Nucleant Particles Research Articles

The effect of the size distribution of inoculant particles on as-cast grain size in aluminium alloys

A model based on free-growth control of grain initiation can quantitatively predict the grain size in aluminium alloys inoculated with commercial refiner as a function of refiner addition level, solute content in the melt and cooling rate. This model is applied to predict the performance of refiners with hypothetical size distributions of nucleant particles. The distributions are log-normal, like those in commercial refiners, but the mean particle diameter and the relative spread of the distribution are varied to map the performance of the refiner in giving minimum grain size, maximum efficiency, or minimum sensitivity of grain size to cooling rate. The optimal nucleant particle size distribution is a compromise between these aims and depends on the alloy and processing conditions. This mapping can assist in choosing between available refiners or in developing improved refiners. Existing commercial refiners are highly effective, but their nucleant particle size distribution leaves room for improvement.

Instability of TiC particles in aluminium melts inoculated with an Al-Ti-C grain refiner

Microstructural observations on super purity aluminium inoculated with an Al-Ti-C refiner confirm that the nucleant TiC particles present in the refiner are thermodynamically unstable on holding in the melt. The rate of conversion of TiC to Al4C3 is characterised using measurements of the titanium content of the aluminium; the conversion is slow enough not to impede grain refinement under normal conditions, but it is strongly temperature dependent. The mechanism of the conversion is examined and shown to be consistent with a progressive rather than a sudden fading of refiner performance. Based on estimates of the evolving size distribution of TiC particles in the melt, grain size predictions are made and shown to be in qualitative agreement with the fading of refinement seen experimentally.

The Variable Potency of TiB<sub>2</sub> Nucleant Particles in the Grain Refinement of Aluminium by Al-Ti-B Additions

The Variable Potency of TiB<sub>2</sub> Nucleant Particles in the Grain Refinement of Aluminium by Al-Ti-B Additions

Effects and mechanisms of grain refinement in aluminium alloys

Grain refinement plays a crucial role in improving characteristics and properties of cast and wrought aluminium alloys. Generally Al-Ti and Al-Ti-B master alloys are added to the aluminium alloys to grain refine the solidified product. The mechanism of grain refinement is of considerable controversy in the scientific literature. The nucleant effects i.e. which particle and its characteristics nucleate α-Al, has been the subject of intensive research. Lately the solute effect i.e. the effect of dissolved titanium on grain refinement, has come into forefront of grain refinement research. The present paper attempts to review the literature on the nucleant effects and solute effects on grain refinement and addresses the importance of dissolved titanium in promoting nucleation of α-Al on nucleant particles.

A model of grain refinement incorporating alloy constitution and potency of heterogeneous nucleant particles

A model for the determination of relative grain size is developed based on the assumption that nucleant substrates are activated by constitutional undercooling generated by growth of an adjacent grain. The initial rate of development of constitutional undercooling is shown to be equivalent to the growth restriction factor and is a useful approximation for the effect of solute on grain size when potent nucleant substrates are present. However, when the nucleants are of a poor potency then a calculation of the fraction solid necessary to develop the constitutional undercooling required for nucleation needs to be performed. Using the model, trends in grain size observed experimentally by the addition of titanium to pure aluminium and to a typical casting alloy, AlSi7Mg0.3, can be predicted. It was also found that summing the individual growth restriction factors of each solute element in a multi-component alloy can grossly overestimate the actual value of the growth restriction factor for the alloy.

The role of solute in grain refinement of magnesium

The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of grain refinement of magnesium alloys is included in this article to provide an update on research in this field.

Grain Formation in AlSi7Mg0.35 Foundry Alloy at Low Superheat

Hypoeutectic Al-Si alloys represent the most widely used alloy system for cast aluminium applications. This system has a unique behaviour with respect to grain formation where an increase in silicon content results in a transition to larger grain sizes after a minimum at an intermediate concentration. As a result of the already large solute content, grain refinement by solute additions is inefficient and nucleant particles from the common aluminium grain refiners are not as effective as in wrought alloys. However, casting conditions, such as a low pouring temperature, that promote the formation of wall crystals (ie. crystals nucleated in the thermally undercooled layer at or next to mould walls) are very effective in yielding a small grain size. This paper presents results of an investigation of the effect of low superheat and mould preheat temperature on grain size. It was found that pouring temperature controls the effectiveness of the wall mechanism while mould preheat has little effect until high preheat temperatures at which a large increase in grain size occurs. The observed changes in grain size are explained in terms of the balance between nucleation rate and survival rate of crystal nuclei resulting from changes in superheat and mould temperature.

Grain refinement of aluminum alloys: Part II. Confirmation of, and a mechanism for, the solute paradigm

In Part I of this article, the literature underpinning both the nucleant and solute paradigms was explained, and the validity of the paradigm shift toward the solute paradigm, as a more complete understanding of grain refinement, was presented. In this Part II, experimental work is presented which confirms the validity of the solute paradigm. TiB2 particle additions were found to refine the columnar zone of pure aluminum; however, an equiaxed structure was only observed when a small amount of titanium was added as solute. The potency of nucleant particles was confirmed by thermal analysis, which showed that additions of TiB2 to pure aluminium removed the nucleation undercooling. Upon the addition of more TiB2 particles and titanium as solute, the grain size continued to decrease until an apparent minimum grain size was achieved, past which little further refinement occurs. That the segregating ability of solute elements in general is essential for grain refinement, and not only that of titanium in particular, was confirmed by comparison of the Al-2Si and Al-0.05Ti systems. Finally, a mechanism of grain refinement is presented that incorporates both nucleant particles and solute segregation as essential for effective grain refinement. The solute is required to form a constitutionally undercooled zone in front of the growing solid/liquid interface to facilitate further nucleation on the substrates present. The potency of the nucleants dictates the probability of nucleation occurring for a given degree of constitutional undercooling.

Metallographic investigation of TiC nucleants in the newly developed Al-Ti-C grain refiner

For grain refinement of aluminium and its alloys, a new aluminium-based master alloy containing TiC nucleants was developed a few years ago, by reacting carbon with Al-Ti binary alloy melt. However, as the melt was held at the usual melting temperatures of below 1273 K, the resultant master alloy lost its grain-refining efficiency, and so this phenomenon was called the “poisoning” effect. Nevertheless, a superheating treatment of the melt at higher temperatures (> 1523 K) rejuvenated the nucleant particles. The present investigation, dealing with electron diffraction of carbide particles extracted from the poisoned master alloy, revealed massive formation of Al4C3 and Ti3AlC, of which Al4C3 appears to account for the poisoning of TiC nucleants. On the other hand, subsequent electron diffraction studies on the rejuvenated nucleants confirmed that they were essentially composed of uncontaminated TiC.

Microcrystalline surface layers created by laser alloying

Laser alloying has been used to create hard surface layers on a ferritic stainless steel. Alloying elements were introduced very simply by painting the steel surface with a suitably charged paint. The materials obtained were examined by electron metallographic techniques and analysed by energy-dispersive spectroscopy and diffraction. Suitable control of laser conditions makes it possible to regulate the depth of substrate melting and the alloy composition. A second melting was necessary to homogenize the material and simultaneously to select the grain size. Microstructures obtained were typically equiaxed with grains of size 0.25–1 μm. The formation of this microstructure is analysed in terms of the existence of an undercooled molten pool containing nucleants for the heterogeneous nucleation of b.c.c. iron. Such microstructures are created because crystal growth requires diffusion and large undercoolings and because suitable nucleant particles are present.

Study on the agglomeration of nucleating agents in amorphous and semicrystalline polymer systems

The agglomeration of an organic nucleating agent, dibenzylidene sorbitol, in polystyrene and polycarbonate has been studied with the help of small-angle light scattering. Characteristic V v patterns (with parallel polars) show that the intensity of the scattering increases with an increase in the level of nucleating agent. The angular distribution of the intensity has been analysed on the basis of Debye-Bueche theory and the average sizes of the agglomerated nucleant particles have been determined. The size of the agglomerated particles is then related to the effect of nucleant on the superstructure when incorporated in semicrystalline polymers such as poly(ethylene terephthalate).

A simple model for grain refinement during solidification

Assuming 1. (i) a spatially isothermal melt 2. (ii) a heterogeneous nucleation rate according to the classical theory 3. (iii) spherical diffusion controlled growth, the number of heterogeneous nucleation events that occurs during the initial stage of a liquid → solid reaction has been computed as a function of nucleant particle density using Al-Ti as the model system. The influence of cooling rate, substrate activity, and alloy constitution has been investigated, the last by comparison with the refinement obtainable in the systems Al-Zr and Al-Cr. The predictions of the model are in acceptable accord with observed behaviour considering the necessary assumptions of the model.