Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy
© Zhu et al.; licensee Springer. 2012
Received: 1 December 2011
Accepted: 4 April 2012
Published: 8 June 2012
Zirconium (Zr) is an important alloying element to Mg-Zn-based alloy system. In this paper, we report the formation of the β-type precipitates on the nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy during ageing at 180°C. Scanning transmission electron microscopy examinations revealed that the nanoscale Zr-rich α rods/laths are dominant in the Zr-rich core regions of the as-quenched sample after a solution treatment at 430°C. More significantly, these Zr-rich particles served as favourable sites for heterogeneous nucleation of the Zn-rich β-type phase during subsequent isothermal ageing at 180°C. This research provides a potential route to engineer precipitate microstructure for better strengthening effect in the Zr-containing Mg alloys.
KeywordsMg alloys Zn-rich precipitates nanoscale Zr-rich particles heterogeneous nucleation electron microscopy
Mg-Zn-based alloys have attracted considerable attention due to their pronounced age-hardening effect [1–5]. The key strengthening precipitates in this alloy system have been considered as two types of Zn-rich precipitates, the rod-like β1′ precipitates perpendicular to the (0001)α plane and the plate-like β2′ precipitates parallel to the (0001)α plane [1–5]. Hardening by precipitation of β-type precipitates is believed to be the main strengthening mechanism of Mg-Zn-based alloys .
Recently, a peak-aged Mg-6Zn-0.5Cu-0.6Zr cast alloy has been reported to possess excellent mechanical properties with an ultimate tensile strength of 266.3 MPa, a 0.2% proof yield strength of 185.6 MPa and an elongation of 16.7% . Both the strength and ductility of the newly designed Mg-6Zn-0.5Cu-0.6Zr alloy are superior to those of the traditional Mg-6Zn-xCu-0.5Mn alloys [5, 6]. Since Zr-rich particles may form after a solution treatment in Zr-containing Mg alloys [2, 7, 8], the present research aims to unveil the effect of these pre-existing nanoscale Zr-rich particles on the formation of the subsequent β-type precipitates of the Mg-6Zn-0.5Cu-0.6Zr alloy during age hardening.
The alloy with a nominal composition of Mg-6Zn-0.5Cu-0.6Zr (wt.%) for this study was prepared by melting high-purity Mg and Zn with Mg-28.78 wt.% Cu and Mg-31.63 wt.% master alloys, in a steel crucible and by casting into a permanent mould under an Ar atmosphere. Samples sectioned from the ingot were solution-treated for 24 h at 430°C. To investigate the microstructural evolution of the Zr-rich and Zn-rich precipitates, the water-quenched samples were subsequently aged in an oil bath for 20 and 120 h at 180°C. Thin foil specimens for scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) were prepared by a twin-jet electropolisher using a solution of 10.6 g LiCl, 22.32 g Mg(ClO4)2, 200 ml 2-butoxi-ethanol and 1,000 ml methanol at about −45°C and 70 V. The STEM study was conducted using a JEOL 2200FS microscope (JEOL Ltd., Tokyo, Japan) equipped with a high-angle annular dark field (HAADF) detector and a Bruker energy dispersive X-ray spectrometer (EDXS) detector (Bruker AXS, Karlsruhe, Germany). The conventional TEM analysis was carried out using a JEOL 3000F microscope equipped with an Oxford EDXS detector (Oxford Instruments, Oxfordshire, UK).
Results and discussion
Calculated misfit values between β 1 ′-MgZn 2 /β 1 ′-Mg 4 Zn 7 and δ-Zn 2 Zr 3 phases
Spacing or length (nm)
It is a significant finding that the Zr-rich phases can act as the precursor phase for the heterogeneous nucleation of Zn-rich β-type strengthening phases in the Mg alloy, given that the Zr-rich core region is a major microstructural feature of Zr-containing Mg alloys [7, 8]. By effectively engineering Zr-rich α rods in the Zr-rich cores of Mg alloys using a solution treatment, the formation of α β1′ rods could be promoted according to the heterogeneous nucleation mechanism revealed by this research.
In summary, we have demonstrated that the nanoscale Zr-rich α rods/laths were predominant in Zr-rich core regions of the Mg-6Zn-0.5Cu-0.6Zr (wt.%) alloy after a solution treatment at 430°C. The nanoscale Zr-rich particles served as a precursor phase for the heterogeneous nucleation of the Zn-rich β-type strengthening precipitates during subsequent isothermal ageing at 180°C. These results are important for controlling Zr-rich particles in the Zr-rich core regions for enhancing the overall strength of the Mg alloy.
scanning transmission electron microscopy
transmission electron microscopy
high-angle annular dark field
energy dispersive X-ray spectrometer.
This project was supported by the National Basic Research (973) Program of China (no. 2009CB623704), Nature Science Foundation of Guangdong Province (no. 07006483), the Doctoral Scientific Research Foundation of the University of South China (no. 2011XQD26), China Scholarship Council and the Australian Research Council Centre of Excellence for Design in Light Metals. ZL acknowledges the funding support by the Australian Research Council (DP0881700). The authors also acknowledge the facilities and scientific and technical assistance from AMMRF at the University of Sydney, particularly, Dr. Dave Mitchell and Dr. Ting-Yu Wang.
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