Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy

Abstract

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 [0001]_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.

Background

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 [1].

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% [5]. 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.

Methods

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

Figure 1 shows the ${\left[1\overline{2}10\right]}_{\alpha }$ HAADF image and the corresponding EDXS map of the as-quenched sample after a solution treatment at 430°C for 24 h. Most particles in the Mg matrix are predominantly rods/laths elongated along the [0001]_α direction, with a length of 50 to approximately 200 nm, although a few particles are elongated along other directions. The rod/lath morphology of these particles was confirmed by further large-angle tilting experiments. The Zr map, Zn map and a combined Zr and Zn map, as shown in Figure 1b,c and d, reveal that all rod-like particles in bright contrast in Figure 1a are enriched with Zn and Zr. This is in good agreement with the previous reports showing that Zr-rich phases exist in various Zr-containing Mg-Zn-based alloys after a solution treatment [2, 7, 8]. EDXS analysis detected no enrichment of Cu in the Zr-rich particles.

Figure 1

Alloy quenched after a solution treatment at 430°C for 24 h. The incident electron beam was parallel to ${\left[1\overline{2}10\right]}_{\alpha }$. (a) HAADF image, (b) Zr EDXS map, (c) Zn EDXS map and (d) a combined EDXS map of Zr and Zn.

In order to investigate the effect of these pre-existing Zr-rich particles on the formation of Zn-rich strengthening precipitates during subsequent isothermal ageing, HAADF imaging and EDXS mapping were conducted on samples aged at 180°C for different time. The ${\left[1\overline{2}10\right]}_{\alpha }$ HAADF image of the 20-h-aged sample, as shown in Figure 2a, reveals that a dispersion of particles was mostly elongated along the [0001]_α direction, with only one marked β₂′ perpendicular to the [0001]_α direction. After tilting a large angle of approximately 51° to the ${\left[0\overline{1}11\right]}_{\alpha }$ zone axis (Figure 2e), all particles observed in Figure 2a were found to be separate without overlapping with each other. The β₂′ precipitate, marked in Figure 2a, is a plate containing a brighter core, which corresponds to an enrichment of Zr (Figure 2b). The Zr map, Zn map and a combined Zr and Zn map, as shown in Figure 2b,c, and d, demonstrate that most of the elongated particles were composites containing a Zn-rich part and a Zr-rich segment. Careful examinations of the EDXS maps and the HAADF image confirmed that each Zr-rich segment was located either at the end or in the middle of an individual elongated precipitate. Therefore, we conclude that those Zr-rich segments of the precipitates are, in fact, the remains of the Zr-rich particles initially present in the as-quenched condition. We further deduce that these Zr-rich particles served as a precursor phase for the heterogeneous nucleation of Zn-rich β₁′ precipitates ([0001]_α rods) and β₂′ precipitates ((0001)_α plates) in the Zr-rich core regions of the Mg alloy during subsequent ageing.

Figure 2

Alloy aged at 180°C for 20 h. The incident electron beam was parallel to ${\left[1\overline{2}10\right]}_{\alpha }$ in (a-d) and ${\left[0\overline{1}11\right]}_{\alpha }$ in (e), respectively. (a) ${\left[1\overline{2}10\right]}_{\alpha }$ HAADF image, (b) Zr EDXS map, (c) Zn EDXS map, (d) a combined EDXS map of Zr and Zn and (e) ${\left[0\overline{1}11\right]}_{\alpha }$ HAADF image.

Figure 3 shows the HAADF image and the corresponding EDXS mapping result of the 120-h-aged sample. Both the length of [0001]_α β₁′ rods and the thickness of (0001)_α β₂′ plates grew significantly with the ageing time. The Zr map, Zn map and a combined Zr and Zn map, as shown in Figure 3b,c and d, indicate that many β₁′ rods and the β₂′ plate contain a Zr-rich segment. The sizes of Zr-rich segments observed in the 120-h-aged sample are smaller than those observed in the 20-h-aged sample. It appears that the size of the Zn-rich segments gradually increased at the expense of the Zr-rich segments during the isothermal ageing. After tilting approximately 36° from the ${\left[1\overline{2}10\right]}_{\alpha }$ beam direction, a ${\left[1\overline{5}43\right]}_{\alpha }$ HAADF image (Figure 3e) further confirms the existence of the Zr-rich segments in the Zn-rich precipitates. All experimental evidences above indicate that the heterogeneous nucleation on the pre-existing Zr-rich particles is significantly important for the formation of Zn-rich precipitates (β₁′ and β₂′) in the Zr-rich core regions of the Mg alloy during ageing at 180°C.

Figure 3

Alloy aged at 180°C for 120 h. The incident electron beam was parallel to ${\left[1\overline{2}10\right]}_{\alpha }$ in (a-d) and ${\left[1\overline{5}43\right]}_{\alpha }$ in (e), respectively. (a) ${\left[1\overline{2}10\right]}_{\alpha }$ HAADF image, (b) Zr EDXS map, (c) Zn EDXS map, (d) a combined EDXS map of Zr and Zn and (e) ${\left[1\overline{5}43\right]}_{\alpha }$ HAADF image.

To explore the crystallographic characteristics of these Zr-rich [0001]_α rods, we examined the as-quenched microstructure using TEM with the beam parallel to the [0001]_α direction, as shown in Figure 4. Most of the Zr-rich particles (>80%) of the as-quenched sample in Figure 4a have a low aspect ratio in the range of 1:1 to approximately 1:3 and a thickness in the range of 6 to approximately 12 nm with their long side, which is less than 25 nm, parallel to the $<11\overline{2}0{>}_{\alpha }$ directions. They are Zr-rich [0001]_α rod/lath particles observed previously by STEM examinations (Figure 1a). The rest of the Zr-rich particles (<20%), marked with black arrows in Figure 4a, are thin rods with aspect ratios of 1:3 to approximately 1:20 and a thickness of 2 to approximately 5 nm, with their long axis approximately 23° away from the $<11\overline{2}0{>}_{\alpha }$ directions. They are similar to the type C Zr-rich rods reported by Gao et al [8]. In contrast, the size and aspect ratio of the dominant Zr-rich [0001]_α rods/laths in the end-on view are significantly different from the Zr-rich $<11\overline{2}0{>}_{\alpha }$ rods reported by Gao et al [8]. This difference is possibly due to the different alloy systems and the heat treatment techniques.

Figure 4

The nanoscale Zr-rich [0001] _α rod-like precipitates in the solution-treated alloy. (a) [0001]_α TEM micrograph and the EDXS spectrum (inset), and (b) micro-beam electron diffraction pattern.

Chemical microanalysis of these [0001]_α rods using EDXS indicated that the atomic ratio of Mg:Zn:Zr was about 51:19:30 (inset, Figure 4a), suggesting that these [0001]_α rods were Zr-rich precipitates with a Zn:Zr ratio close to 2:3. The corresponding micro-beam diffraction patterns (Figure 4b) confirm that these Zr-rich [0001]_α rods have a tetragonal structure similar to that of Zn₂Zr₃ δ phase (a = b = 7.633 Å, c = 6.965 Å, α = β = γ = 90 [8, 9]). The orientation relationship (OR) implied by the superimposed precipitate and matrix patterns was such that ${\left[1\overline{1}0\right]}_{\delta }//{\left[0001\right]}_{\alpha }$${\left(110\right)}_{\delta }//{\left(1\overline{1}00\right)}_{\alpha }$ and ${\left(001\right)}_{\delta }//{\left(\overline{1}\overline{1}20\right)}_{\alpha }$. By combing the commonly reported OR between β₁′-MgZn₂[3, 10] /β₁′-Mg₄Zn₇[11, 12] and α-Mg matrix with the OR of the δ-Zn₂Zr₃ phase determined in this work, the possible ORs and the crystallographic disregistries between δ phase and β₁′ phase were determined and listed in Table 1. The inter-planar misfits between the matching planes ${\left(001\right)}_{\delta -Z{n}_{2}Z{r}_3}//{\left(0001\right)}_{{\beta }_1\prime }-MgZ{n}_2$${\left(001\right)}_{\delta -Z{n}_{2}Z{r}_3}//{\left(\overline{12}70\right)}_{{\beta }_1\prime }-M{g}_{4}Z{n}_7$${\left(110\right)}_{\delta -Z{n}_{2}Z{r}_3}//{\left(630\right)}_{{\beta }_1\prime }-M{g}_{4}Z{n}_7$ and the directional misfits along the matching directions ${\left[1\overline{1}0\right]}_{\delta }-Z{n}_{2}Z{r}_3//{\left[11\overline{2}0\right]}_{{\beta }_1\prime -MgZ{n}_2}$${\left[1\overline{1}0\right]}_{\delta }-Z{n}_{2}Z{r}_3//{\left[001\right]}_{{\beta }_1\prime -M{g}_{4}Z{n}_7}$ were calculated as 2.5%, 5.4%, 5.1% and 3.2%, 1.8%, which are less than the critical values of 6% and 10% given in the edge-to-edge matching model [13]. The low lattice mismatch between these two phases explains why β₁′ rods form directly on the end plane (001)_δ of the Zr-rich rods, as shown in Figures 2 and 3. The presence of the initial Zr-rich phases can provide much lower activation energy barrier and a favourable crystallographic correlation for the nucleation of the subsequent Zn-rich precipitates according to the classical nucleation theory [14].

Table 1 Calculated misfit values between β ₁ ′-MgZn ₂ /β ₁ ′-Mg ₄ Zn ₇ and δ-Zn ₂ Zr ₃ phases

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 [0001]_α rods in the Zr-rich cores of Mg alloys using a solution treatment, the formation of [0001]_α β₁′ rods could be promoted according to the heterogeneous nucleation mechanism revealed by this research.

Conclusions

In summary, we have demonstrated that the nanoscale Zr-rich [0001]_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.