The Study Of Nucleation And Growth Mechanism About TiC And TiAl₃ With Commercial Aluminium

Grain refinement has extremely important significance to the aluminium alloy, but up to now,people does not still reach consensus on the mechanism,The aluminium and their alloy of the refinement mechanism is very complicated,still is in the stage probing,developing,and much problem needed to solved.The effects of the holding time of remelting on the microstructure and grain refining performanc of Al-Ti-C master alloy with the same composition and different structural morphologies prepared by thermal explosion method of self-propagating high-temperature synthesis(SHS)in aluminum liquid with Al,Ti,C powder have been observed.The results show that two kinds of the Al-Ti-C are composed by TiAl₃,TiC and Al.The holding time of remelting has an remarkably impact on the Al-Ti-C microstructure.With the extension of holding time of remelting,TiAl₃ aggregated and grown up,and TiC gathered,but TiAl₃ and TiC have a strong performance of stability and there is no other miscellaneous such as Al₄C₃ in the process of remelting.The Al-Ti-C master alloys which have been remelted still have a certain grain refining capacity,which is attributed to the degree of microstructure heredity.Al-TiAl₃ and Al-TiC grain refiners were directly synthesized by thermal explosion method of self-propagating high-temperature synthesis(SHS)in aluminum liquid in this paper.The second phase particles TiC and TiAl₃ on their refining performances for commercially aluminum were researched in Al-TiC and Al-TiAl₃ mast alloys.The results show that.TiC and TiAl₃ phase can alone as theα-Al nucleation phase,the two nucleation capabilities are poor,but TiC particle exhibits better nucleation and higher resistance to grain refining fading than TiAl₃ phase. When the second phase particles TiC and TiAl₃ common as theα-Al nucleation phase and have a relatively quantity,they demonstrated better nucleation and higher resistance to grain refining fading.The experiments show that the ratio of Ti/C is 1.8:1 was the best component on their refining performances in the seven groups grain refiner.The conclusion is the second phase particles TiC and TiAl₃ can not be separated from the Al-Ti-C grain refiner,and relatively quantity of TiC and TiAl₃ phases are the guarantee of excellent grain refinement refining capacity.Theα-Al core and distribution of TiC in aluminum melt in the presence of solute TiAl₃ was stdied by SEM,XRD,EPMA and other analytical tools when the Al-TiC and Al-TiAl₃ master alloys refining industry aluminum,and the nucleation mechanism of Al-Ti-C grain refiner were discussed.The results show that:TiC crystallites alone do not nucleateα-Al effectively,most of them was pushed into the grain boundaries by theα-Al dendrites at the end of local solidification..On the other hand,When dissolved TiAl₃ was present,most of the TiC particles were found to occur within the grains and nucleation occurs on the particles and there is a "Ti transition zone" between TiC and aluminum interface.In the present study,precipitation of second phases for Al-Ti-C master alloy were studied when the Al-TiC grain refiner was introduced into commercial aluminium melts alone,and in the presence of Al-TiAl₃.The results show that TiAl₃ has an important impact on precipitation of TiC particles.Most TiC particles deposit at the bottom of Al samples when only present TiC particles,while when TiAl₃ was present,the deposition of TiC particles was changed,and becomes more and more slow.Even after long time,there was only a small amount of precipitation in TiC. The refinement effect fading of Al-Ti-C master alloy is caused by the TiC precipitation,stirring TiC particles can re-dispersion distribution,increase its refining effect.Based on the result of this paper,the refining mechanism of Al-Ti-C was built.Al-Ti-C grain refiner of nucleation mechanism is TiAl₃ release Ti atoms in aluminum melt,Ti atoms congregate to the TiC surface based on the activity gradient of Ti in the melt and TiC inoculants,and the quantity of Ti atoms on the interface between TiC and aluminum melt have a remarkable increase.A Ti-rich zone forms around TiC particles,ultimately forming "TIC/aluminum interface Ti transition zone".Ti transition zone formation,not only improve the structure of adaptability and improved wettability between TiC and Al,but also reduces the surface tension of TiC particles,which making a lot of TiC particles can be uniformly distributed in the aluminum melt,which becomeα-Al core and heterogeneous nucleation in the aluminum melt solidification.