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.
Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys
A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB2, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB2 particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.
Phase formation during mechanical alloying in the Ti–Si system
Abstract Mechanical alloying of Ti–Si powder mixture or TiH 1.924 –Si powder mixture was performed by high-energy ball milling, in which the Ti to Si mole ratio was 1:2. The phase formations in both mixtures during milling were investigated by X-ray diffraction. Mechanical alloying a mixture of elemental powders led to the formation of TiSi 2 equilibrium compound, while when starting from Si + TiH 1.924 , a metastable phase was formed. The metastable phase completely transforms to TiSi 2 when the mechanically alloyed powder is annealed at temperatures above 800 °C. Two different pathways of phase transitions during mechanical alloying are attributed to the existence of hydrogen atoms, i.e., when starting from Si + TiH 1.924 compared to Si + Ti, the kinetics for the formation of the TiSi 2 intermetallic compound slows down. The kinetics related to the different phase transitions depending on the starting powder mixture are discussed.