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Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing treatment
Author’s Accepted Manuscript Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing treatment Bizheng Wang, Chuming Liu, Yonghao Gao, Shunong Jiang, Zhiyong Chen, Zhen Luo www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30804-3 http://dx.doi.org/10.1016/j.msea.2017.06.038 MSA35176
To appear in: Materials Science & Engineering A Received date: 13 February 2017 Revised date: 18 May 2017 Accepted date: 10 June 2017 Cite this article as: Bizheng Wang, Chuming Liu, Yonghao Gao, Shunong Jiang, Zhiyong Chen and Zhen Luo, Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing t r e a t m e n t , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing treatment Bizheng Wanga, Chuming Liua, Yonghao Gaoa,b, Shunong Jiangc, Zhiyong Chena, Zhen Luod a
School of Material Science and Engineering, Central South University, Changsha 410083, China
b.
State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083,China c d
School of Civil Engineering, Central South University, Changsha 410083, China
Hunan Nonferrous Metals Vocational And Technical College, Zhuzhou 412006, China *
Corresponding Author: Yonghao Gao,
Email: [email protected]
Abstract: Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) bulk sample with exceptional high strength is successfully fabricated by multidirectional forging (MDF) and subsequent ageing treatment. The microstructure evolution and mechanical properties are investigated in this study. Results show that grain refining process during MDF is closely related to the dynamic recrystallization (DRX) behavior and a microstructure consisting of ultra-fine grains (~ 0.8 μm) and dynamic precipitates of Mg5(GdY) is obtained when forged by 15 passes (Σ ε=4.5). After peak-ageing treatment, densely distributed β' phase precipitates out from the matrix. Ag is mainly segregated in the dynamic precipitates. The forging-T5 sample (15 passes) exhibits a room mechanical property of 391 MPa in YS, 448 MPa in UTS and 3.9 % in elongation to failure. The strength retains 339 MPa in YS and 411 MPa in UTS when tested at 200 ℃. The remarkable mechanical performance can be ascribed to the intensive grain refinement by MDF process, dynamic precipitates as well as precipitate strengthening by β' phase. Keywords: Mg-Gd-Y-Ag-Zr alloy; Multidirectional forging; Ageing; Mechanical property 1. Introduction Due to the urgent demand for weight reduction of aircrafts for better fuel efficiency and economic profit, Mg-Gd-Y series, which is characteristic of low
density, high specific strength and good creep resistance [1, 2] has attracted increasing interest from aerospace industries for applications to various structural components. If with higher strength, it will undoubtedly get more attention from the above-mentioned field. In fact, to be a wrought magnesium alloy with remarkable ageing response, Mg-Gd-Y series is usually strengthened through the combination of hot deformation and subsequent ageing treatment [3-6]. Thus, to obtain higher strength, lots of efforts have been devoted to optimizing the hot working processes [7-9] or/and enhancing the age hardening response [10-13]. Referring to hot working methods, forging is a common one to fabricate large bulk samples. And compared with other forging types [9, 14-16], multidirectional forging (MDF) has greater potential in producing ultra-fine grains [17, 18] and is thus very attractive for fabricating large scale wrought magnesium products with both high strength and ductility. Hence, many researches have been performed and encouraging results are obtained. Xing et.al [19] reported that grains were refined to 0.36 μm in AZ31 processed by MDF at decreasing temperature, combining UTS and elongation of 530 MPa and 15 %, respectively. Miura et.al [20] also obtained a fine-grain structure (0.8 μm) for AZ61, with an UTS of 530 MPa and elongation of 10 %. Contrary to the strong concern for application of MDF to Mg-Al alloys [19-24], few studies concerning MDF has been carried out on Mg-Gd-Y series. Tang et al [25] conducted the pioneering work on Mg-10Gd-4.8Y-0.6Zr alloy and obtained the finest grains with an average size of 9.3 μm after 6 MDF passes, i.e. accumulated strain of 1.8. But further grain refinement and property improvement were restricted due to the frequent intermediate annealing. In an analogous study, a GW94 alloy with submicron equiaxed grains and an elongation of 21.7 % was produced by multi-axial forging (MAF) to an accumulative strain of 3.2 [26], implying that ultra-fine grains, accompanied by substantial property improvement, may also be obtained for Mg-Gd-Y series by large strain MDF process without intermediate annealing. As for approaches to enhance the ageing response of Mg-Gd-Y series,
Ag micro-alloying seems to be a very effective one. In previous works, Ag has been proven to be effective in accelerating precipitation kinetics and enhancing ageing response of ZK60 [27] and AZ80 [28]. Besides, Ag addition also significantly enhances the mechanical properties of Mg-Gd-Y-Zr through forming plate γ′′ as well as promoting the precipitation of β' phase [29, 30]. Aiming to achieve higher strength, this work creatively conducts a combination of large strain MDF process and ageing on Mg-Gd-Y-Ag-Zr alloy. The microstructure evolution and variation of mechanical properties are investigated on emphasis. It may provide guidance in applying MDF to the fabrication of large-sized high performance Mg-RE alloys for industrial use. 2. Experimental procedure Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) alloy ingot with a diameter of 325 mm was fabricated by semi-continuous casting. Cuboids with a dimension of 60 × 45 × 40 mm3 were cut from the central part of the ingot and then solution treated at 525 ℃ for 7 h, followed by quenching into water at room temperature. Multidirectional forging (MDF) was conducted on a 315 t hydraulic press machine with an initial speed of 12.5 mm/s. Prior to forging, both the specimens and anvils were preheated to 475 ℃ and held for 2 h in an electric resistance furnace. A mixture of graphite and oil was used as lubricant to reduce the friction force on the sample/anvil interface.
Fig. 1 Schematic of MDF procedure and sampling: (a) tri-directional forging (b) bio-directional forging (c) sampling position.
Fig. 1 presents the schematic of MDF procedure. Firstly, as shown in Fig. 1a, samples were tri-directionally forged for initial 9 passes and that forged by 1, 3 and 6 passes were reserved and shown here. Then, for samples undergoing 12 or higher passes, they were bio-directionally forged for the final 6 passes (Fig. 1b). The true strain per pass was approximately 0.3 calculated by ε=ln (h0/h1), where h0 and h1 were the height before and after forging. All the samples were quenched into water at room temperature immediately after forging. Ageing treatment was carried out in an air furnace with temperature fluctuation below 1 ℃. Hardness tests were conducted on a Vickers’ Hardness testing machine with a load of 4.9 N and dwelling time of 15 s. To ensure the reliability, at least 8 indentations were made for each test and the average value was reported. Tensile specimens with a gauge dimension of Φ5 25 mm2 were machined from the center of the samples along Z direction. Tensile tests were carried out on an Instron 3369 testing machine at a constant cross head speed of 1 mm/min. For each condition, 3 parallel specimens were tested and the average value was shown here. Specimens for microstructure characterization were machined from the central part of deformed sample by Spark-Erosion Wire Cutting (Fig. 1c), with the observation plane parallel to the final forging direction. The microstructure was examined by Leica optical microscope (OM), FEI Quanta 200F scanning electron microscope (SEM) at 20 kV equipped with an energy dispersive spectroscopy system (EDS) and FEI Tecnai G20 transmission electron microscope (TEM) operated at 200 kV. The phase constitution was detected on Rigaku D/Max 2500 X-ray diffractometer with a CuKα radiation (λ=0.154 nm). Samples for both OM and SEM examination were mechanically ground and polished, followed by etching in a solution of 4.2 g picric acid + 10 ml acetic acid + 70 ml ethanol + 10 ml water. Foils for TEM observation were ground to a thickness of 100 μm, and then thinned to perforation by twin-jet electrolytic polishing.
3.
Results
3.1 Phase constitution of as-cast and as-homogenized samples
Fig. 2 Phase characterization for as-cast and as-homogenized samples: (a) BSE image of as-cast sample (b) BSE image of as-homogenized sample (c) XRD patterns (d) optical micrograph of as-homogenized sample.
The phase constitution and morphology of as-cast and as-homogenized samples are shown in Fig. 2 and the corresponding EDS results are listed in Table 1. For as-cast alloy (Fig. 2a), the microstructure is composed of α-Mg matrix and island-like eutectic compound at grain boundaries. The eutectic phase is detected to be Mg24Y5 type by the peaks marked “■” in XRD pattern (Fig. 2c). Ag is mainly segregated in grain boundary and eutectic phase rather than forming any other Ag-containing new phase. After solution treatment, most eutectic compounds dissolve into matrix and only some bright particles remain at grain boundaries (Fig. 2b). Regarding XRD pattern of as-homogenized sample, peaks representing Mg24Y5 disappear while some new peaks appear, which shows a good consistence with the observation result in Fig. 2b and implies that new peaks marked “▲” correspond to the particles at grain boundaries. From the
EDS result of point D, these bright particles are identified to be RE-rich cuboid-shaped compound, which is considered to occupy an f c c structure and come into being during solution treatment [31, 32]. Equiaxed grains with an average size of 55 µm are obtained after homogenization (Fig. 2d). Table 1. Phase compositions in as-cast and as-homogenized states. Point Mg(wt.%) Gd(wt.%) Y(wt.%) Ag(wt.%) Zr(wt.%) A 94.24 3.06 1.87 0 0.83 B 81.53 12.34 4.57 1.10 0.46 C 67.41 22.88 8.81 0.90 0 D 19.30 36.03 43.86 0 0.81 E 87.87 7.74 3.59 0.32 0.80 3.2 Microstructure evolution during MDF process Fig. 3 exhibits the optical micrographs of samples forged by different passes. After 1 pass deformation (Fig. 3a), initial grains are elongated along the direction perpendicular to the forging direction, and fine grains with an average size of 6 μm appear along the initial grain boundary, showing a typical DRX characteristic. After 3 passes forging (Fig. 3b), some initial grains have been fully invaded by the DRXed ones while the others still keep un-recrystallized, implying that deformation is quite inhomogeneous in each grain under such a strain level. In addition, the average size of fine DRXed grains decreases from 6 μm to 3.4 μm. When it is increased to 6 passes (Fig. 3c), residual coarse grains are divided into smaller parts by shear deformation and the DRXed area is heavily etched. To clarify the microstructure more clearly, results of TEM observation and XRD detection are presented in Fig. 4. The border part between coarse grain and DRXed region is shown in Fig.4a. Different to coarse grain part, fine DRXed grains is surrounded by many dark spherical particles. This may be the reason for the heavily etched morphology of DRXed region shown in optical micrograph. The EDS results of typical parts (Table 2) reveal that Ag is mainly segregated in the dark particle but little found in the DRXed grain. From the comprehensive analysis of XRD pattern (Fig. 4c) and the micro-area diffraction of particle encircled by white dashed line
(inserted in Fig. 4a, B//[011]), these particles are identified to be dynamic precipitates of Mg5(GdY) [33]. For the fully recrystallized region (Fig. 4b), a mixture of ultra-fine grains (~ 0.5 μm) and dynamic precipitates are observed. These precipitates are mainly distributed at the DRXed grain boundaries with varied sizes from 20 nm to 470 nm. Their effect on grain growth will be discussed in the next section. Forged by 15 passes (Fig. 3d), coarse grains are almost fully refined by DRX. The average grain size is appropriately estimated to be 0.8 μm by gates lines transect, showing a remarkable refining effect by MDF process. Meanwhile, dynamic precipitates, which remain to be Mg5(GdY) detected by XRD pattern (Fig.4c), either band along the flowing direction or locate at grain boundaries. After that, the microstructure undergoes little change regardless of increased strain level (Fig. 3e).
Fig. 3 Optical micrograph of samples forged by different passes: (a) 1 passes (b) 3 passes (c) 6 passes (d) 15 passes (e) 18 passes.
Fig. 4 Results of TEM observation and XRD detection for sample forged by 6 passes: (a) TEM observation of border part between coarse grain and DRXed region (b) bright filed image of fully DRXed region (c) XRD patterns.
Table 2. EDS results of points in Fig. 4a. Point Mg(wt.%) Gd(wt.%) Y(wt.%) Ag(wt.%) Zr(wt.%) H
87.7
7.9
4.1
0.3
0
I
43.6
46.6
8.1
1.7
0
J
91.4
6.0
3.5
0.1
0
3.3 Ageing response Fig. 5 shows the age-hardening curve at 200 ℃ for sample forged by 15 passes. The hardness experiences a rapid rise in the initial period of ageing and reaches the peak when aged for 40 h. It increases from 107 HV for initial state to 134 HV for peak-aged state, where an increment by 25% is achieved. After reaching the peak, the hardness undergoes a drop and then keeps stable for a quite long time. Based on the ageing curve, 200 ℃/40 h is selected for peak-ageing treatment. TEM micrographs of peak-aged sample (15 passes) are shown in Fig. 6. From the bright field image at lower magnification (Fig. 6a), ultra-fine grains (below 1 μm) and dynamic precipitates with varied sizes are observed, which confirms the analysis of the microstructure shown in Fig. 3d. Meanwhile, precipitates, formed during ageing treatment, are densely distributed within the whole grain. These precipitates exhibit an elliptical shape and are uniformly distributed along [0001]Mg direction, which are confirmed to be β' phase { c b c o, a=0.64 nm, b=2.22 nm, c=0.52 nm, [001]β′ ∥[0001]Mg , (100)β′ ∥(21̅1̅0)Mg [32]} due to the occurrence of extra spots at 1/4, 1/2 and 3/4 position between (0000)Mg and (011̅0)Mg , as shown by black arrow in Fig. 6c. There is no other phase formed by Ag addition during ageing treatment.
Fig. 5 Ageing curve at 200 ℃ for sample forged by 15 passes.
Fig. 6 TEM observation of peak-aged sample (15 passes): (a) bright field image with low magnification (b) bright field image of ageing precipitation with high magnification (c) corresponding SAED pattern of Fig.6(b), incident beam B//[21̅1̅0]Mg.
3.4 Mechanical properties Fig. 7 shows the engineering stress-strain curves for samples tested at various conditions. The corresponding mechanical properties are listed in Table 3. The YS, UTS and elongation of sample forged by 15 passes are 301 MPa, 366 MPa and 10 %, respectively. Compared to the as-homogenized condition, a substantial enhancement in mechanical properties is obtained after MDF process. Meanwhile, the sample forged by 18 passes exhibits a quite similar mechanical property to that of 15-pass processed one. After peak-ageing treatment, the strength of the 15-passes processed sample further increases to 391 MPa in YS and 448 MPa in UTS, but the elongation decreases to 3.9%. Curves of peak-aged sample (15 passes) tested from RT to 300 ℃ are shown
in Fig. 7b. With increasing temperatures, both YS and UTS decrease. However, it retains 331 MPa in YS, 411 MPa in UTS when tested at 200 ℃, which is much higher than those of many conventional magnesium alloys and some Mg-RE alloys [34, 35]. When it is tested above 200 ℃, the strength dramatically drops while the elongation remarkably increases.
Fig. 7 Engineering stress-strain curves for (a) samples tested at RT (b) peak-aged samples (15 passes) tested from RT to elevated temperatures.
Table 3. Corresponding mechanical properties of samples tested at various conditions (given in parenthesis is the standard deviation). State YS/MPa UTS/MPa Elongation/% As-homogenized 165 (±4) 219 (±3) 5.5 (±0.1) As-forged (15 passes) 301 (±12) 366 (±7) 10.0 (±0.4) As-forged (18 passes) 309 (±3) 364 (±3) 10.2 (±0.8) Peak-aged (tested at RT) 391 (±4) 448 (±4) 3.9 (±0.6) Peak-aged (tested at 200℃) 339 (±2) 411 (±2) 3.7 (±0.4) Peak-aged (tested at 250℃) 251 (±1) 297 (±8) 14.5 (±3.7) Peak-aged (tested at 300℃) 62 (±1) 88 (±4) >100 4. Discussion Due to the poor workability at ambient temperature, wrought magnesium alloys, including Mg-Gd-Y series, are usually deformed at elevated temperature, where large strain can be accommodated by multiple slips of dislocation [36]. Meanwhile, dynamic recrystallization (DRX) behavior, which involves nucleation and growth of new grain, plays a very important role in grain refining during hot deformation and is deeply affected by deformation conditions, such as temperature, strain and strain rate [37, 38]. In the present work, the relative high
temperature suppresses the occurrence of twinning or shear deformation during the initial forging stage. DRX is restricted around the coarse grain boundaries because only these regions provide enough stored energy and pile-up of dislocations for nucleation of new grain. For increased deformation resistance resulted from temperature falling during continued forging, shear deformation throughout the coarse grain largely occurs and provides more interfaces for occurrence of DRX. At the same time, with increased forging passes, the volume fraction of DRXed grains is enhanced while their average size is reduced. The enhanced volume fraction can be attributed to the increased strain level by forging that provides abundant stored energy for nucleation of more grains. With regard to the average size of DRXed grains, Mabuchi et al [39] suggests that it is governed by Zener-Hollomon parameter: 𝑚 Z𝑑𝑟𝑒𝑐 =𝐴
(1) 𝑄
Z = 𝜀̇𝑒𝑥𝑝 ( ) 𝑅𝑇
(2)
where drec is the average size of DRXed grains, A is constant, 𝜀̇ is strain rate, Q is apparent activation energy, R is gas constant and T is absolute temperature. Equations (1) and (2) imply that average size of DRXed grains will decrease when the temperature is declined. Fig.8 exhibits the plot of average DRXed grain size and final forging temperature versus number of forging passes. Obviously, the final forging temperature decreases gradually with increased forging passes, which results in the reduction of DRXed grain size. However, the dramatic drop in average size of samples forged by more than 6 passes can also be related to another reason, i.e. the occurrence of dynamic precipitates (DPs). Xiao et al. [33] reported that these DPs are prone to form within DRXed area under the condition of 300-400 ℃ and continuous strain. And they will have a pinning effect on the migration of grain boundaries, which suppresses the growth of freshly formed DRXed grains. After 15 MDF passes (Ʃ ɛ=4.5), initial grains are almost fully refined by DRX. Since then, the microstructure maintains dynamically stable between work hardening and dynamic recrystallization
softening. And the average size of DRXed grains also keeps unchanged. Similar phenomenon has also been reported elsewhere [21, 40]. To explain this, a second critical value of strain is proposed, beyond which increased strain has little effect on further grain refinement [38]. Obviously, the second critical value in this work is approximately 4.5 at a final forging temperature of ~ 360 ℃.
Fig.8 Plot of average DRXed grain size and final forging temperature versus number of forging passes
As for ageing behavior, a decomposition sequence of S.S.S.S→β′′ (D019) →β' (c b c o) →β1 (f c c) →β (f c c) is proposed for Mg-Gd-Y-Zr alloy [41]. At the initial stage, small spherical β′′ phase uniformly precipitates out within the matrix. Subsequently, lens-like β' phase are formed on {112̅0}Mg plane by a possible isomorphic transformation from β′′ phase. The size and volume fraction of β' phase increase with increasing ageing time, which causes the rapid increase in hardness value. Under peak-ageing condition, β' becomes the predominant strengthening phase (Fig. 6b) and keeps thermally stable at 200 ℃ for a long time. According to recent researches, Ag remarkably enhances the ageing response of Mg-Gd-Y-Zr alloy. Firstly, addition of a new element often reduces the solid solubility of other solute atoms, and thus increases the amount of precipitates. Secondly, Ag promotes the formation of basal plate γ′′ phase, which improves both the mechanical properties [29, 42] and creep resistance [43]. However, it seems not effective in producing new phase by Ag addition in this work, which might be correlated to its segregation characteristic. From the results of Table 2, Ag is largely segregated in dynamic precipitate but little found
in DRXed grain. Similar phenomenon has also been reported in our previous research on AZ80+Ag [44]. Although Ag addition does not produce new phase, it really contributes a lot in promoting precipitation and property improvement. On one hand, segregation of Ag promotes the occurrence of dynamic precipitates during forging, which is of significance in producing ultra-fine grains. On the other hand, compared with the Ag-free alloy, the advanced time for reaching peak of hardness test [29] and close hardening effect [11] regardless of occurrence of dynamic precipitates imply that it also accelerates the precipitation of β' phase during ageing treatment. Compared to as-homogenized sample, an increment of ～140 MPa in strength and ～ 4.5 % in elongation is achieved for as-forged ones. The comprehensive enhancement in mechanical properties can be ascribed to the remarkable grain refinement, occurrence of dynamic precipitates and work hardening from increased dislocation density after MDF process. And the similarity in mechanical properties between samples forged by 15 and 18 passes should be a result of microstructural similarity between them. After peak-ageing treatment, the UTS of peak-aged sample is increased to 448 MPa and a remarkable increment of ～ 229 MPa is achieved compared with that of as-homogenized one. Except for grain refinement and dynamic precipitates, the peak-aged sample is also strengthened by the dense precipitation of β' phase at the expense of diminished work hardening effect during ageing treatment. Thus, the contribution of β' phase in strength improvement is more than 80 MPa. In fact, β' phase on {112̅0}Mg planes acts the most effective barrier to prevent basal dislocation slip [45], and thus intensively strengthens the matrix. However, just for its strong suppression on basal slip, cracks are easy to take place at the interface between matrix and fine precipitates, which causes the sharp decrease in elongation to failure of peak-aged sample. When tested at 200 ℃, these particles are stable enough to retain the high strength. However, they are thought to be less effective in impeding non-basal and cross slip activated at high
temperature, which can be one of the reason of the steep decrease in strength when tested above 200 ℃. 5.
Conclusions In the present work, Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) alloy is subjected
to MDF process and subsequent ageing treatment. The microstructure evolution and corresponding variation of mechanical properties are investigated. The main conclusions have been drawn as follows: (1) DRX behavior plays a vital role in grain refining process during MDF. When the temperature is high for initial MDF passes, DRXed grains only appear along initial grain boundaries and their average size decreases with gradually decreasing temperature. With increasing forging passes, nucleation sites for new grains also extend into interior shear deformation region and the growth up of newly formed DRXed grains is intensely retarded by dynamic precipitates. Ultimately, a homogeneous microstructure consisting of ultra-fine grains and dynamic precipitates of Mg5(GdY) is obtained when forged by 15 passes. (2) The main strengthening phase for peak-aged condition is metastable β' phase. Ag addition does not result in the formation of new phase but contributes a lot to dynamic precipitation during forging and ageing precipitation of β' phase. (3) The YS, UTS and elongation of as-forged and peak-aged samples (15 passes) are 301 MPa, 366 MPa and 10.0 %, 391 MPa, 448 MPa and 3.9 %, respectively. When tested at elevated temperatures, the strength of peak-aged sample drops dramatically while the elongation increases remarkably. However, the strength retains 339 MPa in YS and 411 MPa in UTS when tested at 200 ℃. Acknowledgment This work is supported by National Basic Research Program of China (973 Program) under project Grant no. 2013CB632200. The authors wish to express the sincere appreciation to Prof. Ding Daoyun for providing writing assistance to the article and the anonymous reviewers for supplying helpful comments.
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PII: DOI: Reference:
S0921-5093(17)30804-3 http://dx.doi.org/10.1016/j.msea.2017.06.038 MSA35176
To appear in: Materials Science & Engineering A Received date: 13 February 2017 Revised date: 18 May 2017 Accepted date: 10 June 2017 Cite this article as: Bizheng Wang, Chuming Liu, Yonghao Gao, Shunong Jiang, Zhiyong Chen and Zhen Luo, Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing t r e a t m e n t , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.06.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure evolution and mechanical properties of Mg-Gd-Y-Ag-Zr alloy fabricated by multidirectional forging and ageing treatment Bizheng Wanga, Chuming Liua, Yonghao Gaoa,b, Shunong Jiangc, Zhiyong Chena, Zhen Luod a
School of Material Science and Engineering, Central South University, Changsha 410083, China
b.
State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083,China c d
School of Civil Engineering, Central South University, Changsha 410083, China
Hunan Nonferrous Metals Vocational And Technical College, Zhuzhou 412006, China *
Corresponding Author: Yonghao Gao,
Email: [email protected]
Abstract: Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) bulk sample with exceptional high strength is successfully fabricated by multidirectional forging (MDF) and subsequent ageing treatment. The microstructure evolution and mechanical properties are investigated in this study. Results show that grain refining process during MDF is closely related to the dynamic recrystallization (DRX) behavior and a microstructure consisting of ultra-fine grains (~ 0.8 μm) and dynamic precipitates of Mg5(GdY) is obtained when forged by 15 passes (Σ ε=4.5). After peak-ageing treatment, densely distributed β' phase precipitates out from the matrix. Ag is mainly segregated in the dynamic precipitates. The forging-T5 sample (15 passes) exhibits a room mechanical property of 391 MPa in YS, 448 MPa in UTS and 3.9 % in elongation to failure. The strength retains 339 MPa in YS and 411 MPa in UTS when tested at 200 ℃. The remarkable mechanical performance can be ascribed to the intensive grain refinement by MDF process, dynamic precipitates as well as precipitate strengthening by β' phase. Keywords: Mg-Gd-Y-Ag-Zr alloy; Multidirectional forging; Ageing; Mechanical property 1. Introduction Due to the urgent demand for weight reduction of aircrafts for better fuel efficiency and economic profit, Mg-Gd-Y series, which is characteristic of low
density, high specific strength and good creep resistance [1, 2] has attracted increasing interest from aerospace industries for applications to various structural components. If with higher strength, it will undoubtedly get more attention from the above-mentioned field. In fact, to be a wrought magnesium alloy with remarkable ageing response, Mg-Gd-Y series is usually strengthened through the combination of hot deformation and subsequent ageing treatment [3-6]. Thus, to obtain higher strength, lots of efforts have been devoted to optimizing the hot working processes [7-9] or/and enhancing the age hardening response [10-13]. Referring to hot working methods, forging is a common one to fabricate large bulk samples. And compared with other forging types [9, 14-16], multidirectional forging (MDF) has greater potential in producing ultra-fine grains [17, 18] and is thus very attractive for fabricating large scale wrought magnesium products with both high strength and ductility. Hence, many researches have been performed and encouraging results are obtained. Xing et.al [19] reported that grains were refined to 0.36 μm in AZ31 processed by MDF at decreasing temperature, combining UTS and elongation of 530 MPa and 15 %, respectively. Miura et.al [20] also obtained a fine-grain structure (0.8 μm) for AZ61, with an UTS of 530 MPa and elongation of 10 %. Contrary to the strong concern for application of MDF to Mg-Al alloys [19-24], few studies concerning MDF has been carried out on Mg-Gd-Y series. Tang et al [25] conducted the pioneering work on Mg-10Gd-4.8Y-0.6Zr alloy and obtained the finest grains with an average size of 9.3 μm after 6 MDF passes, i.e. accumulated strain of 1.8. But further grain refinement and property improvement were restricted due to the frequent intermediate annealing. In an analogous study, a GW94 alloy with submicron equiaxed grains and an elongation of 21.7 % was produced by multi-axial forging (MAF) to an accumulative strain of 3.2 [26], implying that ultra-fine grains, accompanied by substantial property improvement, may also be obtained for Mg-Gd-Y series by large strain MDF process without intermediate annealing. As for approaches to enhance the ageing response of Mg-Gd-Y series,
Ag micro-alloying seems to be a very effective one. In previous works, Ag has been proven to be effective in accelerating precipitation kinetics and enhancing ageing response of ZK60 [27] and AZ80 [28]. Besides, Ag addition also significantly enhances the mechanical properties of Mg-Gd-Y-Zr through forming plate γ′′ as well as promoting the precipitation of β' phase [29, 30]. Aiming to achieve higher strength, this work creatively conducts a combination of large strain MDF process and ageing on Mg-Gd-Y-Ag-Zr alloy. The microstructure evolution and variation of mechanical properties are investigated on emphasis. It may provide guidance in applying MDF to the fabrication of large-sized high performance Mg-RE alloys for industrial use. 2. Experimental procedure Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) alloy ingot with a diameter of 325 mm was fabricated by semi-continuous casting. Cuboids with a dimension of 60 × 45 × 40 mm3 were cut from the central part of the ingot and then solution treated at 525 ℃ for 7 h, followed by quenching into water at room temperature. Multidirectional forging (MDF) was conducted on a 315 t hydraulic press machine with an initial speed of 12.5 mm/s. Prior to forging, both the specimens and anvils were preheated to 475 ℃ and held for 2 h in an electric resistance furnace. A mixture of graphite and oil was used as lubricant to reduce the friction force on the sample/anvil interface.
Fig. 1 Schematic of MDF procedure and sampling: (a) tri-directional forging (b) bio-directional forging (c) sampling position.
Fig. 1 presents the schematic of MDF procedure. Firstly, as shown in Fig. 1a, samples were tri-directionally forged for initial 9 passes and that forged by 1, 3 and 6 passes were reserved and shown here. Then, for samples undergoing 12 or higher passes, they were bio-directionally forged for the final 6 passes (Fig. 1b). The true strain per pass was approximately 0.3 calculated by ε=ln (h0/h1), where h0 and h1 were the height before and after forging. All the samples were quenched into water at room temperature immediately after forging. Ageing treatment was carried out in an air furnace with temperature fluctuation below 1 ℃. Hardness tests were conducted on a Vickers’ Hardness testing machine with a load of 4.9 N and dwelling time of 15 s. To ensure the reliability, at least 8 indentations were made for each test and the average value was reported. Tensile specimens with a gauge dimension of Φ5 25 mm2 were machined from the center of the samples along Z direction. Tensile tests were carried out on an Instron 3369 testing machine at a constant cross head speed of 1 mm/min. For each condition, 3 parallel specimens were tested and the average value was shown here. Specimens for microstructure characterization were machined from the central part of deformed sample by Spark-Erosion Wire Cutting (Fig. 1c), with the observation plane parallel to the final forging direction. The microstructure was examined by Leica optical microscope (OM), FEI Quanta 200F scanning electron microscope (SEM) at 20 kV equipped with an energy dispersive spectroscopy system (EDS) and FEI Tecnai G20 transmission electron microscope (TEM) operated at 200 kV. The phase constitution was detected on Rigaku D/Max 2500 X-ray diffractometer with a CuKα radiation (λ=0.154 nm). Samples for both OM and SEM examination were mechanically ground and polished, followed by etching in a solution of 4.2 g picric acid + 10 ml acetic acid + 70 ml ethanol + 10 ml water. Foils for TEM observation were ground to a thickness of 100 μm, and then thinned to perforation by twin-jet electrolytic polishing.
3.
Results
3.1 Phase constitution of as-cast and as-homogenized samples
Fig. 2 Phase characterization for as-cast and as-homogenized samples: (a) BSE image of as-cast sample (b) BSE image of as-homogenized sample (c) XRD patterns (d) optical micrograph of as-homogenized sample.
The phase constitution and morphology of as-cast and as-homogenized samples are shown in Fig. 2 and the corresponding EDS results are listed in Table 1. For as-cast alloy (Fig. 2a), the microstructure is composed of α-Mg matrix and island-like eutectic compound at grain boundaries. The eutectic phase is detected to be Mg24Y5 type by the peaks marked “■” in XRD pattern (Fig. 2c). Ag is mainly segregated in grain boundary and eutectic phase rather than forming any other Ag-containing new phase. After solution treatment, most eutectic compounds dissolve into matrix and only some bright particles remain at grain boundaries (Fig. 2b). Regarding XRD pattern of as-homogenized sample, peaks representing Mg24Y5 disappear while some new peaks appear, which shows a good consistence with the observation result in Fig. 2b and implies that new peaks marked “▲” correspond to the particles at grain boundaries. From the
EDS result of point D, these bright particles are identified to be RE-rich cuboid-shaped compound, which is considered to occupy an f c c structure and come into being during solution treatment [31, 32]. Equiaxed grains with an average size of 55 µm are obtained after homogenization (Fig. 2d). Table 1. Phase compositions in as-cast and as-homogenized states. Point Mg(wt.%) Gd(wt.%) Y(wt.%) Ag(wt.%) Zr(wt.%) A 94.24 3.06 1.87 0 0.83 B 81.53 12.34 4.57 1.10 0.46 C 67.41 22.88 8.81 0.90 0 D 19.30 36.03 43.86 0 0.81 E 87.87 7.74 3.59 0.32 0.80 3.2 Microstructure evolution during MDF process Fig. 3 exhibits the optical micrographs of samples forged by different passes. After 1 pass deformation (Fig. 3a), initial grains are elongated along the direction perpendicular to the forging direction, and fine grains with an average size of 6 μm appear along the initial grain boundary, showing a typical DRX characteristic. After 3 passes forging (Fig. 3b), some initial grains have been fully invaded by the DRXed ones while the others still keep un-recrystallized, implying that deformation is quite inhomogeneous in each grain under such a strain level. In addition, the average size of fine DRXed grains decreases from 6 μm to 3.4 μm. When it is increased to 6 passes (Fig. 3c), residual coarse grains are divided into smaller parts by shear deformation and the DRXed area is heavily etched. To clarify the microstructure more clearly, results of TEM observation and XRD detection are presented in Fig. 4. The border part between coarse grain and DRXed region is shown in Fig.4a. Different to coarse grain part, fine DRXed grains is surrounded by many dark spherical particles. This may be the reason for the heavily etched morphology of DRXed region shown in optical micrograph. The EDS results of typical parts (Table 2) reveal that Ag is mainly segregated in the dark particle but little found in the DRXed grain. From the comprehensive analysis of XRD pattern (Fig. 4c) and the micro-area diffraction of particle encircled by white dashed line
(inserted in Fig. 4a, B//[011]), these particles are identified to be dynamic precipitates of Mg5(GdY) [33]. For the fully recrystallized region (Fig. 4b), a mixture of ultra-fine grains (~ 0.5 μm) and dynamic precipitates are observed. These precipitates are mainly distributed at the DRXed grain boundaries with varied sizes from 20 nm to 470 nm. Their effect on grain growth will be discussed in the next section. Forged by 15 passes (Fig. 3d), coarse grains are almost fully refined by DRX. The average grain size is appropriately estimated to be 0.8 μm by gates lines transect, showing a remarkable refining effect by MDF process. Meanwhile, dynamic precipitates, which remain to be Mg5(GdY) detected by XRD pattern (Fig.4c), either band along the flowing direction or locate at grain boundaries. After that, the microstructure undergoes little change regardless of increased strain level (Fig. 3e).
Fig. 3 Optical micrograph of samples forged by different passes: (a) 1 passes (b) 3 passes (c) 6 passes (d) 15 passes (e) 18 passes.
Fig. 4 Results of TEM observation and XRD detection for sample forged by 6 passes: (a) TEM observation of border part between coarse grain and DRXed region (b) bright filed image of fully DRXed region (c) XRD patterns.
Table 2. EDS results of points in Fig. 4a. Point Mg(wt.%) Gd(wt.%) Y(wt.%) Ag(wt.%) Zr(wt.%) H
87.7
7.9
4.1
0.3
0
I
43.6
46.6
8.1
1.7
0
J
91.4
6.0
3.5
0.1
0
3.3 Ageing response Fig. 5 shows the age-hardening curve at 200 ℃ for sample forged by 15 passes. The hardness experiences a rapid rise in the initial period of ageing and reaches the peak when aged for 40 h. It increases from 107 HV for initial state to 134 HV for peak-aged state, where an increment by 25% is achieved. After reaching the peak, the hardness undergoes a drop and then keeps stable for a quite long time. Based on the ageing curve, 200 ℃/40 h is selected for peak-ageing treatment. TEM micrographs of peak-aged sample (15 passes) are shown in Fig. 6. From the bright field image at lower magnification (Fig. 6a), ultra-fine grains (below 1 μm) and dynamic precipitates with varied sizes are observed, which confirms the analysis of the microstructure shown in Fig. 3d. Meanwhile, precipitates, formed during ageing treatment, are densely distributed within the whole grain. These precipitates exhibit an elliptical shape and are uniformly distributed along [0001]Mg direction, which are confirmed to be β' phase { c b c o, a=0.64 nm, b=2.22 nm, c=0.52 nm, [001]β′ ∥[0001]Mg , (100)β′ ∥(21̅1̅0)Mg [32]} due to the occurrence of extra spots at 1/4, 1/2 and 3/4 position between (0000)Mg and (011̅0)Mg , as shown by black arrow in Fig. 6c. There is no other phase formed by Ag addition during ageing treatment.
Fig. 5 Ageing curve at 200 ℃ for sample forged by 15 passes.
Fig. 6 TEM observation of peak-aged sample (15 passes): (a) bright field image with low magnification (b) bright field image of ageing precipitation with high magnification (c) corresponding SAED pattern of Fig.6(b), incident beam B//[21̅1̅0]Mg.
3.4 Mechanical properties Fig. 7 shows the engineering stress-strain curves for samples tested at various conditions. The corresponding mechanical properties are listed in Table 3. The YS, UTS and elongation of sample forged by 15 passes are 301 MPa, 366 MPa and 10 %, respectively. Compared to the as-homogenized condition, a substantial enhancement in mechanical properties is obtained after MDF process. Meanwhile, the sample forged by 18 passes exhibits a quite similar mechanical property to that of 15-pass processed one. After peak-ageing treatment, the strength of the 15-passes processed sample further increases to 391 MPa in YS and 448 MPa in UTS, but the elongation decreases to 3.9%. Curves of peak-aged sample (15 passes) tested from RT to 300 ℃ are shown
in Fig. 7b. With increasing temperatures, both YS and UTS decrease. However, it retains 331 MPa in YS, 411 MPa in UTS when tested at 200 ℃, which is much higher than those of many conventional magnesium alloys and some Mg-RE alloys [34, 35]. When it is tested above 200 ℃, the strength dramatically drops while the elongation remarkably increases.
Fig. 7 Engineering stress-strain curves for (a) samples tested at RT (b) peak-aged samples (15 passes) tested from RT to elevated temperatures.
Table 3. Corresponding mechanical properties of samples tested at various conditions (given in parenthesis is the standard deviation). State YS/MPa UTS/MPa Elongation/% As-homogenized 165 (±4) 219 (±3) 5.5 (±0.1) As-forged (15 passes) 301 (±12) 366 (±7) 10.0 (±0.4) As-forged (18 passes) 309 (±3) 364 (±3) 10.2 (±0.8) Peak-aged (tested at RT) 391 (±4) 448 (±4) 3.9 (±0.6) Peak-aged (tested at 200℃) 339 (±2) 411 (±2) 3.7 (±0.4) Peak-aged (tested at 250℃) 251 (±1) 297 (±8) 14.5 (±3.7) Peak-aged (tested at 300℃) 62 (±1) 88 (±4) >100 4. Discussion Due to the poor workability at ambient temperature, wrought magnesium alloys, including Mg-Gd-Y series, are usually deformed at elevated temperature, where large strain can be accommodated by multiple slips of dislocation [36]. Meanwhile, dynamic recrystallization (DRX) behavior, which involves nucleation and growth of new grain, plays a very important role in grain refining during hot deformation and is deeply affected by deformation conditions, such as temperature, strain and strain rate [37, 38]. In the present work, the relative high
temperature suppresses the occurrence of twinning or shear deformation during the initial forging stage. DRX is restricted around the coarse grain boundaries because only these regions provide enough stored energy and pile-up of dislocations for nucleation of new grain. For increased deformation resistance resulted from temperature falling during continued forging, shear deformation throughout the coarse grain largely occurs and provides more interfaces for occurrence of DRX. At the same time, with increased forging passes, the volume fraction of DRXed grains is enhanced while their average size is reduced. The enhanced volume fraction can be attributed to the increased strain level by forging that provides abundant stored energy for nucleation of more grains. With regard to the average size of DRXed grains, Mabuchi et al [39] suggests that it is governed by Zener-Hollomon parameter: 𝑚 Z𝑑𝑟𝑒𝑐 =𝐴
(1) 𝑄
Z = 𝜀̇𝑒𝑥𝑝 ( ) 𝑅𝑇
(2)
where drec is the average size of DRXed grains, A is constant, 𝜀̇ is strain rate, Q is apparent activation energy, R is gas constant and T is absolute temperature. Equations (1) and (2) imply that average size of DRXed grains will decrease when the temperature is declined. Fig.8 exhibits the plot of average DRXed grain size and final forging temperature versus number of forging passes. Obviously, the final forging temperature decreases gradually with increased forging passes, which results in the reduction of DRXed grain size. However, the dramatic drop in average size of samples forged by more than 6 passes can also be related to another reason, i.e. the occurrence of dynamic precipitates (DPs). Xiao et al. [33] reported that these DPs are prone to form within DRXed area under the condition of 300-400 ℃ and continuous strain. And they will have a pinning effect on the migration of grain boundaries, which suppresses the growth of freshly formed DRXed grains. After 15 MDF passes (Ʃ ɛ=4.5), initial grains are almost fully refined by DRX. Since then, the microstructure maintains dynamically stable between work hardening and dynamic recrystallization
softening. And the average size of DRXed grains also keeps unchanged. Similar phenomenon has also been reported elsewhere [21, 40]. To explain this, a second critical value of strain is proposed, beyond which increased strain has little effect on further grain refinement [38]. Obviously, the second critical value in this work is approximately 4.5 at a final forging temperature of ~ 360 ℃.
Fig.8 Plot of average DRXed grain size and final forging temperature versus number of forging passes
As for ageing behavior, a decomposition sequence of S.S.S.S→β′′ (D019) →β' (c b c o) →β1 (f c c) →β (f c c) is proposed for Mg-Gd-Y-Zr alloy [41]. At the initial stage, small spherical β′′ phase uniformly precipitates out within the matrix. Subsequently, lens-like β' phase are formed on {112̅0}Mg plane by a possible isomorphic transformation from β′′ phase. The size and volume fraction of β' phase increase with increasing ageing time, which causes the rapid increase in hardness value. Under peak-ageing condition, β' becomes the predominant strengthening phase (Fig. 6b) and keeps thermally stable at 200 ℃ for a long time. According to recent researches, Ag remarkably enhances the ageing response of Mg-Gd-Y-Zr alloy. Firstly, addition of a new element often reduces the solid solubility of other solute atoms, and thus increases the amount of precipitates. Secondly, Ag promotes the formation of basal plate γ′′ phase, which improves both the mechanical properties [29, 42] and creep resistance [43]. However, it seems not effective in producing new phase by Ag addition in this work, which might be correlated to its segregation characteristic. From the results of Table 2, Ag is largely segregated in dynamic precipitate but little found
in DRXed grain. Similar phenomenon has also been reported in our previous research on AZ80+Ag [44]. Although Ag addition does not produce new phase, it really contributes a lot in promoting precipitation and property improvement. On one hand, segregation of Ag promotes the occurrence of dynamic precipitates during forging, which is of significance in producing ultra-fine grains. On the other hand, compared with the Ag-free alloy, the advanced time for reaching peak of hardness test [29] and close hardening effect [11] regardless of occurrence of dynamic precipitates imply that it also accelerates the precipitation of β' phase during ageing treatment. Compared to as-homogenized sample, an increment of ～140 MPa in strength and ～ 4.5 % in elongation is achieved for as-forged ones. The comprehensive enhancement in mechanical properties can be ascribed to the remarkable grain refinement, occurrence of dynamic precipitates and work hardening from increased dislocation density after MDF process. And the similarity in mechanical properties between samples forged by 15 and 18 passes should be a result of microstructural similarity between them. After peak-ageing treatment, the UTS of peak-aged sample is increased to 448 MPa and a remarkable increment of ～ 229 MPa is achieved compared with that of as-homogenized one. Except for grain refinement and dynamic precipitates, the peak-aged sample is also strengthened by the dense precipitation of β' phase at the expense of diminished work hardening effect during ageing treatment. Thus, the contribution of β' phase in strength improvement is more than 80 MPa. In fact, β' phase on {112̅0}Mg planes acts the most effective barrier to prevent basal dislocation slip [45], and thus intensively strengthens the matrix. However, just for its strong suppression on basal slip, cracks are easy to take place at the interface between matrix and fine precipitates, which causes the sharp decrease in elongation to failure of peak-aged sample. When tested at 200 ℃, these particles are stable enough to retain the high strength. However, they are thought to be less effective in impeding non-basal and cross slip activated at high
temperature, which can be one of the reason of the steep decrease in strength when tested above 200 ℃. 5.
Conclusions In the present work, Mg-8.0Gd-3.7Y-0.3Ag-0.4Zr (wt. %) alloy is subjected
to MDF process and subsequent ageing treatment. The microstructure evolution and corresponding variation of mechanical properties are investigated. The main conclusions have been drawn as follows: (1) DRX behavior plays a vital role in grain refining process during MDF. When the temperature is high for initial MDF passes, DRXed grains only appear along initial grain boundaries and their average size decreases with gradually decreasing temperature. With increasing forging passes, nucleation sites for new grains also extend into interior shear deformation region and the growth up of newly formed DRXed grains is intensely retarded by dynamic precipitates. Ultimately, a homogeneous microstructure consisting of ultra-fine grains and dynamic precipitates of Mg5(GdY) is obtained when forged by 15 passes. (2) The main strengthening phase for peak-aged condition is metastable β' phase. Ag addition does not result in the formation of new phase but contributes a lot to dynamic precipitation during forging and ageing precipitation of β' phase. (3) The YS, UTS and elongation of as-forged and peak-aged samples (15 passes) are 301 MPa, 366 MPa and 10.0 %, 391 MPa, 448 MPa and 3.9 %, respectively. When tested at elevated temperatures, the strength of peak-aged sample drops dramatically while the elongation increases remarkably. However, the strength retains 339 MPa in YS and 411 MPa in UTS when tested at 200 ℃. Acknowledgment This work is supported by National Basic Research Program of China (973 Program) under project Grant no. 2013CB632200. The authors wish to express the sincere appreciation to Prof. Ding Daoyun for providing writing assistance to the article and the anonymous reviewers for supplying helpful comments.
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