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Precipitation modification in cast Mg–1Nd–1Ce–Zr alloy by Zn addition
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Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition Yiyuan Zhou a, Penghuai Fu a,b,∗, Liming Peng a, Dan Wang a, Yingxin Wang a, Bin Hu c, Ming Liu c, Anil K. Sachdev d, Wenjiang Ding a a National
Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, PR China b Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd, Shanghai 201615, PR China c General Motors China Science Laboratory, Shanghai 201206, PR China d General Motors Research & Development Center, Warren, MI 48090, USA Received 26 November 2018; received in revised form 26 January 2019; accepted 20 February 2019 Available online xxx
Abstract The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions were studied. Precipitates in cast Mg–1Nd–1Ce-Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2 Zn is added, the disk-shaped precipitates are refined and very fine basal precipitates form additionally. When 0.5 Zn is added, the basal precipitates become the main strengthening phase. Further increasing the Zn addition to 1.0%, only spare basal precipitates and point-like precipitates exist. The 0.5 Zn addition alloy has the highest strength at room temperature, whose yield strength, ultimate tensile strength and elongation in T6 condition are 136 MPa, 237 MPa and 9%, respectively. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Mg alloy; Precipitate; Rare earth; Zn addition; Mechanical properties; Mg wheel.
1. Introduction A recent push by the automotive industry to reduce the fuel consumption and the cost of automobiles is providing enhanced motivation for the study of lightweight cast materials for structural components. Magnesium (Mg) alloys containing rare earth elements were frequently investigated recently and some of high strength Mg-Gd based alloys [1–4], such as Mg-Gd-Ag-Zr [5–7], Mg-Gd-Zn-Zr [8–10], Mg-Gd-Y-Zr [11–13] alloys were developed, which are very attractive to the automobile manufacturers. However, high content of rare earth elements increases the cost of these alloys, which would limit their applications in the automotive industry. Therefore, ∗ Corresponding author at: National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (P. Fu).
in order to promote the application of Mg alloys in automobiles, high strength and relatively low-cost Mg alloys are needed. Rational use of Neodynium (Nd, one of rare earth (RE) elements) could make it possible to develop a high strength and relatively low-cost Mg alloy. Nd is one of light rare earth elements, with maximum solubility in solid Mg of 3.6 wt% (all compositions in wt% except otherwise stated) at eutectic temperature of 545 °C. Mg-Nd binary alloys already have significant age hardening effect [14–21], whose precipitation sequence were recently updated to be [18]: SSSS → GP Zones (N, V, hexagons) → β"’ → β 1 (Mg3 Nd) → β (Mg12 Nd) →β e (Mg41 Nd5 ), where β"’ includes the β’ phase and ranges in composition from xNd = 0.125 to xNd ≈ 0.166. When Zn element is added into Mg-Nd binary alloys, basal precipitates are introduced, such as the basal GP zones (hexagonal, a = 0.556 nm) in Mg–2.4RE–0.4Zn–0.6Zr alloy (where 2.4RE includes 1.3Ce, 0.6La, 0.4Nd and 0.1Pr) [22], and
https://doi.org/10.1016/j.jma.2019.02.003 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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Table 1 Chemical compositions of Mg-1Nd-1Ce-xZn-0.5Zr alloys (wt%). Ce
Nd
Zn
Alloys
Average
Variance
Average
Variance
Average
Mg-1Ce-1Nd-Zr Mg-1Ce-1Nd-0.2Zn-Zr Mg-1Ce-1Nd-0.5Zn-Zr Mg-1Ce-1Nd-1.0Zn-Zr
1.09 1.13 1.18 1.16
0.003 0.004 0.012 0.004
1.03 1.10 1.17 1.17
0.004 0.002 0.010 0.004
– 0.19 0.56 0.96
the basal γ " phase (hexagonal, a = 0.556 nm, c = 1.563 nm) in Mg–2.8Nd–1.3 Zn (wt%) alloy [23]. The addition of 0.5 Zn to Mg–3Nd alloy was reported to increase its peak-aged hardness [24]. Generally, at the present stage, the effect of Zn addition on the precipitation sequence is still not characterized clearly. Mg–3Nd–0.2Zn-Zr (NZ30K) [25–26] was recently developed, whose typical mechanical properties are: yield tensile strength of 140 MPa, ultimate strength of 305 MPa and elongation of 10% [25]. Due to its acceptable strength and ductility, NZ30K alloy was used to cast thick-wall components, such as car wheels, engine blocks and engine covers. However, as the cost of Nd element increases significantly in recent years, the NZ30K alloy becomes expensive, which stops its application in automobiles. In the present study, in order to reduce the cost of MgNd based alloys, the content of Nd element is set to be 1%. 1% Ce is added to make up the loss of strength due to the reduced precipitation strengthening by the decrease of Nd element. Ce element is much cheaper and Ce addition could provide both dispersion strengthening and precipitation strengthening [27,28]. The effects of different Zn addition (0, 0.2, 0.5, 1.0%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions are systematically studied. 2. Experimental Alloys of nominal composition Mg–1Nd–1Ce-xZn-Zr (wt%) (all compositions in wt% except otherwise stated), where x = 0, 0.2, 0.5 and 1, were prepared from purity Mg, Zn and Mg–90Nd, Mg–90Ce, Mg–30Zr master alloys by melting in an electrical resistance furnace under the protection of the mixture gas of SF6 , CO2 and air, then cast in permanent mould at pouring temperature of 730 ± 5 °C and mould temperature of 150 ± 20 °C. The actual chemical compositions was determined by an inductively coupled plasma analyzer (ICP) and listed in Table 1. Specimens cut from the cast ingots were first solution-treated at 540 °C for 10 h and quenched into hot water at ∼70 °C, then subsequently aged at 200 °C in an oil-bath. Vickers hardness testing was taken using 5 kg load and holding time of 30 s. Each hardness value was averaged from at least five individual measurements. Tensile samples were cut into rectangular tensile specimens with the dimensions of 10 mm width, 2 mm thickness and 25 mm gauge length by an electric-sparking wire-cutting machine. Tensile testing was carried out on a Zwick/Roell-20 kN material test machine at a cross head speed of 0.5 mm/min at
Zr Variance
Average
Variance
Mg
0.005 0.017 0.009
0.4 0.5 0.49 0.5
0.016 0.004 0.005 0.005
Bal. Bal. Bal. Bal.
room temperature. Three tensile samples were tested for each condition. Specimens were etched in a 4 vol% nital or in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol and observed in Zeiss Axio observer A1 optical microscope (OM). The grain sizes of the samples were measured with the linear intercept method according to the ASTM E112-12, in which each value was the average of at least five individual OM images under 200 times magnification. For the scanning electron microscopy (SEM) observation, the as-cast and solution-treated metallographic specimens were used in the polished condition to obtain credible data of chemical composition and phase constitution using Phenom XL equipped with an energy dispersive X-ray spectrometer (EDS) and a backscatter electron detector (BSE). The volume fractions of the intermetallic compounds at the grain boundaries of the as-cast and solution-treated specimens were measured using SEM-BSE images taken from at least five random fields. Thin foil for transmission electron microscope (TEM) observation were prepared by punching 3 mm diameter discs, then mechanical grounding to ∼50 μm, finally polishing using the Ion Polishing System (PIPs 691, 7° under 3 keV until perforation followed by −3°/+2° under 1 keV for 15 min and 0.5 keV for another 15 min). TEM observations were performed using a JEOL-2010 TEM operating at 200 kV. Phase analyses were carried out with X-ray diffractometer (XRD, copper target Rigaku D/max 2550 V) under a scanning speed of 2°/min. Fracture surface was investigated in a SEM.
3. Results 3.1. Microstructure of as-cast and solution-treated alloys Fig. 1 shows the microstructure of the as-cast and solutiontreated Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. The as-cast alloys are mainly composed of α-Mg matrix and eutectic compounds, which distribute along the grain boundaries. From the XRD patterns in Fig. 2(a), the eutectic compounds are Mg12 (RE, Zn) phase. The peak positions change with the Zn addition, indicating the lattice constant of Mg12 (RE, Zn) phase varies with the Zn addition. The EDS analysis in Table 2 indicates the eutectic compounds (Point 1) in the as-cast 1 Zn addition alloy contain about 8.83% Nd, 10.62% Ce and 7.30% Zn. The volume fractions of the eutectic compound in as-cast alloys increase with the addition of Zn element and shown in Table 3.
Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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Fig. 1. Microstructure of as-cast and solution treated Mg-1Nd-1Ce-xZn-Zr alloys: (a) (e) x = 0; (b) (f) x = 0.2; (c) (g) x = 0.5; (d) (h) x = 1. (a)–(d) as-cast alloys, (e)–(h) solution-treated alloys. Point 1 in Fig. 1(d) and Point 2 in Fig. 2(h) are the positions of the EDS analysis. Table 2 Results of EDS analysis in Figs. 1 and 3.
Position Point 1 in Fig. 1(d) Point 2 in Fig. 1(h) Point 3 in Fig. 3(b)
wt% wt% wt%
Table 3 Volume fraction of eutectic compounds.
Element
Alloys
0Zn (%)
0.2Zn (%)
0.5Zn (%)
1.0Zn (%)
Mg
Nd
Ce
Zn
Zr
6.9
8.5
13.2
13.3
73.25 70.32 55.70
8.83 9.40 –
10.62 16.26 –
7.30 4.01 14.30
– – 30.00
OM images of as-cast alloys OM images of solution-treated alloys Fracture surface for solution-treated alloys Fracture surface for peak-aged alloys
2.6
2.9
3.3
3.8
3.3
9.8
11.1
13.7
4.3
10.8
12.7
17.6
After 540 °C × 10 h solution treatment, part of the eutectic compounds dissolved into α-Mg matrix, while some residual eutectic compounds still locate along the grain boundaries (Fig. 1(e)–(l)). As shown in Table 3, compared with the ascast alloys, the volume fractions of the eutectic compound in solution-treated alloys largely decrease during solution treatment, which also increase with the addition of Zn element. From the XRD patterns in Fig. 2(b), the residual eutectic compounds are still Mg12 (RE, Zn) phase. Taking 1Zn addition alloy for example, the chemical composition of the residual eutectic compounds changes during the solution treatment
(Table 2). Compared with the as-cast alloy (Point 1), the content of Ce element obviously increases while the content of Zn element decreases in the residual eutectic compounds (Point 2). Besides the residual eutectic compounds, fine particles (indicated by arrows in Fig. 2(f)–(h)) are also observed in the solution-treated alloys, which are more obvious in Zn addition alloys. The typical morphologies of these particles at grain interiors are shown in Fig. 3. According to the EDS analysis
Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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a
Fig. 3. Particles at grain interiors in solution-treated Mg-1Nd-1Ce-0.5Zn-Zr alloys. 100 90
0Zn 0.2Zn 0.5Zn 1.0Zn
b
Grain Growth Rate (%)
80 70 60 50 40 30 20 10 0 0
5
10
15
20
Solution time (h) Fig. 4. Grain growth rate of cast Mg-1Nd-1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0, wt%) alloys during solution treatment at 540 °C.
the 1Zn addition alloy only grow ∼29%, and the other three alloys grow ∼50% (0.5Zn), ∼68% (0.2Zn), ∼72% (0Zn), respectively. It seems that the residual eutectic compounds at grain boundaries can effectively hinder the growth of grains during solution treatment. Fig. 2. XRD curves of as-cast (a) and (b) solution treated Mg-1Nd-1Ce-xZnZr (x = 0, 0.2, 0.5, 1.0, wt%) alloys.
in TEM, the particles are Zr-containing phases, which were frequently observed in Zn and Zr containing alloys after solution treatment, such as the Mg–3Nd–0.2Zn-Zr [25,26] and Mg-Ca-Zn-Zr [29] alloys. Fig. 4 presents the grain growth rate of cast Mg–1Nd–1CexZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys during solution treatment. Generally, longer solution time leads to higher grain growth rates and coarser grains. The grain growth rate decreases with the increase of the Zn addition. The 1 Zn addition alloy has the largest grain growth resistance during solution treatment. After solution treatment at 540 °C for 20 h, the grains of
3.2. Age hardening behavior Fig. 5 shows the age hardening curves of the solutiontreated Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys isothermally aged at 200 °C for different aging time. Generally, with the increase of aging time, the hardness first increases then decreases. The characteristic values of the age hardening curves are shown in Table 4. It could be found that the hardness of the solution-treated alloys increases with the increase of the Zn addition. The 1.0Zn addition alloy has the highest hardness in solution-treated condition (52.4 HV), which is 6.5–9.7 HV higher than the other three alloys. The age hardening ability (HV, HV peak aged - HV solution-treated ) of the 0Zn, 0.2Zn and 0.5Zn addition alloys are almost the same, ∼15 HV, while that of the 1.0Zn addition alloy is much
Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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Y. Zhou, P. Fu and L. Peng et al. / Journal of Magnesium and Alloys xxx (xxxx) xxx 65 0 Zn 0.2 Zn 0.5 Zn 1.0 Zn
Hardness (HV)
60
55
50
45
Mg-1Nd-1Ce-xZn-Zr alloys 200ºC isothermal aging curves
As-quenched 40 1
10
100
1000
Aging time (h) Fig. 5. Age hardening curves of cast Mg-1Nd-1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0, wt%) alloys at 200 °C. Table 4 Characteristic values of the age hardening curves of Mg-1Nd-1Ce-xZn-Zr alloys. Alloys
HV
Mg-1Nd-1Ce-Zr Mg-1Nd-1Ce-0.2Zn-Zr Mg-1Nd-1Ce-0.5Zn-Zr Mg-1Nd-1Ce-1.0Zn-Zr
42.7 45.2 45.9 52.4
HV = HV
peak-aged
- HV
solution-treated
HV 57.7 60.3 60.3 59.4
peak-aged
HV Peak aging time (h) 15.0 15.1 14.4 7.0
8 8 16 16
solution-treated .
lower, only 7 HV, dropped by half. The peak aging time increases with the increase of the Zn addition. 0 Zn and 0.2 Zn addition alloys take 8 h to the peak hardness while 0.5Zn and 1.0Zn addition alloys get to the peak hardness by 16 h. That is to say, the Zn addition tends to delay the arrival of the peak hardness. Fig. 6 shows the TEM images and corresponding selected area electron diffraction (SAED) patterns of cast Mg–1Nd– 1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C. In the Zn-free alloy, two types of precipitates are observed, the disk-shaped and the point-like precipitates, as shown in Fig. 6(b). The disk-shaped precipitates (indicated by arrows) can be seen from both [0001]α and [112¯ 0]α direction, while the point-like precipitates (indicated by circles) can only be seen from [112¯ 0]α direction. From Fig. 6(a), it could be seen that the disk-shaped precipitates locate on {112¯ 0}α planes. As indicated by the arrow in Fig. 6(a), some of the disk-shaped precipitates are bent on {112¯ 0}α planes. Compared with the previous studies on Mg-Nd [16–20] and Mg-Ce [27,28] alloys, the disk-shaped precipitates have similar morphologies to the β"’ phase in dilute Mg-Nd alloy [18], which are not perfectly ordered β’, but instead consist of Nd zig-zag rows interleaved by strips of Nd hexagons, consisting of different fractions of β’ (orthorhombic, Mg7 Nd) and β" (D019 , Mg3 Nd). Therefore, in the present study, the disk-shaped
5
precipitates are identified as β"’. The point-like precipitates clearly observed in the present study are not reported in literature. Since there are no additional diffraction points and clear image of the point-like precipitates here, they are just called as point-like precipitates at the present stage. The morphologies of precipitates are significantly modified by the addition of 0.2Zn, as shown in Fig. 6(c) and (d). Compared with the Zn-free alloy (Fig. 6(a) and (b)), the size of the precipitates observed from [0001]α direction is significantly reduced while the density increases (Fig. 6(c)). From Fig. 6(d), it could be found that besides the disk-shaped β’" phases and the point-like phases, very fine plate-like phases on the basal planes with very high density are observed. Similar basal precipitates were reported in Mg–2.8Nd–1.3Zn alloy [23] and Mg–2.4RE–0.4Zn–0.6Zr (where 2.4RE includes 1.3Ce, 0.6La, 0.4Nd and 0.1Pr) alloy [22], where basal γ " phase (hexagonal, a = 0.556 nm, c = 1.563 nm) and an ordered GP zone structure (hexagonal, a = 0:556 nm) were identified respectively. As the basal precipitates in the present study is very fine, much more close to those in Mg–2.4RE–0.4Zn– 0.6Zr alloy [22], the fine plate-like precipitates are now recognized GP Zones. When 0.5Zn is added, the precipitates are further modified. The basal precipitates become the main strengthening phase (Fig. 6(f)), whose number density is much higher than the 0.2Zn addition alloy (Fig. 6(d)). The additional streaks, extending parallel to [0001]α , are observed in the SAED pattern in Fig. 6(f), which confirms that the basal plate-like precipitates are very thin in c-axis direction. Inspection of [0001]α zone axis SAED patterns in Fig. 6(e), reveals unambiguously the existence of additional reflections at 1/3{112¯ 0}α and 2/3{112¯ 0}α positions. These diffraction patterns are similar to those observed in Mg–2.4RE–0.4Zn–0.6Zr alloy [22], and it is further confirmed that they could be indexed as ordered GP zones. There are almost no prismatic precipitates and the contents of the point-like precipitates significantly decrease (Fig. 6(f)). The size of the precipitates observed from [0001]α direction is further reduced and the their number density also significantly decreases. When 1.0Zn is added, nearly no precipitates could be observed from [0001]α direction and only spare basal precipitates and point-like precipitates (indicated by the circle in Fig. 6(h)) can be observed from [112¯ 0]α direction. Compared with the 0.5Zn addition alloy, the number density of precipitates in the 1.0 Zn addition alloy significantly drops. Table 5 summaries the precipitates constitutes of cast Mg– 1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C. In the Zn-free alloy, the disk-shaped precipitates on prismatic planes and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the disk-shaped and point-like precipitates, the very fine basal precipitates form. When 0.5Zn is added, the basal precipitates become the main strengthening phase, and the contents of the prismatic and the point-like precipitates significantly decrease. While the Zn addition increases to 1.0%, only spare basal precipitates and point-like precipitates exist.
Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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Fig. 6. TEM micrographs and corresponding SAED patterns of cast Mg-1Nd-1Ce-xZn-Zr alloys at peak-aged conditions: (a) and (b) 0Zn, (c) and (d) 0.2Zn, (e) and (f) 0.5Zn, (g) and (h) 1Zn. Images of (a), (c), (e) and (g) are along [0001]α axis zone and images of (b), (d), (f) and (h) are along [1120¯ ]α axis zone. Table 5 Precipitate constitutes in peak-aged Mg-1Nd-1Ce-xZn-Zr alloys. Alloys
Prismatic precipitates
Point-like precipitates
Basal precipitates
Mg-1Nd-1Ce-Zr Mg-1Nd-1Ce-0.2Zn-Zr Mg-1Nd-1Ce-0.5Zn-Zr Mg-1Nd-1Ce-1.0Zn-Zr
Large-size, plenty Small-size, plenty Small-size, small amount –
Plenty Small amount Small amount Small amount
– Large amount Large amount Small amount
“-” None.
3.3. Mechanical properties and fracture behaviors Fig. 7 presents the mechanical properties of cast Mg–1Nd– 1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys in solution-treated and peak-aged conditions. In solution-treated condition, as the Zn addition increases from 0 to 1.0%, the yield strength (YS) increases from 65 MPa to 92 MPa and the ultimate tensile strength (UTS) increases from 164 MPa to 194 MPa. Meanwhile, the elongation enhances when trace amount of Zn is added into alloys from 0 to 0.5%. While the Zn addition further increases to 1.0%, the elongation drops almost by half, from 21.5% to 12.6%. In the peak-aged condition, when the Zn addition increases from 0 to 0.5%, the YS increases from 92 MPa to 136 MPa, and the UTS increases from 208 MPa to 237 MPa, while the elongation first increases from 10% (0Zn) to 12.3% (0.2Zn), and then decreases to 9.0% (0.5Zn). When
the content of Zn addition further increases to 1.0%, both of the YS and UTS decrease, while the elongation increases. 0.5Zn addition alloy has the highest strength: YS of 136 MPa, UTS of 237 MPa, elongation of 9.0%. Compared with the solution-treated alloys, the peak-aged alloys apparently have higher YS and UTS. As shown in Table 6, the increments of the YS (YS, YS peak-aged - YS solution-treated ) are 27 MPa, 41 MPa, 49 MPa and 19 MPa for the 0, 0.2, 0.5 and 1.0Zn addition alloys, respectively. While the corresponding UTS increments (UTS) are 44 MPa, 52 MPa, 49 MPa and 16 MPa. The drops of the elongation are 7.3%, 8.3%, 12.5% and 0.9%, respectively. It seems that the higher increment of the YS corresponds well with the higher drop of the elongation. The 0.5Zn addition alloy has the highest increment of the YS and the highest drop of the elongation, while the 1.0 Zn addition alloy has the lowest increment of the
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residual eutectic compounds on the fracture surface are calculated and list in Table 3, which increases with the increase of the Zn addition. Different from the calculated results based on OM images, such as the solution treated condition, the volume fractions calculated based on the fracture surface is significantly higher, especially those of Zn addition alloys, which indicates that the alloys tend to fracture through the residual eutectic compounds or along the interface between the eutectic compounds and the α-Mg matrix. Fig. 9 shows the optical microstructure of ruptured samples perpendicular to the fracture surface of the solutiontreated and peak-aged Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. In the Zn-free alloy, the secondary cracks tend to form at grain interiors, as shown in Fig. 9(a) and (e) in both solution-treated and peak-aged conditions. When the Zn element is added, the secondary cracks tend to appear along the grain boundaries. Fine cracks through the residual eutectic compounds are frequently observed. Similar fractured residual eutectic compounds can also be observed on the fracture surface, as shown in Fig. 8(n). Therefore, during tensile deformation, the residual eutectic compounds fracture to initiate micro-cracks and these micro-cracks facilitate the macroscopic fracture of the alloys (Fig. 9(f)). According to the OM images in solution-treated and peak-aged conditions in Fig. 9, the micro-cracks tend to propagate into grain interiors in the solution-treated alloys after the initiation, while along the grain boundaries in the peak-aged ones. Such phenomenon is consistent well with the less content of the cleavage planes on the fracture surfaces (Fig. 8).
a
b
Fig. 7. Tensile properties of cast Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys in solution-treated condition(a) and (b) peak-aged condition.
4. Discussion 4.1. Point-like phase in Mg–1Nd–1Ce-Zr alloy
Table 6 Strengthening effect comparison in peak-aged Mg-1Nd-1Ce-xZn-Zr alloys. Alloy YS (YS
- YS (MPa) UTS (UTS peak-aged - UTS solution-treated ) (MPa) Elongation (Elongation peak-aged Elongation solution-treated ) (%) YS(YS - YS 0 Zn ) (MPa) peak-aged
0Zn
0.2Zn
0.5Zn
1.0Zn
27
41
49
19
44
52
49
16
−7.3
−8.3
−12.5
−0.9
0
14
22
−8
solution-treated )
YS and the lowest drop of the elongation. Therefore, the precipitates deteriorate the ductility of the alloys, and the higher strengthening effect corresponds to the higher drop of the ductility. Fig. 8 shows the typical fracture surface images of the solution-treated and peak-aged Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. The residual eutectic compounds can be clearly observed in backscattering electron micrographs while the cleavage planes are more distinguished in second electron micrographs. In the Zn-free alloy, many cleavage planes are presented, while the amount of cleavage planes decreases with the increase of the Zn addition. The volume fractions of the
In Mg–1Nd–1Ce-Zr alloy (the Zn-free alloy), besides the prismatic β"’ precipitates, the point-like phases are observed (Fig. 6(b)), which were not reported in both Mg-Nd and MgCe based alloys in literature. The precipitation behavior of an Mg–0.5at% Ce alloy was once systematically studied by HAADF-STEM [28], where very thin disk-shaped precipitates on the prismatic planes (called as GP-zones) were reported when the alloy was aged at 180 °C for 70 h. Besides the diskshaped precipitates, if the TEM images of Figs. 2(c), 3(a) and 5(a) in Reference [28] are checked carefully, similar point-like clusters can also be observed from both [0001]α and [112¯ 0]α directions, even though they were not mentioned in the reference [28]. Since the HAADF-TEM imaging is capable to give a bright contrast at positions of heavy atoms, these point-like clusters in Mg–0.5at% Ce alloy should be rich in Ce element. Recently, in a Mg–3at% Ce alloy [30] solutiontreated and aged at 200 °C for 3 h, similar point-like precipitates were reported, which were confirmed to have fcc structure with higher Ce content than the prismatic precipitates. Therefore, in the present study, the point-like precipitates are probably also formed by the segregation of Ce element and its detail structure needs to be revealed in future.
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Fig. 8. SEM images of the fracture surfaces of solution-treated (a)–(h) and peak-aged (i)–(p) Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys: (a) (b) 0Zn-T4, (c) (d) 0.2Zn-T4, (e) (f) 0.5Zn-T4, (g) (h) 1.0Zn-T4 and (i) (j) 0Zn-T6, (k) (l) 0.2Zn-T6, (m) (n) 0.5Zn-T6, (o) (p) 1.0Zn-T6. (a)(c)(e)(g)(i)(k)(m)(o) are second electron images and (b)(d)(f)(h)(j)(l)(n)(p) are corresponding BSE images. In BSE images, the residual eutectic compounds are clearly observed in white color.
4.2. Influence of Zn addition on the precipitates It could be found in Table 5 and Fig. 6 that, the precipitates constitution of cast Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C are significantly modified by the Zn addition. In the Zn-free alloy (Mg– 1Nd–1Ce-Zr alloy), the disk-shaped β"’ precipitates on prismatic planes and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the point-like precipitates and the refined diskshaped β’" precipitates, very fine precipitates form on the basal planes. When 0.5Zn is added, the basal precipitates become the main strengthening phase in the alloy. There are almost no prismatic precipitates and the number
density of the point-like precipitates significantly decreases. When the content of Zn addition increases to 1.0%, only very spare basal precipitates and point-like precipitates exist. At the present stage, the effects of Zn addition on the precipitation sequence of Mg-Nd alloys are still lack of clear characterization. Since the effect of Zn addition on the precipitation sequence of Mg-Gd alloys were well studied, some illumination could be got from the precipitation sequence review of Mg-Gd-Zn alloys [31–33]. In MgGd binary alloys, the precipitate evolution was reported in the following sequence [2]: SSSS → GP zone → β" (DO19 , Mg3 Gd) →β (cbco, Mg7 Gd) →β 1 (fcc, Mg3 Gd) →β (fcc, Mg5 Gd). All of the precipitates locate on the prismatic planes. When Zn element is added, basal precipitates and/or a
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Fig. 9. Optical microstructure of ruptured cast Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys, perpendicular to the fracture surface, in solutiontreated (a)–(d) and peak-aged (e)–(h) conditions.
novel phase with long period stacking ordered (LPSO) structure were introduced. In Mg–1Gd–0.4Zn–0.2Zr (at%) (Mg– 6.1Gd–1.0Zn–0.7Zr, wt%) alloy [31], the precipitation process during isothermal aging at 200 °C and 250 °C was reported to involve the formation of meta-stable γ " (space group P6¯ 2 m, a = 0.0556 nm and c = 0.444 nm) and γ ’ (space group P3¯ m1, a = 0.321 nm and c = 0.780 nm), both of which form as plate shaped particles on (0001)α . When the content of Zn addition increases, bulk LPSO phases form, such as those in Mg–2Gd–1 Zn (at%) (Mg–11.5Gd–2.4 Zn, wt%) [32], Mg–2Gd–1.2Y–1Zn–0.2Zr (at%) (Mg–11.1Gd–3.8Y– 2.3Zn–0.7Zr, wt%) [33] alloys. Therefore, during the aging or solidification process, Gd and Zn atoms tend to cosegregate to form new basal phases. It was suggested by J.F. Nie that the co-segregation of Gd and Zn atoms would minimize the elastic strain compared with individual atoms of Gd or Zn [31], which could promote the formation of Gd & Zn containing phases, both the precipitates and the LPSO phases. More significant influence of Zn addition on the precipitates happens in the present Mg–1Nd–1Ce-xZn-Zr alloys. As shown in Fig. 6(c) and (d), even 0.2 Zn addition has significantly influence on the precipitates constitutes: (1) Plenty of very fine precipitates form on the basal planes, which do
not exist in the Zn-free alloy; (2) The size of the prismatic phase is largely reduced while the number density increases. It is probably because that the atomic radius of Ce and Nd is 0.183, 0.182 nm, respectively, and much larger than Gd atom (0.178 nm). The co-segregation of Ce and Nd with Zn could minimize the elastic strain further compared with Gd. On the other hand, the maximum solid solubility of Ce (0.6%) and Nd (3.6%) is much lower than Gd (23.1%). Therefore, only a small amount of Zn addition into Mg–1Nd–1Ce-Zr alloy can modify the precipitates such significantly. When the Zn addition increases to 0.5%, nearly no prismatic phases exist and the basal precipitates become the main strengthening phase. When the Zn addition further increases to 1.0%, only spare basal precipitate exists. Compared with the Zn-free, 0.2Zn and 0.5Zn addition alloys, the total amount of precipitates in the 1.0Zn addition alloy is much less. Therefore, it could be speculated that the Zn addition could improve the thermal stability of eutectic (Mg, Zn)12 RE compounds, or reduce the solid solubility of RE elements in the α-Mg matrix, which in turn decreases the RE solution content in α-Mg matrix. As a result, at the solution temperature even as high as 540 °C, limited eutectic compounds in the 1Zn addition alloy dissolved into the matrix, which further leads to very spare basal precipitates during the aging process.
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Table 7 Typical tensile properties of aging harden-able cast Mg alloys at room temperature. Series
Alloy
Thermal condition
Tensile properties RT YS (MPa)
UTS (MPa)
Elongation (%)
Mg-Al
AZ91D AZ63 ZC63 ZK61 WE43 NZ30K AM-SC1 Mg-1Nd-1Ce-0.5Zn-Zr
S-T6 S-T6 S-T6 S-T6 S-T6 P-T6 P-T6 P-T6
145 122 125 195 162 142 130 136
270 232 210 310 250 305 206 237
6.0 5.5 4.0 10.0 2.0 11.0 3.7 9.0
Mg-Zn Mg-RE
Reference
[34]
[25] [35] Present study
S: Sand mould casting; P: Permanent mould casting.
4.3. Strengthening effect of different precipitates As described above, compared with the solution-treated alloys, the increments of the YS (YS, YS peak-aged - YS solution-treated, in Table 6) for the peak-aged 0, 0.2, 0.5 and 1.0Zn addition alloys are 27 MPa, 41 MPa, 49 MPa and 19 MPa, respectively. The increments of the YS come from the transformation from the solid solution atoms in the solutiontreated condition to the precipitates in the peak-aged condition. Therefore, compared with the Zn-free alloy, small amount of Zn addition (0.2, 0.5%) increases the YS by 14 and 22 MPa (Table 6), respectively. Such increments of the YS are due to the modification effect of the precipitates (Fig. 6 and Table 5). That is to say, the additional fine basal precipitates and the refined prismatic precipitates introduced by the 0.2Zn addition, and transformation of the main precipitates from the disk-shaped β"’ and the point-like precipitates in the Zn-free alloy to the finer basal precipitates in the 0.5Zn addition alloy, improve the YS of the alloys. Therefore, the introduction of the basal precipitates by small amount of Zn addition (0.2% and 0.5%) is beneficial to the YS. However, when the amount of Zn addition is as high as 1.0%, there are only very limited basal precipitates and pointlike precipitates and the YS is even lower than the Zn-free alloy. Therefore, the amount of Zn addition in Mg–1Nd–1CeZr alloy should be less than 1.0%, and 0.5Zn addition is the best based on the present study.
become much larger, which lead to unacceptable mechanical properties. Therefore, AM50 and AM60 alloys cannot be used to cast car wheels. Due the lower cooling rate in the wheel casting process, aging harden-able cast Mg alloys, used in T6 condition (solution treatment + aging treatment) and mainly strengthened by the precipitates, are necessary. Table 7 lists the typical tensile properties of aging harden-able cast Mg alloys at room temperature. Only AZ91D, ZK61 and NZ30K alloys are close to the minimum requirement of car wheels (130 MPa–210 MPa-7%). Unfortunately, as there are no effective grain refinement methods, the grains of AZ91D Mg cast wheels are very coarse, which leads to very poor mechanical properties. The elongation of AZ91D alloy in cast wheels is usually less than 3.0%, which makes it less available for car wheels. ZK61 alloy has very poor hot tearing resistance, and hot cracks form very easily during the wheel casting. Therefore, before the present study, the NZ30K alloy is the limited chosen for Mg cast wheels [36]. As discussed in introduction part, the high cost of Nd element retards the wheel application of NZ30K alloy. The typical mechanical properties of the present Mg–1Nd–1Ce–0.5Zn-Zr alloy are: 136 MPa–237 MPa-9%, higher than the minimum requirement of car wheels (130 MPa–210 MPa-7%). As the cost of the Mg–1Nd–1Ce–0.5Zn-Zr alloy is much lower than the NZ30K alloy, it could probably promote the application of Mg wheel in automobiles. 5. Conclusion
4.4. Mg alloys suitable for wheel casting (thick-wall casting) According to their chemical compositions, commercial Mg cast alloys can be classified into Mg-Al, Mg-Zn and MgRE based alloys. Mg-Al based cast alloys, such as AZ91D, AM50, AM60, are the most widely used Mg alloys due to their good castability and low cost. AM50 and AM60 Mg alloys have little aging harden-ability and they can only be cast in high pressure die casting process (HPDC), where acceptable mechanical properties are obtained mainly due to the fine grains produced under the high cooling rate. Car wheels are one of typical thick-wall castings, and usually cast by gravity or low pressure casting in permanent moulds under much lower cooling rate compared with HPDC. Under such lower cooling rate, the grains of AM50 and AM60 alloys
The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg– 1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions are studied. The following conclusions can be drawn. (1) Precipitates in cast Mg–1Nd–1Ce-Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the disk-shaped and point-like precipitates, the very fine basal precipitates form. When 0.5Zn is added, the basal precipitates become the main strengthening phase, and the contents
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of the prismatic precipitates and the point-like precipitates significantly decrease. While the Zn addition increases to 1.0%, only spare basal precipitates and pointlike precipitates exist. (2) Compared with the Zn-free alloy, small amount of Zn additions improves the yield strength (YS) in T6 condition. The additional fine basal precipitates and the refined prismatic precipitates introduced by the 0.2Zn addition lead to 14 MPa enhancement of YS, while the transformation of the main precipitates from the prismatic disk-shaped precipitates and the point-like precipitates in the Zn-free alloy to the finer basal precipitates in the 0.5 Zn addition alloy leads to 22 MPa enhancement of YS. Further increasing the Zn addition to 1.0 wt%, leads to lower YS than the Zn-free alloy. (3) Mg–1Nd–1Ce–0.5Zn-Zr alloy has the highest strength in T6 condition: yield strength of 136 MPa, ultimate tensile strength of 237 MPa and elongation of 9%, which is higher than the minimum requirement of car wheels (130 MPa–210 MPa-7%). Therefore, Mg–1Nd– 1Ce–0.5Zn-Zr alloy could be used to cast Mg wheels. Acknowledgments This work was supported by National Key Research and Development Program of China (2016YFB0301000 & 2016YFB0701204), Shanghai Rising-Star Program (15QB1402700), National Natural Science Foundation of China (NSFC) (51671128 & 51771113) and Special Fund of Jiangsu Province for the Transformation of Scientific and Technological Achievements (BA2016039). References [1] P. Fu, L. Peng, H. Jiang, W. Ding, C. Zhai, China Foundry 11 (2014) 277–286. [2] J.-F. Nie, Metall. Mater. Trans. A 43 (2012) 3891–3939. [3] F. Pan, M. Yang, X. Chen, J. Mater. Sci. Technol. 32 (2016) 1211–1221. [4] J. Zhang, S. Liu, R. Wu, L. Hou, M. Zhang, J. Magnes. Alloy. 6 (2018) 277–291. [5] Y. Zhang, Y. Wu, L. Peng, P. Fu, F. Huang, J. Alloy. Compd. 615 (2014) 703–711. [6] K. Yamada, H. Hoshikawa, S. Maki, et al., Scr. Mater. 61 (2009) 636–639. [7] Q. Wang, J. Chen, Z. Zhao, et al., Mater. Sci. Eng. A 528 (2010) 323–328.
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Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition Yiyuan Zhou a, Penghuai Fu a,b,∗, Liming Peng a, Dan Wang a, Yingxin Wang a, Bin Hu c, Ming Liu c, Anil K. Sachdev d, Wenjiang Ding a a National
Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, PR China b Shanghai Light Alloy Net Forming National Engineering Research Center Co., Ltd, Shanghai 201615, PR China c General Motors China Science Laboratory, Shanghai 201206, PR China d General Motors Research & Development Center, Warren, MI 48090, USA Received 26 November 2018; received in revised form 26 January 2019; accepted 20 February 2019 Available online xxx
Abstract The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions were studied. Precipitates in cast Mg–1Nd–1Ce-Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2 Zn is added, the disk-shaped precipitates are refined and very fine basal precipitates form additionally. When 0.5 Zn is added, the basal precipitates become the main strengthening phase. Further increasing the Zn addition to 1.0%, only spare basal precipitates and point-like precipitates exist. The 0.5 Zn addition alloy has the highest strength at room temperature, whose yield strength, ultimate tensile strength and elongation in T6 condition are 136 MPa, 237 MPa and 9%, respectively. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: Mg alloy; Precipitate; Rare earth; Zn addition; Mechanical properties; Mg wheel.
1. Introduction A recent push by the automotive industry to reduce the fuel consumption and the cost of automobiles is providing enhanced motivation for the study of lightweight cast materials for structural components. Magnesium (Mg) alloys containing rare earth elements were frequently investigated recently and some of high strength Mg-Gd based alloys [1–4], such as Mg-Gd-Ag-Zr [5–7], Mg-Gd-Zn-Zr [8–10], Mg-Gd-Y-Zr [11–13] alloys were developed, which are very attractive to the automobile manufacturers. However, high content of rare earth elements increases the cost of these alloys, which would limit their applications in the automotive industry. Therefore, ∗ Corresponding author at: National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (P. Fu).
in order to promote the application of Mg alloys in automobiles, high strength and relatively low-cost Mg alloys are needed. Rational use of Neodynium (Nd, one of rare earth (RE) elements) could make it possible to develop a high strength and relatively low-cost Mg alloy. Nd is one of light rare earth elements, with maximum solubility in solid Mg of 3.6 wt% (all compositions in wt% except otherwise stated) at eutectic temperature of 545 °C. Mg-Nd binary alloys already have significant age hardening effect [14–21], whose precipitation sequence were recently updated to be [18]: SSSS → GP Zones (N, V, hexagons) → β"’ → β 1 (Mg3 Nd) → β (Mg12 Nd) →β e (Mg41 Nd5 ), where β"’ includes the β’ phase and ranges in composition from xNd = 0.125 to xNd ≈ 0.166. When Zn element is added into Mg-Nd binary alloys, basal precipitates are introduced, such as the basal GP zones (hexagonal, a = 0.556 nm) in Mg–2.4RE–0.4Zn–0.6Zr alloy (where 2.4RE includes 1.3Ce, 0.6La, 0.4Nd and 0.1Pr) [22], and
https://doi.org/10.1016/j.jma.2019.02.003 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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Table 1 Chemical compositions of Mg-1Nd-1Ce-xZn-0.5Zr alloys (wt%). Ce
Nd
Zn
Alloys
Average
Variance
Average
Variance
Average
Mg-1Ce-1Nd-Zr Mg-1Ce-1Nd-0.2Zn-Zr Mg-1Ce-1Nd-0.5Zn-Zr Mg-1Ce-1Nd-1.0Zn-Zr
1.09 1.13 1.18 1.16
0.003 0.004 0.012 0.004
1.03 1.10 1.17 1.17
0.004 0.002 0.010 0.004
– 0.19 0.56 0.96
the basal γ " phase (hexagonal, a = 0.556 nm, c = 1.563 nm) in Mg–2.8Nd–1.3 Zn (wt%) alloy [23]. The addition of 0.5 Zn to Mg–3Nd alloy was reported to increase its peak-aged hardness [24]. Generally, at the present stage, the effect of Zn addition on the precipitation sequence is still not characterized clearly. Mg–3Nd–0.2Zn-Zr (NZ30K) [25–26] was recently developed, whose typical mechanical properties are: yield tensile strength of 140 MPa, ultimate strength of 305 MPa and elongation of 10% [25]. Due to its acceptable strength and ductility, NZ30K alloy was used to cast thick-wall components, such as car wheels, engine blocks and engine covers. However, as the cost of Nd element increases significantly in recent years, the NZ30K alloy becomes expensive, which stops its application in automobiles. In the present study, in order to reduce the cost of MgNd based alloys, the content of Nd element is set to be 1%. 1% Ce is added to make up the loss of strength due to the reduced precipitation strengthening by the decrease of Nd element. Ce element is much cheaper and Ce addition could provide both dispersion strengthening and precipitation strengthening [27,28]. The effects of different Zn addition (0, 0.2, 0.5, 1.0%) on the microstructure and mechanical properties of cast Mg–1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions are systematically studied. 2. Experimental Alloys of nominal composition Mg–1Nd–1Ce-xZn-Zr (wt%) (all compositions in wt% except otherwise stated), where x = 0, 0.2, 0.5 and 1, were prepared from purity Mg, Zn and Mg–90Nd, Mg–90Ce, Mg–30Zr master alloys by melting in an electrical resistance furnace under the protection of the mixture gas of SF6 , CO2 and air, then cast in permanent mould at pouring temperature of 730 ± 5 °C and mould temperature of 150 ± 20 °C. The actual chemical compositions was determined by an inductively coupled plasma analyzer (ICP) and listed in Table 1. Specimens cut from the cast ingots were first solution-treated at 540 °C for 10 h and quenched into hot water at ∼70 °C, then subsequently aged at 200 °C in an oil-bath. Vickers hardness testing was taken using 5 kg load and holding time of 30 s. Each hardness value was averaged from at least five individual measurements. Tensile samples were cut into rectangular tensile specimens with the dimensions of 10 mm width, 2 mm thickness and 25 mm gauge length by an electric-sparking wire-cutting machine. Tensile testing was carried out on a Zwick/Roell-20 kN material test machine at a cross head speed of 0.5 mm/min at
Zr Variance
Average
Variance
Mg
0.005 0.017 0.009
0.4 0.5 0.49 0.5
0.016 0.004 0.005 0.005
Bal. Bal. Bal. Bal.
room temperature. Three tensile samples were tested for each condition. Specimens were etched in a 4 vol% nital or in a solution of 12 g picric acid + 80 ml acetic acid + 80 ml water + 350 ml ethanol and observed in Zeiss Axio observer A1 optical microscope (OM). The grain sizes of the samples were measured with the linear intercept method according to the ASTM E112-12, in which each value was the average of at least five individual OM images under 200 times magnification. For the scanning electron microscopy (SEM) observation, the as-cast and solution-treated metallographic specimens were used in the polished condition to obtain credible data of chemical composition and phase constitution using Phenom XL equipped with an energy dispersive X-ray spectrometer (EDS) and a backscatter electron detector (BSE). The volume fractions of the intermetallic compounds at the grain boundaries of the as-cast and solution-treated specimens were measured using SEM-BSE images taken from at least five random fields. Thin foil for transmission electron microscope (TEM) observation were prepared by punching 3 mm diameter discs, then mechanical grounding to ∼50 μm, finally polishing using the Ion Polishing System (PIPs 691, 7° under 3 keV until perforation followed by −3°/+2° under 1 keV for 15 min and 0.5 keV for another 15 min). TEM observations were performed using a JEOL-2010 TEM operating at 200 kV. Phase analyses were carried out with X-ray diffractometer (XRD, copper target Rigaku D/max 2550 V) under a scanning speed of 2°/min. Fracture surface was investigated in a SEM.
3. Results 3.1. Microstructure of as-cast and solution-treated alloys Fig. 1 shows the microstructure of the as-cast and solutiontreated Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. The as-cast alloys are mainly composed of α-Mg matrix and eutectic compounds, which distribute along the grain boundaries. From the XRD patterns in Fig. 2(a), the eutectic compounds are Mg12 (RE, Zn) phase. The peak positions change with the Zn addition, indicating the lattice constant of Mg12 (RE, Zn) phase varies with the Zn addition. The EDS analysis in Table 2 indicates the eutectic compounds (Point 1) in the as-cast 1 Zn addition alloy contain about 8.83% Nd, 10.62% Ce and 7.30% Zn. The volume fractions of the eutectic compound in as-cast alloys increase with the addition of Zn element and shown in Table 3.
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Fig. 1. Microstructure of as-cast and solution treated Mg-1Nd-1Ce-xZn-Zr alloys: (a) (e) x = 0; (b) (f) x = 0.2; (c) (g) x = 0.5; (d) (h) x = 1. (a)–(d) as-cast alloys, (e)–(h) solution-treated alloys. Point 1 in Fig. 1(d) and Point 2 in Fig. 2(h) are the positions of the EDS analysis. Table 2 Results of EDS analysis in Figs. 1 and 3.
Position Point 1 in Fig. 1(d) Point 2 in Fig. 1(h) Point 3 in Fig. 3(b)
wt% wt% wt%
Table 3 Volume fraction of eutectic compounds.
Element
Alloys
0Zn (%)
0.2Zn (%)
0.5Zn (%)
1.0Zn (%)
Mg
Nd
Ce
Zn
Zr
6.9
8.5
13.2
13.3
73.25 70.32 55.70
8.83 9.40 –
10.62 16.26 –
7.30 4.01 14.30
– – 30.00
OM images of as-cast alloys OM images of solution-treated alloys Fracture surface for solution-treated alloys Fracture surface for peak-aged alloys
2.6
2.9
3.3
3.8
3.3
9.8
11.1
13.7
4.3
10.8
12.7
17.6
After 540 °C × 10 h solution treatment, part of the eutectic compounds dissolved into α-Mg matrix, while some residual eutectic compounds still locate along the grain boundaries (Fig. 1(e)–(l)). As shown in Table 3, compared with the ascast alloys, the volume fractions of the eutectic compound in solution-treated alloys largely decrease during solution treatment, which also increase with the addition of Zn element. From the XRD patterns in Fig. 2(b), the residual eutectic compounds are still Mg12 (RE, Zn) phase. Taking 1Zn addition alloy for example, the chemical composition of the residual eutectic compounds changes during the solution treatment
(Table 2). Compared with the as-cast alloy (Point 1), the content of Ce element obviously increases while the content of Zn element decreases in the residual eutectic compounds (Point 2). Besides the residual eutectic compounds, fine particles (indicated by arrows in Fig. 2(f)–(h)) are also observed in the solution-treated alloys, which are more obvious in Zn addition alloys. The typical morphologies of these particles at grain interiors are shown in Fig. 3. According to the EDS analysis
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a
Fig. 3. Particles at grain interiors in solution-treated Mg-1Nd-1Ce-0.5Zn-Zr alloys. 100 90
0Zn 0.2Zn 0.5Zn 1.0Zn
b
Grain Growth Rate (%)
80 70 60 50 40 30 20 10 0 0
5
10
15
20
Solution time (h) Fig. 4. Grain growth rate of cast Mg-1Nd-1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0, wt%) alloys during solution treatment at 540 °C.
the 1Zn addition alloy only grow ∼29%, and the other three alloys grow ∼50% (0.5Zn), ∼68% (0.2Zn), ∼72% (0Zn), respectively. It seems that the residual eutectic compounds at grain boundaries can effectively hinder the growth of grains during solution treatment. Fig. 2. XRD curves of as-cast (a) and (b) solution treated Mg-1Nd-1Ce-xZnZr (x = 0, 0.2, 0.5, 1.0, wt%) alloys.
in TEM, the particles are Zr-containing phases, which were frequently observed in Zn and Zr containing alloys after solution treatment, such as the Mg–3Nd–0.2Zn-Zr [25,26] and Mg-Ca-Zn-Zr [29] alloys. Fig. 4 presents the grain growth rate of cast Mg–1Nd–1CexZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys during solution treatment. Generally, longer solution time leads to higher grain growth rates and coarser grains. The grain growth rate decreases with the increase of the Zn addition. The 1 Zn addition alloy has the largest grain growth resistance during solution treatment. After solution treatment at 540 °C for 20 h, the grains of
3.2. Age hardening behavior Fig. 5 shows the age hardening curves of the solutiontreated Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys isothermally aged at 200 °C for different aging time. Generally, with the increase of aging time, the hardness first increases then decreases. The characteristic values of the age hardening curves are shown in Table 4. It could be found that the hardness of the solution-treated alloys increases with the increase of the Zn addition. The 1.0Zn addition alloy has the highest hardness in solution-treated condition (52.4 HV), which is 6.5–9.7 HV higher than the other three alloys. The age hardening ability (HV, HV peak aged - HV solution-treated ) of the 0Zn, 0.2Zn and 0.5Zn addition alloys are almost the same, ∼15 HV, while that of the 1.0Zn addition alloy is much
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Hardness (HV)
60
55
50
45
Mg-1Nd-1Ce-xZn-Zr alloys 200ºC isothermal aging curves
As-quenched 40 1
10
100
1000
Aging time (h) Fig. 5. Age hardening curves of cast Mg-1Nd-1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0, wt%) alloys at 200 °C. Table 4 Characteristic values of the age hardening curves of Mg-1Nd-1Ce-xZn-Zr alloys. Alloys
HV
Mg-1Nd-1Ce-Zr Mg-1Nd-1Ce-0.2Zn-Zr Mg-1Nd-1Ce-0.5Zn-Zr Mg-1Nd-1Ce-1.0Zn-Zr
42.7 45.2 45.9 52.4
HV = HV
peak-aged
- HV
solution-treated
HV 57.7 60.3 60.3 59.4
peak-aged
HV Peak aging time (h) 15.0 15.1 14.4 7.0
8 8 16 16
solution-treated .
lower, only 7 HV, dropped by half. The peak aging time increases with the increase of the Zn addition. 0 Zn and 0.2 Zn addition alloys take 8 h to the peak hardness while 0.5Zn and 1.0Zn addition alloys get to the peak hardness by 16 h. That is to say, the Zn addition tends to delay the arrival of the peak hardness. Fig. 6 shows the TEM images and corresponding selected area electron diffraction (SAED) patterns of cast Mg–1Nd– 1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C. In the Zn-free alloy, two types of precipitates are observed, the disk-shaped and the point-like precipitates, as shown in Fig. 6(b). The disk-shaped precipitates (indicated by arrows) can be seen from both [0001]α and [112¯ 0]α direction, while the point-like precipitates (indicated by circles) can only be seen from [112¯ 0]α direction. From Fig. 6(a), it could be seen that the disk-shaped precipitates locate on {112¯ 0}α planes. As indicated by the arrow in Fig. 6(a), some of the disk-shaped precipitates are bent on {112¯ 0}α planes. Compared with the previous studies on Mg-Nd [16–20] and Mg-Ce [27,28] alloys, the disk-shaped precipitates have similar morphologies to the β"’ phase in dilute Mg-Nd alloy [18], which are not perfectly ordered β’, but instead consist of Nd zig-zag rows interleaved by strips of Nd hexagons, consisting of different fractions of β’ (orthorhombic, Mg7 Nd) and β" (D019 , Mg3 Nd). Therefore, in the present study, the disk-shaped
5
precipitates are identified as β"’. The point-like precipitates clearly observed in the present study are not reported in literature. Since there are no additional diffraction points and clear image of the point-like precipitates here, they are just called as point-like precipitates at the present stage. The morphologies of precipitates are significantly modified by the addition of 0.2Zn, as shown in Fig. 6(c) and (d). Compared with the Zn-free alloy (Fig. 6(a) and (b)), the size of the precipitates observed from [0001]α direction is significantly reduced while the density increases (Fig. 6(c)). From Fig. 6(d), it could be found that besides the disk-shaped β’" phases and the point-like phases, very fine plate-like phases on the basal planes with very high density are observed. Similar basal precipitates were reported in Mg–2.8Nd–1.3Zn alloy [23] and Mg–2.4RE–0.4Zn–0.6Zr (where 2.4RE includes 1.3Ce, 0.6La, 0.4Nd and 0.1Pr) alloy [22], where basal γ " phase (hexagonal, a = 0.556 nm, c = 1.563 nm) and an ordered GP zone structure (hexagonal, a = 0:556 nm) were identified respectively. As the basal precipitates in the present study is very fine, much more close to those in Mg–2.4RE–0.4Zn– 0.6Zr alloy [22], the fine plate-like precipitates are now recognized GP Zones. When 0.5Zn is added, the precipitates are further modified. The basal precipitates become the main strengthening phase (Fig. 6(f)), whose number density is much higher than the 0.2Zn addition alloy (Fig. 6(d)). The additional streaks, extending parallel to [0001]α , are observed in the SAED pattern in Fig. 6(f), which confirms that the basal plate-like precipitates are very thin in c-axis direction. Inspection of [0001]α zone axis SAED patterns in Fig. 6(e), reveals unambiguously the existence of additional reflections at 1/3{112¯ 0}α and 2/3{112¯ 0}α positions. These diffraction patterns are similar to those observed in Mg–2.4RE–0.4Zn–0.6Zr alloy [22], and it is further confirmed that they could be indexed as ordered GP zones. There are almost no prismatic precipitates and the contents of the point-like precipitates significantly decrease (Fig. 6(f)). The size of the precipitates observed from [0001]α direction is further reduced and the their number density also significantly decreases. When 1.0Zn is added, nearly no precipitates could be observed from [0001]α direction and only spare basal precipitates and point-like precipitates (indicated by the circle in Fig. 6(h)) can be observed from [112¯ 0]α direction. Compared with the 0.5Zn addition alloy, the number density of precipitates in the 1.0 Zn addition alloy significantly drops. Table 5 summaries the precipitates constitutes of cast Mg– 1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C. In the Zn-free alloy, the disk-shaped precipitates on prismatic planes and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the disk-shaped and point-like precipitates, the very fine basal precipitates form. When 0.5Zn is added, the basal precipitates become the main strengthening phase, and the contents of the prismatic and the point-like precipitates significantly decrease. While the Zn addition increases to 1.0%, only spare basal precipitates and point-like precipitates exist.
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Fig. 6. TEM micrographs and corresponding SAED patterns of cast Mg-1Nd-1Ce-xZn-Zr alloys at peak-aged conditions: (a) and (b) 0Zn, (c) and (d) 0.2Zn, (e) and (f) 0.5Zn, (g) and (h) 1Zn. Images of (a), (c), (e) and (g) are along [0001]α axis zone and images of (b), (d), (f) and (h) are along [1120¯ ]α axis zone. Table 5 Precipitate constitutes in peak-aged Mg-1Nd-1Ce-xZn-Zr alloys. Alloys
Prismatic precipitates
Point-like precipitates
Basal precipitates
Mg-1Nd-1Ce-Zr Mg-1Nd-1Ce-0.2Zn-Zr Mg-1Nd-1Ce-0.5Zn-Zr Mg-1Nd-1Ce-1.0Zn-Zr
Large-size, plenty Small-size, plenty Small-size, small amount –
Plenty Small amount Small amount Small amount
– Large amount Large amount Small amount
“-” None.
3.3. Mechanical properties and fracture behaviors Fig. 7 presents the mechanical properties of cast Mg–1Nd– 1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys in solution-treated and peak-aged conditions. In solution-treated condition, as the Zn addition increases from 0 to 1.0%, the yield strength (YS) increases from 65 MPa to 92 MPa and the ultimate tensile strength (UTS) increases from 164 MPa to 194 MPa. Meanwhile, the elongation enhances when trace amount of Zn is added into alloys from 0 to 0.5%. While the Zn addition further increases to 1.0%, the elongation drops almost by half, from 21.5% to 12.6%. In the peak-aged condition, when the Zn addition increases from 0 to 0.5%, the YS increases from 92 MPa to 136 MPa, and the UTS increases from 208 MPa to 237 MPa, while the elongation first increases from 10% (0Zn) to 12.3% (0.2Zn), and then decreases to 9.0% (0.5Zn). When
the content of Zn addition further increases to 1.0%, both of the YS and UTS decrease, while the elongation increases. 0.5Zn addition alloy has the highest strength: YS of 136 MPa, UTS of 237 MPa, elongation of 9.0%. Compared with the solution-treated alloys, the peak-aged alloys apparently have higher YS and UTS. As shown in Table 6, the increments of the YS (YS, YS peak-aged - YS solution-treated ) are 27 MPa, 41 MPa, 49 MPa and 19 MPa for the 0, 0.2, 0.5 and 1.0Zn addition alloys, respectively. While the corresponding UTS increments (UTS) are 44 MPa, 52 MPa, 49 MPa and 16 MPa. The drops of the elongation are 7.3%, 8.3%, 12.5% and 0.9%, respectively. It seems that the higher increment of the YS corresponds well with the higher drop of the elongation. The 0.5Zn addition alloy has the highest increment of the YS and the highest drop of the elongation, while the 1.0 Zn addition alloy has the lowest increment of the
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residual eutectic compounds on the fracture surface are calculated and list in Table 3, which increases with the increase of the Zn addition. Different from the calculated results based on OM images, such as the solution treated condition, the volume fractions calculated based on the fracture surface is significantly higher, especially those of Zn addition alloys, which indicates that the alloys tend to fracture through the residual eutectic compounds or along the interface between the eutectic compounds and the α-Mg matrix. Fig. 9 shows the optical microstructure of ruptured samples perpendicular to the fracture surface of the solutiontreated and peak-aged Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. In the Zn-free alloy, the secondary cracks tend to form at grain interiors, as shown in Fig. 9(a) and (e) in both solution-treated and peak-aged conditions. When the Zn element is added, the secondary cracks tend to appear along the grain boundaries. Fine cracks through the residual eutectic compounds are frequently observed. Similar fractured residual eutectic compounds can also be observed on the fracture surface, as shown in Fig. 8(n). Therefore, during tensile deformation, the residual eutectic compounds fracture to initiate micro-cracks and these micro-cracks facilitate the macroscopic fracture of the alloys (Fig. 9(f)). According to the OM images in solution-treated and peak-aged conditions in Fig. 9, the micro-cracks tend to propagate into grain interiors in the solution-treated alloys after the initiation, while along the grain boundaries in the peak-aged ones. Such phenomenon is consistent well with the less content of the cleavage planes on the fracture surfaces (Fig. 8).
a
b
Fig. 7. Tensile properties of cast Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys in solution-treated condition(a) and (b) peak-aged condition.
4. Discussion 4.1. Point-like phase in Mg–1Nd–1Ce-Zr alloy
Table 6 Strengthening effect comparison in peak-aged Mg-1Nd-1Ce-xZn-Zr alloys. Alloy YS (YS
- YS (MPa) UTS (UTS peak-aged - UTS solution-treated ) (MPa) Elongation (Elongation peak-aged Elongation solution-treated ) (%) YS(YS - YS 0 Zn ) (MPa) peak-aged
0Zn
0.2Zn
0.5Zn
1.0Zn
27
41
49
19
44
52
49
16
−7.3
−8.3
−12.5
−0.9
0
14
22
−8
solution-treated )
YS and the lowest drop of the elongation. Therefore, the precipitates deteriorate the ductility of the alloys, and the higher strengthening effect corresponds to the higher drop of the ductility. Fig. 8 shows the typical fracture surface images of the solution-treated and peak-aged Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys. The residual eutectic compounds can be clearly observed in backscattering electron micrographs while the cleavage planes are more distinguished in second electron micrographs. In the Zn-free alloy, many cleavage planes are presented, while the amount of cleavage planes decreases with the increase of the Zn addition. The volume fractions of the
In Mg–1Nd–1Ce-Zr alloy (the Zn-free alloy), besides the prismatic β"’ precipitates, the point-like phases are observed (Fig. 6(b)), which were not reported in both Mg-Nd and MgCe based alloys in literature. The precipitation behavior of an Mg–0.5at% Ce alloy was once systematically studied by HAADF-STEM [28], where very thin disk-shaped precipitates on the prismatic planes (called as GP-zones) were reported when the alloy was aged at 180 °C for 70 h. Besides the diskshaped precipitates, if the TEM images of Figs. 2(c), 3(a) and 5(a) in Reference [28] are checked carefully, similar point-like clusters can also be observed from both [0001]α and [112¯ 0]α directions, even though they were not mentioned in the reference [28]. Since the HAADF-TEM imaging is capable to give a bright contrast at positions of heavy atoms, these point-like clusters in Mg–0.5at% Ce alloy should be rich in Ce element. Recently, in a Mg–3at% Ce alloy [30] solutiontreated and aged at 200 °C for 3 h, similar point-like precipitates were reported, which were confirmed to have fcc structure with higher Ce content than the prismatic precipitates. Therefore, in the present study, the point-like precipitates are probably also formed by the segregation of Ce element and its detail structure needs to be revealed in future.
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Fig. 8. SEM images of the fracture surfaces of solution-treated (a)–(h) and peak-aged (i)–(p) Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys: (a) (b) 0Zn-T4, (c) (d) 0.2Zn-T4, (e) (f) 0.5Zn-T4, (g) (h) 1.0Zn-T4 and (i) (j) 0Zn-T6, (k) (l) 0.2Zn-T6, (m) (n) 0.5Zn-T6, (o) (p) 1.0Zn-T6. (a)(c)(e)(g)(i)(k)(m)(o) are second electron images and (b)(d)(f)(h)(j)(l)(n)(p) are corresponding BSE images. In BSE images, the residual eutectic compounds are clearly observed in white color.
4.2. Influence of Zn addition on the precipitates It could be found in Table 5 and Fig. 6 that, the precipitates constitution of cast Mg–1Nd–1Ce-xZn-Zr (x = 0, 0.2, 0.5, 1.0) alloys peak-aged at 200 °C are significantly modified by the Zn addition. In the Zn-free alloy (Mg– 1Nd–1Ce-Zr alloy), the disk-shaped β"’ precipitates on prismatic planes and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the point-like precipitates and the refined diskshaped β’" precipitates, very fine precipitates form on the basal planes. When 0.5Zn is added, the basal precipitates become the main strengthening phase in the alloy. There are almost no prismatic precipitates and the number
density of the point-like precipitates significantly decreases. When the content of Zn addition increases to 1.0%, only very spare basal precipitates and point-like precipitates exist. At the present stage, the effects of Zn addition on the precipitation sequence of Mg-Nd alloys are still lack of clear characterization. Since the effect of Zn addition on the precipitation sequence of Mg-Gd alloys were well studied, some illumination could be got from the precipitation sequence review of Mg-Gd-Zn alloys [31–33]. In MgGd binary alloys, the precipitate evolution was reported in the following sequence [2]: SSSS → GP zone → β" (DO19 , Mg3 Gd) →β (cbco, Mg7 Gd) →β 1 (fcc, Mg3 Gd) →β (fcc, Mg5 Gd). All of the precipitates locate on the prismatic planes. When Zn element is added, basal precipitates and/or a
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Fig. 9. Optical microstructure of ruptured cast Mg-1Nd-1Ce-xZn-0.5Zr (x = 0, 0.2, 0.5 and 1.0, wt%) alloys, perpendicular to the fracture surface, in solutiontreated (a)–(d) and peak-aged (e)–(h) conditions.
novel phase with long period stacking ordered (LPSO) structure were introduced. In Mg–1Gd–0.4Zn–0.2Zr (at%) (Mg– 6.1Gd–1.0Zn–0.7Zr, wt%) alloy [31], the precipitation process during isothermal aging at 200 °C and 250 °C was reported to involve the formation of meta-stable γ " (space group P6¯ 2 m, a = 0.0556 nm and c = 0.444 nm) and γ ’ (space group P3¯ m1, a = 0.321 nm and c = 0.780 nm), both of which form as plate shaped particles on (0001)α . When the content of Zn addition increases, bulk LPSO phases form, such as those in Mg–2Gd–1 Zn (at%) (Mg–11.5Gd–2.4 Zn, wt%) [32], Mg–2Gd–1.2Y–1Zn–0.2Zr (at%) (Mg–11.1Gd–3.8Y– 2.3Zn–0.7Zr, wt%) [33] alloys. Therefore, during the aging or solidification process, Gd and Zn atoms tend to cosegregate to form new basal phases. It was suggested by J.F. Nie that the co-segregation of Gd and Zn atoms would minimize the elastic strain compared with individual atoms of Gd or Zn [31], which could promote the formation of Gd & Zn containing phases, both the precipitates and the LPSO phases. More significant influence of Zn addition on the precipitates happens in the present Mg–1Nd–1Ce-xZn-Zr alloys. As shown in Fig. 6(c) and (d), even 0.2 Zn addition has significantly influence on the precipitates constitutes: (1) Plenty of very fine precipitates form on the basal planes, which do
not exist in the Zn-free alloy; (2) The size of the prismatic phase is largely reduced while the number density increases. It is probably because that the atomic radius of Ce and Nd is 0.183, 0.182 nm, respectively, and much larger than Gd atom (0.178 nm). The co-segregation of Ce and Nd with Zn could minimize the elastic strain further compared with Gd. On the other hand, the maximum solid solubility of Ce (0.6%) and Nd (3.6%) is much lower than Gd (23.1%). Therefore, only a small amount of Zn addition into Mg–1Nd–1Ce-Zr alloy can modify the precipitates such significantly. When the Zn addition increases to 0.5%, nearly no prismatic phases exist and the basal precipitates become the main strengthening phase. When the Zn addition further increases to 1.0%, only spare basal precipitate exists. Compared with the Zn-free, 0.2Zn and 0.5Zn addition alloys, the total amount of precipitates in the 1.0Zn addition alloy is much less. Therefore, it could be speculated that the Zn addition could improve the thermal stability of eutectic (Mg, Zn)12 RE compounds, or reduce the solid solubility of RE elements in the α-Mg matrix, which in turn decreases the RE solution content in α-Mg matrix. As a result, at the solution temperature even as high as 540 °C, limited eutectic compounds in the 1Zn addition alloy dissolved into the matrix, which further leads to very spare basal precipitates during the aging process.
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Table 7 Typical tensile properties of aging harden-able cast Mg alloys at room temperature. Series
Alloy
Thermal condition
Tensile properties RT YS (MPa)
UTS (MPa)
Elongation (%)
Mg-Al
AZ91D AZ63 ZC63 ZK61 WE43 NZ30K AM-SC1 Mg-1Nd-1Ce-0.5Zn-Zr
S-T6 S-T6 S-T6 S-T6 S-T6 P-T6 P-T6 P-T6
145 122 125 195 162 142 130 136
270 232 210 310 250 305 206 237
6.0 5.5 4.0 10.0 2.0 11.0 3.7 9.0
Mg-Zn Mg-RE
Reference
[34]
[25] [35] Present study
S: Sand mould casting; P: Permanent mould casting.
4.3. Strengthening effect of different precipitates As described above, compared with the solution-treated alloys, the increments of the YS (YS, YS peak-aged - YS solution-treated, in Table 6) for the peak-aged 0, 0.2, 0.5 and 1.0Zn addition alloys are 27 MPa, 41 MPa, 49 MPa and 19 MPa, respectively. The increments of the YS come from the transformation from the solid solution atoms in the solutiontreated condition to the precipitates in the peak-aged condition. Therefore, compared with the Zn-free alloy, small amount of Zn addition (0.2, 0.5%) increases the YS by 14 and 22 MPa (Table 6), respectively. Such increments of the YS are due to the modification effect of the precipitates (Fig. 6 and Table 5). That is to say, the additional fine basal precipitates and the refined prismatic precipitates introduced by the 0.2Zn addition, and transformation of the main precipitates from the disk-shaped β"’ and the point-like precipitates in the Zn-free alloy to the finer basal precipitates in the 0.5Zn addition alloy, improve the YS of the alloys. Therefore, the introduction of the basal precipitates by small amount of Zn addition (0.2% and 0.5%) is beneficial to the YS. However, when the amount of Zn addition is as high as 1.0%, there are only very limited basal precipitates and pointlike precipitates and the YS is even lower than the Zn-free alloy. Therefore, the amount of Zn addition in Mg–1Nd–1CeZr alloy should be less than 1.0%, and 0.5Zn addition is the best based on the present study.
become much larger, which lead to unacceptable mechanical properties. Therefore, AM50 and AM60 alloys cannot be used to cast car wheels. Due the lower cooling rate in the wheel casting process, aging harden-able cast Mg alloys, used in T6 condition (solution treatment + aging treatment) and mainly strengthened by the precipitates, are necessary. Table 7 lists the typical tensile properties of aging harden-able cast Mg alloys at room temperature. Only AZ91D, ZK61 and NZ30K alloys are close to the minimum requirement of car wheels (130 MPa–210 MPa-7%). Unfortunately, as there are no effective grain refinement methods, the grains of AZ91D Mg cast wheels are very coarse, which leads to very poor mechanical properties. The elongation of AZ91D alloy in cast wheels is usually less than 3.0%, which makes it less available for car wheels. ZK61 alloy has very poor hot tearing resistance, and hot cracks form very easily during the wheel casting. Therefore, before the present study, the NZ30K alloy is the limited chosen for Mg cast wheels [36]. As discussed in introduction part, the high cost of Nd element retards the wheel application of NZ30K alloy. The typical mechanical properties of the present Mg–1Nd–1Ce–0.5Zn-Zr alloy are: 136 MPa–237 MPa-9%, higher than the minimum requirement of car wheels (130 MPa–210 MPa-7%). As the cost of the Mg–1Nd–1Ce–0.5Zn-Zr alloy is much lower than the NZ30K alloy, it could probably promote the application of Mg wheel in automobiles. 5. Conclusion
4.4. Mg alloys suitable for wheel casting (thick-wall casting) According to their chemical compositions, commercial Mg cast alloys can be classified into Mg-Al, Mg-Zn and MgRE based alloys. Mg-Al based cast alloys, such as AZ91D, AM50, AM60, are the most widely used Mg alloys due to their good castability and low cost. AM50 and AM60 Mg alloys have little aging harden-ability and they can only be cast in high pressure die casting process (HPDC), where acceptable mechanical properties are obtained mainly due to the fine grains produced under the high cooling rate. Car wheels are one of typical thick-wall castings, and usually cast by gravity or low pressure casting in permanent moulds under much lower cooling rate compared with HPDC. Under such lower cooling rate, the grains of AM50 and AM60 alloys
The effects of different Zn addition (0, 0.2, 0.5, 1.0 wt%) on the microstructure and mechanical properties of cast Mg– 1Nd–1Ce-Zr alloy in as-cast, solution-treated and 200 °C peak-aged conditions are studied. The following conclusions can be drawn. (1) Precipitates in cast Mg–1Nd–1Ce-Zr alloy are significantly modified by the Zn addition. In the Zn-free alloy, the disk-shaped prismatic precipitates and the point-like precipitates are the main strengthening phases. When 0.2Zn is added into the alloy, besides the disk-shaped and point-like precipitates, the very fine basal precipitates form. When 0.5Zn is added, the basal precipitates become the main strengthening phase, and the contents
Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003
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of the prismatic precipitates and the point-like precipitates significantly decrease. While the Zn addition increases to 1.0%, only spare basal precipitates and pointlike precipitates exist. (2) Compared with the Zn-free alloy, small amount of Zn additions improves the yield strength (YS) in T6 condition. The additional fine basal precipitates and the refined prismatic precipitates introduced by the 0.2Zn addition lead to 14 MPa enhancement of YS, while the transformation of the main precipitates from the prismatic disk-shaped precipitates and the point-like precipitates in the Zn-free alloy to the finer basal precipitates in the 0.5 Zn addition alloy leads to 22 MPa enhancement of YS. Further increasing the Zn addition to 1.0 wt%, leads to lower YS than the Zn-free alloy. (3) Mg–1Nd–1Ce–0.5Zn-Zr alloy has the highest strength in T6 condition: yield strength of 136 MPa, ultimate tensile strength of 237 MPa and elongation of 9%, which is higher than the minimum requirement of car wheels (130 MPa–210 MPa-7%). Therefore, Mg–1Nd– 1Ce–0.5Zn-Zr alloy could be used to cast Mg wheels. Acknowledgments This work was supported by National Key Research and Development Program of China (2016YFB0301000 & 2016YFB0701204), Shanghai Rising-Star Program (15QB1402700), National Natural Science Foundation of China (NSFC) (51671128 & 51771113) and Special Fund of Jiangsu Province for the Transformation of Scientific and Technological Achievements (BA2016039). References [1] P. Fu, L. Peng, H. Jiang, W. Ding, C. Zhai, China Foundry 11 (2014) 277–286. [2] J.-F. Nie, Metall. Mater. Trans. A 43 (2012) 3891–3939. [3] F. Pan, M. Yang, X. Chen, J. Mater. Sci. Technol. 32 (2016) 1211–1221. [4] J. Zhang, S. Liu, R. Wu, L. Hou, M. Zhang, J. Magnes. Alloy. 6 (2018) 277–291. [5] Y. Zhang, Y. Wu, L. Peng, P. Fu, F. Huang, J. Alloy. Compd. 615 (2014) 703–711. [6] K. Yamada, H. Hoshikawa, S. Maki, et al., Scr. Mater. 61 (2009) 636–639. [7] Q. Wang, J. Chen, Z. Zhao, et al., Mater. Sci. Eng. A 528 (2010) 323–328.
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Please cite this article as: Y. Zhou, P. Fu and L. Peng et al., Precipitation modification in cast Mg–1Nd–1Ce-Zr alloy by Zn addition, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2019.02.003