Grain refinement of Al and its alloys through inoculation is a common practice in commercial foundries because it is the most convenient, practical and low-cost approach to achieve fine as-cast microstructures. Grain refinement not only delivers enhanced casting soundness and mechanical properties, but also facilitates subsequent mechanical forming processes, and ensures consistently better performance of the final products. Over the past few decades, the mechanism underlying grain refinement in cast Al alloys has been comprehensively studied and many theories/models have been proposed. It is now generally agreed that two essential components are required for effective grain refinement, i.e. numerous potent nuclei and sufficient effective solute. Although most observations of grain refinement have been successfully explained in terms of these two aspects, a number of important details in grain refinement are still disputed, as no universally accepted mechanism can fully satisfy all the laboratory and industrial observations. One of the major problems is a lack of understanding of the role of peritectics in grain refinement of Al alloys, since previous studies focused on the Al-Ti-B and Al-Ti-C systems. Therefore, further investigation is required to clarify the grain refinement mechanisms underlying the peritectics. It is believed that the outcomes will provide new insight into the understanding of grain refinement mechanisms in cast Al alloys.
In this thesis the grain refinement of a series of peritectic alloys including Al-Ti, Al-V, Al-Zr and Al-Nb alloys, together with a number of eutectic alloys including Al-Cu, Al-Mg, and Al-Si, has been studied in order to provide in-depth understanding of the role of peritectics in grain refinement of Al alloys. It is observed that peritectic alloys are generally associated with significant grain refinement whereas only slight grain size reduction is obtained in eutectic alloys. Using an analytical approach by plotting the measured grain size data against the inverse growth restriction factor, 1/Q (Q is a quantitative measurement of the growth restriction effect), the mechanisms underlying grain refinement in the peritectic and eutectic alloys have been elucidated. It is found that the limited grain refinement in eutectic alloys is mainly attributed to the lower solute segregating power of eutectic-forming solutes, which causes less constitutional supercooling. However, peritectic-forming solutes not only induce grain refinement by generating larger constitutional supercooling but, more importantly, they introduce numerous nucleant particles which promote grain refinement via enhanced heterogeneous nucleation.
To further understand the factors that govern the heterogeneous nucleation on nucleant particles, both the crystallography and the particle size distributions of pro-peritectic compounds, Al3Zr and Al3Nb, which are potential nucleant particles, in cast Al-Zr and Al-Nb alloys, have been comprehensively investigated. The crystallographic features between these pro-peritectic particles and Al matrix, including atomic matching and orientation relationships, have been evaluated using the edge-to-edge matching (E2EM) model. The model predictions have then been experimentally verified using Electron Backscatter Diffraction (EBSD) in Scanning Electron Microscopy (SEM) and Convergent Beam Kikuchi Line Diffraction Patterns (CBKLDP) in Transmission Electron Microscopy (TEM). It is found that the pro-peritectic Al3Zr and Al3Nb particles have favourable crystallographic matching and possess reproducible Orientation Relationships (ORs) with the Al matrix, indicating the high potency of such particles as effective nucleants. Furthermore, the size distributions of pro-peritectic Al3Zr and Al3Nb particles have also been examined and analysed. The results agreed well with the free growth model. This implies that some of the in-situ formed pro-peritectic Al3Zr and Al3Nb particles have the right size to promote heterogeneous nucleation.
In order to further understand the distinct difference between peritectic-forming and eutectic-forming solutes in terms of their grain refinement efficiency in Al alloys, the thermodynamic driving force for solidification of different binary Al alloys has been calculated. Variations of the Gibbs free energy difference between solid and liquid phases of Al alloys with the solute content and supercooling have been investigated. It is found that, at the same level of Q value and supercooling, the solidification driving force of peritectic alloys is greater than that of eutectic alloys. Compared with the eutectic alloys, the higher solidification driving force in peritectic alloys implies that peritectic alloys are not only associated with a higher primary nucleation rate at the same thermal supercooling, but also have a higher secondary nucleation rate within the constitutional undercooling zone, even at the same growth restriction level (the same Q-values). Therefore, smaller grains are normally obtained in peritectic alloys.
In summary, the investigations presented in this thesis have advanced the understanding of the role of peritectics in grain refinement of Al alloys. Based on the results, it is concluded that the grain refinement in cast Al alloys is a result of the combined effect of nucleant potency, constitutional supercooling, peritectic reaction and thermodynamic driving force. No single factor can define the grain refining efficiency. As an effective grain refiner, the relevant solute added in Al melt must fulfil the following criteria: (1) the solute must be able to quickly build up a large constitutional supercooling, which is expressed by the growth restriction factor; (2) the solute must provide a large solidification driving force for a given constitutional supercooling; (3) the intermetallic compounds formed between the solute and Al are potent heterogeneous nucleation sites (to be the active nucleant). For the third criterion, it is essential to fulfil the crystallographic requirements of effective nucleants in terms of the Edge To Edge Matching (E2EM) model. Generally such solutes are peritectic-forming solutes with Al.