| In situ aluminum matrix composites are featured with such advantages as high specific strength and specific modulus,and this is because that the reinforced particles can nucleate and grow in-situ in the aluminum matrix via chemical reaction and have thermodynamically stable phase.The materials are expected to find wide application in aviation,aerospace,transportation and other industries.However,there remain some problems to be solved for this kind of materials(such as particle cluster,foundry defect and coarseness of matrix grains,etc.),which result in the fact that the mechanical properties are not so significantly improved.Although the conventional processing means,such as rolling,extruding and forging,etc.can effectively eliminate defects and refine the grains,these means have limited effects on improving particle cluster.Aiming at solving the above-mentioned problems,this paper prepared Al3Ti/Al(A356),ZrB2/6061Al and ZrB2/2024Al composite materials by means of direct melt reaction method/mixed-salt reaction,and mainly studied the cluster structure and formation mechanical of reinforced particles of different sizes.On this basis,this paper revealed the influence of particle cluster on the microstructure and performance of in situ aluminum matrix composites.To break the clusters and disperse particles,we the employed friction stir processing(FSP)for secondary processing for the above-mentioned composite materials,gave quantitative description to the particles,second phases,and the evolutionary process of grain size of the composite materials,and analyze the influence of FSP paths,content of reinforced particles and second phase change to the microstructure and performance of FSP composite materials.The morphology and distribution of endogenetic Al3Ti particles are related to the reaction system,melt temperature and matrix alloy components.When the melt was stirred for 15min at 750?,and the Ti-Al system didn't have complete reaction,and we also found in the matrix that the core-shell structure of Ti that was wrapped by Al3Ti resultant layer in addition to Al3Ti,indicating that the Al3Ti was formed through solid-liquid interfacial reaction mechanism;under the same conditions,the K2TiF6-Al system had complete reaction,and the Al3Ti nucleated and grew at villiaumite/alumimum bi-layer melt interface mainly through dissolution-precipitation reaction mechanism.For the K2TiF6-Al system,when the melt temperauter was between 700 and 800?,the Al3Ti was block-shaped(10?20?m)and distributed in the grains;however,at 900?,Al3Ti was rod-shaped,with the length-diameter ratio was as high as 10-20,and presented transgranular distribution;the water quenching test revealed the evolutionary process of Al3Ti morphology,i.e.,near-spherical(900?)?block-shaped(850?)?stub-shaped(800?)?rod-shaped(750?),proving that the morphological change of Al3Ti mainly occurred during the solidification process of high-temperature melt.In the 6vol.%Al3Ti/A356 composite materials prepared with the K2TiF6-Al system,only a small amount of Al3Ti was found in the grains,most of which were intertextured with Si to form clusters and distributed at grain boundaries/dendritic arms.This was because that the viscocity of the Al-Si alloy melt decreased,making the Al3Ti particles repelled by solid-liquid interface,and finally segregated into clusters.With the increase of FSP paths,the microstructural evenness of Al3Ti/A356 composite material increased constantly,and in the meantime,the size of Si and Al3Ti particles decreased.The morphological change of Si was determined by the thermomechanical effect,and the accumulated plastic strain continuously crushed the eutectic Si and the repeated thermal cycles prompted the precipitation of micro/sub-micro Si from in the grains;the particle corners of Al3Ti were passivated mainly under the effect of stress field,and the particle diameter was refined to 6.9?m after FSP-4p.With the increase of the volume fraction of Al3Ti and Si,the particle/matrix interface had increasing amount of mobile dislocation sources,and the dynamic recrystallization accelerated.In the meantime,these particles supressed the growth of recrystallization grains,and the size of the matrix grains can finally be refined to 0.8?m.The strength and elongation of FSP-Al3Ti/A356 composite materials improved with the increase of paths,and the main reinforcing mechanism includes:load transfer,refined crystalline strengthening,Orowan strengthening and CTE strengthening.At FSP-1p,Orowan strengthening made the biggest contribution to the yield strength,and with the increase of paths,the refined crystalline strengthening mechanism becomes the main source of contribution,and the Orowan strengthening effect reduced,which was mainly due to the decrease of nano Si particles in the grains.1?3vol.%ZrB2/6061Al nano composite materials were successfully prepared using the K2ZrF6-KBF4-Al system,and the endogenetic ZrB2 particles were averaged to be 130±50nm in size,and in polygonal or near-hexagonal shape,and they were evenly distributed on the matrix in the form of flocculent clusters.The appearance of clusters was related to the reaction paths—the 600rpm powerful stirring caused Al to directly react with the emulsifying salt droplets,and the ZrB2 precipitated and grew from the nucleation of the droplets to finally form nano particle clusters.The morphology of clusters was determined by the volume fraction of ZrB2.When the ZrB2 content was ?2vol.%,Class ?(<10?m)and Class ?(20-60?m)clusters appeared,which were respectively found in the grains and at the grain boundaries/dendritic arms.However,when the ZrB2 content was increased to 3vol.%,Class ?(?120?m)clusters appeared in the grains.Among them,the Class ? clusters were strengthened and plasticized through dispersion strengthening and plastic glide band;the Class ?clusters were mainly strengthened and plasticized through refined crystalline strengthening and equal-axis ductile voids;and Class ? clusters were prone to cracks due to stress concentration,which had adverse influence on plasticity,but they could be strengthened through load transfer.Special stratified structure appeared at the stir zone after FSP-1p,and this was closely related to the rheological behavior of the material which was mainly determined by the geometrical shape of the stirring pin.Where,the thread grooves could effectively crush the clusters and disperse nano particles to form a homogeneity zone.With the increase of FSP paths,the structural homogeneity of 2vol.%ZrB2/6061A nano composite materials improved constantly,as indicated by the expanding homogeneity zone and grain refinement;and the stiffness,strength and elongation also increased correspondingly.Since the dispersed nano particles can effectively pin dislocations and sub-boundaries,the grain size of FSP-4p composite materials reduced with the increase of the volume fraction of ZrB2.In the meantime.the texture intensity of B fibers weakened,showing that the composite materials are mainly subject to the shear force generated by the threads of the stirring pin.K2ZrF6-KBF4-Al system was adopted to successfully prepare 2vol.%ZrB2/2024 A1 nano composite materials,and the endogenetic ZrB2 particles are evenly distributed on the matrix in the form of clusters.Since 2024Al had a high content of solute elements,the cluster structure was complicate.There were clusters of ZrB2 particles wrapped by Cu and Mg,with dispersed ZrB2 nano particles around.The morphology and distribution of ZrB2 clusters in the composite materials were improved by controlling the cooling rate,which were very effective in constraining the grain growth.When the cooling rate was 68.4?/s,the?10?m clusters intertextured with ?+S phases at the interphase boundary to form fine cellular dendritic crystals.With the decline of cooling rate,the clusters were pushed to the grain boundaries/dendritic arms where local segregation occurred,and therefore,the restritive effect on the grain grwoth was weakened,and the size of corresponding dendrites and dendrite arms was enlarged.Due to the T6 heat treatment,the ?+S phases were re-dissolved and precipitated into the semi-coherent S' phase,and the morphology of the clusters remained unchanged.The yield strength of the composite materials and the matrix alloy were insensitive to the cooling rate because that the second phase(?+S)and clusters at the grain boundaries/dendritic arms didn't have direct effect to the in-grain dislocation multiplication and movement.With the increase of cooling rate,the static toughness of the matrix alloy increased,but that of the composite materials didn't show impovement until 68.4?/s.After T6 heat treatment,the precipitated phase improved the toughness and plasticity of composite materials and matrix alloy to different degrees,while the clusters had significant strengthening and plasticizing effect on the matrix,as indicated by the improvement on static toughness.For 2vol.%ZrB2/2024Al composite materials,even stir zone can be obtained rapidly through water-cooled continuous FSP-2P,and its rheological behavior was different from that of ZrB2/6061Al and mainly determined by the intrinsic properties of matrix alloy(such as thermo-plasticity,high-temperature strength and friction coefficient,etc.)The quantity of ZrB2 nanodispersion particles and Al2Cu precipitated phases in the FSP composite materials depended on the initial status of the sheet material.The part of ? phase in the casting was broken into particles of 1-2?m in size,and part of them were re-dissolved and precipitated into Al2Cu nanophase;however,the T6 state precipitated completely in the form of Al2Cu and ?" transient phase.The latter had the smallest matrix grain size and highest proportion of LAGBs(36.8%)due to its possession of increased amount of pin dislocations and(sub)grain boundary mass points.The strength of the FSP composite materials with initial status of T6 state was obviously higher than those whose initial state is casting state,but there was not much difference in elongation.Compared with FSP matrix alloy,the FSP composite materials had significantly increased strength but the elongation decreased slightly. |
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