Research On In-situ Reaction Mechanism Of TiAl₃ Phase With Various Aspect Ratios In Aluminum Matrix Composites

The Al-12Si based in-situ composites with different titanium raw materials were prepared by melt contact reaction method.The formation mechanism of in-situ TiAl₃phase in Al-Si alloy was studied by changing the parameters during in-situ reaction system process.The effect of adding alloy element B to the in-situ reaction system on the formation mechanism of TiAl₃phase in Al-Si melt was discussed.The microstructure of the composite material and the phase composition,morphology,size and distribution in the melt of the in-situ phase TiAl₃were analyzed by the optical microscope?OM?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and data processing software,the main conclusions are as follows:The morphology and size of TiAl₃ in Al-12Si composite prepared by different titanium raw materials in Ti-Al in-situ reaction system are obviously different.The size of in-situ TiAl₃phase prepared by Ti powders is smaller than that prepared by sponge titanium particles.The TiAl₃phase is acicular,and its aspect ratio is reduced from 45 to 16 when Ti powder is used as the raw material,the TiAl₃phase is long strip and massive,and its aspect ratio is reduced from 65to 25 when sponge titanium is used as the raw material.The synthesis mechanism of in-situ TiAl₃in Al-12Si composites is as follow:the in-situ Al-Ti intermetallic compound formed in the composite contains Si when the matrix alloy contains Si,which is solid solution in TiAl₃crystals,that Si atoms replace Al atoms into TiAl₃phase by substitution,when Si atoms diffuse into the TiAl₃crystal structure,and finally form TiAl Si ternary phase,and TiAl Si phase still maintains TiAl₃crystal structure.The morphology of in-situ TiAl₃ phase are short rod-liked by adding B element to Al-12Si composites.The size of TiAl₃phase increases with the contindeous adding of B,and the aspect ratio increases from 3.5 to 4.3,but the size of TiAl₃is still significantly smaller than that without B.The formed Ti B₂is distributed in a polygon in the matrix.The Ti B₂particles is uniform and dispersed with the increase of B content.Based on the thermodynamic calculation and experimental results analysis of in-situ powder system?Ti-Al-B?,the effect of the synthesis mechanism of in-situ TiAl₃phase in composites by adding B to the in-situ reaction system?Ti-Al?is as follows:Ti B₂has a larger forming ability in the melt than TiAl₃,so B and Ti react preferentially,consuming part of Ti in melt,and then excess Ti in actual reaction continues to bond with Al to form TiAl₃.Because B and Ti are combined rapidly in the melt,the purpose of restraining more Ti from reacting with Al to form TiAl₃is achieved,so the preferential formation of Ti B₂effectively hinders the formation and growth of TiAl₃.The effects of Si content in the Al-Si alloys modified by two modifiers on the in situ TiAl₃synthesis mechanism were investigated.The experimental results show that the content of Si in the matrix has a significant effect on the size of TiAl₃.The in-situ phase TiAl₃in Al-x Si?7,12,24,28wt.%?composite is long strip shape under the condition of phosphate modification.When Si?7wt.%,the TiAl₃phase aspect ratio is 16.5;when Si=12wt.%,and the TiAl₃aspect ratio is down to 13.5;when Si=24wt.%,the TiAl₃aspect ratio is 13.0;when Si=28wt.%,the aspect ratio of TiAl₃is the lowest,which is 8.0.Under the condition of Al-P master alloy modification,TiAl₃morphology in Al-7Si composite is block,aspect ratio is 4.TiAl₃in the remaining 3 kind of Si-content alloys is uniformly distributed,the morphology is strip-liked.The ratio of length to diameter of TiAl₃decreases from 13.5 to 7.5 with the increasing of Si content.Through the analysis of the microstructure of hypoeutectic,eutectic and hypereutectic Al Si alloy,the reason of the influence of Si content on the synthesis mechanism of in-situ TiAl₃is obtained:when Al reaches the Al-Ti reaction layer in free state,the contact between Al and Ti can directly formed TiAl₃and Al Ti.Because the formation energy of TiAl₃is less than that of Al Ti,TiAl₃is first generated in the melt.When TiAl₃is generated and then spread out of the Al-Ti reaction layer into the Al liquid,the free Ti in the melt adheres to the generated TiAl₃and contineous to grow.During the growth of TiAl₃,the melt temperature continues to rise,and more Si atoms diffuse into the TiAl₃crystal structure due to the increase in Si content,diluting the concentration of Ti at the growth front of TiAl₃,thus inhibiting the growth of TiAl₃along the length direction.However,due to the high content of Si in hypereutectic Al-Si alloys,the percentage of Si atoms in TiAl₃increases as Si solid dissolves into TiAl₃crystal,and the resulting Al-Ti intermetallics become more stable TiAl Si ternary compound.Adding 0.25wt.%mixed RE?La-Ce?to Al-12Si composites with different mass fraction?4,6wt.%?Ti powder,the in-situ formed phases include TiAl₃and Al Ce,La Si rare earth phases,and the morphology of rod-liked TiAl₃changes from long to short after the action of mixed RE.After adding 0.25wt.%La-Ce,the aspect ratio of TiAl₃decreased,and that of TiAl₃decreased from 17 to 11 in 4wt.%Ti powder system;that of TiAl₃decreased from 22 to 10 in 6wt.%Ti powder system.The effect of adding mixed RE to the matrix alloy on the in-situ TiAl₃phase synthesis mechanism is that the RE phase can inhibit the growth of TiAl₃and lead its structure tiny.The reasons that hinder the growth of TiAl₃phase are mainly due to the addition of RE elements.Firstly,the Ti powder in the melt is preferentially combined with Al to form TiAl₃.The TiAl₃gradually melts again as the melt temperature continues to elevate,and Al and the mixed RE added combined to generated RE phases,which hinders the growth of TiAl₃.Secondly,the RE phase are easy to precipitate at the interface of?-Al and TiAl₃particles,thereby suppressing the segregation and aggregation of TiAl₃around the?-Al matrix,and obtaining a small amount of TiAl₃ phase with small size.