Performance Comparison of Al–Ti Master Alloys with Different Microstructures in Grain Refinement of Commercial Purity Aluminum
Three types of Al–5Ti master alloys were synthesized by a method of thermal explosion reaction in pure molten aluminum. Performance comparison of Al–5Ti master alloy in grain refinement of commercial purity Al with different additions (0.6%, 1.0%, 1.6%, 2.0%, and 3.0%) and holding time (10, 30, 60 and 120 min) were investigated. The results show that Al–5Ti master alloy with blocky TiAl3 particles clearly has better refining efficiency than the master alloy with mixed TiAl3 particles and the master alloy with needle-like TiAl3 particles. The structures of master alloys, differing by sizes, morphologies and quantities of TiAl3 crystals, were found to affect the pattern of the grain refining properties with the holding time. The grain refinement effect was revealed to reduce markedly for master alloys with needle–like TiAl3 crystals and to show the further significant improvement at a longer holding time for the master alloy containing both larger needle–like and blocky TiAl3 particles. For the master alloy with finer blocky particles, the grain refining effect did not obviously decrease during the whole studied range of the holding time.
Observations on the microstructure and performance of an AlTiC grain-refining master alloy
Abstract AlTiC grain-refining master alloys are of increasing importance in aluminium casting because they are believed to introduce a smaller volume fraction of insoluble particles into the melt than conventional AlTiB master alloys are. The main microstructural features of an Al-6wt.%Ti-0.02wt.%C master alloy have been examined using X-ray diffraction metallic phases were identified, namely titanium aluminide and titanium carbide. The titanium aluminides were found to contain small amounts of dissolved vanadium and to possess a range of morphologies from well faceted to rough and irregular. A combination of microanalysis and electron diffraction was used to show that clusters of submicron particles, rich in Ti but with virtually no Al, were TiC crystals. Individual particles were found to be single crystals with an octahedral morphology. A simple thermodynamic analysis of the stability of TiC in the Al-rich corner of the AlTiC phase diagram is presented which confirms the stability of TiC during master alloys synthesis. Using standard test procedures, the AlTiC alloy proved to be an effective grain refiner when added to commercial purity aluminium, and it is proposed that undissolved TiC particles were most likely to have been the heterogeneous nuclei.
Effect of interfacial energies on heterogeneous nucleation
Abstract From experimental evidence it has previously been shown that heterogeneous nucleation is non-reciprocal and therefore that the balance of interfacial energies is a controlling factor. If σNL > σNS + σSL the nucleus S can grow from the liquid L on the substrate N. When nucleation takes place the contact angle θ is small and there is epitaxy between the nucleant and the nucleus that reduces the σNS to very low values so that, as a first approximation, heterogeneous nucleation can be expected when σNL > σSL. Interfacial energies σSL are roughly proportional to the melting point, and high melting point phases usually nucleate lower melting phases. The contrary may happen if the low melting phase has a low symmetry crystal. Nucleation can take place on high index faces of this crystal because, although its average σNL is lower than the σSL of a higher melting point, high symmetry crystal, the σNL of its high index faces may be higher than the σSL.MST/845