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Research Paper

Biodegradable Mg–Zn–Y alloys with long-period stackingordered structure: Optimization for mechanical properties

Xu Zhao, Ling-ling Shi, Jian Xu�

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road,

Shenyang 110016, China

a r t i c l e i n f o

Article history:

Received 6 August 2012

Received in revised form

20 November 2012

Accepted 22 November 2012

Available online 5 December 2012

Keywords:

Magnesium alloys

Biodegradable

Alloying

Mechanical properties

Long period stacking order

nt matter & 2012 Elsevie.1016/j.jmbbm.2012.11.01

hor. Tel.: þ86 24 [email protected] (J. Xu).

a b s t r a c t

To optimize the mechanical properties for biodegradable orthopedic implant, microstructures

and tensile properties of Mg–Zn–Y alloys containing long period stacking ordered (LPSO)

phase were investigated. For the as-cast Mg100�3x(Zn1Y2)x (1rxr3) alloys, volume fraction of

18R LPSO phase increases with increasing the contents of Zn and Y. Mg97Zn1Y2 alloy exhibits

the optimal combination of strength and plasticity. Substitution of bioactive element Ca for Y

in the Mg97Zn1Y2 does not favor the formation of LPSO phase, but involving the formation of

Mg2Ca phase. By micro-alloying with Zr as grain refinement agent, morphology of a-Mg in the

Mg96.83Zn1Y2Zr0.17 alloy is changed into the equiaxial shape, together with a significant

refinement in grain size to 30 mm. It brings about an improvement not only in strength but

also in plasticity, in contrast to the Zr-free alloy. In comparison with the as-cast state, warm-

extruded alloys manifest significantly improved properties not only in strength but also in

plasticity due to the refinement of a-Mg grain by dynamic recrystallization and the alignment

of LPSO phase along extrusion direction.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Over the past decade, in terms of corrosion features in physio-

logical environment, a considerable attention has been paid to

the magnesium alloys as biodegradable material (Hort et al.,

2010; Li et al., 2008; Staiger et al., 2006; Witte, 2010; Witte et al.,

2008). In comparison with the stainless steels and titanium

alloys, biodegradable magnesium alloys are identified as revo-

lutionizing biometals (Yun et al. (2009)). Their potential applica-

tions are expected at least in several aspects such as vascular

stent (Erbel et al., 2007; Mani et al., 2007), orthopedic implants (Li

et al., 2008; Willbold et al., 2011; Witte et al., 2005) and tissue

engineering scaffold (Geng et al., 2009). It should be noted that,

r Ltd. All rights reserved.6

; fax: þ86 24 23971215.

from the perspective of mechanical properties, requirement for

the properties of alloy is significantly dependent on the applica-

tion target. Considering the orthopedic application, in contrast

to the stainless steels and Ti alloys, key advantage of magne-

sium alloys is degradable and absorbable in human body, rather

than permanently resident. Then, a second surgery is not

necessary, to remove the temporary device after the tissue

healed well. Meanwhile, magnesium alloys have low mass

density of 1.7–2.0 g/cm3 and elastic modulus of about 40 GPa,

which are well matched with those of natural bone. It is

expected to limit the osteolysis due to the stress shielding effect

induced by severe mismatch in modulus (Nagels et al., 2003;

Staiger et al., 2006). Compared with biodegradable polymers

dx.doi.org/10.1016/j.jmbbm.2012.11.016

dx.doi.org/10.1016/j.jmbbm.2012.11.016

dx.doi.org/10.1016/j.jmbbm.2012.11.016

mailto:[email protected]

dx.doi.org/10.1016/j.jmbbm.2012.11.016


currently used in clinical practice (Bostman, 1991), magnesium

alloys manifest significant advantages including the high

strength and ductility, and the reliability for load-bearing

implant application. In addition, magnesium alloys are conve-

nient for processing of the machining and sterilization. In the

sense of long-term safety, degradation products in physiological

environment of magnesium alloys are alkaline rather than acid

as the polymer does. As revealed recently (Witte et al., 2005),

degraded magnesium ion has a significant effect to stimulate

the growth of new bone tissue, to make damaged bone tissue

healing more quickly. For orthopedic implants such as screws,

pins and plates for fixation, the desirable implants should have

a strength as high as possible and an acceptable ductility,

together with a controllable degradation rate. As an important

issue, alloying elements in the alloy and corrosion products

must be non-toxic during degradation in the body. Currently,

commercially-available magnesium alloys with high strength,

such as AZ31, AZ91 and WE43, contain toxic elements in some

extents. For instance, aluminum which plays a role for solid-

solution strengthening a-Mg phase is a neurotoxicant, led to a

risk for Alzheimers disease (El-Rahman, 2003). Toxicity of rare-

earth elements remains in debate. It is believed that accumula-

tion of cerium or praseodymium in the body probably causes

severe hepatotoxicity (Nakamura et al., 1997). In addition, fast

degradation rate in physiological environment brings about the

problems of hydrogen release and loss of mechanical integrity

before healing. Therefore, it is of interest to design new

magnesium alloys to meet the requirement of each special

clinical application.

On the other hand, magnesium alloys with long period

stacking ordered (LPSO) phase are reported to have excellent

mechanical properties, and the major LPSO structure are 18R

and 14H structure (Kawamura and Yamasaki, 2007). The LPSO

phase has a similar a-axis with hcp Mg, but its stacking

periodicity is lengthened along the c-axis (Luo and Zhang,

2000). Therefore, in the electron diffraction spots of LPSO

phase, it presents a series of additional spots besides the

spots of hcp Mg. Moreover, the LPSO phase is also a chemi-

cally ordered structure, in which the solute atoms are

enriched in four atomic layers on the closely packed plane

at six-period intervals for 18R structure or seven for 14H

structure. So far, LPSO phase has been found to be formed in

ternary Mg–Zn–RE (RE¼Y, Gd, Tb, Dy, Ho, Er, Tm) alloy

(Kawamura and Yamasaki, 2007). Strengthening mechanism

of LPSO phase was explained according to following issues.

First of all, the presence of LPSO phase in a-Mg matrix plays a

role of increasing the critical resolved shear stress of basal

slip and activates non-basal slip (Matsuda et al., 2005a). Next

the interface between LPSO and Mg is coherent along both

the basal and prismatic plane (Shao et al., 2010). Finally,

kinking bands of LPSO phase, which forms during the

deformation processes, are considered as an additional con-

tributor for strengthening (Hagihara et al., 2010b; Shao et al.,

2010). Obviously, volume fraction of LPSO phase is important

to determine the mechanical properties of the alloy. As

indicated by Hagihara work (Hagihara et al., 2010a), compres-

sive yield strength for Mg89Zn4Y7 alloy with 85%–90% of

volume fraction of LPSO phase can reach to nearly 500 MPa,

which is nearly three fold higher than conventional Mg

alloys. As proposed empirically, the reason responsible for

the formation of LPSO phase is attributed to large atomic size

ratio of alloying element, such as rare earth, against Mg and

negative heat of mixing between the atomic pairs (Kawamura

and Yamasaki, 2007). In the sense of biocompatibility, it is

interesting to note that alloying elements in the Mg–Zn–Y

alloys with LPSO phase are biocompatible. Zinc is one of the

most abundant nutritionally essential elements in human

body, and the appropriate daily intake of zinc is 14 mg/d,

(Goldhaber, 2003). Yttrium is also biocompatible with a

tolerant daily intake of 4.2 mg/d (Zhang et al., 2000). As a

result, Mg–Zn–Y alloys with LPSO phase are basically biocom-

patible. However, the concentration of Y should be mini-

mized since it is not an essential element for human body.

In order to minimize the potential long-term toxicity in the

alloys, element calcium is selected to substitute for Y,

because not only it is a major component in human bone

but also Ca ion combined with Mg ion can promote the bone

healing (Li et al., 2008). To favor the formation of LPSO phase,

Ca atom is expected to be equivalent to the Y atom. Also, heat

of mixing between Zn and Ca is very negative (DHmix¼�22 kJ/

mol) (Takeuchi and Inoue, 2005). Furthermore, zirconium is

the most effective grain refiner for Mg alloy to strengthen the

alloy through grain refinement. At the same time, Zr is a

biocompatible element (Eisenbarth et al., 2004). Therefore,

Zr is also a good candidate element for designing new

biodegradable magnesium alloys.

In the current work, Mg97Zn1Y2 alloy was selected as the

base composition due to its high strength reported previously

and good biocompatibility. Firstly, by tuning the concentra-

tion of Zn and Y in the alloy, the effect of volume fraction of

LPSO phase on mechanical properties was investigated.

Secondly, substitution of Ca for Y in Mg97Zn1Y2 alloy on the

formation of LPSO phase was examined, as well as the

influence on mechanical properties. Based on the alloys with

optimal mechanical properties, influence of grain refinement

by micro-alloying with Zr on mechanical properties was

examined. Finally, selecting the typical alloys for warm-

extrusion, the microstructure and mechanical properties of

the extruded alloys are investigated.

2. Experimental

Commercial pure Mg (99.99%) and Zn (99.999%) with high

purity, and intermediate alloys of Mg–30%Y (mass percen-

tage, wt%) and Mg–30%Zr (wt%) were used as starting materi-

als. Alloy ingots with nominal composition in quantities of

about 100 g were induction-melted in a graphite crucible

under an inert atmosphere. The molten alloys were then cast

into the steel mould. In addition, ingots in quantities of 5–6

kg were prepared as well for the warm extrusion. For these

large-sized ingots, the pure elements and intermediate alloys

were melted in electrical resistance furnace using mild steel

crucible protected by mixture gases of N2 and CO2. The alloy

melt was held for 30 min at 973 K and then poured into a steel

mould with a cavity diameter of 130 mm, which is preheated

to 473 K. Warm extrusion of as-cast ingots were performed at

723 K, using an extrusion ratio of 42:1 and extrusion rate of

3 mm/s at atmosphere.

j o u r n a l o f t h e m e c h a n i c a l b e h a v i o r o f b i o m e d i c a l m a t e r i a l s 1 8 ( 2 0 1 3 ) 1 8 1 – 1 9 0 183

For the observation of optical microscopy (OM) and scan-

ning electron microscopy (SEM), samples cut from the ingots

were mechanically polished and then etched with a solution

of the picric acid of 5 g, acetic acid of 5 ml, distilled water of

10 ml and ethanol of 100 ml. OM observation was conducted

on the Leica optical microscope. SEM characterization was

performed on a Quanta 600 (FEI, the Netherlands) micro-

scope. Crystalline phases in the alloys were identified using

X-ray diffraction (XRD), at a Rigaku D/max 2400 diffract-

ometer (Tokyo, Japan) with monochromated Cu Ka radiation.

Average grain size of the alloys was measured using the

linear intercept method. Volume fraction of the LPSO phase

in microstructure of the alloys was estimated by image

analyzer. Transmission electron microscopy (TEM) was used

for characterizing the morphology and phase identification

with JEM-2000FXII (Tokyo, Japan). For TEM observation, disks

with a thickness of about 300 mm were cut from bulk sample,

then ground to a thickness of about 50 mm, and finally

thinned into foil with argon ion milling.

Mechanical properties of the alloys were evaluated by

standard tensile test at room temperature. Cylindrical tensile

bars with a diameter of 3 mm and gauge length of 15 mm

were cut from as-cast ingots or extruded bar along the

extrusion direction. For the as-extruded specimens, loading

axis was taken to be parallel to the extruded direction.

Tensile tests were carried out on a Zwick/Roell Z050 Testing

System with a strain rate of 1�10�3 s�1. An extensometer

with a gauge length of 15 mm was used to measure the strain

response. At least three samples were tested for each state.

3. Results

3.1. Formation of LPSO phase in as-cast alloys

Fig. 1 shows the XRD patterns of as-cast alloys with a

nominal composition of Mg100�3x(Zn1Y2)x (x¼1, 2, 3). As

indicated, all of the alloys are identified as two phases,

a-Mg and Mg12YZn phase. With increasing the Zn and Y

contents in the alloys as a fixed ratio of 1:2 for Zn:Y,

diffraction intensities of Mg12YZn phase gradually increase,

Fig. 1 – XRD patterns of as-cast Mg100�3x(Zn1Y2)x (x¼1, 2, 3)

alloys.

indicating the increase of volume fraction of Mg12YZn phase.

As the representative, Fig. 2(a) displays a TEM bright field

image of the Mg97Zn1Y2 alloy. In terms of the contrast, a

phase as a bright region marked as A, and a lamellae phase as

a dark region marked as B are observed. For the areas of A

and B, corresponding selected area electron diffraction

(SAED) patterns taken along the /1 1 2 0S direction are

shown in Fig. 2(b) and (c), respectively. From the SAED pattern

in Fig. 2(b), region A is identified to be a-Mg with hcp

structure. However, as shown in Fig. 2(c), additional reflection

spots array at positions n/6 (n is an integer) of the (0 0 0 2)Mg

plane, indicating that the Mg12YZn phase (region B) is a

typical structure of 18R LPSO (Zhu et al., 2010).

Fig. 3(a)–(d) show backscattered SEM images of as-cast

Mg100�3x(Zn1Y2)x (x¼1, 2, 3) alloys. In all cases, microstructure

in the alloys consists of a-Mg phase with dark contrast, and

18R LPSO phase with bright contrast. At x¼1, as seen in

Fig. 3(a), fraction of LPSO phase is estimated to be about

25 vol%. Dendritical LPSO phase precipitates in the area

between secondary dendrite arm of a-Mg dendrite. Average

grain size of a-Mg dendrite is around 150 mm, with secondary

dendrite arm spacing of about 25 mm. At x¼2, fraction of

LPSO phase increases up to �48 vol%. Grain size of a-Mg

remains at about 110 mm, without distinct secondary dendrite

as at x¼1. LPSO phase grows up into irregular block-like

shape. At x¼3, fraction of LPSO phase increases up to

�66 vol%. Grain size of a-Mg is reduced to be about 80 mm.

LPSO phase becomes the matrix and no secondary

dendrite forms.

Fig. 2 – (a) TEM bright field image of Mg97Zn1Y2 alloy. (b) and

(c) SAED patterns along the /1 1 2 0S direction for the areas

marked with A and B, respectively.

Fig. 3 – SEM images of Mg100�3x(Zn1Y2)x alloys. (a) x¼1, (b) x¼2, (c) x¼3, (d) in high-magnification for x¼3.

Fig. 4 – XRD patterns of as-cast Mg97Zn1Y2-yCay (y¼0, 0.5, 1,

1.5) alloys.

Fig. 5 – (a) TEM bright field image of Mg97Zn1Y1Ca1 alloy.

(b), (c) and (d) SAED patterns along the /1 1 2 0S direction

for the areas marked with A, B and C respectively.


Based on the Mg97Zn1Y2 alloy, element Ca is used to

substitute Y in the alloys. Fig. 4 shows XRD patterns of as-

cast Mg97Zn1Y2�yCay (y¼0, 0.5, 1, 1.5) alloys. With increasing

Ca content in the alloys, intensity of diffraction lines for

Mg12YZn LPSO phase is mitigated. It is accompanied by

gradual increase in intensity of diffraction lines for Mg2Ca

phase. In the case of the y¼0.5 and y¼1, Mg2Ca phase co-

exists with LPSO phase. Nevertheless, LPSO phase is no

longer detectable at y¼1.5, whereas Mg2Ca phase exists as

the second phase in the alloy only. At y¼0.5 and y¼1,

microstructures of two alloys are roughly similar. Fig. 5(a)

displays a TEM bright-field image of Mg97Zn1Y1Ca1 alloy.

As observed in Fig. 5(a), three phases present different contrast,

marked as A for bright region, B for lamella-structure region

and C for spherical-structure region, respectively. Fig. 5(b)–(d)

show SAED patterns of these three phases taken along the

/1 1 2 0S direction. Similar to the case of Ca-free Mg97Zn1Y2

alloy (see Fig. 2), region A and B is identified as a-Mg phase and

18R LPSO phase, respectively. Moreover, region C with spherical

feature is identified as Mg2Ca phase. It indicates that substitu-

tion of Ca for Y in the alloy does not have an effect to promote

the formation of LPSO phase, even though atomic size of Ca

Fig. 6 – Optical micrographs of as-cast Mg97Zn1Y2 alloy (a) and (b) and Mg96.83Zn1Y2Zr0.17 alloys (c) and (d). (b) and (d) are

magnification micrographs.

Fig. 7 – Engineering stress–stain curves in tension for as-

cast Mg97Zn1Y2 and Mg96.83Zn1Y2Zr0.17 alloys.


atom is large and heat of mixing between Ca and Mg is very

negative.

Furthermore, minor amount of Zr is added in the Mg97

Zn1Y2 alloy as grain refiner. For a comparison Fig. 6(a)–(d)

show optical micrographs of as-cast Mg97Zn1Y2 and Mg96.83

Zn1Y2Zr0.17 alloys. As indicated, after micro-alloying with Zr,

morphology of a-Mg grain changes from dendrite to equiaxed

shape, together with the refinement of grain size from 150 mm

down to 30 mm. In contrast to the Zr-free alloy, LPSO phase

distributes at grain boundary of a-Mg phase Zr-containing

alloy. The volume fraction and grain size of LPSO phase

remain unchanged. In terms of the XRD pattern (not shown

here), Mg96.83Zn1Y2Zr0.17 alloy also consists of two phases, a-

Mg phase and Mg12YZn phase, which is the same as the

Mg97Zn1Y2 alloy, as seen in Fig. 1. In other words, micro-

alloying with Zr addition only plays a role of grain refinement

for a-Mg matrix.

3.2. Mechanical property of as-cast alloys

As the representative, Fig. 7 displays engineering stress-strain

curves of as-cast Mg97Zn1Y2 and Mg96.83Zn1Y2Zr0.17 alloys in

tension. In contrast to the Zr-free alloy, Mg96.83

Zn1Y2Zr0.17 alloy exhibits higher yield strength and larger

plasticity. As a result, adding Zr as grain refiner has a

significant effect to the strength and plasticity.

Fig. 8 shows a plot of the strength and elongation against

content of the Zn and Y as well as volume fraction of LPSO

phase in Mg100�3x(Zn1Y2)x (x¼1, 2, 3) series as-cast alloys.

As indicated, with increasing the Zn and Y contents, tensile

yield strength (sy) of the alloy increases from 136 MPa at x¼1

to 157 MPa at x¼3, increased by �15%. Ultimate tensile

strength (UTS) of the alloy first increases from 218 MPa at

x¼1 to 236 MPa at x¼2 with an increase of�9%, and then

decreases to 221 MPa at x¼3. On the contrary alloy elongation

is drastically reduced, from 7% at x¼1 to 1% at x¼3. Conse-

quently, the LPSO phase does have a significant effect of

strengthening the magnesium alloys, but accompanied by

degradation in plasticity.

Fig. 9 shows a plot of the strength and elongation against

Ca concentration in Mg97Zn1Y2�yCay (y¼0, 0.5, 1) series

as-cast alloys. As the Ca content increases in the alloys, yield

strength increases from 136 MPa for the Ca-free alloy up to

146 MPa at 0.5% Ca with an increase of �7%, but subsequently

decreases to 116 MPa at y¼1. The UTS is slightly reduced,

from 218 MPa at y¼0 to 205 MPa with a reduction of 5% for

Fig. 8 – Strength and elongation versus volume fraction (Vf)

of 18R LPSO phase in Mg100�3x(Zn1Y2)x (x¼1, 2, 3) alloys.

Fig. 9 – Strength and elongation versus content of Ca in

Mg97Zn1Y2�yCay (y¼0, 0.5, 1) alloys.

Fig. 10 – Optical micrographs of as-extruded Mg97Zn1Y2

alloy, taken from the (a) transverse section and (b) the

longitudinal section along the extrusion direction.

Fig. 11 – XRD patterns of as-extruded Mg97Zn1Y2 alloy, taken

from the (a) transverse section and (b) the longitudinal

section along the extrusion direction.


Mg97Zn1Y1Ca1 alloy. The elongation decreases from 7% to 5%

without remarkable change on further increasing the Ca

content. When the Ca content is less than 0.5%, the strength

increase, associated with deterioration in the plasticity.

When the Ca content is higher than 1%, deterioration in both

strength and plasticity is severe.

3.3. Microstructure and mechanical propertyof as-extruded alloys

To further improve the mechanical properties, microstructure

and tension properties of the warm-extruded Mg97Zn1Y2

alloys with and without Zr addition were examined.

Fig. 10(a)–(b) show optical micrographs of as-extruded

Mg97Zn1Y2 alloy, taken from the transverse and longitudinal

sections along the direction of extrusion, respectively. As

seen in Fig. 10(a), subjected to the warm-extrusion, distribu-

tion of LPSO phase becomes more uniform, in the form of

plate-like or block-like shapes at the transverse section.

As shown in Fig. 10(b), the LPSO phase aligns along the

direction of extrusion at the longitudinal section. Moreover,

the a-Mg grains are significantly refined due to dynamic

recrystallization, with average grain size of about 17 mm. In

contrast, the grains of LPSO phase distribute in the fiber-like

form with a thickness of 5–15 mm and length of about 100 mm.

Fig. 11 shows the XRD patterns of as-extruded Mg97Zn1Y2

alloy, taken from the transverse and longitudinal sections

along the direction of extrusion. As seen in the pattern (a),

intensities of (1 0 _1_1 0) diffraction plane is stronger at the

transverse section, whereas stronger one become (0 0 0 2) basal

plane at the longitudinal section in the pattern (b). As a result,

it is evident that warm-extrusion processing introduces a basal


texture in the alloy. Since dynamic recrystallization happens

mainly in the a-Mg phase, crystallographic orientation of a-Mg

grains is randomly arranged. Thus, formation of the texture is

probably caused by the deformation of LPSO phase. In fact,

such a texture was also found by Yamasaki et al. (2011), which

was explained by that strong texture easily happens in the

LPSO phase owing to their crystal rotation during extrusion.

Similar microstructure and texture are observed also in the as-

extruded Mg96.83Zn1Y2Zr0.17 alloy.

Fig. 12 shows a diagram of a comparison of the as-extruded

alloys with the as-cast alloys for the tensile strength and

elongation. In comparison with the as-cast alloys, extrusion

deformation significantly enhances both of the strength and

plasticity in two states for the alloys with and without Zr.

Subjected to the extrusion, tensile yield strength of Mg97

Zn1Y2 alloy increases by �25%, whereas the UTS increases by

�33%, together with an increase by a factor of three in

elongation. For the as-extruded alloy, no obvious difference

between Mg97Zn1Y2 and Mg96.83Zn1Y2Zr0.17 alloys is present.

4. Discussion

4.1. Formation of LPSO phase

As well documented, LPSO structure in Mg alloys are present

in several forms including 18R, 14H, 10H and 24R (Matsuda

et al., 2005b). The 18R type of LPSO structure usually appears

in the ingot cooled by conventional rates because it is an

equilibrium phase at room temperature, while the 14H type

of structure is usually present after solid-solution treatment

(Kawamura and Yamasaki, 2007; Zhu et al., 2010). In the

current case, we found that the 18R type of LPSO structure is

formed in the as-cast Mg97Zn1Y2 alloy. Increasing the alloying

elements of Zn and Y does not change the solidification path

in all Mg100�3x(Zn1Y2)x alloys, L-a-MgþLPSO.

From the view of atomic size, Ca (rCa¼0.197 nm) is compar-

able to Y (rY¼0.182 nm), with an atom radius larger than that

of Mg (rMg¼0.16 nm) atom (Takeuchi and Inoue, 2005). Mean-

while, the Ca has negative heat of mixing with the Zn and

Mg. However, our results indicate that Ca addition in the alloy

does not play a role to enhance the formation of LPSO phase.

Complete substitution of Ca for Y in the alloy gives rise to the

Fig. 12 – Strength and elongation of Mg97Zn1Y2 and Mg96

disappearance of LPSO phase, as shown in Fig. 4. Thus, our

findings do not support the suggestion that the formation of

LPSO phase is associated with large atom size mismatch with

Mg and negative heat of mixing between the components

(Amiya et al., 2003). Recently, Kawamura and Yamasaki (2007)

proposed that adding the elements with large solid solubility

in the Mg probably favor to the formation of LPSO phase in

Mg–Zn–RE alloys. It is noteworthy that lower solid solubility

of Ca in Mg is consistent with this suggestion. Consequently,

the LPSO phase formed in Mg97Zn1Y1.5Ca0.5 and Mg97

Zn1Y1Ca1 alloys is mainly due to the presence of Y. In these

Ca-containing alloys, solidification path is the same as in the

Ca-free alloy besides the formation of Mg2Ca intermetallics.

In the microstructure, the LPSO phase and Mg2Ca phase

alternatively arrange in the lamellar form.

During solidification of the melt, dendrite grains of a-Mg

with low solute concentration precipitate as primary phase,

and then the remaining melts with a high concentration of

solutes solidify as the LPSO phase into the gap of dendrite

arms. a-Mg grains grow into large dendrite with secondary

dendrite arm in the alloy only in the case of lower fraction of

LPSO phase. With increasing the concentration of alloying

elements, melt composition shifts towards the eutectic point.

It suppresses the formation of secondary dendrite, and the

precipitation of LPSO phase is in large-sized block as the

matrix.

4.2. Strengthening effect via grain refinement

As well known, grain refinement is conventional approach

to strengthen the alloys without sacrificing the ductility.

Relationship between strength and grain size is empirically

described as Hall–Petch relation ss¼s0þkd�1/2. In this equa-

tion, the value of k represents the extent of enhancement in

yield strength with the reduction of grain size. In the

magnesium alloys, strengthening effect is more sensitive to

the grain size, because the k value of magnesium is high

(k¼2–15 MPa/mm1/2) (Barnett et al., 2004). For as-cast alloys,

alloying with minor addition Zr has significant effect to refine

the grain size. Refinement validity of a given element can be

evaluated by growth restriction factor (GFR) (Lee et al., 2000).

.83Zn1Y2Zr0.17 alloys in as-cast and as-extruded state.


Zr element of GFR¼38 has been recognized to be the most

effective grain refiner for the Mg alloys.

In the as-cast Mg96.83Zn1Y2Zr0.17 alloy, micro-alloying with

Zr element leads to a transition of a-Mg dendrites into

equiaxed crystals in the microstructure. Because Zr atoms

in the melt provide abundant nucleation sites for a-Mg grain,

it made a-Mg easier to form as equiaxed crystals. In contrast

to the primary dendrite in the Mg97Zn1Y2 alloy, grain size of

a-Mg in Mg96.83Zn1Y2Zr0.17 alloy is refined from 150 mm to

30 mm. It is such microstructure that yields a combination of

strength and plasticity in Mg96.83Zn1Y2Zr0.17 alloy, superior to

the Zr-free Mg97Zn1Y2 alloy.

As shown in Fig. 12, both the strength and plasticity are

significantly improved after warm-extrusion. During the

extrusion, plastic deformation at the interface between

the Mg matrix grain and LPSO phase is incompatible, since

the LPSO phase exhibits strong plastic anisotropy (Hagihara

et al., 2010b). It yields a large stress concentration at the

interface. Then, dynamically recrystallization only occurs

within the ‘‘soft’’ a-Mg grains as the deformation zones.

Subject to the recrystallization in as-extruded Mg97Zn1Y2

alloy, a-Mg grains manifest equiaxed shape and small grain

size of about 30 mm. It brings about the improvement of the

mechanical properties after extrusion deformation. Our

results indicate that warm-extrusion is more crucial to

improve the plasticity, as indicated by an elongation of 30%

for the extruded alloy.

4.3. Effect of LPSO phase on the strength and plasticityof alloys

In the Ca-containing alloys, although Mg2Ca phase precipi-

tates as additional secondary phase besides LPSO phase,

volume fraction of a-Mg is almost equal to the case of

Mg97Zn1Y2 alloy. No obvious difference in grain size of a-Mg

dendrite is observed between the alloys with and without Ca.

As indicated in Section 3.2, the mechanical properties of

Ca-containing alloys are inferior to those of Mg97Zn1Y2 alloy.

In other words, it indicates that the effect of strengthening

and toughening from Mg2Ca phase are incompetent to LPSO

phase, which mainly result from the good deformability of

LPSO phase. Plastic deformation in the LPSO phase operates

via two routes, (0 0 0 1) basal slip and kinking deformation

(Hagihara et al., 2010b). In as-cast alloys, crystal orientation

of LPSO grains is random. Then, it is certain that some LPSO

grains with a large Schmid factor for the (0 0 0 1) /1 1 2 0Sbasal slip exist. Consequently, these basal slips of LPSO phase

are responsible for its large plasticity.

As noticed, effects of strengthening and toughening of

LPSO phase depend on the fraction of LPSO phase in the

alloy, as shown in Fig. 7. As shown by Hagihara et al. (2010a)

grain size effect of LPSO phase on the strength in Mg alloys

also follows to the classical Hall–Petch relation. It means that

the morphology and grain size of LPSO phase are key factors

on the mechanical properties. In the Mg97Zn1Y2 alloy, LPSO

phase precipitates in the form of thin plate-like phase

dispersed in the secondary dendrite. For the Mg91Zn3Y6 alloy,

however, LPSO phase assembles to thick block-like shape as

the matrix, together with large length and thickness. Conse-

quently, global properties of the alloys are controlled by a

combination of the grain size and fraction of LPSO phase.

With increasing the fraction of LPSO phase, yield strength of

the alloys increases slightly, while the plasticity dramatically

decreases.

Since the LPSO phase has a large ratio of critical resolved shear

stress (CRSS) of basal slip to non-basal slip (Yamasaki et al.,

2011), twin deformation and dynamic recrystallization is difficult

under extrusion deformation. As a result, LPSO phase has a

strong propensity to create the basal texture, in which the c axis

of LPSO grains is perpendicular to the direction of extrusion.

Such unique texture plays an important role responsible for the

deformation of LPSO phase. Under the tension loading condition,

loading axis of the specimen is parallel to the extrusion direction

of ingot. Then, the loading axis is parallel to the (0 0 0 1) basal

plane of LPSO phase. It results in that the Schmid factor for the

(0 0 0 1) /1 1 2 0S basal slip is negligible, and that the operation

of basal slip is suppressed in the as-extrusion specimen, and

kinking deformation occurs. Hagihara et al. (2010b) investigated

the deformation mode of LPSO phase in the alloys prepared by

directional solidification during compression test, and proposed

the kinking-deformation mechanism of LPSO phase. Elastic

buckling in LPSO phase, which occurred in the region with high

stress, induces the creation of basal dislocation pairs with an

opposite Burgers vector, and their motion in opposite directions

leads to the formation of deformation kink bands. In addition,

Yamasaki et al. (2011) suggested that kinking deformation

contributes to the major deformation in tensile loading. Under

the kinking deformation, crystal orientation of LPSO grain

changed. In this way, the basal slipping can be operated after

large plastic deformation, which is responsible for the large

plasticity of the alloy.

4.4. Comparison with other biodegradable magnesium alloys

Fig. 13(a) and (b) display diagrams to summarize the yield

strength and elongation of some typical magnesium alloys

for biomedical applications in the as-cast and extruded state,

respectively. As indicated, in all cases, strength and elonga-

tion of the as-extruded alloys are higher than those as-cast

alloys, except for as-extruded Mg–1Ca binary alloy. As indi-

cated in Fig. 13(a), elongation of as-cast alloys is less than

15%, and T6 treated WE43 alloy manifests the highest yield

strength at 185 MPa together with elongation of �7%

(Avedesian, 1999). Our Mg96.83Zn1Y2Zr0.17 alloy exhibits good

comprehensive mechanical properties, comparable to the as-

extruded Mg–1Ca binary alloy. For the group of as-extruded

alloys, as seen in Fig. 13(b), ZYbK520 alloy (Gunde et al., 2011)

has the highest yield strength of 350 MPa. Nevertheless, the

current Mg97Zn1Y2 alloy has the largest elongation of 30%,

which can rival to ZW21 alloy (Hanzi et al., 2009). In addition,

the alloy has high uniform elongation of 21%.

As shown in Section 3.2, in the alloys with Ca substitution,

there is only a modest reduction of strength and a rather

more significant reduction in elongation. Considering the

advantages in biocompatibility, such a change in mechanical

properties is acceptable. Finally, besides the mechanical

properties, degradation performances under simulated phy-

siological condition for the Mg–Zn–Y alloys with LPSO micro-

structure are investigated as well in the additional work, to be

presented elsewhere.

Fig. 13 – Schematic diagram of a survey of yield strength and

elongation for several typical biodegradable magnesium

alloys in as-cast state (a) and as-extruded state (b). For a

comparison, compositions of our two Mg-Zn-Y alloys are

expressed as weight percentage here, i.e., Mg-7Y-2.5Zn

(Mg97Zn1Y2) and Mg-7Y-2.5Zn-0.6Zr (Mg96.83Zn1Y2Zr0.17).

(Gunde et al., 2011; Hanzi et al., 2009; Li et al., 2008; Liu et al.,

2007; Witte et al., 2005; Zhang et al., 2008; Zhang and Yang,

2008; Zhang et al., 2009; Zhang et al., 2010).


5. Conclusions

In as-cast Mg100�3x(Zn1Y2)x (1rxr3) alloys, volume fraction of

18R LPSO phase increases with increasing the contents of Zn

and Y, which has a strong effect on plasticity and little effect

on yield strength. Substitution of bioactive element Ca for Y

in the Mg97Zn1Y2 does not favor the formation of LPSO phase,

but involving the formation of Mg2Ca phase. It indicates that

large atom-size mismatch and negative heat of mixing

between alloying elements and Mg are not necessary issues

as previously claimed. By micro-alloying with Zr as grain

refinement agent, morphology of a-Mg in the Mg96.83

Zn1Y2Zr0.17 alloy is changed into the equiaxial shape, together

with a significant refinement in grain size to 30 mm. It brings

about an improvement not only in strength but also in

plasticity, in contrast to the alloy without Zr. In comparison

with the as-cast state, warm-extruded alloys manifest sig-

nificantly improved properties not only in strength but also in

plasticity due to the refinement of a-Mg grain by dynamic

recrystallization and alignment of LPSO phase along the

extrusion direction. The yield strength and elongation of

warm-extruded Mg97Zn1Y2 alloy is 170 MPa and 30%, respec-

tively, which is a promising candidate as degradable ortho-

pedic implants.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China under Grant No. 51001099.

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