain boundaries and inhibit the static recrystallization of thealloys,while the bimodal particle size distribution of Mg₁₇Al₁₂ phases in AZ91have both inhibitory and promoting effects on static recrystallization.?3?The effect of annealing temperatures on the recrystallization kinetics of AZ91alloys was studied.It was found that as the annealing temperature increased,theinfluence of bimodal particle size distribution of Mg₁₇Al₁₂ phases in AZ91 onrecrystallization was changed from inhibition to promotion.It was revealed thatthe coarse Mg₁₇Al₁₂ phase in the AZ91 alloy is more likely to induce PSN duringhigh temperature annealing,thereby counteracting the inhibition of the relativelyrecrystallization of fine Mg₁₇Al₁₂ and increasing the recrystallization rate.?4?The aging precipitation behavior of Mg₁₇Al₁₂ phases in AZ61 alloys wereinvestigated during different aging temperatures and time.It was found that onlycontinuous precipitates are formed in AZ61 alloys after aging of a supersaturatedsolid solution at 180oC,while both discontinuous and continuous precipitates canoccur simultaneously after aging of a supersaturated solid solution at 300oC and400oC.The effect of pre-aging on the deformed microstructure of AZ61 alloyswas studied.It was found that the proportion of{101?2}twins in the cold-rolledAZ61 alloy after cold rolling decreased with the increasing of the pre-aging timeat 180oC,which demonstrated that the Mg₁₇Al₁₂ phases precipitated during aginghave an inhibitory effect on the formation of twins.?5?The effect of pre-aging precipitation of Mg₁₇Al₁₂ phase on static recrystallizationof cold rolled AZ61 alloy and its mechanism were investigated.With the increaseof Mg₁₇Al₁₂ phases,the recrystallization rate increased first and then decreased.Itwas revealed that dislocations easily accumulated around the Mg₁₇Al₁₂ phasesduring the rolling process,which provided driving force for staticrecrystallization.In the later stage of annealing,with the continuous consumptionof storage energy,the inhibition of Mg₁₇Al₁₂ phases on static recrystallization wasgradually obvious,resulting in slower recrystallization of the alloy.