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General Rule of Phase Decomposition in Zn-Al Based Alloys (II) ---On Materials Transactions, Vol. 45, No. 11 (2004) pp. 3083 to 3097 #2004 The Japan Institute of Metals OVERVIEW General Rule of Phase Decomposition in Zn-Al Based Alloys (II) —On Effects of External Stresses on Phase Transformation— Yao hua Zhu Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, P.R. China Microstructural changes and phase transformation of Zn-Al based alloys (ZA alloys) were systematically investigated during various thermal and thermo-mechanical processes using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe micro-analysis (EMPA), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and differential scanning calorimeter (DSC) etc. techniques. Phase decompositions of the alloys were studied under various thermal and thermo-mechanical circumstances. General rule of phase decomposition (II) (On effects of external stresses on phase transformation) was summarized with explanations from point view of Gibbs free energy. (Received April 19, 2004; Accepted September 6, 2004) Keywords: ageing, deformation, stress induced phase decomposition, zinc-aluminium based alloys 1. Introduction Zn-Al based alloys have been commercially accepted for many years, since the first casting alloy was developed at the New Jersey Zinc Company in 1922. The most popular die casting alloy used, ZAMAK 4 was the 898 alloy discovered in their series of alloy development. A new family of hyper- eutectic Zn-Al based alloys with high aluminum and copper contents was developed based on the ZAMAK alloys in North America and China in the 1970’s. The copper content was up to 3% (in mass%) and aluminum content were selected as about 8, 12, 22 and 27% (in mass%). The mechanical and physical properties of these alloys are much Fig. 1 Phase diagram of binary Zn-Al alloy. improved. This new family of the alloy has been candidates as substitutions for traditional bushing alloys, such as bronze and aluminum alloys. To meet the growing demands for application of these alloys in industry, extensive studies on 350°C 280°C microstructural changes and phase transformations which occur during various thermal and thermo-mechanical proc- esses are required. 2. General Rule of Phase Decomposition in Zn-Al Based Alloy Since the 1970’s, a systematic investigation of phase 270°C 250°C relationships in Zn-Al based alloys (ZA alloys) has been carried out. The studies began with the establishment of phase diagrams of alloy systems of Zn-Al binary, Zn-Al-Cu and Zn-Al-Si ternary and Zn-Al-Cu-Si quaternary alloy systems.1–4) This was followed by studies of the phase transformations which occurred in solution-treated alloys of Fig. 2 Isothermal sections of Zn-Al-Cu phase diagram. different compositions, i.e., aluminum-rich, monotectoid and eutectoid alloys.5–20) The phase diagram of Presnyakov et al. modified by þ T0 ¼¼ þ " at 285C Goldak and Parr has been adopted for representation of the 1) þ " ¼¼ þ at 276 C Zn-Al binary phase diagram, shown in Fig. 1. The phase 0 diagrams of both Zn-Al-Cu and Zn-Al-Cu-Si systems are þ " ¼¼ T þ at 268 C shown in Fig. 2.2–4) For alloys of composition in various ranges in Zn-Al, Zn- The phase relationships in equilibrium state in Zn-Al Al-Si, Zn-Al-Cu, and Zn-Al-Cu-Si systems, the mechanisms alloys containing copper and/or silicon have been estab- of the phase transformations occurring during post quench- lished as follows:2–4) aging have been investigated and previously published.12–20) 3084 Y. h. Zhu Table 1 The phases involved in the investigations. X-ray diffraction (XRD) intensity of (111) and (200) crystal 0 : Al rich fcc phase planes of the s phase decreased, accompanying the 0 0 0 : Zn rich fcc phase formation of three phases T, " and , i.e., s ! T þ ": hcp phase CuZn4 " þ , after quench-aging at room temperature for 16 min. : Zn rich hcp phase The 0 phase decomposed at the grain boundaries during : Si rich phase of bcc structure s T0: distorted bcc structure, Zn10Al35Cu55 (in mass%) quenching, and this discontinuous precipitation was devel- 0 E: Rare earth containing Zn-rich phase oped along the grain boundaries in the s phase after aging 0 s: Supersaturated Al rich fcc phase at room temperature for 10 and 20 min, as shown in 0 s: Supersaturated Zn rich fcc phase 0 0 back-scattered microscopy (BSEM) images, Figs. 4a, b and T: Al rich eutectoid terminal fcc phase derived from s 12,14,22,31) 0 c. and s phases 00 3.1.2 Aging characteristics of furnace cooled (FC) m: The first transition phase 00 00 : The Al rich matrix phase in equilibrium with m eutectoid ZA alloy 0 m: The second transition phase Furnace cooling is a slow cooling process, which is often 0 0 : The Al rich matrix phase in equilibrium with m involved in various advanced metallurgical processes. It is of f : Stable or final stable Al rich fcc phase 0 considerable interests in studies of the structural evolution of s: Supersaturated Zn rich hcp phase 0 alloys during the metallurgical processes. E: Supersaturated Zn rich hcp phase in extruded Zn-Al alloys 0 FC: Supersaturated Zn rich hcp phase in furnace cooled Zn-Al alloy The eutectoid Zn-Al based alloy (ZnAl22Cu2) was solu- 0 T: Metastable Zn rich hcp phase tion-treated at 350 C for 4 days, then cooled inside the f : Stable or final stable Zn rich hcp phase furnace chamber to ambient temperature. The FC alloy specimens were aged at 100, 150 and 170C. BSEM examination was carried out on the FC and aged specimens. The phases involved in the investigations are listed in The BSEM images of the FC and 100C-aged specimens are Table 1. shown in Figs. 5a–c.29) Two phases, (Zn-rich phase of fcc Based on the studies of phase diagrams, i.e., equilibrium structure) and " (CuZn4 with hexagonal close packed, i.e., phase relationships, and phase transformations during vari- hcp structure), were observed in the FC specimen. During 0 ous thermal holdings, an intrinsic co-relationship between furnace cooling, the phase became an unstable s phase 0 equilibriums and non-equilibrium phase transformations, i.e., and decomposed into various metastable phases: , " and FC a general rule of phase decomposition in Zn-Al based alloys (Zn-rich hcp phase). The Al-rich and Zn-rich metastable was summarized for the first time.3,21) It indicated that phases decomposed further into fine and coarse lamellae due equilibrium phase transformations were generally possible to different diffusion rate of Al and Zn atoms. The " phase did during post quench-aging, i.e., non-equilibrium processes. not decompose and appeared as the original particles in the Among these possible equilibrium phase transformations, FC alloy specimen. The resultant microstructure of the FC those at higher temperature occurred at early stage of aging, alloy specimen consisted of the coarse and fine lamellae, and 0 while those at relatively lower temperature always happened the light-contrast FC and the " phase particles, as shown in during prolonged aging. Fig. 5a. X-ray diffractograms of the as FC alloy specimen 0 showed the co-existence of the three phases: , " and FC, 3. Phase Transformations in Various Thermal and Fig. 6.29) Thermo-Mechanical Processes During aging at 100C, more precipitates of phase were 0 observed in the FC phase, as arrows ‘‘!’’ indicated in 3.1 Phase decomposition in isothermal holding Fig. 5. The dark-contrast phase precipitates grew at the Further studies extended the work to cover various phase boundaries after aging at 100C for 10 h, and clearly complex non-equilibrium and more practical processes in developed after aging at 100C for 30 h. This was detected as the Zn-Al based alloys with various chemical compositions: a discontinuous precipitation in the X-ray diffractograms, 22–35) 0 eutectoid and monotectoid alloys. X-ray diffraction Fig. 6. The (0002) diffraction peak of the FC phase 0 (XRD), scanning electron microscopy (SEM), electron probe decreased in height at 36.8 degree, whilst a metastable T micro-analysis (EPMA), transmission electron microscopy phase appeared at a lower 2 ¼ 36:5 degree, accordingly 0 0 (TEM), electron back-scattered diffraction (EBSD) and d-spacing values of the FC phase and the T phase were differential scanning calorimeter (DSC) etc. techniques were 0.2437 nm and 0.2456 nm, respectively. After aging at 100C 0 applied in the studies. for 20 h, the original (0002) peak of the FC phase decreased 0 0 3.1.1 Decomposition of s phase in solution treated ZA greatly, and that of the T phase remained apparently at the 0 alloys lower 2 side of the FC phase. This decomposition of the 0 The Zn-rich phase with face centered cubic (fcc) FC phase was followed by another phase transformation. 0 structure becomes an unstable s phase at ambient temper- After aging for 20 h, both X-ray diffraction peaks from ature. Because more than 70% zinc-rich " and phases are ð10110 Þ and (0002) crystal planes of the " phase decreased in 0 12) formed from decomposition of the s phase, it is important height at 2 ¼ 37:6 degree and 2 ¼ 42:1 degree, relatively. to understand well the mechanism of the decomposition of Meanwhile the (110) diffraction peak of T0 phase increased, 0 the s phase. Shown in Figs. 3a and b are X-ray diffracto- as shown in Fig. 6. It was recognized as a four-phase grams of both as quenched and the aged specimen of the transformation: þ " ! T0 þ or decomposition of the eutectoid Zn-Al based alloy, respectively.
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