http://support.spring8.or.jp/Report_JSR/PDF_JSR_24B/2012B1430.pdf

2012B1430

BL20XU

In-situ Synchrotron X-ray Observations at SPring-8 BL20XU ofSolidification in Hypoeutectic Al-Si Alloys1
Kazuhiro Nogitaa,b, Hideyuki Yasudac, Arvind Prasadb, Stuart D. McDonalda,b, Tomoya Nagirac,
Noriaki Nakatsukac, Kentaro Uesugid, David H. StJohnbab

Nihon Superior Centre for the Manufacture of Electronic Materials,
School of Mechanical and Mining Engineering, The University of Queensland,c
Department of Adaptive Machine Systems, Osaka University,dJapan Synchrotron Radiation Research Institute, SPring-8

This paper demonstrates how recent advances in synchrotron technology have allowed for the X-ray
imaging of real-time solidification in Al-Si alloys, despite the small difference in atomic number of theseelements. The experiments performed at the SPring-8 synchrotron, involved imaging the solidification of
Al-1wt%Si and Al-4wt%Si alloys under a low-temperature gradient and a cooling rate of around 0.3°C/s.
The nucleation and growth of the primary aluminium grains as well as the onset of eutectic solidification
were clearly observed. In the alloys containing Al-4wt%Si, contrast was sufficient to characterise thenucleation rate and growth velocity of the aluminium grains. The importance of improving observation ofsolidification in the Al-Si system by increasing the time resolution during critical events is discussed.
Keywords:

Solidification, Synchrotron X-ray imaging, Al-Si alloy

1. Introduction
Advances in understanding the solidification of metallic alloys have traditionally been limited by theintrinsic difficulties of direct observation. The analysis of solidification by metallographic characterisationof solid samples requires assumptions, which have often been supported by supplementary experiments
such as thermal analysis or quenching along with observations in transparent organic systems. Advances insynchrotron analysis have provided the opportunity to use X-rays to directly view solidification processes
in-situ, including the development of microstructure and the formation of defects. As these techniques
become more sophisticated it is possible to make observations in systems that have previously shown
insufficient difference in the contrast between solid and liquid phases. One such system is the Al-Si system,
which as a binary system, has been the subject of only limited investigations using synchrotron techniques.
Alloys based on the Al-Si system are the most commonly used aluminium foundry alloys and accountfor 85-90% of all cast aluminium products [1]. They have a low density, excellent strength-to-weight ratio,
good corrosion resistance, high electrical and thermal conductivity and are economical to recycle. There are
two major solidification reactions that contribute to the microstructure of these alloys. The first is thesolidification of primary aluminium grains and the second is the solidification of the aluminium-silicon
eutectic. Although the system has been the subject of intensive research for many decades, in-situ
solidification observations would make an important contribution to understanding how the castmicrostructures develop during solidification. However, similarities in the atomic mass of Al and Si alongwith the liquid/solid density of Al-Si limit the contrast obtainable by synchrotron X-ray techniques.
The binary Al-Si system is a simple eutectic system containing no intermetallic compounds and ischaracterised by the eutectic reaction at approximately 12.5Si and 577 °C [2]. The density of Al-Si alloys
in the liquid, ρl, and solid, ρs, varies with composition and temperature and is described by Equations 1 and2 [3] where T is temperature and C, wt% Si. Using these equations and assuming infinite diffusion in theliquid and none in the solid, the density difference at the solid-liquid interface on initial solidification willvary between 5.5% and 7% in hypoeutectic Al-Si alloys.

1

Amended from a submission to Materials Characterisationρl= 2559.1 – 0.2820 T + 3.371 C

Equation 1ρs= 2688.6 – 0.1990 T – 2.295 CEquation 2Owing to the similar atomic numbers of Al and Si, this alloy system is not suited for X-ray
radiography studies. One approach to address this issue has been to make trace element additions toenhance visualization. For instance, Mathiesen et al. studied eutectic formation in Al-8Si and Al-9Si alloys
with Cu added to enhance X-ray absorption contrast [4]. The precipitation of Fe bearing intermetallic
phases in Al-Si-Cu alloys has also been successfully examined using synchrotron techniques [5]. Jung etal., in their study of dendrite fragmentation of columnar grains in Al-7Si binary alloys observed that theminimal difference in the atomic numbers of Al and Si causes “no contrast” between the solid and theliquid [6]. To circumvent the visualization problem, they subtracted the previous image from the current toobtain some contrast, based on the work by Nguyen et al [7]. Their experimental conditions included
samples that were 150-300 µm thick samples, with an X-ray beam energy of 17.5 keV. The optics in theirsystem were chosen so as to obtain a large enough field of view (15x15 mm) with a good resolution (7.46µm) and they used a maximum image capturing rate of 0.3 Hz. Other attempts have been made to study
in-situ Al-Si solidification phenomenon using synchrotron radiation. These include those of Faraji et al. instudies on the nucleation behavior of different grain refiners in commercial A356 alloy using 3DXRD [8].
Srirangam et al. [9] have studied the structure of the liquid in Al-Si alloys with and without Sr additions
using XRD from a synchrotron source. Wang et al. studied the formation of Fe rich β-intermetallics in
Al-7.5Si-3.5Cu-0.8Fe alloys using synchrotron radiation [10]. Likewise, Terzi et al. studied β-phaseformation in Al-8Si-4Cu-0.8Fe using synchrotron radiation [11]. More generally, synchrotron techniques
are becoming increasingly important in materials characterization, as outlined by Rowenhorst and Voorhees[12] in a review of the advantages of using 3D techniques for materials characterisation.
In the casting of metals it is desirable to refine the grain size, often through the addition of inoculants.
While some progress has been made in understanding the role of inoculants, the challenge facing thecasting industry is the fact that >90% of added inoculants do not take part in the nucleation phenomena.
Recently, one of the authors of this paper, StJohn, [13] proposed the Interdependence Theory which
predicts the formation of a Nucleation Free Zone (NFZ) ahead of a growing grain. This NFZ is predicted toexist even in the presence of inoculants. Thus, the theory predicts the presence of unused inoculants. Agreater understanding of the Interdependence theory could be obtained through the in-situ observation ofsolidification phenomena and synchrotron X-ray experiments are one of the main techniques capable ofobtaining this data. This paper investigates thefeasibility of observing the nucleation and growth of
Al grains in binary Al-Si alloys.
2. Materials and Methods
Alloys were prepared by melting appropriatequantities in an induction furnace and both alloys had
an addition of 0.1% (nominal) Ti added in the form ofan Al-3Ti-1B master alloy. Thermal analysis samples
were taken from the melt using a procedure describedin detail elsewhere [14].
Synchrotron X-ray radiography experiments
were performed on beam line BL20XU of theSPring-8 synchrotron (see Figure 1) with an energy of
16keV, using an experimental technique that wasdeveloped in previous research [15]. A planar
undulator was used as a light source and the radiation
was monochromatised with a Si double crystal
monochromator. An image detector consisting of asingle crystal phosphor screen and a CCD camera wasused. The sample detector distance was less than 1 m.
A graphite heater was used and BN plates shieldedradiation from the heater, creating a window (roughly10 mm×10 mm) at the centre for the X-ray beam.

Figure 1．(a) Schematic of the experimental
setup for the real-time observationof solidification [16], (b) vacuum
chamber and (c) furnace at theSPring-8 BL20XU beam line.

Specimen cells were made of Al2O3 window plates and BN retainer plates. Al-Si samples of composition
shown in Table 1, with dimensions of 7 mm×7 mm×0.1 mm were placed between the two Al2O3 window
plates with a thickness of 0.15 mm. The specimen and the window plates were retained by the BN plates.
Specimens were inserted into the furnace and melted at around 20°C superheat for the Al-1Si sample andaround 40°C superheat for the Al-4Si sample. The imposed thermal gradient led to a temperature differenceof ~1 K across the field of view and the cooling rate was approximately 0.3°C/min. Images were captured
at a frame rate of approximately 1Hz and the pixel size of the measurement is around 1 μm x 1 μm.
Since the X-ray absorption coefficients of Al and Si are nearly the same, the solute rejection at thesolidifying front does not cause any clear contrast between the solid / liquid interface. Thus, it is required todetect the contrast induced by the density difference between the solid and the liquid phases. Polished
Al-Si specimens, with no coating and a thickness of 100μm were inserted into the specimen cell.
Observation was performed under a vacuum (10-3 torr). Although a thin oxide film was produced on thespecimen surface, the melt flow compensated for the solidification shrinkage. Consequently, the thicknessof the specimen was kept constant throughout solidification. The constant thickness was advantageous for
detecting the solid / liquid interface.
3. Results and Discussion
The X-ray images of solidification as
captured during the experiments are shown inFigure 2 (a) – (e) for both alloys. The labeled
times in Figure 2 are measured from the time atwhich nucleation was first observed (tN=0) andwould roughly correspond to the liquidus
temperature of the given alloy. Although times
could be recorded accurately, due to the small
sample size it was not practical to embed athermocouple in the samples and exact
temperatures cannot be reported. Very rapidly
after grain nucleation (within a few seconds), thetotal number of grains for each sample is fixedand no further nucleation was observed. It wasnoted that the number of grains was slightly
higher in the Al-1Si sample and this may relate
to the higher concentrations of Ti and B. For
the first few seconds the newly formed grains
are highly mobile and some rotation and driftingof the grain location under gravity is observed.
Around five seconds after nucleation, the grains
had formed a coherent network and no further
grain movement was noted. The effect of solutesegregation in halting grain growth was apparent
in many instances where the growth of adjacentdendrite arms appeared to slow before
impingement on their immediate neighbours.
The commencement of eutectic solidification
was observed as a clear increase in contrast(bright white region) between dendrite arms andadjacent grains as seen in Figure 2(e). It isnoted that the time of eutectic solidificationcommencement is different for these alloys due
Figure 2．Selected X-ray stills of solidification forto the larger non-equilibrium freezing range in
the Al-1Si alloy, left, and the Al-4Si
the alloy with lower silicon concentration.
alloy, right. Time measurements are
Between the onset of nucleation, just prior to
relative to the first observation ofFigure 2(a) and eutectic solidification, Figure 2nucleation.
(e), significant coarsening of the secondary
dendrite arm spacing occurs, although the rate of coarsening is faster earlier in solidification.

4. Conclusions
This research was performed to determine the feasibility of using synchrotron based X-radiographic
techniques to directly observe the nucleation and growth of primary Al grains in binary Al-Si alloys. We
have demonstrated the successful capture of solidification events including grain nucleation, coarsening,
and the onset of eutectic solidification. The growth interface was detected by the density difference
between the solid and liquid. The resolution that was obtained should be possible for most aluminium
alloys. Quantitative measurements from the images can be used to obtain nucleation frequency and dendrite
tip velocities. It is apparent from the results that the use of synchrotron X-ray techniques must be tailored tothe phenomena of most interest. For research into grain nucleation and refinement, the timing of nucleationand the spatial distribution of grains is critical and this would benefit from a large field of view and a highdata collection rate around the primary reaction when the microstructure evolves rapidly, even in samples
cooled relatively slowly. On the contrary if eutectic solidification is the main focus, then a higher
magnification is required. The results of this research will be used in the ongoing development of grainrefiners for Al-Si alloys and also contribute to improved synchrotron observation techniques.
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