Transition Zone (Phase) On The Interface Of TiB₂,TiC In Aluminum Alloys

The behaviors of TiB₂ and TiC particles in pure aluminum or aluminum alloy melt and the nucleation mechanisms were studied by using eletron-probe micro-analyzer (EPMA), X-ray diffraction (XRD), differential scanning calorimeter (DSC), optical micrograph (OM) and scanning electron microscope (SEM), etc.The results indicate that there are obvious deposit after TiB₂ or TiC particles added into the pure aluminum melt, and the factual depositing speed is much faster than the theoretical one. The viscosity of the pure aluminum melt increases greatly after addition of a little amount of high-effective Al-Ti-B(C) master alloy. And the viscosity has a corresponding relationship with refinement efficiency, i.e. theα-Al grain size of solidified sample is finer when the viscosity of the melt is higher, indicating that viscosity can be used to approximately estimate the refinement efficiency of Al-Ti-B(C) refiners in production to a certain extent, which also make it possible for controlling directly the final refinement efficiency in the state of melt. Furthermore, a little AT is enough for the aluminum melt to nucleate after the addition of a little amount of Al-5Ti-1B master alloy. It is found that the above phenomena all have a relationship with the change of the melt structure.TEM analysis shows that there is a Ti-rich transition zone on the interface between TiB₂ or TiC particles and aluminum matrix in theα-Al grain centre in the rapidly solidified ribbon. Since the rapidly solidified ribbon can conserve the instantaneous structure of the melt mostly, the Ti-rich transition zone in the solidified sample originates from the Ti-rich transition zone in the melt. Based on this, a new nucleation mechanism called as "Ti-rich transition zone on the interface of TiB₂ or TiC particles" was proposed, which explained many important phenomena in the refinement process.Based on "Ti-rich transition zone on the interface of TiB₂ or TiC particles" mechanism and the heredity of Al-Ti-B master alloy, the mathematics expression of"Effective heredity gene count" is improved as follows: N_eff^h =δ_n[TiB2]N_TiB2^h or N_eff^h =k(e^{1-δ_h[TiAl3]}-1)N_TiB2^h . Therefore, the "effective heredity gene count" of Al-Ti-Bmaster alloy can be estimated based on the distribution and amount of TiB₂ particles and the morphology and size of TiAl₃ compounds before refinement experiment.The thermal stability of TiC and TiAl₃ compounds is various in the presence of Si, Mg or Cu element in aluminum melt, which results to different effects on Ti-rich transition zone on the interface of TiC particles: Si reacted with TiC to produce Al₄C₃ or SiC, which packed around the TiC particles or replaced them completely. While TiAl₃ reacted with Si forming a new TiAl_2.xSi_0.4 compound, therefore there is lack of active Ti in the melt. These both resulted in the serious destruction of Ti-rich transition zone on the interface of TiC particles, which caused the serious fading. Mg element has no influence on the stability of TiC and TiAl₃, so a little TiC particles reacted with Al to form Al₄C₃ particles with the holding time prolonged, which caused the destruction of little Ti-rich transition zone on the interface of TiC particles, with a slight fading. However, TiC particles were relatively stable in the presence of Cu, while TiAl₃ phase reacted with Cu to produce a new phase Ti(Al,Cu)₂, which is responsible for the destruction of partial Ti-rich transition zone on the interface of TiC particles. These indicate that the good refinement efficiency will occur only when TiC particles are stable, together with many active Ti atoms in melt.The phosphorous modification effect can be significantly improved and the size of the primary Si can be refined from 40μm to approximately 20μm only after the addition of 0.2%Al-5Ti-1B or Al-8Ti-2C master alloy in the P modified near-eutectic Al-Si alloy. EPMA shows there is an A1P transition phase around or near to TiB₂ compound in the center of primary Si, while there are A1P transition phases around or near to Al₄C₃ compound in the center of primary Si owing to the instability of TiC.Crystal lattice correspondence analysis shows that TiB₂ or Al₄C₃ has a good lattice matching coherence relationship with A1P, and the disregistry of (112) crystal face of TiB₂ and (442) crystal face of AlP is 7.27%, while that of (110) crystal face of Al₄C₃ and (311) crystal face of AlP is just 5% calculated by Turnbull-Vonnegut equation. So AlP crystals can easily nucleate on TiB₂ or Al₄C₃ interface and have a like-peritectic coupling with them to form quantities of coupling compounds. Because the amount of TiB₂ or Al₄C₃ particles is large, the coupling particles are abundant, which provides a large number of nuclei for the primary silicon, so the quantity of primary Si increases significantly, and accordingly the size decreases. This is a proposed mechanism for the improvement of A1P modification efficiency in the near-eutectic Al-Si alloy after the addition of a small amount of TiB₂ or TiC particles.