Equiaxed dendritic solidification and grain refiner potency characterised through in situ X-radiography

Acta Materialia 95 (2015) 83–89

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Equiaxed dendritic solidification and grain refiner potency characterised through in situ X-radiography A.G. Murphy a,⇑, W.U. Mirihanage b, D.J. Browne a, R.H. Mathiesen b a b

School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland Department of Physics, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

a r t i c l e

i n f o

Article history: Received 16 April 2015 Revised 29 April 2015 Accepted 30 April 2015

Keywords: Al–Cu Equiaxed Dendritic In situ X-radiography Solidification

a b s t r a c t Results of near-isothermal equiaxed solidification experiments on the Al–Cu system performed in situ with laboratory-based radiography are presented. The ability to align the X-ray beam axis both perpendicular and parallel to the gravity vector provides limited control over buoyant grain motion normally associated with terrestrial solidification. The latter alignment case provides solidification conditions where the impact of gravity during solidification is strongly reduced. Exploiting this experimental configuration, direct observations of the effect of buoyancy on grain motion, nucleation density, and average grain size are presented. Detailed measurements of solid area fraction evolution were extracted from image sequences, demonstrating alloy-characteristic temperature dependence, allowing for comparison with theoretical predictions. Finally, measurements of nucleation density and dendrite growth velocities, as a function of applied cooling rates and alloy composition, were made and are discussed here with reference to relevant theoretical models. This work represents the first such study where solidification conditions without significant effects from gravity were achieved, for both solid and liquid phases, in ground-based experiments. Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

1. Introduction Adding Al–Ti–B master alloy to a liquid melt prior to pouring into a mould generally results in a fine equiaxed structure in aluminium castings. McCartney [1], Greer et al. [2] and Quested [3] have published comprehensive reviews of grain refinement mechanisms along with the competing nucleation theories, i.e. boride nucleation, peritectic nucleation, duplex nucleation. Fundamentally, Al–Ti–B master alloy, composed of TiB2 and TiAl3 particles in an a-Al matrix, is added in the liquid melt [4]. The peritectic TiAl3 dissolves into the melt thereby increasing the titanium concentration, which causes an increase in the so-called growth restriction factor [5]. The TiB2 particles are stable in the melt and act as the nucleation substrates for a-Al. Greer et al. [6,7] developed the free-growth model for heterogeneous nucleation on a TiB2 particle suspended in a liquid melt. Once a critical particle undercooling is reached, dictated by individual particle diameters, free growth of primary solid on the particle commences. The efficacy of grain refiner, i.e. the ratio of the number of grains nucleated to the number of particles added, is of paramount interest for the casting industry. Inoculant addition levels ⇑ Corresponding author. http://dx.doi.org/10.1016/j.actamat.2015.04.060 1359-6454/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

must be kept to a minimum as boride particles are considered inclusions in later stages of production. Industrial levels of grain inoculation are usually between 1 and 10 kg/tonne [2], with typical grain refiner efficiencies reported at less than 1% [6]. Maxwell and Hellawell [8] determined that, for a spatially isothermal melt under diffusion-limited growth, the latent heat released from earlier nucleated grains reduces the undercooling, thereby preventing further nucleation by recalescence. This effect has been recently simulated by Mirihanage and Browne [9] and Hunt and Fan [10]. Considering the measured log-normal size distribution of TiB2 particles [11], less prevalent larger particles nucleate solid at lower undercoolings thereby preventing further nucleation on smaller particles. However, very large particles are more susceptible to sedimentation with increased melt holding. Schaffer and Dahle [12] investigated the effects of TiB2 particles settling during the melt holding period, observing faster than predicted particle settling rates, suggesting possible particle agglomeration in the melt. Quested and Greer [13] suggested inactive particle impingement on growing grains to be the likely source of inoculant particle deactivation (fading). The addition of certain solutes to the melt, e.g. zirconium or silicon, has also been shown to poison TiB2 particles, further reducing grain refiner efficiency [5].

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Experimental validation of nucleation and solidification theory has traditionally relied on metallographic measurements made on as-cast microstructures post-mortem. The as-cast microstructure, however, provides little information on the interrelated dynamic solidification phenomena that are dominant during the early stages of solidification, e.g. thermosolutal grain impingement, grain buoyancy, sedimentation, etc. In situ X-radiography and X-ray tomography on metallic alloys have now become mainstays of solidification science, allowing real-time observations of dynamic mushy-zone phenomena from micro to meso length-scales [14,15]. A fundamental feature of most in situ X-ray monitored solidification experiments performed to date, adding further complexity to any subsequent analysis, is inherent gravity-induced thermosolutal convection and grain buoyancy/sedimentation as a consequence of beam/sample configuration, i.e. horizontal beam and vertical sample [16,17]. Recent advances in compact microfocus X-ray source and detector technology have allowed solidification experiments, with spatiotemporal resolutions comparable to those reported in synchrotron-based studies, to be performed in home-laboratories [18,19] as well as under microgravity conditions on-board parabolic flights [20] and sounding rockets [21]. This article describes the results of extensive terrestrial-based near-isothermal in situ solidification experiments of Al–Ti–B inoculated and non-inoculated Al–Cu alloy samples using the XRMON-Gradient Furnace (GF) and associated X-ray diagnostics [21]. 2. Equipment & experiments A full account of the X-ray diagnostic equipment used, as well as the sample manufacturing technique developed for this work have previously been presented [21,22]. Only a brief description of the pertinent experimental parameters and equipment configuration will be given herein. Fig. 1(a) schematically illustrates the experimental configuration employed in this work, comprising a microfocus X-ray source (Viscom XT9100-T), solidification furnace (XRMON-GF), and a specialised digital camera (Vosskuhler 11000) equipped with a Scint-X scintillator. A spatial field-of-view (FOV) of 4327  2882 lm was obtained, giving a virtual pixel size of 2.16 lm at the CCD, with a maximum spatial resolution of 7 lm, which is a function of the source size, the chosen geometric magnification, and the scintillator pitch [19]. Optimal operating parameters for the X-ray source, providing the best compromise between sample transmission and absorption contrast for the

(a)

chosen alloy system, was calibrated to 50 keV and 60 lA. Images were recorded continuously during solidification experiments at a rate of 1.5 Hz, and all experiments were performed in a relatively inert N2-rich atmosphere of 65% O2. The Al-based alloy compositions used were 10 and 20 wt.%Cu, inoculated with 0.0, 0.1, and 0.5 wt.%Al–Ti–B (5/1) grain refiner (GR) master alloy, respectively. Sample dimensions were 50  5 mm with a thickness of 200 ± 10 lm (L  W  D, Fig. 1). The XRMON-GF was operated in calibrated isothermal mode, in that the heater elements were adjusted until equiaxed nucleation originated close to the centre of the FOV thereby indicating the realisation of a reasonably flat temperature profile [23]. Solidification was initiated through application of a constant cooling rate at both heater locations simultaneously, with constant cooling rates ranging from 0.025 to 1.0 K/s. The compact nature of the system allowed experiments to be performed with the sample aligned both perpendicular (horizontal sample orientation, Fig. 1(b)) and parallel (vertical sample orientation, Fig. 1(c)) to the gravity vector. With the sample oriented horizontally (Fig. 1(b)) gravity acts through the thin sample dimension (D), thereby dramatically limiting the potential effects of grain buoyancy (Fg) within the sample. Experimental conditions, therefore, were somewhat comparable to those observed under microgravity conditions using the same experimental configuration [20]. Conversely, with the sample oriented vertically (Fig. 1(c)), which has traditionally been the case with synchrotron-based in situ X-radiography [16,24], gravity-induced thermosolutal convection and buoyant grain motion have largely been unavoidable. 3. Results and discussion The gradient furnace configuration only allows for control of the imposed temperature profile along the sample length (L, Fig. 1). Thus in this work, an aberrant lateral temperature gradient across the FOV could not be completely eliminated by fine-tuning the control system. Fig. 2(a) shows the results of the lowest temperature gradient achieved using an Al–20 wt.%Cu–0.1 wt.% GR sample solidified at a constant cooling rate of 0.5 K/s. Equiaxed nucleation originated in the centre of the y-axis and to the extreme left of x-axis shown in Fig. 2(b). Subsequent nucleation continued radially outwards covering the entire domain, suggesting the existence of a cold spot towards the left side of the FOV. The magnitude of the lateral temperature gradient was quantified through offline image analysis. Fig. 2(b) shows the temporal evolution of equiaxed

(b)

(c)

Fig. 1. Schematic illustration of laboratory-based in situ X-ray monitoring system and sample alignment configuration. (a) X-ray diagnostics. (b) Horizontal sample configuration, g. (c) Vertical sample configuration, g;.

A.G. Murphy et al. / Acta Materialia 95 (2015) 83–89

(a)

85

(b)

Fig. 2. Al–20 wt.% Cu–0.1 wt.%GR sample solidified horizontally at a constant cooling rate of 0.5 K/s. (a) In situ image taken at time, t  40 s, where t = 0 was chosen as some arbitrary time prior to the onset of equiaxed nucleation. (b) Temporal evolution of equiaxed nucleation across the FOV calculated using Voronoi tessellation of grain nucleation centres. The colourbar indicates the spatiotemporal location of nucleation, from commencement to cessation, within the FOV. The thermal gradient along the xaxis direction was measured at 1.5 K/mm. See also supplementary video 1 in Appendix A. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

nucleation within the FOV extracted from the image sequence. The black dots indicate the equiaxed nucleation centre locations. A Voronoi tessellation of all 290 nucleation centres was performed giving the colourised envelopes shown in Fig. 2(b); the colour-scale indicates the recorded nucleation time. Approximately 22 s elapsed between the onset and end of equiaxed nucleation. Assuming a constant cooling rate within the sample, and a uniform distribution of inoculant particles requiring a low undercooling for activation, the position and time of each nucleation event was correlated with changing temperature and, thus, the lateral temperature gradient along the FOV x-axis was measured at 1.5 K/mm. For the purposes of this work, the measured temperature gradient was considered to be sufficiently low so as to adequately simulate near-isothermal solidification.

and thus remained stationary throughout solidification. Both grains were consequently of similar size and morphology to those observed during horizontal solidification, Fig. 3(a). Furthermore, the stationary grains acted as impediments to buoyant grains nucleated upstream (below the FOV), thus, as free floating grains nucleated and rose upwards, coherency developed early preventing further primary grain growth resulting in a higher grain density. Note, buoyant grain motion in the horizontally oriented samples could not be eliminated entirely, and was still observed along the thin dimension of the sample prior to the grain size becoming comparable to the sample thickness. Grain rotation was also observed in the FOV as grains came to rest on the upper surface of the sample after nucleating freely from within the melt volume.

3.1. Grain buoyancy

3.2. Solid (area) fraction and growth velocity measurements

Fig. 3 shows two images demonstrating the effect of grain buoyancy during solidification. Fig. 3(a) and Fig. 3(b) show the results of the sample solidified in the horizontal (Fig. 1(b)) and vertical orientations (Fig. 1(c)), respectively. In both cases, samples were solidified at a constant cooling rate of 0.025 K/s. Buoyant grain motion, as a consequence of sample orientation, had a dramatic effect on both average grain size and grain nucleation density at low cooling rates. Grain densities of 4 grains/mm3 and 80 grains/mm3 together with average grain sizes of 800 lm and 200 lm were measured from Fig. 3(a) and (b), respectively – a 20-fold increase in grain density in the FOV because of buoyant grain motion. For direct comparison, consider the equiaxed grains indicated (arrows) in Fig. 3(b). Both grains nucleated embedded on the sample wall

Independent of the grain density, the temperature-dependent apparent solid fraction should be comparable between samples at the same undercooling. To correlate this aspect of solidification with in situ X-radiography, images from Al–20 wt.%Cu samples with 0.1 and 0.5 wt.%GR were extracted at an undercooling DT = 10 K. DT was calculated under the assumption that the sample temperature at the time of first observed equiaxed nucleation (tTL) was equal to the alloy liquidus temperature, TL. Thus _  tTL), where t is the image time and T_ is the applied coolDT = T(t ing rate. Measurements were taken from samples oriented both horizontally and vertically, for cooling rates from 0.025 to 0.5 K/s. The apparent solid area fraction was calculated by measuring the solid grain envelopes, which were calculated

Fig. 3. Al–20 wt.%Cu–0.1 wt.%GR alloy solidifying in the (a) horizontal orientation (g), and in the (b) vertical orientation (g;), respectively, at a constant cooling rate of 0.025 K/s. The arrows highlight equiaxed grains that nucleated on a stationary particle trapped in the sample wall. Both images were recorded at an undercooling of DT  8 K. See also supplementary videos 2 and 3, respectively, in Appendix A.

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automatically using a manually selected global grey-level threshold. Little deviation from the average value of gS = 0.49 ± 0.05 was observed at this undercooling, irrespective of sample orientation. Fig. 4 illustrates the operation of an automated algorithm developed specifically to measure solid evolution from in situ X-ray image sequences. The dashed red line defines a manually selected internal region-of-interest (ROI), outside which any growth was ignored. This was to account for any CCD edge effects or any permanent artefacts that could not be removed through image enhancement. The blue contours indicate the extracted grain envelope temporal evolution during solidification. Also shown are four pre-selected primary dendrite arm growth axes, denoted by the star-dashed lines. By supplying the grain nucleation centre and the growth direction, the algorithm can track grain growth by calculating the intersection between the grain envelope and the specified line segment. Fig. 5(a) shows the recorded primary dendrite arm growth for line segments, A1, A2, A3, and A4 in Fig. 4. Fig. 5(b) shows that after nucleation growth velocities decrease monotonically for all grains. This deceleration was as a result of solutal diffusion field interaction between neighbouring grains, which suggests that individual grains are aware of surrounding grains very early after nucleation. Note, due to imaging resolution limitations, equiaxed grains had already grown larger than 50 lm by the time they became measurable within the FOV. This observation is in contrast to the work of McFadden et al. [25] on Al–

12 wt.%Ge alloys, and Bogno et al. [26] on Al–10 wt.%Cu alloys, where an initial acceleration regime was observed, before the onset of solutal impingement. However, in both cases cited the sample was oriented vertically, resulting in gravity-induced grain motion and solute sedimentation in the FOV. In this work, using the horizontal sample orientation, solutal mixing in the thin sample dimension (D, Fig. 1(b)) is expected to be complete, and the solutal field surrounding individual grains is assumed to remain relatively uniform and unperturbed throughout solidification. McFadden et al. [25] and Bogno et al. [26] were able to compare the measured dendrite tip growth velocities with analytical models of dendrite tip growth kinetics by measuring quantitative solute concentration profiles ahead of the dendrite tips and updating the undercooling accordingly. Similar comparisons in this work were not possible due to the use of polychromatic X-rays preventing quantitative measurements of local solute concentration variation in the liquid. Using the algorithm, solid area fraction measurements were extracted from three individual Al–20 wt.%Cu–0.1 wt.%GR samples, performing 8 melt-solidification cycles per sample. Cooling rates used were in the range 0.025 to 1.0 K/s, resulting in measurements from 23 solidification experiments. To allow direct comparison, area fraction curves were normalised to the alloy liquidus temperature, TL = 602.6 °C, when the measured area fraction became non-zero. The calculated average area fraction for one such

1000

25

(a) 800

A1 A2 A3 A4

Growth Velocity (µm/s)

Primary Arm Length (µm)

Fig. 4. Image analysis algorithm showing extracted grain area envelopes overlaid on FOV. Blue contour lines show the temporal grain envelope evolution. Star-dashed line profiles indicate equiaxed grain nucleation and primary arm growth direction. Al–20 wt.% Cu–0.1 wt.%GR alloy, cooling rate of 0.025 K/s. The dashed red boundary represents the measurement ROI. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

600 400 200 0

(b)

A1 A2 A3 A4

20 15 10 5 0

0

100

200 Time (s)

300

0

100

200

300

Time (s)

Fig. 5. (a) Measured dendrite arm lengths versus time for a constant cooling rate of 0.025 K/s. (b) Equiaxed primary arm velocity profiles. A1, A2, A3, and A4 refer to primary arms indicated in Fig. 4.

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1.0

d¼

Area Fraction

0.8 0.6 0.4 0.2 0.0

Sample Mean Scheil (area) 520

540

560

580

600

620

o

Temperature ( C) Fig. 6. Measured average solid area fraction (solid red line) with 1 SD (solid black envelope). 2D Scheil (area) prediction (dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

sample is shown in Fig. 6, along with a 1r standard deviation (SD) from the mean. Over the 23 melt-solidification cycles performed the overall variance recorded was quite small (SD, Fig. 6), and can be attributed to changes in 2D projections of solid area for the same 3D solid volume, i.e. grain rotation and misorientation. Also shown for comparison is the 2D (area) Scheil predicted temperature dependent solid fraction [27] (dashed line), which was calculated as the squared cube root of the volume averaged value. The measured area fraction showed little agreement with the theoretical Scheil predictions. This is due to the surface area equivalent of a volume-averaged quantity not exactly describing the real dendrite surface. Critically, a dendrite has a different surface/volume ratio than a sphere, which is the intrinsic morphology consistent with the Scheil predictions, and thus would only hold for averages over many isothermal dendrites or ideally for a spherical solid. Preliminary work into a full correlation of the 2D solid area fraction to the 3D solid volume has already been performed, using microgravity-based solidification experiments, demonstrating reasonably good agreement [20]. 3.3. Equiaxed nucleation and inoculant potency The efficacy of the grain refiner was quantified using measured nucleation densities as a function of applied cooling rates. Fig. 7(a) clearly demonstrates the effect of grain refiner addition level compared to solute concentration, showing the calculated equivalent grain diameter measurements, for Al–10 wt.%Cu and Al–20 wt.%Cu, for refiner addition levels 0.0 to 0.5 wt.%GR. Eq. (1) was used to calculate the equivalent grain diameter, d.

(a)



6 pN 0

87

1=3 ð1Þ

where N0 is the grain density, calculated as the number of nucleated grains counted in the FOV volume (2.49 mm3). A change in the copper concentration, for the same level of grain inoculation, only had a minor influence on the measured average grain diameter, showing a slight increase at lower cooling rates (<0.2 K/s). Reducing the inoculant addition level, however, resulted in a significant increase in the average grain diameter along with significantly more sensitivity to changes in the applied cooling rates. In all of the cases, where inoculation level was P0.1 wt.%GR, as the cooling rate increased up to and beyond 1.0 K/s, grain size decreases to a minimum value of 200 lm, which was also seen in the work of Mangelinck-Nöel, et al. [28] on grain refined Al–Ni alloys. Fig. 7(b) shows the grain density measurements along with the calculated equivalent grain diameter plotted against applied cooling rate for all three horizontally oriented samples tested. The error bars indicate 1 SD from the mean. No significant variance is seen when examining the grain density, however, significant variance in grain diameter is observed at low cooling rates, i.e.<0.2 K/s. This was due to the average grain size becoming comparable to the FOV dimension, as well as greater than twice the sample thickness. For further comparison, Fig. 8 shows the calculated grain diameter from (a) a Voronoi tessellation and (b) the measured separation distance between nucleation centres using Delaunay triangulation. In the case of the Voronoi tessellation the grain diameter was calculated based on the area of the generated cell. The Delaunay triangulation, however, represents the direct straight-line distance between nucleation centres of nearest neighbour grains, i.e. the grains most likely to influence subsequent nucleation. Close inspection of Fig. 8(a) and (b) shows significantly more variation in separation distances than for average diameters. Critically, the nucleation centre of an individual grain need not be close to the centroid of the grain area, illustrated by the grain labelled A4 in Fig. 4. This is further illustrated in Fig. 8(c) where the histograms calculated for both methods are compared for a high cooling rate of 1.0 K/s. Both methods calculate similar average distances, although they differ markedly with respect to the deviation. While this point may be of little relevance to the actual as-cast grain structure, for direct numerical models of solidification [29], i.e. those which model nucleation based on random distribution of grain refiner particles and log-normal or normal particle diameters, this demonstrates that particles can nucleate solid much closer to neighbouring particles than the measured equivalent grain diameter suggests. Mangelinck-Noël et al. [28] calculated the active particle distribution for an Al–3.5 wt.%Ni alloy inoculated with Al–Ti–B grain

(b)

Fig. 7. (a) Calculated grain diameter, based on grain density and measured solid area fraction, for Al–10 wt.%Cu and Al–20 wt.%Cu with 0.0 wt.%GR to 0.5 wt.% GR grain refiner inoculation levels. (b) Grain density variation (N0) with cooling rate and equivalent grain diameter (d) variation with cooling rate. Measured from 3 No. Al–20 wt.%Cu– 0.1 wt.%GR samples. Error bars denote 1 SD from the average value. Magnitude of the applied cooling rate is shown.

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(a)

(b)

(c)

Fig. 8. Al–20 wt.%Cu–0.1 wt.%GR. (a) Voronoi tessellation method of calculating average diameter distribution. (b) Nucleation centre separation distance measurements. (c) Histogram comparison for both methods at a cooling rate of 1.0 K/s. ‘Diameter’ and ‘Distance’ refer to the Voronoi equivalent diameter (a) and Delaunay separation distance (b), respectively. Magnitude of applied cooling rate is shown.

Grain Density (mm-3)

250 200 150 100 50 0

0

2

4

6

8

10

12

14

Undercooling (K) Fig. 9. Cumulative grain density as a function of undercooling for Al–20 wt.%Cu– 0.1 wt.% GR alloy samples tested. Error bars denote 1 SD from the mean value.

refiner addition. Performing vertically upwards solidification, equiaxed nucleation was promoted by increasing the pulling velocity in the presence of a constant low thermal gradient. The level of undercooling was calculated using LGK growth kinetics [30], providing a measure of grain density versus undercooling. To make a similar comparison in this work it was necessary to use the applied cooling rate and the measured thermal gradient to determine the _ T, where V is the growth equivalent growth velocity, i.e. V = T/G velocity and GT is the thermal gradient. Fig. 9 shows the measured cumulative grain density function versus the calculated undercooling. Mangelinck-Noël et al. [28] attempted to fit the experimental measurements to both a normal and log-normal particle size distribution, after Greer et al. [6], by postulating the underlying shape of the observed distribution, i.e. the maximum grain density. However, the upper limit chosen (130 mm3) was significantly lower than maximum observed in this work (>200 mm3). The methodology used by Mangelinck-Noël et al. [28] to calculate the active particle density, was based on converting the predicted grain area into the corresponding grain volume by multiplying the area by the sample thickness; the grain density was thereafter calculated as the inverse of the minimum equivalent grain volume. The reason for the choice of this methodology is unclear, considering, with in situ X-radiography, the number of grains nucleating in a representative 3D volume can be observed directly in the FOV, providing the exact grain density/active particle density measurements. In this work, it is likely that the maximum grain density measured, Fig. 7(b), was, in fact, close to the upper limit. However, further experiments are required, at higher cooling rates, to determine the exact active particle distribution. 4. Conclusions Microgravity-like solidification conditions were achieved in the laboratory, in that equiaxed grain motion was severely limited

through manipulation of the sample orientation with respect to gravity. However, grain buoyancy and rotation could not be eliminated entirely, thus full microgravity conditions are still necessary for true diffusion-limited solidification. At low cooling rates, buoyant grain motion caused a significant increase in nucleation density combined with a decrease in the average grain size. Stationary grains, in comparison, maintained a nucleation separation distance during solidification, which was inversely proportional to the applied cooling rates, causing increasing solute concentration in the intergranular liquid deactivating other available inoculant particles. Thus, increased inoculant particle efficiency may be achieved through buoyant grain motion promoting early post-nucleation grain coherency. In horizontally oriented samples, solid (area) fraction was independent of applied cooling rates and showed relatively little variance across the samples tested, suggesting 2D area measurements to be characteristic of the alloy system. Measurements of primary dendrite arm growth velocities showed no acceleration regime during early grain growth due to early post-nucleation solutal interaction between growing grains. Thus, for direct comparison with analytical growth kinetics models solutal interaction needs to be considered, requiring quantitative measurements of liquid-solute concentrations ahead of the dendrite tips. In this work, however, a detailed calibration of the laboratory-based polychromatic microfocus X-ray source and detector setup is required to account for beam hardening effects prevalent during phase transformation, which currently prevent quantitative measurements of both solid and liquid concentration profiles. The experimental data presented can be used for comparison and validation of numerical models. Acknowledgements The authors wish to acknowledge the financial support of the European Space Agency (ESA) under the PRODEX program, (Contract Nos. 90392 and 4000110414). This work is part of the ESA-MAP (Microgravity Applications Promotion) project XRMON. Thanks are due to Dr Mark Gibson and Mr Daniel East of CSIRO, Australia, for supply of materials. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.actamat.2015.04. 060. References [1] D.G. McCartney, Int. Mater. Rev. 34 (1989) 247. [2] A.L. Greer, P.S. Cooper, M.W. Meredith, W. Schneider, P. Schumacher, J.A. Spittle, A. Tronche, Adv. Eng. Mater. 5 (2003) 81.

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