Grain refinement of aluminium and its alloys is common industrial practice. The field has been extensively investigated by many workers over the past 50 years, not only to develop efficient grain refiners for different aluminium alloys, but also to achieve an understanding of the mechanism of grain refinement. The present review confines itself to the literature on grain refinement by heterogeneous nucleation and alloying. Initially, the fundamentals of grain refinement by inoculants are outlined. The types of grain refiner, Al-Ti-B master alloys in particular, and their methods of manufacture are next discussed. The grain refining tests to assess the efficiency of the grain refiners and the grain refining behaviour of aluminium alloys are also discussed in brief. The performance of a grain refiner, as well as the response of an aluminium alloy to grain refinement, is influenced by the microstructure of the grain refiner as controlled by the process parameters involved in its preparation and the alloying elements present in the aluminium alloy. The roles of these factors, and particularly the roles of poisoning elements such as Si, Cr, Zr, Li, are reviewed. The paper also reviews the mechanisms of grain refinement, the fading and poisoning phenomena, and the trends in the development of new grain refiners for aluminium alloys containing poisoning elements.

Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying

Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum alloys. Recently, there has been a shift in understanding of the grain-refinement paradigm from purely being concerned with the nucleation event, called here the “nucleant paradigm,” to also being concerned with the effect of solute elements, or, the “solute paradigm,” on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement.

Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a review of the literature

Grain structure is an important and readily observable feature in cast aluminium alloys. Three different types of grain morphology are possible, namely, columnar, twinned columnar, and equiaxed. Inoculants in the form of master alloys are used to promote the formation of a fully equiaxed grain structure and this is termed grain refinement. Initially, fundamental aspects of solidification are outlined in order that the principles of grain refining using master alloys can be understood. Techniques for the commercial production and testing of common Al–Ti-based master alloys are then discussed briefly. The exact mechanisms by which grain refinement occurs are not yet fully understood and experimental and theoretical studies on the problem are critically reviewed with particular emphasis on (a) the role of solute titanium, (b) the thermodynamics of Al–Ti-based alloy systems, and (c) the nature of heterogeneous nuclei. Finally, current and future trends in the use of grain refining alloys are summarised.

Grain refining of aluminium and its alloys using inoculants

We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion...

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Criteria for developing castable, creep-resistant aluminum-based alloys - A review

Despite the commercial importance of grain refinement and the volume of scientific studies on this topic, its mechanism is still unclear. There are several theories as to how and why commercial grain refiners (Al-Ti and Al-Ti-B) work, but careful analysis shows that no clear consensus has yet emerged. In the present study, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique. Experimental findings indicate that TiB2 crystallites alone do not nucleate α-Al. On the other hand, in the presence of dissolved Ti even below the peritectic level, an interfacial TiAl3 layer is formed at the TiB2/melt interface which subsequently nucleates the α-Al. The theoretical and practical implications of grain refinement phenomena are discussed in the light of the present experimental findings. A ‘duplex’ nucleation mechanism is proposed based on solute segregation to the substrate/melt interface.

Mechanism of grain refinement in aluminium

The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl3 + (Al, Ti)B2 was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB2 and A1B2 form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium alloys expands the field of primary crystallization of TiAl3 toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found only in alloys in which TiAl3 is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum, equilibrium is seldom attained and in commercial practice grain refinement by TiAl3 is found also outside its equilibrium field of primary crystallization.

J. A. Marc Antonio

Papers

Grain refinement in aluminum alloyed with titanium and boron