Microstructure evolution of a high-strength low-alloy Zn–Mn–Ca alloy through casting, hot extrusion and warm caliber rolling

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Microstructure evolution of a high-strength low-alloy Zn–Mn–Ca alloy through casting, hot extrusion and warm caliber rolling Zhang-Zhi Shi a, b, *, Hui-Yan Li a, b, Jun-Yi Xu a, b, Xi-Xian Gao a, b, Xue-Feng Liu a, b, ** a

Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China b Key Laboratory for Advanced Materials Processing of Ministry of Education, University of Science and Technology Beijing, Beijing, 100083, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Zn alloys Mechanical properties Microstructure Plastic deformation

Through casting, hot extrusion and then warm caliber rolling, biodegradable Zn-0.8Mn-0.4Ca alloy exhibits yield strength (YS) of 245 MPa, ultimate tensile strength (UTS) of 323 MPa, and elongation to failure (EL) of 12%. Its strengths are better than most of the existing biodegradable Zn alloys with ELs of 10–15%. So the alloy can be considered as a more competitive candidate for making bone screws or intravascular stents. Ca has a high ef­ ficiency of forming CaZn13 phase, the equilibrium volume fraction of which reaches 10.23 vol% with the minor addition of 0.43 wt% Ca. The CaZn13 particles not only impede dislocation motion, but also stimulate recrys­ tallization of Zn grains, resulting in a considerable strengthening effect. The average size of Zn dendrites in the as-cast alloy reaches 289 μm, while that of Zn grains in the as-rolled alloy is 5.0 μm. On the other hand, the minor Ca addition severely decreases the ductility of the Zn-0.8Mn base alloy due to the large size of CaZn13 particles inherited from the as-cast microstructure. Although their average size is reduced to 12.9 μm after the caliber rolling, it is about five times of that of MnZn13 particles. For design and fabrication of Ca-containing Zn alloys in future, refinement of CaZn13 phase should be a matter of vital concern.

1. Introduction Zinc (Zn) has drawn attention in recent years as a biodegradable metal since it shows some advantages over biodegradable Mg and Fe in medical applications [1]. Zn corrodes with a rate between Mg and Fe, with biocompatible corrosion products and without releasing of hydrogen. A two year implantation of Zn-0.8Cu alloy stents in coronary arteries of white pigs demonstrates no thrombosis response, mild and gradually decreasing inflammatory response and sufficient structural supporting [2]. The melting point of Zn is low, which makes it easy to be fabricated through conventional or advanced forming techniques [3–5]. However, this also leads to low recrystallization temperatures of Zn, resulting in a very limited work hardening response in various biode­ gradable Zn alloys [6–8]. Ca is the most abundant mineral element in human body, which is

therefore naturally selected as an alloying element of pure Zn and its alloys in order to adjust their properties [9–12]. Ca addition can modify mechanical properties and biocompatibility of pure Zn [13]. As-extruded Zn–1Ca (in wt.% by default) alloy exhibits yield strength (YS) of about 200 MPa, ultimate tensile strength (UTS) of about 240 MPa, and elongation to failure (EL) slightly less than 8% [13]. Ca addition accelerates corrosion rate of pure Zn from 23 μm/y (0% Ca) to 74 μm/y (2% Ca) in Hank’s solution [10]. According to Zn–Ca phase diagram [14], the solid solubility of Ca in Zn is negligible. Thus, minor Ca additions can lead to the formation of coarse CaZn13 dendrites, which should be blamed for the accelerated corrosion rates. A high-strength low-alloy (HSLA) Zn-0.8Mn-0.4Ca alloy has been developed recently [11]. In as-extruded state, its YS, UTS and EL reach 253 MPa, 343 MPa and 8%, respectively. Its YS and UTS are 26.5% and 42.9% higher than the aforementioned Zn–1Ca alloy, respectively.

* Corresponding author. Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, School of Materials Science and Engineering, Uni­ versity of Science and Technology Beijing, Beijing, 100083, China. ** Corresponding author. Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, School of Materials Science and Engineering, Uni­ versity of Science and Technology Beijing, Beijing, 100083, China. E-mail addresses: [email protected] (Z.-Z. Shi), [email protected] (X.-F. Liu). https://doi.org/10.1016/j.msea.2019.138626 Received 16 September 2019; Received in revised form 1 November 2019; Accepted 1 November 2019 Available online 4 November 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Zhang-Zhi Shi, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2019.138626

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Although a preliminary investigation on the microstructure of this HSLA Zn–Mn–Ca alloy has been done, much more details are to be unveiled. In addition, its ductility requires to be improved in order to meet re­ quirements of medical applications better. In the present study, warm caliber rolling will be employed to further adjust its mechanical prop­ erties. Microstructure evolution of the alloy through casting, hot extrusion and warm caliber rolling will be studied in details, which will benefit alloy design and fabrication of Ca-containing Zn alloys.

decreases its YS and UTS moderately to 244.9 MPa and 322.6 MPa, respectively. For biodegradable Zn alloys with ELs of 10–15%, the asrolled alloy exhibits higher YS and UTS than most of the reported biodegradable Zn alloys [4,15]. Representative engineering strain-stress curves are shown in Fig. 1a. The curves show first a short work-hardening stage and then a long softening stage until failure. The softening phenomenon has been observed in various Zn alloys [16–19]. It should be noted that it does not necessarily indicate non-uniform deformation or necking. As shown in Fig. 1b, no necking appears in neither the as-extruded nor the as-rolled samples after tensile testing. The low recrystallization temperature of Zn is likely to be responsible for the softening stage, since recrystallization of Zn can even happen during room temperature tensile testing [6]. The extrusion or the rolling makes no change of the phase constitution of the alloy, which consists of Zn, MnZn13 and CaZn13 phases, as shown in Fig. 1c. In the as-cast alloy, there are many coarse CaZn13 dendrites (Fig. 2a). According to Zn–Ca phase diagram [14], Ca can form CaZn13 in molten Zn at high temperatures. The dendritic shape indicates CaZn13 is the primary phase during solidification. After the extrusion, they are broken into smaller pieces with a blocky shape elongated along ED (Fig. 2b). However, large CaZn13 particles still exist after the extrusion, which crack severely after the rolling (Fig. 2c). The cracked CaZn13 particles are possible sources of unstable crack propagation during tensile testing. Fig. 2d shows the sizes (in equivalent diameters) of the second phase particles. The size of MnZn13 particles changes a little during the whole fabrication process, but their shapes changes a lot from strip-shaped to round-shaped. Their average sizes in different states fall in a narrow range of 2.5–2.8 μm. However, CaZn13 particles become smaller after each fabrication step. Their average sizes decrease from 26.0 μm to 17.0 μm after the extrusion, and then further decrease to 12.9 μm after the rolling, which is still about 5 times of that of MnZn13 particles.

2. Material and methods 2.1. Alloy preparation Pure Zn, Mn and Ca were melted and kept at 725 � C for 5 min in a vacuum induction melting furnace in an argon atmosphere, and then cast into a cylindrical graphite mould and cooled naturally to room temperature. The measured composition of the alloy is Zn-0.81Mn0.43Ca (in wt.%), close to its nominal composition of Zn-0.8Mn-0.4Ca. An extruding blank of Φ39.6 � 50 mm was cut from the cast ingot along its axial direction (AD), homogenized at 360 � C for 6 h, and then water quenched. It was extruded at 230 � C with an extrusion ratio of 16:1. After preheated at 100 � C for 10 min, the extruded rod of Φ10 mm was caliber rolled by four continuous passes into a rod of Φ6 mm. The area reduction was 64% in total. Both the extruding direction (ED) and the rolling direction (RD) are paralleled to AD. The rolling temperature is an important parameter, since rolling at room temperature caused serious surface damage of the alloy. 2.2. Mechanical testing and microstructure characterization Tensile test samples were prepared according to ASTM E8. They were tested on a CMT4105 electronic universal testing machine at a strain rate of 2 � 10 3 s 1, at room temperature. Dumbbell-shaped tensile test samples were cut along ED and RD. The dimensions of the samples were chosen according to the sizes of the extruded and the rolled rods. For samples cut from the extruded rod, they were 5 mm in diameter and 25 mm in original gauge length. For those cut from the rolled rod, they were 3 mm in diameter and 15 mm in original gauge length. X-ray diffractometer (XRD, SmartLab, Rigaku, Japan) was used to analyze phases in the alloy. A scanning electron microscope (SEM, Zeiss Merlin, Germany) with electron backscatter diffraction (EBSD) camera was used to characterize microstructure. Samples for EBSD measurements were ground with 1000–5000 grit sandpapers, polished mechanically until obtaining a mirror-like surface, and then further polished with colloidal silica suspension. Then, the surface was immediately flushed with ethanol and blown dry under a cool air flow, ready for EBSD measure­ ments. Step sizes for the EBSD measurements were 0.25–0.60 μm, depending on how fine the microstructures were.

3.2. EBSD microstructure of the as-cast alloy The as-cast alloy consists of Zn dendrites with an average size of about 289 μm, MnZn13 and CaZn13 particles, observed by using an op­ tical microscope in a previous study [11]. EBSD measurements in the present study will provide more details of the microstructure. For clarity and convenience, a second phase particle is designated as ‘P’, while a grain is designated as ‘G’. A particle may consist of one or more grains. In order to differentiate phases, Zn, MnZn13 and CaZn13 are denoted by additional letters ‘z’, ‘m’ and ‘c’, respectively. Crystal structures of the three phases used for indexing EBSD Kikuchi patterns are summarized in Table 2. As shown in Fig. 3a, large CaZn13 particles Pc1~Pc4 align along AD, three of which are connected by small MnZn13 particles. On the right of them, there is a much larger CaZn13 particle Pc5 with holes in it filled with Zn. Fig. 3b is colored according to inverse pole figures (IPFs) of the phases in Fig. 3c. Each of the CaZn13 particles consists of one grain. The grains are in blue, indicating that their < 111> directions are nearly paralleled to AD (Fig. 3c). < 111> of Gc1~Gc5 in Fig. 3b is calculated to be deviated from AD in a narrow range of 9.04� –9.16� . Zn grain Gz1 is divided by the CaZn13 particles into two parts with a misorientation of only 0.58� . MnZn13 particles grow on the CaZn13 particles, at Zn grain boundaries (GBs) or individually in the Zn matrix. The misorientation between Gz3 and Gz4 is calculated to be 0.47� , while that between Gz3 and Gz5 is calculated to be 0.30� , suggesting that they may be parts of one grain. Since heat transfers along directions near AD during solidification of the alloy melt, < 111> are favorable growth directions of CaZn13. CaZn13 has a complex FCC structure as shown in Fig. 3d. Considering straight atom rows [21], atomic density declines in the following sequence: < 111>, <010>, <10–1> (Fig. 3d). The favorable growth direction is the closest-packed straight atomic direction passing through both Zn and Ca atoms. However, for Al and Pb with simple FCC struc­ tures, the favorable dendritic growth directions are <100>, since each one of them is the bisecting direction of a pyramid made of slowest

3. Results 3.1. Mechanical properties and second phases Table 1 lists mechanical properties of the alloy in different states. The as-cast alloy is brittle with very limited EL. After the extrusion, its YS and UTS increase respectively to about 2.3 and 2.9 times, while its EL rea­ ches 8.0%. Further caliber rolling increases its EL to 11.8%, however, Table 1 Mechanical properties of Zn-0.8Mn-0.4Ca alloy in different states. The data of the alloy in as-cast and as-extruded states are collected from a previous study [11]. State

YS (MPa)

UTS (MPa)

EL (%)

As-cast As-extruded As-rolled

112.2 � 3.4 253.4 � 1.3 244.9 � 5.7

120.3 � 6.3 343.2 � 1.6 322.6 � 11.2

0.3 � 0.1 8.0 � 1.4 11.8 � 0.9

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Fig. 1. (a) Engineering strain-stress curves, (b) tensile samples before and after the tests, (c) XRD patterns.

Fig. 2. SEM images at � 2000 magnification of (a) the as-cast alloy, (b) the as-extruded alloy, and (c) the as-rolled alloy. (d) Sizes of the second phase particles. 3

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Table 2 Crystal structures of the phases [20]. Phase

Space group

Pearson symbol

a (nm)

b (nm)

c (nm)

α (� )

β (� )

γ (� )

Zn MnZn13 CaZn13

P63/mmc C2/m Fm-3c

hP2 mC28 cF112

0.2665 1.3483 1.213

0.2665 0.76626 1.213

0.4947 0.5134 1.213

90 90 90

90 127.78 90

120 90 90

growing planes (i.e., the closest-packed {111} planes) and the minimum elastic modulus direction [22]. Zn addition in Al makes its FCC structure more complex and causes a continuous transition of the favorable growth directions from <100> to <110> due to a modification of solid-liquid interfacial energy [23].

value of MnZn13 grains and that of the only CaZn13 grain are 1.19� and 0.16� , respectively. It indicates that they have low dislocation densities. Generally speaking, recrystallized grains have KAM values < 1� [24]. The coarse Zn grains in the as-cast alloys are probably recrystallized after the hot extrusion, since Zn grains in Zn–Mn alloys can recrystallize even at room temperature during tensile testing [6]. As shown in Fig. 4e, the majority (i.e., 84.9%) of Zn grains are newly recrystallized with the maximum intragranular KAM values < 1� . The rest ones are deformed after recrystallization, such as the large grain Gz1 and the elongated grain Gz2 in Fig. 4d. The majority (i.e., 91.4%) of MnZn13 grains have maximum intragranular KAM values < 1� (Fig. 4e). The average size of MnZn13 particles in the as-cast alloy is 2.8 μm (Fig. 2d), comparable to that in the as-extruded alloy. Since MnZn13 (168.2 HV) is much harder than Zn (31.8 HV), KAM values of them < 1� indicate that they are hardly deformed during the extrusion. Fig. 4f shows pole figures of Zn grains. Their {0001} poles (i.e., <0001> directions) distribute randomly around ED with the maximum pole density at 86� from ED, or equivalently at 4� from a radial direction. This indicates that {0001}<2-1-10> basal slipping is the major defor­ mation mode. Similar {0001} fiber texture has been observed in Mg AZ31 rods also with a HCP structure [27]. CaZn13 particles distribute non-uniformly within the alloy (Fig. 2b). Fig. 5a shows an area with more CaZn13 particles, which align along ED. The elongated CaZn13 particle Pc1 in the middle of the figure consists of 15 grains. Excluding the boundary grains that are intersected by map boundaries, the largest one (i.e., Gc1) is 6.6 μm in size, while the smallest one (i.e., Gc2) is 0.9 μm in size. The second largest grain Gc3 consists of two sub-grains. One is Gc3-1 of 6.2 μm in size, and the other is Gc3-2 of 1.6 μm in size. In the vicinity of the sub-GB between them, dislocation density is higher, with KAM values as large as 2.5� (Fig. 5b).

3.3. EBSD microstructure of the as-extruded alloy Fig. 4a shows a representative EBSD map of the as-extruded alloy at � 1000 magnification, consisting of Zn grains, MnZn13 and CaZn13 particles. The MnZn13 particles distribute along ED. Boundary mis­ orientations are illustrated in Fig. 4b, in which {10–12} twin boundaries (TBs), if any, are outlined in red. It is seen that only two limited segments of GBs, as pointed out by arrows, are in red, indicating that essentially no twin has formed. Fig. 4c shows size distribution of the grains in Fig. 4a, including 488 Zn grains and 30 MnZn13 grains. Only one CaZn13 grain is found in Fig. 4a, the size of which is 2.7 μm. The average sizes of Zn and MnZn13 grains are 4.2 � 1.2 μm and 5.4 � 1.3 μm, respectively (Fig. 4c). The majority (i.e., 68.4%) of Zn grains have sizes in the range of 3–5 μm. Only two Zn grains have sizes larger than 8 μm. The size of the largest one is 17.0 μm. The majority (i.e., 73.3%) of MnZn13 grains have sizes in the range of 4–7 μm. Note that there is an acceptable discrepancy be­ tween the data in Figs. 2d and 4c. This is because the data in Fig. 2d are collected from several SEM images with a total area much larger than that in Fig. 4a. Fig. 4d shows the distribution of kernel average misorientation (KAM), which is often used to evaluate local misorientations [6,24–26]. The higher the dislocation density is, the warmer is the color in Fig. 4d. Although the maximum KAM value of Zn grains reaches 3.17� , most Zn grains are in blue with low dislocation densities. The maximum KAM

Fig. 3. EBSD maps of the as-cast alloy: (a) Phase distribution. (b) Grains colored according to IPFs in (c). GBs with misorientation angles larger than 15� are outlined in black, while phase boundaries are outlined in white. The same for boundaries in other EBSD maps afterwards. By default, a grain is colored according to its mean orientation. (c) IPF color coding. (d) Crystal structure of CaZn13. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 4. EBSD maps of the as-extruded alloy: (a) At � 1000 magnification, Zn grains colored ac­ cording to IPF, while MnZn13 in blue and CaZn13 in yellow for clarity. Sub-GBs with mis­ orientations larger than 5� are outlined in grey. GBs and phase boundaries are outlined respec­ tively in black and white as aforementioned. (b) Boundary misorientation map. GBs and sub-GBs of Zn grains are colored according to their asso­ ciated misorientations. {10–12}<-1011> twin boundaries (TBs) of Zn with a misorientation of <-12-10>86� (offset �5� ), if any, are outlined in red. (c) Distribution of grain sizes in equivalent diameters. Grains intersected by map boundaries (i.e., boundary grains) are excluded to avoid data distortion. Grains with GBs >15� are counted. (d) Kernel average misorientation (KAM) map. (e) Distribution of the maximum intragranular KAM values. (f) Pole figures of Zn grains. R1 and R2 are radial directions perpendicular to each other and ED is out of plane. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Deformation bands can be seen in Gc3 and its neighboring grains, indicating that this part of Pc1 is the most severely deformed during the extrusion. The majority of Zn grains are in cold colors with low dislo­ cation density. However, in the large elongated grain Gz3, evidence of localized deformation is seen, which is in warmer colors with KAM values as large as 4.0� (Fig. 5b). Moreover, this grain is twinned much more heavily than its neighboring finer grains (Fig. 5c). The twinning behavior in Gz3 is complicated (Fig. 5d). The grain consists of several sub-grains. Those relevant to the following analysis are sub-grains Gz3-1, Gz3-2 and Gz3-3. The mean orientations of subgrains Gz3-1, Gz3-2 and Gz3-3 are (83.251� , 80.524� , 226.971� ), (87.524� , 79.098� , 216.181� ) and (80.915� , 70.853� , 224.264� ), respectively. The misorientation between Gz3-1 and Gz3-2 is 10.97� , while that between Gz3-1 and Gz3-3 is 10.46� . For any Zn grain, there are six possible {10–12} twin variants, designated by Vi (i ¼ 1–6). They are defined as follows: {-1102}<-110-1> (V1), {-101-2}<-1011> (V2), {-1012}<-101-1> (V3), {-110-2}<-1101> (V4), {0–112}<0-11-1> (V5) and {01–12}<01-1-1> (V6). First, the mean orientations of the sub-grains are used for calculating the orientations of their six possible twin variants (i.e., ideal orientations). The results are listed in Table 3. Δθ is the misorientation between the mean orientation of a measured twin and the ideal orientation of a twin variant. A reasonable criterion is Δθ < 5� for twin variant determination. This works well for 8 of the 14 twins, which are T1, T2, and T9~T14. It can be seen from the local color variation in Fig. 5d that dislocation slipping leads to considerable

intragranular misorientations, which is blamed for the uncertainty of twin variant determination of the rest twins (Table 3). So it is appro­ priate to use representative local orientations in the grain matrix in the vicinity of the twins for further analysis. Then, their corresponding twin variants can be determined (Table 3). A special one is T7, which changes from V5 to V6 from bottom (O5) to head (O6). Excluding T7, only one twin belongs to V2, three twins belong to V5, and nine twins belong to V6. Fig. 5d also shows <0001> pole figures of the twins and the subgrains. It can be calculated that the misorientations between <0001> of the sub-grains and ED are 81.42� –87.57� , which are about paralleled to a radial direction, i.e., R1 in Fig. 5d. While those between ED and <0001> of the twins except T11 are calculated to be 66.85� –84.80� , which are near or about paralleled to another radial direction, i.e., R2 in Fig. 5d. So the majority of the twins turns <0001> of the grain matrix from one radial direction to another. However, the misorientation be­ tween <0001> of T11 and ED is calculated to be 33.97� , which turns <0001> of the grain matrix toward ED due to twinning. Within T8, there exist 10 regions bearing the {10–12} twinning relationship with it, as marked by arrows and ‘1–10’ in Fig. 5d. Their misorientations with respect to Gz3-2 are calculated to be only 2.08� –8.63� , the majority (i.e., 60%) of which are lower than 5� . The appearance of these regions have two possibilities. One is that they are untwinned grain matrix. The other is that they belong to one special variant of {10–12}-{10–12} double twinning, the effect of which is the same as detwinning [28]. 5

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Fig. 5. EBSD maps of the as-extruded alloy: (a) At � 3000 magnification, Zn grains IPF colored. (b) KAM map. (c) Boundary misorientation map of Zn grains, in which TBs are outlined in red. (d) Twinning analysis of Gz3 with a step size of 0.25 μm, which is colored according to IPF and each measured orientation in the grain. Inserted is <0001> pole figure of the sub-grains and the twins. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

majority (i.e., 82.9%) of MnZn13 grains have sizes in the range of 3–6 μm. The majority (i.e., 90.0%) of CaZn13 grains have sizes in the range of 3–6 μm. Compared with those in the as-extruded alloy, MnZn13 and CaZn13 grains are refined, but Zn grains are coarsened mildly. KAM map in Fig. 6d indicates that dislocation density is high in the majority of Zn grains but low in MnZn13 and CaZn13 grains. Quantitative analysis is presented in Fig. 6e. The minority (i.e., 11.6%) of Zn grains have maximum intragranular KAM values < 1� , indicating that they are newly recrystallized grains. The majority (i.e., 71.4%) of Zn grains have maximum intragranular KAM values in the range of 1� –2.5� , indicating that they are deformed grains. In contrast, the majority (i.e., 60%) of MnZn13 grains and the vast majority (i.e., 85.7%) of CaZn13 grains have maximum intragranular KAM values < 1� , indicating that they are hardly deformed. Thus, Zn grains neighboring MnZn13 and CaZn13 particles can have higher dislocation densities due to strain in­ compatibility, as pointed out by arrows in Fig. 6d. Fig. 6f shows pole figures of Zn grains. Their {0001} poles still distribute randomly around RD (//ED), the maximum density of which appears at 89� from RD, or equivalently at 1� from a radial direction. This shows a tendency of {0001} poles turning toward the radial di­ rections during the rolling, indicating the activation of {0001}<2-1-10> basal slipping, agreeing well with the KAM distribution in Fig. 6d. In addition, a proportion of {0001} poles turn toward RD in Fig. 6f, which correspond to the red areas in Fig. 6a. Since {10–12} twinning rotates <0001> of 86� , the red areas should be {10–12} twins. Fig. 7a shows a map of more red areas at a larger magnification, which is twinned more heavily as shown in Fig. 7b. Slip bands of high dislocation densities appear in many grains, especially in the larger ones (Fig. 7b). According to the trace analysis in Fig. 7b, the majority of Zn grains deform through {0001}<2-1-10> basal slip and/or {01–10}<21-10> prismatic slip systems. {10–12} twinning is activated to accom­ modate plastic strain perpendicular to the basal plane. In an elongated grain near the right border, evidence of {2-1-10}<01–10> slipping is detected. In an elongated twinned area near the left border, evidence of {-1011}<2-1-13> pyramidal slipping is detected. The twinned areas in red turn <0001> of grains from a radial direction to RD, resulting in high {0001} pole density around RD (Fig. 7c).

Table 3 Analysis of the 14 twins in grain Gz3. V5 and V6 (or V2 and V3) are two closest neighboring twin variants with a misorientation of 8.0� . Locations of discrete orientations Ok (k ¼ 1–8) are marked in Fig. 5d. Twin

Orientation of grain matrix

Δθ (� )

Variant

T1 T2 T3

Gz3-1 Gz3-1 Gz3-1 O1: (81.153� , 76.194� , Gz3-1 O2: (85.486� , 81.479� , Gz3-1 O3: (87.742� , 74.006� , Gz3-1 O4: (84.590� , 85.617� , Gz3-1 O5: (92.240� , 78.831� , O6: (84.958� , 74.248� , Gz3-1 Gz3-2 O7: (90.226� , 79.395� , O8: (91.863� , 78.520� , Gz3-2 Gz3-2 Gz3-2 Gz3-2 Gz3-2 Gz3-3

9.39 (V5), 3.91 (V6) 10.61 (V5), 2.17 (V6) 4.96 (V5), 5.81 (V6) 10.01 (V5), 2.40 (V6) 7.57 (V5), 5.00 (V6) 6.04 (V5), 3.43 (V6) 8.31 (V5), 7.03 (V6) 7.65 (V5), 1.97 (V6) 4.29 (V5), 3.93 (V6) 2.03 (V5), 9.19 (V6) 12.03 (V5), 6.34 (V6) 2.01 (V5), 6.40 (V6) 9.26 (V5), 1.68 (V6) 10.30 (V5), 6.96 (V6) 3.28 (V5), 4.74 (V6) 3.02 (V5), 6.17 (V6) 4.22 (V5), 6.37 (V6) 7.77 (V5), 0.54 (V6) 1.80 (V5), 8.11 (V6) 4.51 (V2), 6.19 (V3) 6.02 (V5), 2.76 (V6) 6.56 (V5), 2.51 (V6) 12.62 (V5), 4.61 (V6)

V6 V6 V6

T4 T5 T6 T7 T8

T9 T10 T11 T12 T13 T14

224.497� ) 228.501� ) 218.164� ) 226.367� ) 215.755� ) 221.223� ) 216.675� ) 216.397� )

V6 V6 V5 V5/V6 V5

V6 V5 V2 V6 V6 V6

3.4. EBSD microstructure of the as-rolled alloy Fig. 6a shows a representative EBSD map of the as-rolled alloy at � 1000 magnification. MnZn13 and CaZn13 particles distribute along RD. Some of the second phase particles connect with each other, forming a long chain. Boundary misorientations are illustrated in Fig. 6b. {10–12} twins form in several Zn grains. Fig. 6c shows size distribution of the grains in Fig. 6a, including 269 Zn, 35 MnZn13 and 10 CaZn13 grains. Their average sizes in the same sequence are 5.0 � 2.4 μm, 4.5 � 1.2 μm and 5.4 � 2.9 μm, respectively (Fig. 6c). The majority (i.e., 63.6%) of Zn grains have sizes in the range of 3–6 μm. The smallest Zn grain is 1.7 μm in size, while the largest one is 16.7 μm in size. The 6

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Fig. 6. EBSD maps of the as-rolled alloy: (a) At � 1000 magnification, Zn grains colored ac­ cording to IPF. (b) Boundary misorientation map. TBs are outlined in red. (c) Distribution of grain sizes in equivalent diameters. (d) KAM map. (e) Distribution of the maximum intragranular KAM values. (f) Pole figures of Zn grains. For easy comparison, its color bar is set to be in the same range as that in Fig. 4f. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

3.5. Fractographic analysis

fraction of CaZn13 (i.e., wCaZn13) can be calculated by wCaZn13 ¼ wCa/4.5, where wCa is the weight percentage of Ca. Then, vCaZn13 can be calcu­ lated by

The fracture of the as-cast alloy shows a faceted texture (Fig. 8a), which is a transgranular fracture similar to that of the as-cast Zn-0.8Mn alloy [6]. On the fracture facets there locate blocky or dendritic CaZn13 particles and small MnZn13 particles (Fig. 8b), indicating the interfaces between these particles and the Zn matrix are weak. The as-extruded alloy shows a ductile fracture with spherical dimples (Fig. 8c), within which large CaZn13 particles are found (Fig. 8d). The as-rolled alloy shows a similar ductile fracture (Fig. 8e). Large CaZn13 particles and small MnZn13 particles locate in the dimples (Fig. 8f). The interfaces between the second phase particles and the Zn matrix seems to be weak bonding places easy for cracking, especially for large CaZn13 particles.

vCaZn13¼(wCaZn13/ρCaZn13)/[wCaZn13/ρCaZn13þ(100 ωCaZn13)/ρZn],

(1)

where ρCaZn13, the density of CaZn13, is calculated to be 6.62 g/cm3 based on its crystal structure (Table 2 and Fig. 3d), and ρZn, the density of Zn, is 7.14 g/cm3 [29]. When wCa is 0.43 wt%, vCaZn13 reaches 10.23 vol%. Such a consid­ erable quantity of CaZn13 particles stimulate recrystallization of Zn grains during hot extrusion, resulting in a uniform fine-grained micro­ structure. The CaZn13 particles are refined sequentially by hot extrusion and warm caliber rolling. The refinement of both the Zn grains and the CaZn13 particles introduces more obstacles for dislocation moving, which strengthens the alloy. However, some large CaZn13 particles cannot be completely broken into small ones. They serve as crack sources during the tensile testing (Fig. 8f), and thus detrimental to ductility. According to Eq. (1), Ca has a high efficiency of forming CaZn13 phase, since its Zn/Ca atomic ratio is as high as 13. A critical value is wCa ¼ 2.165 wt% when vCaZn13 ¼ 50 vol%. So the matrix of an Zn–Ca alloy with Ca > 2.165 wt% is CaZn13 (105.0 HV) but not Zn (31.8 HV), which makes the alloy very hard and brittle. Indeed, the hardness of as-

4. Discussion 4.1. Effects of Ca on mechanical properties and guidance for alloy design The equilibrium volume fraction of CaZn13 (i.e., vCaZn13) in an Zn–Ca alloy can be calculated based on the lever rule, analogous to the calcu­ lation of that of MnZn13 in an Zn–Mn alloy [29]. According to Zn–Ca phase diagram [14], equilibrium Ca contents in Zn and in CaZn13 are 0 wt% and 4.5 wt% at room temperature. So the equilibrium weight 7

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Fig. 7. The microstructure of the as-rolled alloy: (a) EBSD maps at � 3000 magnification, Zn grains colored according to IPF. (b) KAM map together with TBs and slip plane traces. (c) Pole figures of Zn grains.

cast Zn–3Ca alloy is about 3 times of that of as-cast pure Zn [10]. The larger volume fraction of CaZn13 is, the more difficult is plastic defor­ mation processing, which is vital for eliminating brittleness of as-cast Zn alloys. Cold caliber rolling of the extruded Zn-0.8Mn-0.4Ca alloy has already suffered to surface cracking. Considering implant applications requiring small-sized alloy products such as suture (typically 0.2–0.3 mm in diameter [30,31]), multi-pass drawing is an indispens­ able procedure, which is usually operated at low temperatures. In this case, the ductility of the alloy should be improved so that vCaZn13 is better to be less than 5 vol% and correspondingly wCa < 0.21 wt%. This provide guidance for future design of Ca-containing biodegradable Zn alloys.

x-axis, whose direction can be described as [1–10]s. The subscript ‘s’ refers to the specimen frame, while no subscript refers to the default crystal frame. Using the Euler angles of Gc6, it can be calculated that [1–10]s is about parallel to [010], with a negligible misorientation of 0.71� . Table 4 lists the calculated coordinates of three closely packed di­ rections on the (101) plane of Gc6 in the specimen frame. It can be measured in Fig. 9a that petal 2 orients about 125� from petal 1 so that the direction of petal 2 can be described as [cos 80� , sin 80� , 0]s. It is about parallel to [1 1 1], with an acceptable misorientation of 6.65� , largely due to a projection (or sample cutting) effect. If the projected vector of [1 1 1] on the surface of the sample (i.e., x-y plane) is used for calculation, i.e., [3.63, 20.56, 0]s (Table 4), the misorientation de­ creases to only 0.01� , indicating that petal 2 grows along [1 1 1]. Petal 3 grows as long as petal 2. They seem to be symmetrical around petal 1, indicating that petal 3 grows along [ 1 11]. Using the calculated co­ ordinates of the crystal vectors in the specimen frame in Table 4, it is easy to obtain the angles between their projected vectors and the x-axis, as listed in the θx column in the table. Then, their directions can be illustrated in Fig. 9a, which agrees well with the shape of Gc6. Dis­ regarding the projection effect, the angle between [010] and [1 1 1] is equal to that between [010] and [ 1 11], both of which is 125.26� . The shape indicates that <111> are the most favorable growth directions, and <010> are the second most favorable ones. The projection effect is evaluated systematically in Fig. 9b. The presumed original orientation of the flower-shaped particle is defined by [101]//[-100]s, [010]//[100]s, corresponding to Euler angles (0� , 0� , 0� ). This is when (101) is paralleled to the observation surface (i.e., x-y plane), and [010] is paralleled to x-axis of the specimen frame. There is no projection effect, and the angle between petal 1 and petal 2 (i.e, θ1) and that between petal 1 and petal 3 (i.e., θ2) are equal to 125.26� . Then,

4.2. Shapes of CaZn13 particles CaZn13 particles with a shape similar to a three-petaled flower are common in the as-cast alloy, a representative one of which is shown in Fig. 9a, marked as Pc6. It locates at the GB between Zn grains Gz4 and Gz5. Again, one of its < 111> directions together with those of Pc7 and Pc8 are close to AD, according to their IPF colors. The calculated de­ viations of their < 111> directions from AD lie in a narrow range of 11.82� –12.09� . Pc6, the intact CaZn13 particle in Fig. 9a, consists of one grain, i.e., Gc6. Its mean orientation is defined by Euler angles (313.786� , 142.456� , 269.039� ). The specimen frame (x, y, z) is defined according to the way that electron beam moves during EBSD measure­ ments, i.e., x-axis orients eastward, y-axis orients southward, and z-axis is into the observation surface, as plotted in Fig. 9a. Using the Euler angles, it can be calculated that the z-axis is anti-paralleled to [709] direction of Gc6, which deviates 7.13� from [101] direction. Since the shape of Gc6 shows symmetry, the zone axis of its three petals in Fig. 9a is likely to be parallel to [101]. Petal 1 of Gc6 orients about 45� from the 8

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Fig. 8. SEM image of tensile fractures of the as-cast alloy at low magnification (a) and at high magnification (b), the as-extruded alloy in low magnification (c) and in high magnification (d), the as-rolled alloy in low magnification (e) and in high magnification (f).

if a rotation is applied to the particle, the projection effect will appear. For FCC structure, a basic Euler space of φ12[0� , 90� ], Φ2[0� , 90� ], φ22[0� , 90� ] is taken into consideration [32]. A rotation can be defined by Bunge Euler angles (φ1, Φ, φ2). A step size of 1� is selected for the calculation. Then, various projected shapes can be obtained, some of which are representative and shown in Fig. 9b. The projection effect can keep θ1 ¼ θ2, but higher or lower than 125.26� , such as at an orientation of (0� , 45� , 0� ) or (0� , 70� , 90� ). An extreme case is at (0� , 90� , 0� ), when θ1 ¼ θ2 ¼ 180� and the particle turns into a lath-shaped one. Alterna­ tively, the projection effect can lead to θ1 6¼ θ2, which can increase one of the angles as large as 180� while decrease the other one as small as 0� , such as at (0� , 90� , 40� ), also lath-shaped. The asymmetry of the parti­ cle’s shape can be very large, such as at (0� , 90� , 90� ), or small, such as at

(30� , 30� , 30� ). Both of the angles can be smaller than 125.26� , such as at (0� , 65� , 85� ), or higher than that, such as at (0� , 60� , 5� ). Lath-shaped CaZn13 particles are shown in Fig. 9c, in which Zn grains are marked by Gz, CaZn13 grains are marked by Gc, the rest are MnZn13 grains. The lath-shaped Gc9 and Gc10 have very similar mean orienta­ tions, which are (144.709� , 147.081� , 186.268� ) and (144.712� , 146.763� , 186.588� ), respectively. Their misorientation is calculated to be only 0.45� . Applying the calculation method for Gc6 in Fig. 9a, the projected <010> and <111> directions of Gc9 and Gc10 can be calculated as listed in Table 5 and illustrated in Fig. 9c. The shape of the grains resembling an isosceles trapezoid can be well explained by the projection effect. Due to the same reason, CaZn13 particles of various shapes are observed, as pointed out in Fig. 9d. 9

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Fig. 9. (a) IPF colored map including a CaZn13 particle with a shape of a three-petaled flower. Projected crystal directions of Pc6 are drawn. Inserted is its cor­ responding SEM image in an artificially reduced size. (b) The projection effect of the flower-shaped CaZn13 particles. (c) A typical example of the projection effect, IPF colored map. (d) Various shapes of CaZn13 particles.

5. Conclusions

Table 4 Crystal vectors of Gc6 in the specimen frame (Fig. 9a). θx is the angle between a projected vector on the x-y plane and x-axis. An anticlockwise rotation corre­ sponds to a positive angle, and vice versa. The numbers are rounded to the second place of decimals.

θ2 ¼ 161.88�

This paper investigates microstructure evolution of a high-strength low-alloy biodegradable Zn-0.8Mn-0.4Ca alloy through casting, hot extrusion and warm caliber rolling. Conclusions are drawn as follows: 1. The alloy consists of Zn, MnZn13 and CaZn13 phases. In the as-cast alloy, the average size of CaZn13 particles reaches 26.0 μm, which is about 9 times of that of MnZn13 particles. The second phase particles stimulate recrystallization of Zn grains during plastic deformation pro­ cessing. Grain refinement and fine particles strengthen the alloy, while large unbroken CaZn13 particles accelerate cracking. As a result, the asrolled alloy exhibits high strength and moderate ductility, which are YS of 245 MPa, UTS of 323 MPa, and EL of 12%. 2. FCC structured CaZn13 phase solidifies preferentially along <111> and <010> directions. The most favorable <111> and the second most favorable <010> growth directions result in the formation of flowershaped CaZn13 particles. Due to a projection (or sample cutting) ef­ fect, various shapes of CaZn13 particles can be observed. 3. The extruded alloy exhibits a typical {0001} extrusion texture of HCP metals, with <0001> directions about paralleled to radial di­ rections. {10–12} twinning turns <0001> directions of a considerable proportion of Zn grains toward the rolling direction after the caliber rolling, resulting in a split {0001} rolling texture.

θx ¼ 40.81�

Data availability statement

Petal

Crystal vector

Coordinates in the specimen frame

Projected direction

θx ( � )

1

[010]

[8.51,

45.45

2

[1 1 1]

[3.63, 20.56, 2.35]s

3

[ 1 11]

[-20.65,

[8.51, 8.64, 0]s [3.63, 20.56, 0]s [-20.65, 3.27, 0]s

8.64,

3.27,

0.12]s

2.10]s

79.99 171.00

Table 5 Crystal vectors of Gc9 and Gc10 in the specimen frame (Fig. 4c). The numbers are rounded to the second place of decimals. Grain

Crystal vector

Coordinates in the specimen frame

Projected direction

Angles (� )

Gc9

[100]

[9.20,

θx ¼ 40.56�

[ 111]

[-12.32, 5.76,

[ 1 1 1]

[-6.08, 9.99, 17.45]s

[ 100]

[-9.16, 7.91, 0.76]s

[1 1 1]

[12.28,

[111]

[6.05,

[9.20, 7.87, 0]s [-12.32, 5.76, 0]s [-6.08, 9.99, 0]s [-9.16, 7.91, 0]s [12.28, 5.92, 0]s [6.05, 9.91, 0]s

Gc10

7.87,

0.72]s 16.02]s

5.92, 15.99]s 9.91,

17.51]s

θ1 ¼ 164.49�

θ1 ¼ 164.91

�

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

θ2 ¼ 162.20�

Declaration of competing interest The authors declare that they have no known competing financial 10

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interests or personal relationships that could have appeared to influence the work reported in this paper.

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