Microestrutura Em 3D

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T E C H N I C A L A R T I C L E

Three Dimensional (3D) Microstructural Characterizationand Quantitative Analysis of Solidified Microstructures

in Magnesium-Based Alloys

M. Y. Wang • J. J. Williams • L. Jiang •

F. De Carlo • T. Jing • N. Chawla

Received: 1 December 2011/ Accepted: 7 February 2012/ Published online: 29 February 2012

Springer Science+Business Media, LLC and ASM International 2012

Abstract Magnesium alloys have the attractive combi-

nation of lightweight and strength. An understanding of solidification microstructures in these materials is impor-

tant. An accurate means of quantifying microstructure in

3D is extremely important. In this study, we have used

serial polishing and synchrotron-based x-ray tomography

technique as a means of 3D characterization of the solidi-

fied microstructures of magnesium-based alloys. These

models were also used to conduct quantitative analysis in

3D. The phase fraction and morphologies of intermetallics

and a-Mg matrix phase were obtained. The phase fractions

of b-Mg17Al12 and Al–Mn intermetallics are consistent

with measurements in the literature and calculations based

on the Scheil–Gulliver solidification model. Our 3D recon-

structions also show that the dendrite morphology has

sixfold symmetry. The results of 3D microstructural char-

acterization and analysis will enable a comprehensive

understanding of solidification variables, microstructure,

and properties.

Keywords Solidification Magnesium alloys Three-

dimensional characterization Synchrotron-based x-ray

tomography Serial sectioning

Introduction

Magnesium alloys are of interest as structural materials due

to their lightweight and high strength-to-weight ratio [1].

These alloys are also beginning to find use as biocompatible

implant materials [2–5]. Dendritic microstructures are

observed in a wide range of solidification processes, and

play a vital role in determining the properties of the material

[6–8]. A limited number of studies have focused on the

evolution of solidification patterns and preferred crystallo-

graphic orientations [9–11]. To date, a few studies on

dendritic microstructures in magnesium alloys have been

carried out [12–16]. In order to develop accurate anisotropic

models for the solid–liquid (S/L) interface in hexagonal

close-packed alloys, quantitative experimental character-

ization in three dimensions needs to be carried out. A sig-

nificant fraction of commercial magnesium alloys is based

on hypoeutectic Mg–Al alloys with Al concentration in the

range of 3–9 wt.%. These alloys exhibit a mixture of pri-

mary a-Mg dendrites surrounded by a eutectic network

eutectic with Mg–Al intermetallic precipitates. Thus,

understanding and quantifying the solidified microstructural

features of Mg-rich alloys in binary Mg–Al binary and other

multi-component systems are crucial. Furthermore, a fun-

damental understanding of the microstructures of materials

in three dimensions (3D) is necessary to accurately model

the evolution and formation of their microstructures.

The objective of the present study is to investigate the

morphologies and phase fraction of microstructures of

Mg–9Al alloy processed by directional solidification and a

commercial AZ91D alloy made by high-pressure die casting

(HPDC). A combination of metallographic serial sectioning

and synchrotron-based x-ray phase-contrast tomography

techniques were used to visualize and quantify the volume

fraction of microstructures in 3D.

M. Y. Wang T. JingDepartment of Mechanical Engineering,

Tsinghua University, Beijing 100084, China

M. Y. Wang J. J. Williams L. Jiang N. Chawla (&)

Materials Science and Engineering, School for Engineering

of Matter, Transport, and Energy, Arizona State University,

Tempe, AZ 85287-6106, USA

e-mail: [email protected]

F. De Carlo

Advanced Photon Source, Argonne National

Laboratory, Argonne, IL, USA

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Metallogr. Microstruct. Anal. (2012) 1:7–13

DOI 10.1007/s13632-012-0008-x



Materials and Experimental Procedure

The Mg–9 wt.% Al alloys cast specimens were prepared

from high-purity 99.95% Mg and 99.9999% Al in a heat-

resistant furnace using a boron nitride (BN)-coated mild

steel crucible. Casting was conducted under high-purity

argon gas to minimize any reactions. The alloy was heated

to 800 C, manually stirred, and held for 0.5 h before beingpoured into a high-purity graphite mold. The cast ingots

had a diameter of 10 mm and a length of 150 mm. The

initial cast ingot was then machined to a diameter of 9 mm

and a length of 110 mm and loaded in a high-purity thin-

wall graphite tube (with the inner diameter of 9 mm and

the length of 120 mm) in a Bridgman-type furnace with a

pulling system. The temperature in the furnace was con-

trolled with an accuracy of ±1 K. A water-cooled cylinder

containing Ga–In–Sn liquid metal cooling (LMC) was used

to freeze the sample. The temperature gradient in the

sample was controlled by adjusting the temperature of the

hot zone of the furnace, which was insulated from the LMCcold zone by a refractory ceramic disk. During the exper-

iment, the samples in the graphite tube were melted and

directionally solidified by pulling the crucible assembly at

desired velocities well into the LMC zone. A relatively

high-temperature gradient (GL) of 8 K/mm was obtained

and the pulling rate (V ) was set to 100 lm/s. The samples

obtained in the experiments were sectioned along the lon-

gitudinal surface (parallel to the solidification direction),

polished, and etched to find the S/L interface. The well-

defined steady solidification zone structure below the S/L

was characterized using serial-sectioning technique and

x-ray synchrotron tomography. Commercial AZ91D alloys

were prepared from master alloy ingots using conventional

HPDC to produce the near-net-shaped castings. Details on

the HPDC can be obtained elsewhere [17]. Table 1 lists the

process parameters used for this study.

Serial-Sectioning

In order to carry out serial sectioning experiments, the

Mg–9 wt.% Al samples were mounted and polished using

an automatic polisher to obtain a reproducible and con-

trolled removal rate. The serial sectioning procedure was as

follows. A square region of interest was selected from the

sample by indenting four equally spaced fiducial marks

using a Vickers pyramidal indenter. In general, selecting

the region of interest is a very subjective step in the 3D

reconstruction process. In this study, the size of the

microstructural region of interest was taken as approxi-

mately 500 9 400 lm, and a depth of approximately

150 lm. Fiducial marks were placed on the sample to

define the region of interest, to measure the material loss

during serial sectioning, and to align the indentationsduring reconstruction. This method of quantifying the

material removal has been proven to be quite effective

because the cross sections of the indentations are nearly

square, making it relatively simple to measure the length of

the diagonals [18, 19]. The approximate depth (h) of the

indentation was determined by the following equation:

h = D /2 tan(h /2), where D is the average of the indenta-

tion diagonals ( D1 and D2) on a 2D projection, and h is the

angle between the two diagonals.

When an indentation had nearly disappeared due to

polishing, a new indentation was placed on the specimen,

and both indentations were photographed so as to providecontinuity in the depth removal data. It was important to

develop a polishing procedure to obtain an efficient rate of

material removed, while still maintaining the quality of the

image produced for reconstructing the microstructures. In

addition, the thickness between sections should be based on

the size of the microstructural features. In our case, the

distance between sections was targeted at around 3 lm

based on the relative size of the dendrites and eutectic

network. The polishing routine utilized in this study is

shown in Table 2. After every section, the fresh surface

was etched slightly with 3–5 vol.% nitric acid in ethanol to

give good contrast.

Synchrotron-Based X-Ray Computed Tomography

X-ray microtomography measurements were carried out at

the Advanced Photon Source (APS) at Argonne National

Laboratory (beamline 2-BM) which offers near-video-rate

acquisition of tomographic data at micrometer spatial res-

olution [20]. An x-ray energy of *30 keV provided the

combination of high penetration ability and excellent

phase contrast for 2 mm 9 2 mm 9 2 mm volume of Mg–

9 wt.% Al as-solidified and AZ91D specimens. A CdWO4

Table 1 HPDC process parameters used for producing AZ91D die

castings

T melt, C T die, C V 1, m/s V 2, m/s Pressure,

MPa

700 200 0.25 4.5 70

T liquidus of the AZ91D alloy is *599 C

Table 2 Polishing routine used in the serial sectioning process

Sample Polishing

step

Al2O3 grit

size, lm

Time,

min

Load, N Speed,

rpm

1 I 0.3 10 5 10

II 0.02 3 0 10

2 I 0.3 5 10 20

II 0.02 1.5 5 20

8 Metallogr. Microstruct. Anal. (2012) 1:7–13

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scintillator screen was used to convert the x-rays to visible

light, and acquired with a 2048 9 2048 pixel CoolSnap K4

CCD camera. Typical exposure times ranging between 80

and 200 ms per projection were used. A resolution of

1.4 lm/voxel was obtained. A projection was acquired

every 1/8. Including the readout time and disk input/out-

put, the tomography was completed in about 20 min. The

2D projections were reconstructed in 3D using a filtered-back-projection algorithm.

After x-ray tomography, all the images were segmented

to black and white images. To reconstruct the 3D solid for

visualization, the sections were aligned and stacked in

Matlab, and then the images were segmented using con-

ventional image analysis software (ImageJ, Bethesda, MD).

Separate gray scale values were assigned to each phase,

e.g., a-Mg dendrite matrix phase, b-Mg17Al12 eutectic,

Al–Mn intermetallics, and the porosity. The 3D microstruc-

tures were digitally reconstructed using image reconstruction

software Mimics (Materialise, Ann Arbor, MI), which was

also use for quantitative analysis of the 3D volumetric data.

Results and Discussion

Serial sectioning of the Mg–9 wt.% Al samples was con-

ducted in two different regions. Figure 1a shows an optical

micrograph of the microstructure in one of the areas. A

characteristic mixture of dendritic a-Mg with clearly formed

secondary and tertiary arms and interdendritic eutectic

structure is observed. Figure 1b shows a 3D rendering of the

microstructure, made of a stack of 32 aligned sections for the

region, with a slice spacing of *2.5 lm. The a-Mg den-

drites and b-Mg17Al12 /Mg eutectic both exhibit a highly

tortuous and interconnected distribution which can onlyreally be appreciated in 3D. In order to study the 3D mor-

phologies of b-Mg17Al12 eutectic in more detail, a model

without Mg was constructed as shown in Fig. 1c. In all the

micrographs in Fig. 1, the interdendritic eutectic is clearly

visible in a network-like structure. The dendrite arms have

diameters in the range of 10–30 lm and the interdendritic

network-walls are about 15 lm thick. In addition, the

microstructure structure appears aligned to the vertical axis,

since the growth direction during directional solidification

was parallel to the vertical axis of the sliced surface. In order

to get a larger volume in a faster time, without sacrificing

metallographic surface quality, a somewhat higher load andhigher speed, at shorter time interval, was used. Figure 2

shows the second region that was analyzed using the second

serial polishing variables shown in Table 1. This 3D volume

consisted of 42 slices with a serial spacing of 3.3 lm.

Comparison of Figs. 1 and 2 reveals some slight differ-

ences in the spatial features of a-Mg dendrite matrix in the

Fig. 1 (a) 2D slice, (b) 3D reconstruction of Mg–9 wt.% Al microstructures by serial polishing method, (c) extracted b-Mg17Al12 from the

model in part (b), showing a highly interconnected network

Metallogr. Microstruct. Anal. (2012) 1:7–13 9

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two microstructures. The preferred orientation of the den-

drites is also observed in this volume.

The Mg–9Al alloy was also characterized by x-ray syn-

chrotron tomography. The advantage of x-ray tomography is

that it is non-destructive and enables the analysis of much

larger datasets than that obtained by serial sectioning. Fig-

ure 3a is a sliced image extracted from a 3D raw dataset of the Mg–9 wt.% Al alloy using the synchrotron-based x-ray

tomography technique. It shows good contrast between

a-Mg and b-Mg17Al12 phases. Because the characterized

specimens were taken from the steady solidification region,

the primary solidified dendritic phase has a very large vol-

ume fraction, and so segmentation of a complete dendrite

grain is difficult. In order to get the whole shape of an indi-

vidual dendrite crystal, a 2D livewire segmented algorithm

in ImageJ software was applied to every slice (around 60

slices) image as shown in Fig. 3b. The livewire algorithmtakes advantage of large changes in grayscale value in the

image to identify phase boundaries in the image. Figure 4

shows the three-dimensional reconstruction of a-Mg den-

drites. It appears that thea-Mg grows along 1120h i-orientated

directions in the (0001) basal plane. We also note an almost

perfect sixfold symmetryin thedendrites, givenby a 60 angle

between primary arms. Figure 5 shows the reconstructed

visualization of micro-porosity and b phase. The insert in the

top right corner shows the large magnified shape of microp-

ores. From the 3D reconstructions, the volume fractions of

each phase were measured. These are shown in Table 3.

Figure 6 shows the reconstructed microstructure of AZ91D in the HPDC condition using x-ray computed

tomography. Normal dendrites in AZ91D alloy can be

suppressed to a certain extent, and within the interdendritic

of a-Mg grains b phase as well as Al–Mn phases are dis-

persed close to the grain boundary. The reconstructed

results revealed the 3D distribution of the micropores, high-

density areas of Al–Mn intermetallics particles, and b phase

(Fig. 6b–d). The Mg17Al12 precipitates are quite finer

compared to the coarser Mg–9 wt.% Al microstructure

Fig. 2 a-Mg dendrite matrix

and b-Mg17Al12 from another

region. The microstructural

walls are parallel to the

solidification direction

Fig. 3 X-ray synchrotron tomography data and 2D segmented image

of a-Mg by livewire segmentation algorithm

Fig. 4 An isolated a-Mg dendrite in Mg–9 wt.% Al alloy showing

sixfold symmetry


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obtained by slower cooling in directional solidification. A

uniform, fine, and dispersed distribution of micropores

could be observed in the reconstruction of the AZ91D

material compared to the large shrinkage voids of direc-

tionally solidified Mg–9 wt.% Al. In general, micropores

are the primary defects formed during HPDC of Mg and Al

alloys. So, the quantitative data analysis for micropores is

quite important. In this study, the volume fraction of

micropores was measured as 0.24% in HPDC AZ91D. The

spatial distribution was quite homogeneous.

The volume fractions of the b phase in the as-solidified

two Mg–9Al samples analyzed were measured in two

regions as 11.2 and 12.3%, respectively. The predicted

fraction of b can be computed by the Scheil–Gulliver

solidification model [21, 22]:

C S ¼ kC 0 1 f Sð Þk 1 ð1Þ

where C S is the solid composition at the given tempera-

ture, C 0 is the alloy composition, k is the solute partition

coefficient in terms of the equilibrium phase diagram, or

k = C S / C L, and f S is the mass fraction of the solid phase.

This equation is a non-equilibrium lever rule which

assumes a local equilibrium state of the advancing solidi-

fication front at the solid–liquid interface, i.e., DL = ?

and DS = 0. A value of about 10 vol.% was computed

using this equation, which is close to that measured

experimentally. In our quantitative analysis, the volume

fractions of the b phase and Al–Mn intermetallics are also

comparable to that reported the in the literature, as shown

in Table 3. Using x-ray tomography, relatively large vol-

umes of AZ91 alloy were analyzed. In this alloy, theamount of eutectic b phase measured experimentally was

7.9%. This is in good agreement with that predicted by the

Scheil–Gulliver equation [21, 22], which predicts a value

of 8.3%.

Conclusions

In conclusion, we successfully used the combined method of

serial-sectioning and synchrotron-based x-ray computed

tomography to characterize and quantify microstructures of

Mg–9Al andAZ91D in three-dimensions.The phase fractionand morphologies of intermetallics and a-Mg matrix phase

were obtained. The phase fractions of b-Mg17Al12 and

Al–Mn intermetallics are consistent with measurements in

the literature and calculations based on the Scheil–Gulliver

solidification model. Our 3D reconstructions also show that

the dendrite morphology has sixfold symmetry. More gen-

erally, these quantitative results of phase fraction and shapes

should be used in validating phase-field modeling and as an

input in microstructure-based finite element analysis to

Fig. 5 (a) Shrinkage micro-

porosity and (b) b-Mg17Al12network in as-solidified Mg–

9 wt.% Al

Table 3 Volume fractions of phase in Mg–9Al and AZ91D alloys,

along with comparison with literature data

Phase Phase fractions, %

Mg–9Al

(serial

sectioning)

AZ91D

(x-ray

tomography)

Literature

comparison

for AZ91D

a-Mg dendrite matrix Bal. Bal. Bal.

b-Mg17Al12 11.2, 12.3 7.9 6.5, 11.2 [19],

12.7 [20]

Voids 0.20 0.24

Al–Mn intermetallic 0.18 0.4 [20]


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better understand the structure–property relationships in

these materials.

Acknowledgments MYW and TJ gratefully acknowledge the

financial support for this study by the National Science and Tech-

nology Major Project of China, under Grant No. 2011ZX04014-052;

the National Science Foundation of China, under Grant No.

51175292; and the Doctoral Fund of Ministry of Education of China,

under Grant No. 20090002110031. Use of the Advanced Photon

Source was supported by the U.S. Department of Energy, Office of

Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. MYW also acknowledges the financial support

from the China Scholarship Council during his stay at Arizona State

University.

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