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Precipitates formed in the as-forged Mg–Zn–RE alloy during ageing process at 250 °C
M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 1 7 6–1 8 3
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Precipitates formed in the as-forged Mg–Zn–RE alloy during ageing process at 250 °C W.C. Xu, X.Z. Han, D.B. Shan⁎ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
The precipitate behavior of the Mg–10Gd–2Y–0.5Zn–0.3Zr alloy during forging process and
Received 8 April 2012
subsequent ageing process at 250 °C is studied in the article. Different from the precipitation
Received in revised form
sequence in extruded Mg–Zn–RE alloy, only long period ordered structure and β′ phase
28 August 2012
precipitate on the Mg matrix during forging process. The slight kinking is mainly ascribed to
Accepted 25 September 2012
the insufficient and uneven plastic deformation in the forged alloy, which suggests that the
Keywords:
Zn–RE alloy. Amounts of long period ordered structure and β′ phases precipitate on the matrix
Mg–Zn–RE alloy
with ageing time increasing until 10 h. The peak hardness, 100 HV, of the forged alloy during
Precipitation
ageing process is obtained when the alloy is aged at 250 °C for 10 h. The long period ordered
Forging process
structure and β′ phase are the main strengthening phases in the peak-aged alloy. Therefore,
Ageing process
the strengthening mechanism of the forged alloy is controlled by the comprehensive effects of
deformation mode of the alloy during forging process is different from that of extruded Mg–
grain refinement and dispersed precipitation of long period ordered structure and β′ phase on the matrix. The strength improvement of the peak-aged alloy results from dense precipitation of long period ordered structure and β′ phase on the matrix. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Magnesium alloy has become an attractive structural material in automobile and aerospace fields due to its low density and high specific strength [1,2]. Especially, the Mg alloy containing rare earth (RE) has attracted many attentions around the world because of its high strength both at ambient and elevated temperature. Until now, a series of Mg–Zn–RE alloys with excellent mechanical properties have been developed [3–5]. The precipitation sequence of typical age-hardening Mg–Zn–RE alloy during ageing process has been studied and confirmed as: supersaturated solid solution (S.S.S.S.)→β″ (D019)→β′ (cbco)→β1 (fcc)→β (fcc) [6,7]. The β″ phase with D019 crystal structure precipitates in the Mg matrix at early stage of ageing process and then transforms to β′ phase with increasing ageing time. The β″ and β′ are the main strengthening phases in Mg–Zn–RE alloy and promote the strength of extruded Mg–Zn–RE alloy obviously
during extrusion process and ageing process [8,9]. Another strengthening phase, long period order (LPO) structure, was observed in the Mg–Zn–RE alloy in the year of 2004 [4]. And then, many studies mainly focused on the precipitate behavior of LPO structure and its effect on the strength of Mg–Zn–RE alloy during the recent years. Previous results show that the LPO structure prefers to precipitate on the matrix during extrusion process and contributes to the high strength of extruded Mg–Zn–RE alloys [10,11]. Furthermore, the excellent mechanical properties with a tensile yield strength (TYS) of 610 MPa and elongation of 5% are obtained due to the obvious strengthening effect of LPO structure in the Mg–Zn–RE alloy [12]. Recently, the strengthening effect of LPO structure in Mg–Zn–RE alloy is studied and results show that the synergetic effects of LPO structure and grain refinement due to dynamical recrystallization (DRX) contribute to the super high strength of extruded Mg–Zn–RE alloy [13].
⁎ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel./fax: +86 451 86418732. E-mail addresses: [email protected] (X.Z. Han), [email protected] (D.B. Shan). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.09.009
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 1 7 6–1 8 3
Although the precipitate sequence in Mg–Zn–RE alloy has been studied and confirmed in the casting and extruded alloy, the precipitate behavior during subsequent ageing process at high temperature in the forged alloy is still not clear by now. Furthermore, the strengthening mechanism of LPO structure should be explored in details regarding its remarkable strengthening effect in the Mg–Zn–RE alloy. Therefore, the microstructure evolution and precipitate behavior during forging process and subsequent ageing process at 250 °C are studied in the article. The strengthening mechanisms in the forged and aged alloy containing LPO structure are also discussed.
2.
177
as -forged
as -cast
Experimental procedures
The nominal composition of the Mg–Zn–RE alloy used in this study is Mg–10Gd–2Y–0.5Zn–0.3Zr (wt.%) denoted as GWZK102 in the article. The alloy was prepared from high purity Mg (99.95%), Zn (99.9%), Mg–20Y, Mg–30.6Gd and Mg–30.33Zr (wt.%) master alloys in an electric resistance furnace at about 750 °C under a mixed gas atmosphere of SF6 and CO2. The ingot was homogenized at 510 °C and forged at 470 °C. Artificial ageing process was carried out at 250 °C with different time intervals in an ageing furnace. The hardness of the alloy is tested using a Vickers hardness tester under a load of 300 N and holding time of 30 s. The microstructure of specimens was observed using optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The specimens were etched in 2 and 5 vol.% nitric acid alcohol solution. The characterization of phases was performed by X-ray diffraction analysis (XRD) as well as Tecnai 300 kV transmission electron microscopy (TEM) operated at an accelerating voltage of 300 kV. The thin foils for TEM were prepared with the help of ion polishing system.
3.
Results
3.1.
Microstructures of the As-cast and the Forged Alloy
Fig. 1 presents the patterns of XRD analysis of as-cast alloy and forged alloy. It indicates that the main secondary phases in the as-cast alloy are the eutectic Mg24(Gd,Y,)5 phase and Mg5(Gd,Y) phase. Fig. 2 shows the microstructures of the as-cast alloy and forged alloy. Amounts of coarse Mg24(Gd,Y,)5 phases locate at the triple points of grain and some tiny Zr-rich cores randomly distribute in the Mg matrix of the as-cast alloy, as shown in Fig. 2 (a) and (b). The microstructure of the forged alloy is not uniform with some big and small grains coexisting, which suggests the occurrence of incomplete dynamical recrystallization (DRX) during forging process, as shown in Fig. 2 (c). Another secondary phase with cuboid shape randomly precipitates from the matrix, which is proved to be the Mg5(Gd,Y) (denoted as β′) phase by XRD and SEM analysis, as shown in Figs. 1 and 2 (d). The β′ phase is an important strengthening phase in Mg–Zn–RE alloy and usually precipitates from the matrix during plastic deformation or subsequent ageing process [5]. Fig. 3 presents the lamellar LPO structure in the forged alloy. It indicates that the lamellar phase are parallel to each other within a grain. The corresponding selected area diffraction pattern shows that there are seven extra diffraction spots at the
Fig. 1 – XRD patterns of as-cast and as-forged alloys.
position of n/7(0001)hcp, which suggests the lamellar phase is 14H-type LPO structure with a 14 basal-plane periodicity. Furthermore, three extrandiffraction spots appear between the o central point and the 0110 α plane, which confirms the precipitation of β′ phase in the forged alloy, as shown in Fig. 3 (b). Under relatively low forging temperature, the Mg alloy is hard to deform plastically for insufficient slip systems. Therefore, the lamellar 14H-type LPO structure is forced to kink due to high external stress during forging process, as shown in Fig. 3 (c). Actually, the kinking is an important plastic deformation mode in lamellar structure of the metals with less slips because it can make some grains rotate to appropriate orientations for the activation of slip systems [14]. In the forged alloy, the kinking of 14H-type LPO structure can provide some favorable orientations for slip in certain grains, which increases the deformability compatibility of the alloy during forging process, as shown in Fig. 3 (c). However, the kinking in the forged alloy is less intense compared with that of extruded alloy, in which the kinking angle is almost 90° due to large deformation. The severe kinking is helpful to refine grains or activate more slip systems to facilitate plastic deformation in the extruded Mg–Zn–RE alloy [13]. However, because of the insufficient and uneven plastic deformation in the forged alloy, the slight kinking of the LPO structure has relatively small contribution to grain refinement as well as to the improvement of mechanical properties, such as tensile strength and ductility.
3.2.
Age Hardening Behavior of the Forged Alloy
Fig. 4 and Table 1 show the ageing hardening behavior and the corresponding mechanical properties of the alloy during ageing process. The hardness of the alloy increases quickly at the early stage of ageing process until 10 h and then decreases with increasing ageing time. The peak hardness, 100 HV, is obtained in the alloy aged at 250 °C for 10 h.
3.3.
Microstructures of Aged Alloy
Fig. 5 shows the microstructures of aged alloy with different time intervals. Only a few particles distribute on the α-Mg matrix and some small DRX grains formed during forging
178
M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 1 7 6–1 8 3
a
b Zr rich core
α-Mg matrix eutectic phase
c
d β′
Fig. 2 – Microstructures of as-cast and as-forged alloy, (a) SEM image of as-cast alloy, (b) large magnification of (a). (c) OM microstructure of as-forged alloy, (d) SEM image of as-forged alloy.
process still can be found under the ageing time of 5 h (Fig. 5a). The precipitated particle volume fraction continues to increase with increasing ageing time and reaches a peak value after ageing for 10 h, as shown in Fig. 5 (b). The microstructure becomes coarse in the alloy aged for 60 h, which is a typical over-aged microstructure of the alloy (Fig. 5c). Fig. 6 shows the patterns of XRD analysis of the alloy aged at 5 h and 10 h. The patterns suggest that the β′ phase is the main secondary phase in the alloy aged at 5 h and 10 h. No β″ phase is observed in the aged alloy since the β″ phase has totally transformed to the stable β′ phase during ageing process at 250 °C, which is different from the results of reference [15]. Fig. 7 presents the elliptical precipitates in the alloy aged at 5 h and 10 h. Corresponding SAED pattern suggests that there are some diffraction spots occurring n o extra n o n oat the position of 1/4 0110 α, 1/2 0110 α and 3/4 0110 α of Mg reflections, which is the typical diffraction characteristic of the β′ phase in the Mg–RE alloys, as shown in Fig. 7 (d). The size of β′ phase with long axis of about 65 nm and short axis of 20 nm changes little with increasing ageing time while its volume fraction reaches a peak value in the matrix nafter ageing for 10 h (Fig. 7b). The β′ o phase precipitates along 1120 α of Mg matrix and has three variants around [0001]α zone, as shown in Fig. 7 (a) and (c) [16,17]. The orientation relationships n obetween β′ phase and Mg matrix are [0001]α//[001]β′ and 2110 α//(100)β′. Fig. 8 presents the periodic distribution of the LPO structure by HRTEM and HADDF-STEM images. The bright lines in Fig. 8 (a) are composed
of many lamellas parallel to each other and the FFT inserted in Fig. 8 (b) confirms that the lamellas are 14H-type LPO structure. Fig. 8 (c) shows that a periodicity of the LPO structure is composed of seven basal atomic planes with basal plane spacing of d(002) × 7–1.8 nm, as indicated by white lines. The bright lines due to Z-contrast are also observed with the same regular interval of 1.8 nm (Fig. 8d). In addition, some stacking faults are observed, which are probably irregular intervals between two periodicities of the LPO structure, shown in Fig. 8 (c) and (d). Abe points out that the grain boundary is a favorable place for the formation of LPO structure because of dislocation accumulation [18]. The ageing process at 250 °C can increase the dispersion rate of solute atoms and cause more LPO structures to precipitate on grain boundaries in the peak-aged alloy. The LPO structure can provide large resistance to dislocation sliding inside grain and improve the strength of the peak-aged alloy at 250 °C.
4.
Discussion
Generally, rare earth elements are added in order to improve the precipitating hardening response and further strengthen Mg–Zn–RE alloy. In the present study, the microstructure of as-cast alloy has similar characteristic with other Mg–Zn–RE alloy, whereas the precipitate sequences in the forged alloy are
179
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 1 7 6–1 8 3
a
b
(0110) (0111)
(0001) (0002)
C*
c
Fig. 3 – TEM images and corresponding selected area electron diffraction (SAED) pattern of the LPO phase: h i(a) LPO phase, (b) SAED pattern of the LPO phase in (a), (c) kinking of the LPO phase in the alloy, electron beam is parallel to 2110 . a little different from that of extruded alloy [7]. Only the LPO structure and β′ phase precipitate from the α-Mg(S.S.S.S.) during the forging process. No β″ phase is observed in the forged alloy
mainly because the β″ phase has transformed to the β′ phase at relatively high forging temperature. Also, no stable β phase can be found in the forged alloy, which suggests that the forging process can not provide enough energy for β′ → β phase transformation. Therefore, the parallel precipitation sequences in the forged alloy are as follows: α−MgðS:S:S:SÞ→14 HLPOstructure α−MgðS:S:S:SÞ→β′ðcbcoÞ:
Table 1 – Mechanical properties of GWZK102 alloy. State
Fig. 4 – Ageing hardening behavior of the alloy at 250 °C with different time intervals.
As-cast As-forged T5, 250 °C-5 h T5, 250 °C-10 h T5, 250 °C-60 h
YTS (MPa) UTS (MPa) HV Elongation (%) 114 210 240 253 231
162 308 343 356 334
80 87 98 100 84
4.20 7.50 7.25 7.20 6.73
180
M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 1 7 6–1 8 3
a
b
Secondary phases
c
Fig. 5 – Microstructures of the alloy aged at 250 °C with different time intervals: (a) 5 h, (b) 10 h, (c) 60 h.
Actually, the deformation mechanism in the forged alloy is also different from that of extruded Mg–Zn–RE alloy. The slight kinking in the forged alloy is attributed to the insufficient and uneven plastic deformation during forging process, which can
Fig. 6 – XRD patterns of the alloy aged at 250 °C: (a) 5 h, (b) 10 h.
be verified from the incomplete DRX during the forging process (Fig. 2c). Due to the insufficient and uneven plastic deformation, the microstructure of the forged alloy is coarse, which leads to relatively lower strength of the forged alloy compared with that of extruded alloy. On the other hand, the strength of the forged alloy is improved obviously in comparison to that of the as-cast alloy in the study. The dispersed distribution of β′ and lamellar LPO structure on the matrix contributes to the high strength of the forged alloy. Indeed, the strengthening mechanism of the forged alloy is controlled by the dispersed distribution of LPO structure and β′ phase as well as refined grains due to the DRX. Amounts of the LPO structure and β′ phase precipitate on the matrix with an increasing ageing time until 10 h during ageing process at 250 °C. According to the previous literature [1], the β′ phase will change to the stable β phase during the ageing process at higher temperature or longer time. Nevertheless, the stable and soft β phase can not be found in the present study due to the short ageing time, which is different from the results of reference [6]. This suggests that only the β′ phase and LPO structure distribute on the matrix in the forged alloy during ageing process. Therefore, the precipitate sequences in the aged alloy are nearly the same with that of the forged alloy, whereas the number of LPO structure and β′ phase on the matrix
181
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 1 7 6–1 8 3
a
b
(1210)
(1210)
(1120)
c
(1210) d
(0110) (1100)
(1210)
(1010)
(1100) (1010) (0110)
(1120) Fig. 7 – TEM images and corresponding SAED pattern of β′ phase in aged alloy at 5 h and 10 h: (a), (b) TEM images of β′ phase at 5 h and 10 h, (c) high resolution TEM image of the β′ phase at 5 h, (d) corresponding SAED pattern of the β′ phase at 5 h, electron beam is parallel to [0001].
increases pronouncedly. The LPO structure with lamellar morphology can provide large resistance to dislocation sliding in α-Mg grain or grain boundary, which obviously promotes the strength of the aged alloy. Therefore, the improvement of mechanical properties during ageing process can be ascribed to the dense distribution of the lamellar LPO structure and β′ phase on the Mg matrix.
5.
Conclusions
The precipitates formed in the Mg–Zn–RE alloy during forging process and ageing process have been studied and some conclusions can be obtained as follows: 1. Only the LPO structure and β′ phase precipitate from the α-Mg(S.S.S.S.) during the forging and ageing processes. The β″ and β can not be found in the forged alloy and peak-aged alloy, which are slightly different from that of extruded Mg– Zn–RE alloy. The parallel precipitate sequences in the forged alloy and aged alloy are: α−MgðS:S:S:SÞ→14 HLPOstructure
α−MgðS:S:S:SÞ→β′ðcbcoÞ: 2. The kinking in the as-forged alloy is less intense than that of extruded Mg–Zn–RE alloy, which results from the uneven and insufficient plastic deformation during the forging process. The strengthening mechanism of the forged alloy is controlled by the dispersed distribution of LPO structure and β′ phase on the matrix and grain refinement due to DRX. 3. Densities of β′ phase and LPO structure precipitate on the matrix with an increasing ageing time until 10 h. The LPO structure and β′ phase are the main strengthening phases in the peak-aged alloy. The improvement of mechanical properties of the alloy during the ageing process is mainly ascribed to the dense distribution of LPO structure and β′ phase on the matrix.
Acknowledgment The authors would like to thank the National Natural Science Foundation of China (no. 51275128). The Analytical and Testing Center of the Harbin Institute of Technology is also appreciated for the sample preparation and useful discussion of the TEM
182
M A TE RI A L S CH A RACT ER IZ A TI O N 75 (2 0 1 3 ) 1 7 6–1 8 3
a
b
1.8nm HADDF-STEM c
1.8nm
d
1.8nm
Fig. 8 – HADDF-STEM and high resolution TEM images of the lamellar LPO phase in the alloy aged at 10 h: (a) and (c) HADDF-STEM images of lamellar phase, (b) and (d) HRTEM images of lamellar phase. HRTEM shows some periodicities with 14 closed packing planes and the interval of 1.8 nm. HADDF-STEM significantly exhibits Z-contrast of the same interval of 1.8 nm at some places. The fast Fourier transformation (FFT) is inserted in (b). analyses. Also, we would like to give our thanks to the scientists of the reference [16] for providing of the images in Fig. 2.
REFERENCES
[1] Liu K, Rokhlin LL, Elkinc FM, Tang DX, Meng J. Effect of ageing treatment on the microstructures and mechanical properties of the extruded Mg–7Y–4Gd–1.5Zn–0.4Zr alloy. Mater Sci Eng A 2010;527:828-34. [2] Liu XB, Chen RS, Han EH. Effects of ageing treatment on microstructures and properties of Mg–Gd–Y–Zr alloys with and without Zn additions. J Alloys Compd 2008;465:232-8.
[3] Kawamura Y, Kasahara T, Izumi S, Yamasaki M. Elevated temperature Mg97Y2Cu1 alloy with long period ordered structure. Scr Mater 2006;55:453-6. [4] Itoi T, Seimiya T, Kawamura Y, Hirohashi M. Long period stacking structures observed in Mg97Zn1Y2 alloy. Scr Mater 2004;51:107-11. [5] He SM, Zeng XQ, Peng LM, Gao X, Nie JF, Ding WJ. Microstructure and strengthening mechanism of high strength Mg–10Gd–2Y–0.5Zr alloy. J Alloys Compd 2007;427: 316-23. [6] He SM, Zeng XQ, Peng LM, Gao X, Nie JF, Ding WJ. Precipitation in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250 °C. J Alloys Compd 2006;421:309-13. [7] Yang Z, Li JP, Guo YC, Liu T, Xia F, Zeng ZW, et al. Precipitation process and effect on mechanical properties of Mg–9Gd–3Y– 0.6Zn–0.5Zr alloy. Mater Sci Eng A 2007;454:274-80.
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 1 7 6–1 8 3
[8] Liang SQ, Guan DK, Chen L, Gao ZH, Tang HX, Tong XT, et al. Precipitation and its effect on age-hardening behavior of as-cast Mg–Gd–Y alloy. Mater Des 2011;32:361-4. [9] Chen B, Lin DL, Zeng XQ, Lu C. Microstructure and mechanical properties of ultrafine grained Mg97Y2Zn1 alloy processed by equal channel angular pressing. J Alloys Compd 2007;440:94. [10] Liu K, Zhang JH, Su GH, Tang DX, Rokhlin LL, Elkin FM, et al. Influence of Zn content on the microstructure and mechanical properties of extruded Mg–5Y–4Gd–0.4Zr alloy. J Alloys Compd 2009;481:811-8. [11] Gao Y, Wang QD, Gu JH, Zhao Y, Tong Y, Yin DD. Three-dimensional shear-strain patterns induced by high-pressure torsion and their impact on hardness evolution. J Alloys Compd 2009;477:374-8. [12] Kawamura Y, Hayashi K, Inoue A, et al. Rapidly solidified powder metallurgy Mg97 Zn1 Y2 alloys with excellent tensile yield strength above 600 Mpa. Mater Trans JIM 2001;42: 1171-2. [13] Shao XH, Yang a ZQ, Ma XL. Strengthening and toughening mechanisms in Mg–Zn–Y alloy with a long period stacking ordered structure. Acta Mater 2010;58:4760-71.
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[14] Itoi T, Takahashi K, Moriyama H, Hirohashi M. A high-strength Mg–Ni–Y alloy sheet with a long-period ordered phase prepared by hot-rolling. Scr Mater 2008;59:1155. [15] Gao X, He SM, Zeng XQ, Peng LM, Ding WJ, Nie JF. Microstructure evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C. Mater Sci Eng A 2006;431: 322-7. [16] Han XZ, Xu WC, Shan DB. Effect of precipitates on microstructures and properties of forged Mg–10Gd–2Y–0.5Zn– 0.3Zr alloy during ageing process. J Alloys Compd 2011;509: 8625. [17] Honma T, Ohkubo T, Kamado S, Hono K. Effect of Zn additions on the age-hardening of Mg–2.0Gd–1.2Y–0.2Zr alloys. Acta Mater 2007;55:4137-50. [18] Abe E, Kawamura Y, Hayashi K, Inoue A. Long-period ordered structure in a high-strength nanocrystalline Mg-1at%Zn-2at%Y alloy studied by atomic-resolution Z-contrast STEM. Acta Mater 2002;50:3845-57.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Precipitates formed in the as-forged Mg–Zn–RE alloy during ageing process at 250 °C W.C. Xu, X.Z. Han, D.B. Shan⁎ School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, PR China
AR TIC LE D ATA
ABSTR ACT
Article history:
The precipitate behavior of the Mg–10Gd–2Y–0.5Zn–0.3Zr alloy during forging process and
Received 8 April 2012
subsequent ageing process at 250 °C is studied in the article. Different from the precipitation
Received in revised form
sequence in extruded Mg–Zn–RE alloy, only long period ordered structure and β′ phase
28 August 2012
precipitate on the Mg matrix during forging process. The slight kinking is mainly ascribed to
Accepted 25 September 2012
the insufficient and uneven plastic deformation in the forged alloy, which suggests that the
Keywords:
Zn–RE alloy. Amounts of long period ordered structure and β′ phases precipitate on the matrix
Mg–Zn–RE alloy
with ageing time increasing until 10 h. The peak hardness, 100 HV, of the forged alloy during
Precipitation
ageing process is obtained when the alloy is aged at 250 °C for 10 h. The long period ordered
Forging process
structure and β′ phase are the main strengthening phases in the peak-aged alloy. Therefore,
Ageing process
the strengthening mechanism of the forged alloy is controlled by the comprehensive effects of
deformation mode of the alloy during forging process is different from that of extruded Mg–
grain refinement and dispersed precipitation of long period ordered structure and β′ phase on the matrix. The strength improvement of the peak-aged alloy results from dense precipitation of long period ordered structure and β′ phase on the matrix. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Magnesium alloy has become an attractive structural material in automobile and aerospace fields due to its low density and high specific strength [1,2]. Especially, the Mg alloy containing rare earth (RE) has attracted many attentions around the world because of its high strength both at ambient and elevated temperature. Until now, a series of Mg–Zn–RE alloys with excellent mechanical properties have been developed [3–5]. The precipitation sequence of typical age-hardening Mg–Zn–RE alloy during ageing process has been studied and confirmed as: supersaturated solid solution (S.S.S.S.)→β″ (D019)→β′ (cbco)→β1 (fcc)→β (fcc) [6,7]. The β″ phase with D019 crystal structure precipitates in the Mg matrix at early stage of ageing process and then transforms to β′ phase with increasing ageing time. The β″ and β′ are the main strengthening phases in Mg–Zn–RE alloy and promote the strength of extruded Mg–Zn–RE alloy obviously
during extrusion process and ageing process [8,9]. Another strengthening phase, long period order (LPO) structure, was observed in the Mg–Zn–RE alloy in the year of 2004 [4]. And then, many studies mainly focused on the precipitate behavior of LPO structure and its effect on the strength of Mg–Zn–RE alloy during the recent years. Previous results show that the LPO structure prefers to precipitate on the matrix during extrusion process and contributes to the high strength of extruded Mg–Zn–RE alloys [10,11]. Furthermore, the excellent mechanical properties with a tensile yield strength (TYS) of 610 MPa and elongation of 5% are obtained due to the obvious strengthening effect of LPO structure in the Mg–Zn–RE alloy [12]. Recently, the strengthening effect of LPO structure in Mg–Zn–RE alloy is studied and results show that the synergetic effects of LPO structure and grain refinement due to dynamical recrystallization (DRX) contribute to the super high strength of extruded Mg–Zn–RE alloy [13].
⁎ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China. Tel./fax: +86 451 86418732. E-mail addresses: [email protected] (X.Z. Han), [email protected] (D.B. Shan). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.09.009
M A TE RI A L S C HA RACT ER I ZA TI O N 75 ( 20 1 3 ) 1 7 6–1 8 3
Although the precipitate sequence in Mg–Zn–RE alloy has been studied and confirmed in the casting and extruded alloy, the precipitate behavior during subsequent ageing process at high temperature in the forged alloy is still not clear by now. Furthermore, the strengthening mechanism of LPO structure should be explored in details regarding its remarkable strengthening effect in the Mg–Zn–RE alloy. Therefore, the microstructure evolution and precipitate behavior during forging process and subsequent ageing process at 250 °C are studied in the article. The strengthening mechanisms in the forged and aged alloy containing LPO structure are also discussed.
2.
177
as -forged
as -cast
Experimental procedures
The nominal composition of the Mg–Zn–RE alloy used in this study is Mg–10Gd–2Y–0.5Zn–0.3Zr (wt.%) denoted as GWZK102 in the article. The alloy was prepared from high purity Mg (99.95%), Zn (99.9%), Mg–20Y, Mg–30.6Gd and Mg–30.33Zr (wt.%) master alloys in an electric resistance furnace at about 750 °C under a mixed gas atmosphere of SF6 and CO2. The ingot was homogenized at 510 °C and forged at 470 °C. Artificial ageing process was carried out at 250 °C with different time intervals in an ageing furnace. The hardness of the alloy is tested using a Vickers hardness tester under a load of 300 N and holding time of 30 s. The microstructure of specimens was observed using optical microscope (OM) and scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The specimens were etched in 2 and 5 vol.% nitric acid alcohol solution. The characterization of phases was performed by X-ray diffraction analysis (XRD) as well as Tecnai 300 kV transmission electron microscopy (TEM) operated at an accelerating voltage of 300 kV. The thin foils for TEM were prepared with the help of ion polishing system.
3.
Results
3.1.
Microstructures of the As-cast and the Forged Alloy
Fig. 1 presents the patterns of XRD analysis of as-cast alloy and forged alloy. It indicates that the main secondary phases in the as-cast alloy are the eutectic Mg24(Gd,Y,)5 phase and Mg5(Gd,Y) phase. Fig. 2 shows the microstructures of the as-cast alloy and forged alloy. Amounts of coarse Mg24(Gd,Y,)5 phases locate at the triple points of grain and some tiny Zr-rich cores randomly distribute in the Mg matrix of the as-cast alloy, as shown in Fig. 2 (a) and (b). The microstructure of the forged alloy is not uniform with some big and small grains coexisting, which suggests the occurrence of incomplete dynamical recrystallization (DRX) during forging process, as shown in Fig. 2 (c). Another secondary phase with cuboid shape randomly precipitates from the matrix, which is proved to be the Mg5(Gd,Y) (denoted as β′) phase by XRD and SEM analysis, as shown in Figs. 1 and 2 (d). The β′ phase is an important strengthening phase in Mg–Zn–RE alloy and usually precipitates from the matrix during plastic deformation or subsequent ageing process [5]. Fig. 3 presents the lamellar LPO structure in the forged alloy. It indicates that the lamellar phase are parallel to each other within a grain. The corresponding selected area diffraction pattern shows that there are seven extra diffraction spots at the
Fig. 1 – XRD patterns of as-cast and as-forged alloys.
position of n/7(0001)hcp, which suggests the lamellar phase is 14H-type LPO structure with a 14 basal-plane periodicity. Furthermore, three extrandiffraction spots appear between the o central point and the 0110 α plane, which confirms the precipitation of β′ phase in the forged alloy, as shown in Fig. 3 (b). Under relatively low forging temperature, the Mg alloy is hard to deform plastically for insufficient slip systems. Therefore, the lamellar 14H-type LPO structure is forced to kink due to high external stress during forging process, as shown in Fig. 3 (c). Actually, the kinking is an important plastic deformation mode in lamellar structure of the metals with less slips because it can make some grains rotate to appropriate orientations for the activation of slip systems [14]. In the forged alloy, the kinking of 14H-type LPO structure can provide some favorable orientations for slip in certain grains, which increases the deformability compatibility of the alloy during forging process, as shown in Fig. 3 (c). However, the kinking in the forged alloy is less intense compared with that of extruded alloy, in which the kinking angle is almost 90° due to large deformation. The severe kinking is helpful to refine grains or activate more slip systems to facilitate plastic deformation in the extruded Mg–Zn–RE alloy [13]. However, because of the insufficient and uneven plastic deformation in the forged alloy, the slight kinking of the LPO structure has relatively small contribution to grain refinement as well as to the improvement of mechanical properties, such as tensile strength and ductility.
3.2.
Age Hardening Behavior of the Forged Alloy
Fig. 4 and Table 1 show the ageing hardening behavior and the corresponding mechanical properties of the alloy during ageing process. The hardness of the alloy increases quickly at the early stage of ageing process until 10 h and then decreases with increasing ageing time. The peak hardness, 100 HV, is obtained in the alloy aged at 250 °C for 10 h.
3.3.
Microstructures of Aged Alloy
Fig. 5 shows the microstructures of aged alloy with different time intervals. Only a few particles distribute on the α-Mg matrix and some small DRX grains formed during forging
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a
b Zr rich core
α-Mg matrix eutectic phase
c
d β′
Fig. 2 – Microstructures of as-cast and as-forged alloy, (a) SEM image of as-cast alloy, (b) large magnification of (a). (c) OM microstructure of as-forged alloy, (d) SEM image of as-forged alloy.
process still can be found under the ageing time of 5 h (Fig. 5a). The precipitated particle volume fraction continues to increase with increasing ageing time and reaches a peak value after ageing for 10 h, as shown in Fig. 5 (b). The microstructure becomes coarse in the alloy aged for 60 h, which is a typical over-aged microstructure of the alloy (Fig. 5c). Fig. 6 shows the patterns of XRD analysis of the alloy aged at 5 h and 10 h. The patterns suggest that the β′ phase is the main secondary phase in the alloy aged at 5 h and 10 h. No β″ phase is observed in the aged alloy since the β″ phase has totally transformed to the stable β′ phase during ageing process at 250 °C, which is different from the results of reference [15]. Fig. 7 presents the elliptical precipitates in the alloy aged at 5 h and 10 h. Corresponding SAED pattern suggests that there are some diffraction spots occurring n o extra n o n oat the position of 1/4 0110 α, 1/2 0110 α and 3/4 0110 α of Mg reflections, which is the typical diffraction characteristic of the β′ phase in the Mg–RE alloys, as shown in Fig. 7 (d). The size of β′ phase with long axis of about 65 nm and short axis of 20 nm changes little with increasing ageing time while its volume fraction reaches a peak value in the matrix nafter ageing for 10 h (Fig. 7b). The β′ o phase precipitates along 1120 α of Mg matrix and has three variants around [0001]α zone, as shown in Fig. 7 (a) and (c) [16,17]. The orientation relationships n obetween β′ phase and Mg matrix are [0001]α//[001]β′ and 2110 α//(100)β′. Fig. 8 presents the periodic distribution of the LPO structure by HRTEM and HADDF-STEM images. The bright lines in Fig. 8 (a) are composed
of many lamellas parallel to each other and the FFT inserted in Fig. 8 (b) confirms that the lamellas are 14H-type LPO structure. Fig. 8 (c) shows that a periodicity of the LPO structure is composed of seven basal atomic planes with basal plane spacing of d(002) × 7–1.8 nm, as indicated by white lines. The bright lines due to Z-contrast are also observed with the same regular interval of 1.8 nm (Fig. 8d). In addition, some stacking faults are observed, which are probably irregular intervals between two periodicities of the LPO structure, shown in Fig. 8 (c) and (d). Abe points out that the grain boundary is a favorable place for the formation of LPO structure because of dislocation accumulation [18]. The ageing process at 250 °C can increase the dispersion rate of solute atoms and cause more LPO structures to precipitate on grain boundaries in the peak-aged alloy. The LPO structure can provide large resistance to dislocation sliding inside grain and improve the strength of the peak-aged alloy at 250 °C.
4.
Discussion
Generally, rare earth elements are added in order to improve the precipitating hardening response and further strengthen Mg–Zn–RE alloy. In the present study, the microstructure of as-cast alloy has similar characteristic with other Mg–Zn–RE alloy, whereas the precipitate sequences in the forged alloy are
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a
b
(0110) (0111)
(0001) (0002)
C*
c
Fig. 3 – TEM images and corresponding selected area electron diffraction (SAED) pattern of the LPO phase: h i(a) LPO phase, (b) SAED pattern of the LPO phase in (a), (c) kinking of the LPO phase in the alloy, electron beam is parallel to 2110 . a little different from that of extruded alloy [7]. Only the LPO structure and β′ phase precipitate from the α-Mg(S.S.S.S.) during the forging process. No β″ phase is observed in the forged alloy
mainly because the β″ phase has transformed to the β′ phase at relatively high forging temperature. Also, no stable β phase can be found in the forged alloy, which suggests that the forging process can not provide enough energy for β′ → β phase transformation. Therefore, the parallel precipitation sequences in the forged alloy are as follows: α−MgðS:S:S:SÞ→14 HLPOstructure α−MgðS:S:S:SÞ→β′ðcbcoÞ:
Table 1 – Mechanical properties of GWZK102 alloy. State
Fig. 4 – Ageing hardening behavior of the alloy at 250 °C with different time intervals.
As-cast As-forged T5, 250 °C-5 h T5, 250 °C-10 h T5, 250 °C-60 h
YTS (MPa) UTS (MPa) HV Elongation (%) 114 210 240 253 231
162 308 343 356 334
80 87 98 100 84
4.20 7.50 7.25 7.20 6.73
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a
b
Secondary phases
c
Fig. 5 – Microstructures of the alloy aged at 250 °C with different time intervals: (a) 5 h, (b) 10 h, (c) 60 h.
Actually, the deformation mechanism in the forged alloy is also different from that of extruded Mg–Zn–RE alloy. The slight kinking in the forged alloy is attributed to the insufficient and uneven plastic deformation during forging process, which can
Fig. 6 – XRD patterns of the alloy aged at 250 °C: (a) 5 h, (b) 10 h.
be verified from the incomplete DRX during the forging process (Fig. 2c). Due to the insufficient and uneven plastic deformation, the microstructure of the forged alloy is coarse, which leads to relatively lower strength of the forged alloy compared with that of extruded alloy. On the other hand, the strength of the forged alloy is improved obviously in comparison to that of the as-cast alloy in the study. The dispersed distribution of β′ and lamellar LPO structure on the matrix contributes to the high strength of the forged alloy. Indeed, the strengthening mechanism of the forged alloy is controlled by the dispersed distribution of LPO structure and β′ phase as well as refined grains due to the DRX. Amounts of the LPO structure and β′ phase precipitate on the matrix with an increasing ageing time until 10 h during ageing process at 250 °C. According to the previous literature [1], the β′ phase will change to the stable β phase during the ageing process at higher temperature or longer time. Nevertheless, the stable and soft β phase can not be found in the present study due to the short ageing time, which is different from the results of reference [6]. This suggests that only the β′ phase and LPO structure distribute on the matrix in the forged alloy during ageing process. Therefore, the precipitate sequences in the aged alloy are nearly the same with that of the forged alloy, whereas the number of LPO structure and β′ phase on the matrix
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a
b
(1210)
(1210)
(1120)
c
(1210) d
(0110) (1100)
(1210)
(1010)
(1100) (1010) (0110)
(1120) Fig. 7 – TEM images and corresponding SAED pattern of β′ phase in aged alloy at 5 h and 10 h: (a), (b) TEM images of β′ phase at 5 h and 10 h, (c) high resolution TEM image of the β′ phase at 5 h, (d) corresponding SAED pattern of the β′ phase at 5 h, electron beam is parallel to [0001].
increases pronouncedly. The LPO structure with lamellar morphology can provide large resistance to dislocation sliding in α-Mg grain or grain boundary, which obviously promotes the strength of the aged alloy. Therefore, the improvement of mechanical properties during ageing process can be ascribed to the dense distribution of the lamellar LPO structure and β′ phase on the Mg matrix.
5.
Conclusions
The precipitates formed in the Mg–Zn–RE alloy during forging process and ageing process have been studied and some conclusions can be obtained as follows: 1. Only the LPO structure and β′ phase precipitate from the α-Mg(S.S.S.S.) during the forging and ageing processes. The β″ and β can not be found in the forged alloy and peak-aged alloy, which are slightly different from that of extruded Mg– Zn–RE alloy. The parallel precipitate sequences in the forged alloy and aged alloy are: α−MgðS:S:S:SÞ→14 HLPOstructure
α−MgðS:S:S:SÞ→β′ðcbcoÞ: 2. The kinking in the as-forged alloy is less intense than that of extruded Mg–Zn–RE alloy, which results from the uneven and insufficient plastic deformation during the forging process. The strengthening mechanism of the forged alloy is controlled by the dispersed distribution of LPO structure and β′ phase on the matrix and grain refinement due to DRX. 3. Densities of β′ phase and LPO structure precipitate on the matrix with an increasing ageing time until 10 h. The LPO structure and β′ phase are the main strengthening phases in the peak-aged alloy. The improvement of mechanical properties of the alloy during the ageing process is mainly ascribed to the dense distribution of LPO structure and β′ phase on the matrix.
Acknowledgment The authors would like to thank the National Natural Science Foundation of China (no. 51275128). The Analytical and Testing Center of the Harbin Institute of Technology is also appreciated for the sample preparation and useful discussion of the TEM
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a
b
1.8nm HADDF-STEM c
1.8nm
d
1.8nm
Fig. 8 – HADDF-STEM and high resolution TEM images of the lamellar LPO phase in the alloy aged at 10 h: (a) and (c) HADDF-STEM images of lamellar phase, (b) and (d) HRTEM images of lamellar phase. HRTEM shows some periodicities with 14 closed packing planes and the interval of 1.8 nm. HADDF-STEM significantly exhibits Z-contrast of the same interval of 1.8 nm at some places. The fast Fourier transformation (FFT) is inserted in (b). analyses. Also, we would like to give our thanks to the scientists of the reference [16] for providing of the images in Fig. 2.
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