Solute adsorption and entrapment during eutectic Si growth in A–Si-based alloys

doi:10.1016/j.actamat.2014.09.040

Acta Materialia

Volume 83, 15 January 2015, Pages 187-202

https://doi.org/10.1016/j.actamat.2014.09.040 Get rights and content

Abstract

The solute adsorption and/or segregation as well as the solute entrapment of Sr, Na and Yb atoms during eutectic Si growth in a series of high-purity Al–5 wt.% Si alloys was investigated by multi-scale microstructure characterization techniques, including high-resolution transmission electron microscopy and atomic-resolution scanning transmission electron microscopy. The adsorption of Sr atoms was directly observed along the 〈1 1 2〉_Si growth direction of Si and/or at the intersection of multiple Si twins, which can be used to interpret the poisoning of the twin plane re-entrant edge and impurity induced twinning modification mechanisms, respectively. In contrast, Yb shows a different mechanism compared to the adsorption of Sr atoms. No significant Yb-rich cluster was observed at the intersection of Si twins. However, considerable Yb-rich segregation lines were observed along the 〈1 1 2〉_Si direction, which can be attributed to the solute entrapment caused by a few Si twins through the natural twin plane re-entrant edge and growth mechanism. Active poisoning of the twin plane re-entrant edge and impurity induced twinning growth mechanisms cannot be observed due to the absence of Yb atoms within eutectic Si. Furthermore, the solute entrapment of modifying elements (X, Sr or Yb) together with Al and Si was proposed to interpret the formation of Al₂Si₂X phases or X-rich clusters within eutectic Si. Such types of Al₂Si₂X phases or X-rich clusters were further proposed to be an “artefact” caused by the solute entrapment during eutectic Si growth, rather than an active factor affecting the modification. The observed solute adsorption and entrapment can be used to interpret the different observations in the cases of different modifying elements, including impurity effects and so-called “quenching modification”, thereby elucidating the modification of eutectic Si in Al–Si alloys.

Introduction

The modification of eutectic Si in Al–Si alloys can be dated back to 1921, when the first modification phenomenon was discovered by Pacz [1]. Recent technological developments of electron microscopy, e.g. high-resolution transmission electron microscopy (HRTEM), high-resolution scanning transmission electron microscopy (HRSTEM) and atom probe tomography (APT), make it possible to investigate the modification mechanisms at an atomic scale. To date, it is generally accepted that impurity induced twinning (IIT) [2] and the twin plane re-entrant edge (TPRE) growth mechanism [3], [4], as well as poisoning of the TPRE [5], are valid under certain conditions. The IIT mechanism postulates that the impurities (i.e. Sr and Na atoms) can be adsorbed on the growing {1 1 1}_Si planes, producing frequent multiple Si twins. The TPRE mechanism proposes that Si growth occurs more readily at the re-entrant edge along the 〈1 1 2〉_Si growth direction of Si, whereas poisoning of the TPRE assumes that the modifier retards Si growth by selectively adsorbing at the TPRE, thus deactivating the growth advantage of the TPRE and forcing new growth of the TPRE. However, IIT, TPRE or poisoning of the TPRE cannot be used to interpret all the previous modification observations. Three salient observations highlight the limitations of the existing proposed modification mechanisms.

Firstly, Yb addition into Al–Si alloys has been reported to only refine, rather than modify, the eutectic Si [6], even though the Yb atom has an exactly suitable radius ratio (r_Yb/r_Si = 1.646) according to the IIT mechanism. A similar investigation of the addition of rare earth elements (i.e. La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) has been also reported [7]. Only Eu was found to modify the eutectic Si to a fibrous morphology, while other remaining rare earth elements were found to solely refine the plate-like Si, although these rare earth elements have a similar radius ratio (r/r_Si ∼ 1.646), which is expected to modify eutectic Si according to the IIT mechanism. The observed disagreements strongly indicate that the well-accepted IIT mechanism, based on the atomic radius alone, is not capable of explaining the modification of eutectic Si, and additional mechanisms are still expected to be active.

Secondly, according to the IIT and/or poisoning of the TPRE mechanisms, the presence or adsorption of a single Sr atom at the twin re-entrant edges (for poisoning of the TPRE) or at the intersection of Si twins (for IIT) can result in the formation of parallel (for poisoning of the TPRE) or multiple (for IIT) Si twins, and thereby a modification of eutectic Si. However, instead of a single Sr atom, the formation of an Al₂Si₂Sr phase or Sr-rich cluster (with high Sr contents) within eutectic Si was very often observed [8], [9], which cannot be fully interpreted using IIT or poisoning of the TPRE mechanisms. There is thus a great need to revise the solute adsorption and entrapment of Sr atoms during the modification of eutectic Si, and thereby to elucidate this important melt treatment for Al–Si-based alloys.

Thirdly, the effect of cooling rates or growth rates on the formation of Si twins has not been fully considered in either IIT or poisoning of the TPRE mechanisms. It is well accepted that cooling rate (respectively undercooling) at the interface is one of the most important factors to affect the nucleation and growth of eutectic Si. Increasing cooling rates (i.e. melt-spinning) results in a so-called “quenching modification” [2], [10]. It has been reported that only when the cooling rate is faster than the critical cooling rate, which is dependent on the diffusion coefficient of the modifier, can the modification of eutectic Si be achieved [11]. With further increasing of the undercooling (up to 350–400 K), as reported in Ref. [12], metallic liquids solidification can be so fast that the interface velocity (V) is of the order of, or even greater than, the diffusion speed (V_D) in the bulk liquid, having a high crystal growth velocity up to 100 m s⁻¹. In the case of such a deep undercooling, the approximation of local equilibrium may become unacceptable for the description of solute diffusion. More importantly, increasing cooling rates may change the modes of solute diffusion. The solid–liquid interface is no longer in equilibrium, and it is generally accepted, and has been demonstrated for solid solutions, that increasing cooling rates gives rise to a reduced partitioning, leading to a solid having the same composition as the liquid through a process known as “solute trapping” [13]. It has been also argued [14] that, when the solidifying phase shows a site ordering, the partitioning behaviour can be considerably more complex: rapid solidification might lead to an increased partitioning, a change in the direction of partitioning or an absence of partitioning at solidification rates much lower than expected. The experimental verification of this phenomenon has also been reported by demonstrating inverted partitioning during rapid solidification of the intermetallic compound NiAl [15]. Clearly, increasing cooling rates results in a significant change to the solute diffusion or partitioning behaviour. Furthermore, the solute diffusion or partitioning behaviour of impurities (i.e. Sr and Na) ahead of the solidification front of eutectic Si under higher cooling rates was believed to have a vital effect on the modification of eutectic Si. However, most previous work focuses on the Al–Si alloys produced by conventional casting, i.e. sand-casting or die-casting [2], [8], [16], [17]. An investigation of the solute adsorption and entrapment during Si growth under higher cooling rates (i.e. melt-spun condition) is still required.

In this paper, the solute adsorption and entrapment during eutectic Si growth in a series of high-purity Al–5 wt.% Si alloys with Sr, Na and Yb additions were investigated, with a special focus on the distribution of Sr and Yb atoms within eutectic Si, in particular along the 〈1 1 2〉_Si growth direction of Si and/or at the intersection of multiple Si twins. Furthermore, the factors affecting the modification of eutectic Si were also discussed in terms of eutectic Si growth. For the first time, solute entrapment was proposed to interpret the different observations in the cases of different modifying elements under different cooling conditions.

Section snippets

Experimental material and procedures

A series of Al–5 wt.% Si alloys (wt.% is used through the paper where not specified otherwise) with the additions of Sr, Na and/or Yb were prepared using arc-melting and subsequent melt-spinning. The details on sample preparations (i.e. arc-melting and melt-spinning) are reported in Ref. [18]. The nominal composition is listed in Table 1. It is noteworthy that the Na content (50 ppm in Alloys 3 and 5) was added by Al–5Si–200 ppm Na master alloy manufactured by 5 N (99.998) Al, 5 N Si and 2 N (99.8)

Al–5Si alloy without addition

Fig. 1 shows a typical microstructure in high-purity melt-spun Al–5Si alloy. A Si particle located along the grain boundary is highlighted in Fig. 1a. Viewed from {0 1 1}_Si zone axis (Fig. 1b), the Si particle appears to grow by the natural TPRE mechanism. The marked orientation (Fig. 1a) is fully consistent with that of TPRE, i.e. the typical 〈1 1 2〉_Si growth direction. In order to further elucidate the details within eutectic Si, a HRTEM image is shown in Fig. 1c. Indeed, Si twinning was grown on

Discussion

The addition of Sr (Fig. 2, Fig. 3, Fig. 4, Fig. 10, Fig. 11, Fig. 12) and Na (Fig. 6, Fig. 7) promotes a significantly multiply twinned Si, while the addition of Yb (Fig. 8, Fig. 9) does not promote such a type of multiple Si twins despite its favourable atom size according to the IIT mechanism. Thus, the following discussions are separated in two different cases, with a special focus on the solute adsorption and/or segregation as well as the solute entrapment during eutectic Si growth.

Conclusions

1.
The solute adsorption of Sr and Na atoms along the 〈1 1 2〉_Si growth direction of Si and/or at the intersection of Si twins during Si growth was characterised, which can be used to interpret the well-known poisoning of the TPRE and IIT mechanisms, respectively.
2.
In contrast, the segregation of Yb atoms is distinctly different from the adsorption of Sr along the {1 1 1}_Si growth planes. No significant Yb-rich cluster was observed at the intersection of Si twins. However, considerable Yb-rich

Acknowledgment

J.H.L. acknowledges Prof. G. Dehm for his access to the TEM facility at the Erich Schmidt Institute of Materials Science of the Austrian Academy of Sciences.

References (32)

M. Timpel et al.
Acta Mater.
(2012)
J.H. Li et al.
Acta Mater.
(2014)
A. Kar et al.
Acta Metall. Mater.
(1992)
K. Nogita et al.
Scr. Mater.
(2006)
M. Kuzmin et al.
Surf. Sci.
(2002)
Y. Du et al.
Mater. Sci. Eng. A
(2003)
A. Pacz, US Patent No. 1387900,...
Shu-Zu Lu et al.
Met. Trans. A
(1987)
R.S. Wanger
Acta Metall.
(1960)
R.D. Hamilton et al.
J. Appl. Phys.
(1960)

M.G. Day et al.

Proc. R. Soc. Lond. A

(1968)

J.H. Li et al.

Metall. Mater. Trans. A

(2013)

K. Nogita et al.

Mater. Trans.

(2004)

J.H. Li et al.

Philos. Mag.

(2012)

Z.G. Yang et al.

Acta Metall. Sin.

(1989)

D. Herlach et al.

Metastable Solids from Undercooled Melts

(2007)

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Solute adsorption and entrapment during eutectic Si growth in A–Si-based alloys

Abstract

Introduction

Section snippets

Experimental material and procedures

Al–5Si alloy without addition

Discussion

Conclusions

Acknowledgment

Acta Mater.

Acta Mater.

Acta Metall. Mater.

Scr. Mater.

Surf. Sci.

Mater. Sci. Eng. A

Met. Trans. A

Acta Metall.