| Mg-Zn-Al series magnesium alloys have been considered new type of magnesium alloy with development low-cost and remarkable heat treatment strengthening characteristics.At present,the investigations on the microstructure and mechanical properties of Mg-Zn-Al base alloys and microalloyed Mg-Zn-Al series alloys in as-cast,solution,aging and semi-solid isothermal treatment situations,which are not sufficient enough currently.More particularly,the reports of the desolvent precipitation of G.P.zones,and the mechanism on G.P.zones and precipitated phase strengthening solid phase particle of non-dendritic alloy have been investigated rarely in heat treatment condition.In consideration of above,the microstructure and mechanical properties of new type Mg-Zn-Al and Mg-Zn-Al base alloys microalloyed with Sm/Mn in as-cast,solutionized,aged and semi-solid situations were characterized using optical microscope?OM?,X-ray diffractometer?XRD?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,tensile testing machine and micro-hardness tester,respectively.According to the experimental results,the strengthening mechanism,and effects of Sm/Mn microalloying and heat treatment on the microstructure and mechanical properties of Mg-Zn-Al base alloys were studied in detail.The principal investigated contents and results of the thesis were as follows:The microstructure and mechanical properties of as-cast Mg-6Zn alloys microalloyed with varying Al content?0,1,3,5,7,wt.%?were investigated.It was shown that eutectic phases evolved from isolated island to coarse and continuous reticular structure,meanwhile,the tensile properties of the alloys enhanced firstly,then decreased with increasing Al addition from 0%to 7%.Based on the microstructure and mechanical properties of the studied alloys,the 3%Al-containing alloy was optimized,namely,Mg-6Zn-3Al?ZA63?alloy had an ultimate tensile strength of 214 MPa,elongation of 8.70%.The microstructure and mechanical properties of as-cast ZA63 alloys microallyed with mutating Sm/Mn?0,0.1,0.3,0.5,wt.%?were investigated.It was founded that the Al2Sm and Zn12Sm phases existed in as-cast ZA63 alloys microalloyed with rare earth Sm.The addition of Sm maked the dendrite morphology of ZA63 alloy more developed,which was mainly attributed to the increase of the constitutional undercooling zones in the solid-liquid interface,leading to solid-liquid interface changed instability and generated salient in solid-liquid interface local regions.The growth process of the salient,Sm atoms were constantly discharged into the surrounding liquid phase,promoting the development of the initial smooth interface to cellular dendrites.As a result of this all,the dendritic morphology of grains tended to be more developed,meanwhile,the tensile properties decreased gradually with increasing Sm addition.Nevertheless,the grains of ZA63 alloy were refined by 0.1%Mn micro-alloying,which was primarily ascribed to the constitutional undercooling in the front edge of solid-liquid interface,resulting in the increase of nucleation rate,and the decrease of heterogeneous nucleation power owing to the generation of heterogeneous nucleation substrate??-AlMn?.With increasing Mn addition,the dendritic morphology of?-Mg grains tended to develop,meanwhile,the eutectic microstructures tended to be coarsening.This was mainly due to the interaction between generated?2-Al8Mn5 phase and?phase weakened the substrate function of heterogeneous nucleation of?phase.Among the Mn-bearing alloys,Mg-6Zn-3Al-0.1Mn?ZAM6301?alloy possessed a good combination of ultimate tensile strength?219 MPa?and elongation?7.01%?.Based on the DSC curve characterization of as-cast ZA63 alloy,the process parameters of solution treatment were designed,then the optimized ZA63,Mg-6Zn-3Al-0.1Sm?ZAS6301?and ZAM6301 alloys were solutionized respectively.According to microstructural variation under different solution technique,the process of 350?+28 h was selected as the optimal solution technique for the three alloys.The microstructural evolution of the three alloys was summarized as:??matrix?phase+??eutectic?phase?supersaturated?-Mg solid solution+Mg32?Al,Zn?49 phase in the optimal solution technique.Meanwhile,the values of ultimate tensile strength?241 MPa,245 MPa and 252 MPa?and elongation?11.12%,8.82%and 9.87%?were simultaneously obtained from the three alloys treated at 350? for 28 h,repectively.The microstructure and mechanical properties of ZA63,ZAS6301 and ZAM6301 alloys experienced solution aging treatment were studied.On the basis of aged performance of the three alloys,the processe of 180?+24 h was regarded as better aging technique,the three alloys aged at 180? for 24 h can achieve ultimate tensile strength of 298 MPa,285 MPa and309 MPa,elongation of 9.78%,7.43%and 7.52%,the hardness peak value of 96.3 HV,85.4HV and 97.8 HV,respectively.For precipitated precipitates of the three alloys experienced at180? for 24 h,TEM analysis demonstrated that there were mainly two types strengthening precipitates in the investigated alloys,namely,the first precipitate was a bar-like,the secondary one was a square-shape.The semi-solid specimens of ZA63-xSm/Mn?x=0,0.1,0.3,0.5,wt.%?alloys were prepared through semi-solid isothermal heat treatment at 560?,580? and 600? for 30min,respectively.The semi-solid microstructure and its internal subtle structure of the alloys in same isothermal process were studied to illuminate the effects of Sm/Mn micro-alloying on non-dendritic microstructure and ascertain suitable semi-solid isothermal heat treatment process for the series alloys.It was shown that after Sm micro-alloying ZA63 alloy,the size and shape factor of solid particles in semi-solid microstructure increased gradually with increasing Sm addition,which was mainly ascribed to as-cast developed dendritic microstructure and Sm with larger atomic radius hindered the diffusion of Mg,Zn and Al with smaller atomic radius.However,after Mn microalloying ZA63 alloy,the size and shape factor of solid particles in semi-solid microstructure decreased first and then increased with increasing Mn addition.The near spherical particles with better roundness and homogeneous size distribution,meanwhile with an average size of 98?m and a shape factor of 1.21 were obtained from ZAM6301 alloy treated at 580? for 30 min.According to the size and roundness of the particles,and the transformative degree of the treated alloy specimens,it was optimized that the suitable isothermal heat treatment parameters should be proposed at 580? for 30 min for the series alloys in the experimental conditions of the thesis.The microstructure of non-dendritic ZA63 and ZAM6301 alloys solutionized at 350? for different time was investigated.It was shown that the consolidation of solid particles was achieved by the migration of between the primary?1-Mg particles with the same crystalline structure and the mutual attachment growth between the free?2-Mg and the primary?1-Mg particles under the action of concentration gradient and energy fluctuation.The microstructure and micro-hardness of non-dendritic ZA63 and ZAM6301 alloys experienced pre-aging,single-aging and two-step aging were studied repectively.It was shown that clustered atoms or G.P.zones with different morphologies were generated from ZA63 and ZAM6301 alloys solutionized at 350? for 12 h,followed by aging at 90? for 24h.Furthermore,for the same alloy,the aging hardening effect of two-step aging was much stronger than that of single aging.The reason was that the the generated clustered atoms or G.P.zones during the pre-aging process,which played the role of heterogeneous nucleation effectively,resulting in the increment of incubation nucleation,growth and precipitation rate of precipitates in the process two-step aging.The maximum values of hardness?96.9 HV?and?98.1 HV?were simultaneously obtained from non-dendritic ZA63 and ZAM6301 alloys aged at 180? for 24 h respectively,meanwhile,the maximum values of hardness?98.4 HV?and?100.2 HV?were simultaneously achieved from non-dendritic ZA63 and ZAM6301 alloys experienced pre-aging at 90? for 24 h then aged at 180? for 24 h.The aging hardening response of non-dendritic ZAM6301 alloy was higher than that of ZA63 alloy in the same aging technique,this mainly ascribed to the micro-alloying of 0.1%Mn leaded to the refinement of grains and the decrease of nucleation power of precipitate phase,and the increase of distributive tendency of precipitated phase and lath-like density.The strengthening mechnism of between solid particles of non-dendritic ZA63 and ZAM6301 alloys was mainly summarized as:the synergetic strengthening of internal stress strengthening of G.P.zones or cluster atoms and dislocation crossing/cutting?1'??2'??and Mg32?Al,Zn?49 phases. |
PDF Full Text Request