New studies of nucleation mechanisms in aluminium alloys: implications for grain refinement practice

Schumacher, Peter; Greer, A.L.; Worth, J.H.; Evans, P.V.; Kearns, Martin; Fisher, P. A.; Green, A. H.

doi:10.1179/026708398790301241

Cited by 47 publications

(44 citation statements)

References 7 publications

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“…We find some evidence that Al 3 Ti (112) is a more powerful nucleant than TiB 2 (0001) with signs that continuous growth of fcc structure on top of Al 3 Ti (112) is possible. This can be seen as supporting the accepted view that "free growth" occurs on Al 3 Ti (112) which coats TiB 2 , as suggested by previous experiments [40][41][42][43] . However, these simulations suggest a second possibility, namely that strained Al grows on the Ti-terminated surface of TiB 2 , and it is this surface on which growth occurs.…”

Section: Discussionsupporting

confidence: 89%

Heterogeneous nucleation of solid Al from the melt byTiB2andAl3Ti: Anab initiomolecular dynamics study

et al. 2010

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The nucleation of solid Al from the melt by TiB 2 is well established and is believed to involve the formation of Al 3 Ti. Since the atomic scale mechanisms involved are not fully understood, we look to computer simulation to provide insight. As there is an absence of suitable potentials for all of this complex system we have performed large scale density functional theory molecular dynamics simulations of the nucleation of solid Al from the melt on TiB 2 and Al 3 Ti substrates at undercoolings of around 2K. Using periodic boundary conditions, we find limited ordering and no signs of incipient growth in the liquid Al close to the B-terminated surface of TiB 2 . By contrast, we see fcc-like ordering near the Ti-terminated surface, with growth being frustrated by the lattice mismatch between bulk Al and the TiB 2 substrate. The Al interatomic distances at the Ti-terminated surface are similar to distances found in Al 3 Ti; we suggest that the layer encasing TiB 2 observed experimentally may be strained Al on a Ti-terminated surface rather than Al 3 Ti. For the Al 3 Ti substrate, fcc-like structures are observed on both sides which extend rapidly into the melt. Periodic boundaries introduce unphysical stresses which we removed by introducing a vacuum region to separate the liquid from the solid at one of the interfaces. We see ordering in the Al on both the B-terminated (0001) surface of TiB 2 , and on Al 3 Ti(112), with the ordering able to be stronger on the Al 3 Ti substrate.However, we cannot draw strong conclusions as these simulations need more time to allow long ranged fluctuations in the liquid Al to dampen out. The huge computational cost restricted the range and duration of simulations that was possible.

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Section: Discussionsupporting

confidence: 89%

Heterogeneous nucleation of solid Al from the melt byTiB2andAl3Ti: Anab initiomolecular dynamics study

et al. 2010

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“…Central to the major controversial issues is the determination of factors that control the efficiency of grain refinement. Factors such as the contact angle (Turnbull, 1953) and crystallographic matching (Bramfitt, 1970) between nucleant particles and matrix metals/alloys, the geometry and size of nucleant particles (Greer et al, 2000;Qian, 2007), the interactions between nucleant particles and alloy chemistry (Schumacher et al, 1998;Johnsson, 1994a,b), and the settling of nucleant particles (Jones & Pearson, 1976) have been suggested to affect grain refinement efficiency. Among all these factors, the crystallography between the nucleant particles and matrix and the size of nucleant particles have been considered to be of critical significance in controlling the efficiency of grain refinement.…”

Section: Introductionmentioning

confidence: 99%

Crystallographic study of grain refinement of Al by Nb addition

et al. 2014

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The grain refinement of Al by the addition of a small amount of peritecticforming solute, Nb, has been studied from the crystallographic point of view. Combining the observations of optical microscopy and scanning electron microscopy with the results of energy-dispersive X-ray spectroscopy and X-ray diffraction, it is confirmed that the particles observed at or near the grain centres of refined Al alloys are pro-peritectic Al 3 Nb particles. The crystallographic matching between the Al 3 Nb particles and Al grains has also been evaluated using an edge-to-edge matching model and further verified using electron backscatter diffraction and transmission electron microscopy. It is found that there are reproducible crystallographic orientation relationships between the Al 3 Nb particles and Al grains, and the experimental results are consistent with the predictions of the edge-to-edge matching model. This implies that the properitectic Al 3 Nb particles are favourable nucleation sites for Al grains from the crystallographic point of view. Furthermore, the analysis of the size distribution of Al 3 Nb particles reveals that the Al 3 Nb particles at the grain centres have relatively large particle size, which also corroborates the high potency of Al 3 Nb according to the free growth model. It is therefore concluded that the significant grain refinement resulting from the addition of Nb is predominantly attributed to the in situ formed Al 3 Nb particles which promote grain refinement via enhanced heterogeneous nucleation.

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“…34,35 A major advance in understanding of this process has been made by Greer's group in Cambridge. In an inspired experiment, 36 TiB 2 particles were added to an Al 85 Y 8 Ni 5 Co 2 alloy melt and then rapidly quenched to give an amorphous matrix. The (0001) faces of the TiB 2 particles were found to be covered with thin epitaxial layers of an Al 3 Ti-like phase on which aAl was formed, again in epitaxial orientation.…”

Section: Grain Refinementmentioning

confidence: 99%

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2011

Materials Science and Technology

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New studies of nucleation mechanisms in aluminium alloys: implications for grain refinement practice

Cited by 47 publications

References 7 publications

Heterogeneous nucleation of solid Al from the melt byTiB2andAl3Ti: Anab initiomolecular dynamics study

Heterogeneous nucleation of solid Al from the melt byTiB2andAl3Ti: Anab initiomolecular dynamics study

Crystallographic study of grain refinement of Al by Nb addition

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