Effect of zirconium on the microstructure and mechanical properties of Zn–4%Al hypoeutectic alloy

Effect of zirconium on the microstructure and mechanical properties of Zn–4%Al hypoeutectic alloy

Journal of Alloys and Compounds 592 (2014) 127–134 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 592 (2014) 127–134

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Effect of zirconium on the microstructure and mechanical properties of Zn–4%Al hypoeutectic alloy Hongying Li a,b,⇑, Zhenguo Li a, Yang Liu a, Haofan Jiang a a b

School of Materials Science and Engineering, Central South University, Changsha 410083, PR China Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Central South University, Changsha 410083, PR China

a r t i c l e

i n f o

Article history: Received 21 October 2013 Received in revised form 11 December 2013 Accepted 12 December 2013 Available online 4 January 2014 Keywords: ZnAl4 alloy Zirconium Eutectic structure Mechanical properties

a b s t r a c t The effect of additional Zr in the range of 0–0.3 wt.% on the microstructure and mechanical properties of ZnAl4 alloy was studied. Microstructure and compositions of experimental alloys were analyzed by scanning electron microscopy and energy dispersive spectroscopy. The results show that the morphology of primary g-Zn phase as well as eutectic structure, with different Zr content in ZnAl4 alloys, are considerably distinct, resulting in the remarkable evolution of mechanical properties. Along with the increase of Zr content, the tensile strength of ZnAl4 alloys increases, peaking at 231 MPa with 0.1 wt.% Zr, followed by a decrease thereafter. The elongation, impact toughness and HB hardness show similar evolution character, peaking at 4.80%, 70.37 J/cm2 and 85.19 kgf/mm2, respectively. The most optimized additive content of Zr to ZnAl4 alloy may be 0.1 wt.%, which contributes to abundant eutectic structure and fine g-Zn phase, thereby improved mechanical properties. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Zinc-aluminum alloys have been used for a multitude of engineering applications, especially in die casting application [1]. Nowadays group of alloys based on Zn–Al system could be used for soldering at high temperature [2,3]. They are also competitive alternative materials to cast irons, bronzes, steel fabrications and aluminum alloys in many structural applications due to their high sliding wear resistance, high damping capacity, excellent castability and machinability [4–9]. However, their low strength and poor impact toughness in the as-cast condition limit their use in engineering. To address the increasing demand for high quality and performance die castings, a series of Zn-based engineering cast alloys have been developed, including AG-40A, AC-41A and AC-43A alloy (ASTM). Of these alloys, the ZnAl4 alloy named AG-40A is the most used general purpose zinc die casting alloy, providing an excellent combination of strength, ductility and impact strength. It also provides excellent plating and finishing characteristics. This alloy is the designer’s first choice for die casting applications. Consequently there is keen interest in studies on improving the properties of this alloy. It is reported that the addition of Zr to Al-based alloys has shown beneficial effects on microstructure and properties, which leads to refinement of the grain size, changes in the structural

⇑ Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha 410083, PR China. Tel./fax: +86 0731 88836328. E-mail address: [email protected] (H. Li). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.12.133

morphology and a substantial improvement in the mechanical properties [10–14]. As the primary a phase of ZnAl alloys is an Al-rich solid solution, it is expected that Zr element in Zn–Al alloys will have similar effects to that in aluminium alloys. Some researchers proved that Zr has a very strong and unique effect on the size and shape of primary a-Al phase and transformed the coarse dendrites to very fine grains of ZA27 alloy [15]. Experiments have confirmed that trace Zr can refine the microstructure of Zn–Al eutectoid alloys [16]. Unfortunately, little research has been carried out on the Zr addition in low aluminum Zn–Al alloys, especially in the hypoeutectic ZnAl alloys. Thus, the aim of this research is to study the effect of different Zr content on the microstructure and mechanical properties of ZnAl4 alloy. The possible effect mechanism will also be discussed. 2. Materials and experiment The ZnAl4 alloy was fabricated by using Zn, Al and Mg ingots (purity > 99.9%). The alloy was melted at 750 °C under the CO2 protective atmosphere in a graphite crucible furnace. A certain amount of ZnCl2 (0.1–0.15% of total ingot weight) was then pressed into the molten alloy for the oxide reduction. After 5–10 min stabilization, the molten alloy was poured into stainless steel mold preheated at 250 °C to obtain the ZnAl4 alloy. Similarly, the ZnAl4–xZr alloys were obtained by adding a certain amount of Al–Zr master alloy into the molten ZnAl4 and performing a subsequent solidification. Table 1 presents the normal chemical compositions of the ZnAl4 alloys and the addition amount of Zr. Microstructure, morphologies of fracture surface and compositions of tested alloys were analyzed by scanning electron microscopy (SEM) (Sirion200) equipped with energy dispersive spectroscopy (EDS). Phase identification of the alloys was further confirmed by analyzing X-ray diffraction patterns generated by SIMENSD500 full-automatic X-ray diffractometer, operating at 40 KV and 45A with

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H. Li et al. / Journal of Alloys and Compounds 592 (2014) 127–134 Cu Ka radiation. The XRD patterns were analyzed by Jade6.0 software. The sizes of thickness and spacing of lamellar structure were measured by IPP (image Pro-Plus) soft. Tensile tests were carried out on the specimens with 10 mm gauge diameter and 105 mm gauge length at the strain rate of 0.002 s 1 using a CSS-44100 electronic universal testing machine according to ASTM E8 standard. Charpy impact tests were conducted using standard unnotched impact samples with the dimensions of 10  10  55 mm prepared in accordance with ASTM 23 standard using a NAI500F Pendulum impact testing machine. Hardness test was carried out according to ASTM E10 standard using an HBE-3000 Brinell hardness tester. At least five impressions were made to determine the mean value of the hardness at different locations to circumvent the possible effect of any alloying element segregation.

3. Results and discussion 3.1. Microstructure

Fig. 1. Zn–Al binary alloy phase diagram.

Table 1 Chemical compositions of the tested alloys and the addition amount of Zr (wt.%). Alloy

Al

Mg

Zr

Zn

1 2 3 4 5

3.98 3.98 3.98 3.98 3.98

0.046 0.046 0.046 0.046 0.046

0 0.05 0.1 0.2 0.3

Bal. Bal. Bal. Bal. Bal.

(a)

According to the Zn–Al binary phase diagram as shown in Fig. 1, primary g-Zn phase precipitates from the melt at first during solidification. When the temperature is below 382 °C, eutectic transformation occurs and a b + g eutectic structure is established from the remaining liquid enriched with a solute segregation. The b phase will be present until 275 °C where a eutectoid transformation occurs, generating a + g lamellar eutectic. With continued solidification, the eutectoid decomposition affects only the composition of the b phase inside the eutectic structure, not that of the g phase [4,17,18]. Fig. 2 shows the SEM images of ZnAl4 alloy containing different amount of Zr. According to Fig. 2(a), the microstructure of the alloy

(b)

lamellar eutectic lamellar eutectic eutectoid structure

eutectoid structure

(c)

(d)

lamellar eutectic

lamellar eutectic eutectoid structure eutectoid structure

(e)

lamellar eutectic

eutectoid structure

Fig. 2. SEM micrographs of as-cast ZnAl4 alloy with different amount of Zr (a) 0 wt.%; (b) 0.05 wt.%; (c) 0.1 wt.%; (d) 0.2 wt.% and (e) 0.3 wt.%.

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Fig. 5. The sizes of lamellar thickness and spacing with different Zr addition. Fig. 3. X-ray diffraction patterns of ZnAl4 alloy with different amount of Zr.

without Zr is composed of developed dendrite matrix corresponding to primary g-Zn phase, eutectic interdendritic structure (a + g), and eutectoid structure (black particles) in-between. With the in-

(a)

crease of the addition amount of Zr to ZnAl4 alloy, the kind and morphology of primary g-Zn phase as well as the amount of eutectic structure vary considerably. From Fig. 2(b and c), it can be seen

(b)

5 m

(c)

5 m

(d)

5 m

5 m

(e)

5 m

Fig. 4. Morphologies of lamellar eutectic in ZnAl4 alloy with different amount of Zr (a) 0 wt.%; (b) 0.05 wt.%; (c) 0.1 wt.%; (d) 0.2 wt.% and (e) 0.3 wt.%.

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Fig. 6. The high magnifications SEM micrographs and EDS analysis of the black particle and lamellar eutectic in the alloy: (a) black particle and (b) lamellar eutectic.

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that the size and the volume fraction of primary g-Zn phase decrease, but the amount of eutectic structure increases. When the addition amount of Zr exceeds 0.1 wt.%, there is little change in the volume fraction of eutectic structure in ZnAl4 alloy. However, a small amount of fine g-Zn phase exists in ZnAl4 alloy, as can be seen from Fig. 2(d and e). Fig. 3 shows XRD patterns of ZnAl4 alloys with different Zr content. Due to the fact that a-Al peak is close to Al2ZnZr peak, a close up of the XRD patterns region where peaks of a-Al and Al2ZnZr are closer was provided in order to distinguish these peaks. Besides gZn and a-Al, Al2ZnZr phase can also be identified. Generally, the addition of Zr to Al-based alloys can result in the precipitation of L12-structured Al3Zr phases from the melt [19,20]. However, in present study, the presence of L12-structured Al2ZnZr phase [21,22] is observed from the XRD patterns and EDS analysis (EDS5 in Fig. 7(a)) in this low aluminum ZnAl4 alloy with Zr addition. This may be hypothesized that a Zn atom enters the crystal

nuclei of Al3Zr to replace an Al atom to form new crystal nuclei Al2ZnZr. Besides, the b phase had decomposed and did not appear on the X-ray diffractogram. The X-ray diffractogram of as-cast samples was analyzed by jade6.0, the results show that the lattice constant of face centered cubic a-Al phase is a = b = c = 4.049  10 10 m; the lattice constant of hexagonal close packed (hcp) g-Zn phase is a = b = 2.665  10 10 m, c = 4.947  10 10 m; the lattice constant of face centered cubic Al2ZnZr phase is a = b = c = 4.030  10 10 m. Fig. 4 presents the SEM micrographs of the lamellar eutectic structure in ZnAl4 alloy. It can be observed that the thickness of the lamellar structure in ZnAl4 alloys with Zr is smaller than that in the alloy without Zr. Fig. 5 shows the sizes of lamellar thickness and spacing with different Zr addition, as measured by IPP (image Pro-Plus) software. The thickness of the lamellar structure and the lamellar spacing of eutectic structure is the smallest when the alloy was added with 0.1 wt.% Zr. However, when the addition amount of Zr reaches 0.3 wt.%, the lamellar spacing of eutectic

(a)

EDS 5

EDS 6

EDS 5

EDS 6

EDS 7

(b)

EDS 7

Fig. 7. EDS analysis of ZnAl4 alloy with 0.3 wt.% Zr.

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tice types plays an important role in grain refinement [24]. The addition of Zr to ZnAl4 alloy can result in the precipitation of L12-structured Al2ZnZr phases from the melt. The Al2ZnZr phase is face centered cubic lattice with a = b = c = 4.030  10 10 m, which is compatible with the face centered cubic lattice structure of a-Al phase with a = b = c = 4.049  10 10 m. According to the disregistry model of two-dimensional lattices, the disregistry between Al2ZnZr phase and a-Al phase is about 0.47% on their two low index planes. So the Al2ZnZr phases are response of the effective heterogeneity nucleation substratum for a-Al phase. As a-Al phase mostly exists in the lamellar eutectic in ZnAl4 alloy. Thus, the increase of a-Al phase results in the increasing amount of lamellar eutectic. The variations of the morphologies and sizes of the primary g-Zn phase are due to the increasing of the lamellar eutectic between the interdendritic. When the amount of the lamellar eutectic is increased, the lamellar eutectic is refined simultaneously. The fine lamellar eutectic distribute in the interdendritic regions between the growing dendritic structures of primary g-Zn phase, which seems to inhibit continued dendritic growth [25]. Secondly, the refinement mechanism of primary gZn phase is based on nucleation in the constitutionally supercooled region ahead of the solidification front [26–28]. Because the solubility of zirconium in zinc is quite limited, solute zirconium atoms will be enriched in the liquid near advancing solid–liquid interface during solidification, resulting in constitutional supercooling. Numerous crystal nuclei that can act as the nuclei for primary gZn will be produced heterogeneously. As a result, the primary gZn phase was refined. However, when the addition amount of Zr is excess, the formation of coarse Al2ZnZr phases (Fig. 7(a)) in the eutectic structure can cause the reduction of the ability to inhibiting g-Zn dendritic growth, as shown in Fig. 2(d)–(e). 3.2. Mechanical properties

Fig. 8. Mechanical properties of ZnAl4 alloy with different amounts of Zr.

structure does not change, but the thickness of the lamellar structure increases compared to the alloy with 0.1 wt.% Zr. Fig. 6 presents the high magnifications SEM micrographs and EDS analysis of the black particle and lamellar eutectic in the alloy. The EDS in Fig. 6(a) shows the chemical composition of the black lamellae and white lamellae. It can be seen that the black particle is composed of secondary g-Zn phase (white) and a-Al phase (black). The formation of this black particle (known as divorced eutectic) is due to non-equilibrium solidification. Fig. 6(b) shows the high magnifications SEM micrograph of lamellar eutectic. It can be observed that lamellar eutectic is composed of secondary g-Zn phase (white) and a + g eutectoid structure (white surrounded by black). The eutectoid structure (a + g) is probably associated to a decomposition from the b phase in eutectic structure (b + g) due to the instability of b phase [17]. Fig. 7 details the microstructure of ZnAl4 alloy with 0.3% Zr. It can be observed that microscopic Al2ZnZr and complex (Al, Zn, Zr, Fe) phases exists in the eutectic structure. The formation of coarse A12ZnZr may be due to the aggregation of extra Zr in the ZnAl4 alloy. The presence of complex (Al, Zn, Zr, Fe) phases may be attributed to the reaction of Zr with impurity element Fe. The effect mechanisms of Zr addition on the microstructure of ZnAl4 alloy are discussed as following. Firstly, the grain refinement of Zr can be explained by the heterogeneous nucleation theory [23]. The grain refinement of as-cast alloy is determined by the number of nuclei in unit melt. The nucleating effectiveness of the nuclei depends on the relationship between the lattice types and parameters of the phase and a-Al matrix and the similarity in lat-

The standard AG-40A alloy (detail composition: 3.5–4.3%Al, 0.02–0.05%Mg, max 0.25%Cu, max 0.10%Fe and balance Zn) shows better mechanical properties (TS = 271-283Mpa, HB = 82, Elongation = 5–10%, Impact toughness = 58 J/cm2) compare with the ZnAl4 alloy without additional Zr in present study. It was reported that magnesium is responsible for the formation of eutectoid structure in hypoeutectic ZnAl4 alloys. The Mg-rich ZnAl4 alloy exhibited higher value of strength and hardness and this fact can be associated to the microstructure refinement, especially in the case of eutectoid structure. Copper has a high tendency to form the eutectic interdendritic structure, but suppress the formation of eutectoid structure. The copper content increases the strength and hardness but decreases elongation of hypoeutectic ZnAl4 alloy [4]. Fig. 8 gives the mechanical properties of the as-prepared ZnAl4 alloy fabricated with different Zr addition. Along with the increase of Zr content, the tensile strength of ZnAl4 alloys increases, peaking at 231 MPa with 0.1 wt.% Zr, followed by a decrease thereafter. The elongation, impact toughness and HB hardness show similar evolution character, peaking at 4.80%, 70.37 J/cm2 and 85.19 kgf/ mm2, respectively. The variation of the mechanical properties can be discussed as follows. Firstly, the strength and the hardness of the solid solution of a-Al are higher than that of g-Zn. So the improvements are mainly attributed to the increase of eutectic structure. On the other hand, the lamellar spacing of eutectic structure can be an important factor in prediction of mechanical properties. Osório et al. [29] found that a finer lamellar eutectic provides better mechanical properties than a coarser morphology in ZnAl4 alloy. Secondly, the finer the g-Zn phase in ZnAl4 alloy, the better the mechanical properties is. When the addition amount of Zr reaches 0.3 wt.%, the decrease of mechanical properties of ZnAl4 alloy is due to the occurrence of coarse Al2ZnZr phase in

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(a)

(b)

50 m

50 m

(c)

50 m

Fig. 9. Fracture morphologies of the impact samples of ZnAl4 alloy with different amount of Zr: 0 wt.%; (b) 0.1 wt.%; (c) 0.3 wt.%.

the eutectic structure. The formation of coarse Al2ZnZr phase leads to the decrease of nuclei number for a-Al phases, thus the size of lamellar eutectic increase. Besides, the relative coarse Al2ZnZr phases give a detrimental effect on the mechanical properties of the alloy since the cracks can easily nucleate along the interface between the coarse Al2ZnZr phase and primary g-Zn phase. It can be seen from Figs. 2(c) and 4(c) that the primary g-Zn phase and lamellar eutectic structure in ZnAl4 alloy is the finest. Thus, when the addition amount of Zr is 0.1 wt.%, the value of the mechanical properties of the alloy reaches the maximum. In addition, the Fe content of ZnA1 alloys has a large harmful effect on the impact toughness. It can be observed from Fig. 7(b) that complex (Al, Zn, Zr, Fe) phases existed in the eutectic structure, which decreases the amount of FeAl3 needles, resulting in an improvement in the impact toughness [30]. However, excessive addition of Zr element will cause the impact toughness to decrease. The morphologies of the fracture surfaces of the impact samples are shown in Fig. 9. It can be observed from Fig. 9(a) that the fracture of the ZnAl4 alloy without Zr is apparently cleavage fracture. The coarse primary dendritic g-Zn phase can be distinguished in the fracture surface. The brittle rupture of the alloy is mainly due to the coarse primary g-Zn phase. When the addition amount of Zr increase to 0.1 wt.% Zr. The fracture surface of the alloy shows the presence of small cleavage facets and many tearing ridges exist on the fracture surface as shown in Fig. 9(b), which suggests that the mixing fracture mode of ductile and brittle fracture contributes to the improvement in the impact toughness. The cleavage facets in the alloy with 0.3 wt.% Zr as shown in Fig. 9(c) are coarser compared with them in Fig. 9(b). This is the reason that the alloy with 0.1 wt.% Zr has the best impact toughness.

4. Conclusions The effect of Zr on the microstructure and mechanical properties of ZnAl4 alloy has been investigated. As-cast microstructure

of ZnAl4 alloy consists of coarse primary g phase, a + g eutectic structure and eutectoid structure. The morphologies of primary g-Zn phase as well as eutectic structure are considerably distinct with different Zr content in ZnAl4 alloys, resulting in the remarkable evolution of mechanical properties. Along with the increase of Zr content, the tensile strength, elongation, impact toughness and HB hardness of ZnAl4 alloys increase, peaking at 231 MPa, 4.80%, 70.37 J/cm2 and 85.19 kgf/mm2 with 0.1 wt.% Zr, respectively. The fracture surface of the alloy with 0.1 wt.% Zr presents many fine small cleavage facets and tearing ridge, indicating the mixing fracture mode of ductile and brittle fracture. Acknowledgment The authors are grateful to Nonferrous Metals Science Foundation of HNG-CSU for financial support (Project Z2011-01-002). References [1] H.Y. Li, Y. Liu, X.C. Lu, X.J. Su, J. Mater. Sci. 47 (2012) 5411–5418. [2] T. Gancarz, J. Pstrus´, P. Fima, S. Mosin´ska, J. Alloys Comp. 582 (2014) 313–322. [3] F. Cheng, F. Gao, Y. Wang, Y. Wu, Z. Ma, J. Yang, Microelectron. Reliab. 52 (2012) 579–584. [4] E.M. da Costa, C.E. da Costa, F.D. Vecchia, C. Rick, M. Scherer, C.A. dos Santos, B.A. Dedavid, J. Alloys Comp. 488 (2009) 89–99. [5] E. Jareño, M. Castro, S. Maldonado, F.A. Hernández, J. Alloys Comp. 490 (2010) 524–530. [6] A. Türk, M. Durman, E.S. Kayali, J. Mater. Sci. 42 (2007) 8298–8305. [7] S.Q. Yan, J.P. Xie, Z.X. Liu, J.W. Li, W.Y. Wang, A.Q. Wang, J. Mater. Sci. 44 (2009) 4169–4173. [8] P. Choudhury, K. Das, S. Das, Evolution of as-cast and heat-treated microstructure of a commercial bearing alloy, Mater. Sci. Eng. A 398 (2005) 332–343. [9] F. Wang, B. Xiong, Y. Zhang, H. Liu, Z. Li, X. Li, C. Qu, Mater. Sci. Eng.: A 532 (2012) 100–105. [10] M. Emamy, A. Daman, R. Taghiabadi, M. Mahmudi, Int. J. Cast Metal Res. 17 (2004) 17–22. [11] S. Seyed Ebrahimi, M. Emamy, N. Pourkia, H. Lashgari, Mater. Des. 31 (2010) 4450–4456. [12] S. Seyed Ebrahimi, M. Emamy, Mater. Des. 31 (2010) 200–209. [13] Y. Deng, G. Xu, Z. Yin, X. Lei, J. Huang, J. Alloys Comp. 580 (2013) 412–426.

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