Aging microstructural characteristics of ZA-27 alloy and

SiCp/ZA-27 composite

LI Zi-quan(李子全), ZHOU Heng-zhi(周衡志), LUO Xin-yi(骆心怡), WANG Tao(汪 涛), SHEN Kai(沈 凯)

  College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,

Nanjing 210016, China

Received 15 August 2005; accepted 18 November 2005

Abstract: The aging characteristics of as-quenched microstructures of ZA-27 alloy and SiC_p/ZA-27 composite(ZMC_p) were investigated using SEM, EDS and TEM. The structure, morphology and size of sub-grains in primary dendrite in ZMC_p continuously change during aging process. Little tiny spherical Zn-rich η phase distributes in the dendrite. Amount of transitional α′phase, well coherent with equilibrium α_f phase, in SiC_p-neighboring dendrite edge zone is less than that in dendrite center zone. Both eutectic and peritectic β phase transform into lamellar α and η phases, obeying _η∥[110]_α, and (002)_α∥_η. In the like-eutecticum of ZMC_p, less amount of β phase and decomposition products are found. The size of α phase decomposed from peritectic β phase in ZMC_p is obviously larger than that in the monolithic alloy. The lamella decomposition of β phase beside SiC_p is evidently more rapid than that in the alloy. SiC particulates strongly accelerate neighboring β phase decomposition in aging process.

Key words: SiC_p/ZA-27 composite; aging microstructure; ZA-27 alloy; precipitation; crystallographic orientation relationship

1 Introduction

Alloys, intermetallic compounds and metal matrix composites(MMC), processed by different solid solution and aging treatments, exhibit a variety of microstructures[1－5]. Influence of aging tempera- ture and time on the phase structure, microstructures and mechanical properties of both alloys and MMCs have been widely studied[6－14].

Commercial Zn-Al foundry alloys and Zn alloy matrix composites have been of considerable industrial applications[15, 16]. However, one serious problem with these alloys and composites is a gradual and irreversible expansion that occurs over a period of time at ambient temperature, which worsens in harsh working conditions. This dimensional change is due to the conversion of metastable phases, which are retained by non-equilibrium solidification and solid-state transformation. Therefore, a solution-aging treatment was used in order to gain relative stable structure. Several investigations regarding the aging characteristics of hypo-eutectic and hyper-eutectic Zn-Al alloys have been performed using X-ray diffraction(XRD), optical microscopy(OM), differen- tial scanning calorimetry (DSC) and scanning elec- tronic microscopy(SEM)[17－23]. LI and CHAO[17] researched mechanical properties and aging charac- teristics of zircon-reinforced Zn-4Al-3Cu alloy at  95 ℃. They found that α phase precipitates from η phase at the initial stage of the aging process, and ε phase appears from η phase later on, furthermore the crystallographic orientation relation- ship between α and η phases is _η∥_α and _η∥(111)_α. SHARMA et al[21] investigated effect of aging parameters on the primary dendritic microstructure and properties of ZA-27/aluminite metal matrix composites. They indicated that the aging and precipitation kinetics in the matrix alloy are significantly accelerated due to the presence of particulate reinforcement, and therefore the changes in properties of the composites were explained on the basis of microstructure alterations during aging.

As mentioned above, almost all aging investi- gations of ZA-27 alloy were carried out without con- sidering the inhomogeneity of the microstructure and composition. However, composition distributions in both as-cast ZA-27 alloy and its SiC particulate reinforced composite are extremely inhomogeneous, due to the non-equilibrium solidification in fabrication process, which results in various microstructures in both materials[24]. So the variety of the micro- structures and inhomogeneity of chemical com- positions can not be ignored during the investigations on the aging process of both hypo-eutectic Zn-Al alloys and their composites.

In brief, the purpose of the present work is to investigate the effect of artificial aging conditions on the alterations of matrix microstructures and compo- sitions for both ZA-27 alloy and SiC_p/ZA-27 compo- sites(ZMC_p), as well as reveal the crystallographic orientation relationships between precipitation phases (or decomposition products) in both ZA-27 alloy and ZMC_p during aging processes using SEM, EDS and TEM.

2 Experimental

In present study, a Mg(1.0%, mass fraction) alloyed commercial foundry ZA-27 alloy was selected as the metal matrix of the composites, which had the chemical composition(mass fraction, %): Al 26－28, Cu 1.8－2.2, Mg 1.05－1.15, balance Zn. The ceramic particulate reinforcement was a-SiC particles with the size of 18 mm, received as abrasive grade. SiC_p/ZA-27 composite with SiC_p volume fraction of 10% was manufactured by application of mechanical electro- magnetic combination stirring process(MECSP)[24]. The ZA-27 and ZMC_p samples were aged at  160 ℃ for 5, 15 and 20 h after solutionizing at 360 ℃ for 2 h by cold water chilling. The solutionized and quenched samples were kept at －20 ℃ before arti- ficial aging.

The characteristics of microstructure and preci- pitation phases in both materials after aging treatment were observed using SEM (HITACHI X-650) at the accelerating voltage of 20 kV, EDS (attached to the SEM) and TEM (JEM-2000EX) at the accelerating voltage of 160 kV.

The dimension of SEM specimens for micrography was d10 mm×10 mm. Their surfaces were ground on SiC papers till to 600 grit. Then fine polishing was done using magnesium oxide paste followed by diamond paste on a velvet cloth. Etchant used was alcoholic solutions of nitric acid with volume fraction of 4%. The TEM specimens with 0.2 mm in thickness were firstly mechanically ground and polished to about 20 ?m in thickness, then punched into d 3 mm thin flakes, and finally reduced on a double-jet ion-milling (Gatan 600-TMP).

3 Results and discussion

The typical microstructures of ZA-27 and ZMC_p aged for different durations are shown in Fig.1 and Fig.2, respectively. The both matrix microstructures in cold-water chilling can be divided into three typical types, namely Al-supersaturated phase  (dark, original primary dendrites), like-eutecticum α+β (white zones), and β phase (white strip around the original primary dendrite α) in Fig.1(a) and Fig.2(a), respectively. α phase is aluminum-rich phase, η phase is zinc-rich phase, and β phase, ZnAl, is intermetallic compounds. EDS results of the phases or micro- structures in ZA-27 and ZMC_p aged for 5 h are listed in Tables 1 and 2, respectively. The Zn contents range from 24% to 76% in the different microstructures for ZA-27, and from 29% to 82% for ZMC_p. The results indicate that the composition is extremely inhomo- geneous either in ZA-27 or ZMC_p. A variety of microstructures and difference in the chemical composition are attributed to non-equilibrium casting conditions and can not be eliminated simply by solid solution process at higher temperatures.

Fig.1 Microstructures of ZA-27 alloy aged for different durations: (a) 0 h; (b) 5 h; (c) 15 h

3.1 Decomposition microstructure of super- saturated  phase

As stated above, high temperature over-saturated phase  in both the monolithic alloy and ZMC_p, retained at ambient temperature due to rapid cooling, undergoes decomposition during artificial aging process. The Zn content gradually increases from 24% on the center of primary dendrite in ZA-27 to 29% on the edge, as listed in Table 1. Differently, the Zn content keeps about 29% along the dendrite in ZMC_p (Table 2). The Zn content indicates that in the primary dendrite of ZA-27 and ZMC_p only precipitation reaction can take place according to the Zn-Al phase diagram. Although there is no pronounced difference in micrograph at low magnification for the dendrite (Figs.1 and 2), changes of sub-grain morphology, size and crystal structure are confirmed by TEM in Fig.3, which shows typical microstructure images of the dendrite in the ZMC_p at as-quenched state and aged at 160 ℃ for 5 h. Near SiC particle, dendritic micro- structure in as-quenched condition is composed of d100 nm×300 nm long column over-saturated phase  (a=0.398 0 nm). The microstructure of the dendrite center zone in ZMC_p consists of white region and small amount of elliptical particles under same aging conditions (Fig.3(b)). The white region is characterized as stable Al-rich phase α_f (a=0.401 5 nm) by an electron diffraction pattern in 〈110〉 (Fig.4), which is decomposed from over-saturated phase . The elliptic particles are confirmed as (α= 0.399 2 nm, the transitional phase of α_f), which contains more Zn than equilibrium phase α_f. As no individual diffraction spots are found in Fig.4 and there is very small difference in their crystal lattice constants, α′keeps well coherent relationship with α_f. Lower aging temperature and shorter aging duration result in the incompleteness of the transition from  to α_f. Some separated tiny secondary precipitates can also be found in Fig.5. They are confirmed as Zn-rich  η phase by accurate diffraction technology. The microstructure of dendrite edge zone neighboring SiC particle consists of equilibrium phase α_f, tiny η phase and very little amount of transitional phase α′(Fig.3(c)). The size of α_f in different regions is almost identical (about 500 nm). The results indicate that SiC particles in ZMC_p have little influence on grain growth, whereas facilitate precipitation in the local zone of over-saturated dendrite, which could be attributed to the Zn diffusion accelerating effect due to the tensile stress field in the matrix, because of the difference between matrix and SiC in coefficient of thermal expansion.

Fig.2 Microstructures of SiC_p/ZA-27 composites aged for different durations: (a) 0 h; (b) 5 h; (c) 20 h

Fig.3 Typical microstructure images of dendrite in MMCs aged at 160 ℃: (a) As-quenched, 0 h; (b) At center, 5 h; (c) At edge, 5 h

Table 1 EDS elemental analysis of phases in ZA-27 alloy aged for 5 h (mass fraction, %)

Table 2 EDS elemental analysis of phases in MMCs aged for 5 h (mass fraction, %)

Fig.4 Electron diffraction pattern of α in〈110〉

3.2 Changes of like-eutecticum

Microstructure of the like-eutecticum in ZA-27 alloy continuously changes with aging time (Fig.1). In as–quenched state, β phase (in dark) merely partially decomposes. Fig.6 shows TEM images of black and white lamellar structure of β (Zn 74.3%) decomposition products in the like-eutecticum of ZA-27 alloy aged for 5 h. During aging process, β phase in the like-eutecticum completely decomposes into (α+η) according to Fig.6, in which α and η phases are identified by electron diffraction patterns, and then the decomposition products form fishbone-like structure (η+(α+η)) with existing η phase, as shown in Fig.1. Multi-element lamellar microstructure (α+η), of which the synthesized electron diffraction patterns are shown in Fig.7, coarsened with the increase of aging time. In order to confirm the crystallographic orientation relationship between α and η, the electron diffraction patterns of α and η in [110] and, as well as their synthetic electron diffraction pattern are simulated with the standard crystallographic parameters of both α and η phases by a special simulation program, as shown in Fig.8. The simulated patterns fully agree with the experimental patterns in Fig.7, which indicates that the (002) crystal plane of α nearly parallels to  crystal plane of η. Therefore, it can be surely confirmed that in the  lamellar structure the black noodle-like phase is α, while the continuous white one is η in the bright field image.

Fig. 5 Dark field image of MMCs matrix aged at 160 ℃ for 20 h

Fig.6 Images of lamellar structure of β phase decomposition in like-eutecticum of ZA-27 alloy aged for 5 h: (a) Bright field image;    (b) Dark field image of η; (c) Dark field image of α

Fig.7 Synthetic electron diffraction pattern of α and η phases

Comparing Fig.1 and Fig.2, amount of like- eutecticum in as-quenched composite is visibly less than that in as-quenched monolithic alloy. Less spherical decomposed β phase distributes in like- eutecticum. As shown in Fig.2, little decomposition product continuously precipitates and grows slowly with increase of aging time. It shows that like- eutecticum microstructure in ZMPs is relatively stable with regarding to ZA-27. One of reasons is that the aggregated Zn and lower Al content in the like- eutecticum (Tables 1 and 2) result from extremely high cooling rate and vigorous agitation during solidification process[24]. Therefore, the composi- tional deviation results in that less amount of β phase presented in the like-eutecticum in equili- brium conditions. Another reason is that much more amorphous oxides[24], formed in fabrication process, aggregated in the final solidification region. Con- sequently, efficient amount of alloy elements reduces, resulting in the reduction of β phase as well. Fig.9(a) shows image of lamellar structure of β phase decomposition products in the like-eutecticum of ZMCs aged for 5 h. The distribution of α and η phases are analogous to that in the monolithic alloy. But some spherical precipitates in the decomposed β phase can be observed in bright and dark field images, shown in Figs.9(a) and 9(b). Fig.9(c) shows an electron diffraction pattern of the precipitates in [111], by which the crystal structure of precipitation phase is determined as B₂ structure with lattice constant of a=0.294 nm. The precipitates are confirmed as CuZn, which was not found in as-cast materials[24].

3.3 Decomposition of peritectic β phase

Very small amount of peritectic β phase, around the primary dendrite in a thin-strip shape, is retained in both as-quenched materials (Fig.1(a) and Fig.2(a)). It transforms into lamellar two-phase structures in style of cellular decomposition (grey in Fig.1(b) and Fig.2(b)), and α phase gradually coarsens in the aging process. For the same aging time, the size of α phase in ZMC_p is obviously larger than that in the monolithic. Especially, lamellar decomposition reac- tion takes place more easily in β phase neighboring SiC particles and coarser α phase is gained in peritectic β phase of ZMC_p. This implies that the growth rate of α phase in ZMC_p is evidently higher than that in ZA-27 alloy, and SiC particles strongly hasten the decomposition of β phase.

Fig. 8 Computer-simulated electron diffraction patterns of α and η as well as α+η in _η and [110]_α : (a) _η ; (b) [110]_α ;   (c) _η ∥[110] _α

Fig. 9 TEM images of lamellar structure and electron diffraction pattern of CuZn in like-eutecticum of SiCp/ZA-27 composite aged for  5 h: (a) Bright field image; (b) Dark field image; (c) Electron diffraction pattern of CuZn in [111]

4 Conclusions

1) Existence of SiC_p in matrix facilitates Zn and Cu diffusion and makes SiC-neighboured matrix transformation easier. Either eutectic or peritectic β phase transforms into lamellar α and η phases in style of cellular decomposition, obeying _η∥[110]_α, and (002)_α∥_η.

2) Only precipitation reaction can occur in primary dendrite for both ZA-27 and ZMC_p. Al-supersatured continuously transforms into transitional phase  and η(Zn), then  transforms with well coherent relationship into stable α_f. Amount of  in dendrite edge zone neighboring SiC_p is less than that in dendrite center zone.

3) Aggregated Zn and Cu as well as oxides in the like-eutecticum of ZMC_p result in less amount of β phase and form spherical precipitates CuZn. The like-eutecticum microstructure in ZMC_p is relatively stable.

4) The size of α phase decomposed from peritectic β phase in ZMC_p is obviously larger than that in the monolithic. The growth rate of α phase in ZMC_p is evidently higher than that in ZA-27, and SiC_p strongly accelerate decomposition of peritectic β phase.

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Foundation item: Project(03G52047) supported by the Aviation Science Foundation, China

Corresponding author: LI Zi-quan; Tel: +86-25-84891343; E-mail: ziquanli@nuaa.edu.cn

(Edited by LI Xiang-qun)