The Study On The Synthesis And Grain Refinement Of Al-Ti-B-C Master Alloy

In this thesis, master alloys were prepared by high-energy ultrasound treatment process at low temperature and induction melting process at high temperature respectively. High purity aluminum, sponge Ti and B4C particles were used as reactants, in accordance with the composition ratio of Al-5Ti-1B4C. And the materials analysis facilities, such as X-ray diffractometer, scanning electron microscopy and energy dispersive spectroscopy, were used to observe and identify the existence and morphology of production phases. The effect of different process parameters on the morphology and size distribution of the TiAl3, TiB₂ and TiC particles in master alloys were carefully studied. The thermodynamic and kinetic mechanisms of the Al-5Ti-1B4C reaction system were analyzed by the calculation and experimental results. Finally, the grain refinement performance of the master alloy synthesized by different process were well researched and the refining mechanisms of Al-Ti-B-C master alloy were discussed. The main conclusions can be summarized as follows:â‘´The thermodynamic calculations showed that the reaction, 3Ti + B4C = 2TiB₂ + TiC, has the lowest enthalpyΔrH0 and Gibbs free energy changeΔrG0 in the Al-5Ti-1B4C system. So, this reaction may react in the system spontaneously.⑵There was no evidence of the reaction between Ti and B4C in the high energy ultrasound treatment process at 850℃. Such master alloys synthesized by ultrasound treatment are only composed with Al and TiAl3. The block TiAl3 phase has a average size less than 20μm. With the increase of the times and duration of ultrasound treatment, the block TiAl3 tend to be finely dispersed in the matrix, and become a spherical shape.⑶The wettability of Al/B4C interface is improved significantly by rising reaction temperature. The Al-Ti-B-C quaternary master alloys prepared by induction melting process at 1400℃and 1600℃are composed with Al, TiAl3, TiB₂ and TiC phases. The TiB₂ has plate-like shape with a length of 20μm, the TiC size spans from 2μm to submicron level, and the length of flake TiAl3 increased with the rising reaction temperature. The Al/B4C interface reaction, 3[Ti] + B4C =2TiB₂ + TiC, is the key mechanism to generate TiB₂ and TiC nucleating substrates.â‘·The Al-Ti-B-C quaternary master alloys containing TiAl3, TiB₂ and TiC had an excellent refining performance on commercial pure aluminum, compared with the master alloys only containing TiAl3. The mean grain sizes of commercial pure aluminum were diminished to 189μm and 192μm separately by adding 0.2wt% master alloys synthesized by induction melting process at 1400℃and 1600℃. After holding for 60 min, the diameter of commercial pure aluminum grain was stable around 220μm.⑸The combination effect of heterogeneous nucleation caused by TiB₂ and TiC particles and the grain growth restriction caused by solute Ti is the main grain refinement mechanism of the quaternary Al-Ti-B-C master alloy. In the nucleating process,α-Al starts to form at the surface of TiB₂ and TiC substrates which have a good wettability and lattice match with Al. In the later grain growth stage, the growth inhibition ofα-Al caused by solute Ti is the dominant effect. So, the refinement mechanism of Al-Ti-B-C mater alloy is a coupling effect of the carbon-boride theory and the solute theory.