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Development of extraordinary high-strength-toughness Mg alloy via combined processes of repeated plastic working and hot extrusion
Materials Science & Engineering A 573 (2013) 127–131
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Development of extraordinary high-strength-toughness Mg alloy via combined processes of repeated plastic working and hot extrusion Ke Liu a, Xudong Wang a,b, Wenbo Du a,n a b
School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Beijing Institute of Aeronautical Materials, Beijing 10085, China
a r t i c l e i n f o
abstract
Article history: Received 17 October 2012 Received in revised form 12 February 2013 Accepted 15 February 2013 Available online 24 February 2013
An extraordinary high-strength-toughness Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy has been successfully fabricated by 400 cycles of repeated plastic working process followed by hot extrusion. The sample exhibited an ultimate tensile strength of 500 MPa, tensile yield strength of 455 MPa, companying with a failure elongation of 12.0% at room temperature. The significant enhancement in mechanical properties was attributed to the grain refinement, homogeneous b phase and modified texture. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mg–Gd–Er–Zr alloy Repeated plastic working Severe plastic deformation Ultrafine grain Dynamic precipitate
1. Introduction Magnesium alloys have recently received a great deal of attention because of their low density, high specific strength and recycle. However, compared with traditional steel and Al alloys, the Mg alloys often display poor yield tensile strength (YTS) as well as relatively low elongation (E) at room temperature. Therefore, much work has been done to improve YTS and E simultaneously. Grain refinement is one of the most effective ways to improve YTS and E. For example, the highest-strength magnesium alloy of the Mg97Zn1Y2 alloy produced by a rapidly solidified powder metallurgy (RS P/M) technique is partly attributed to nanometer-scale grains [1,2]. Certainly, the presence of the highest-strength Mg alloy in the wide world displayed a higher specific strength than those of the conventional Ti-6Al-4V alloy and 7075-T6 aluminum alloy, which also promotes our enthusiasm to develop high-performance Mg-RE based alloys by means of other new and effective approach that improves the combination of tensile strength and elongation. Comparatively, these thermal mechanical process, such as hot extrusion (HE), rolling (R) and equal channel angular pressing (ECAP) often conducted to refine grain size, are more attractive in the fields of industry application comparing with the RS P/M processing. In general, the thermal–mechanical processing often leads to a great promotion in YTS via introducing refinement of
n
Corresponding author. Tel./fax: þ 86 10 67392917. E-mail address: [email protected] (W. Du).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.030
grains mainly attributed to the dynamic recrystallization (DRX) [3], relative strong texture [4] and precipitation [5]. Though the presence of the strong texture is beneficial to improve the YTS effectively, the unexpected plastic anisotropy is also induced [6]. For example, a high-strength Mg–14Gd–0.5Zr (wt%) alloy (HEþR) has been reported recently in literature [7], and it indicates that the YTS increased but the E conversely decreases as the intensity of the texture increases. Previously, the modification of texture has been pointed out by Mukai et al. [8], Hantzsche et al. [6], Cottam et al. [9] and Huang et al. [10], which is of importance to improve the E by eliminating plastic anisotropy. Thus, an effective method which can combine ultrafine grain size with modification of texture is a critical issue to improve YTS and E simultaneously [11]. Therefore, an investigation on how to produce a high-strengthtoughness alloy via combining the ultrafine grain size and modification of texture is essential at present. In the present investigation, we report an extraordinary high-strength-toughness Mg alloy of Mg–11.8Gd–1.9Er–0.4Zr (wt%) produced by repeated plastic working (RPW) processing followed by subsequent hot extrusion. The RPW process as one of severe plastic deformation (SPD) methods provided more attractive and meaningful results. The YTS and E of the alloy are 455 MPa and 12%, respectively. The significant enhancement in mechanical properties is attributed to the refinement of grains (the microstructure is dominated by high-angle grain boundaries and average grain size is 0.5 mm), uniform distribution of b phase and modified texture (max intensity of texture 2.5).
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K. Liu et al. / Materials Science & Engineering A 573 (2013) 127–131
and 4.7%, respectively. Moreover, Homma et al. [15] developed a new attractive extruded alloy of Mg–10Gd–5.7Y–1.62Zn–0.65Zr (wt%) with YTS and E of 419 MPa and 3.6%, respectively. Obviously, as far as the tensile properties (YTS and E) are concerned, the values of our present work are tremendously higher than those of the alloys referred above (see Table 1).
2. Experimental procedures The nominal composition of the Mg–11.8Gd–1.9Er–0.4Zr alloy was first produced by gravity casting, and then homogenized at 460 1C for 6 h. Then, the oxidized surface of the alloy was removed by a sander machine. The clean block alloy was machined into chips with a size of 1–5 mm by a storage tank crusher. The chips were compacted and mixed via the RPW machine in order to produce a compact with a diameter of 35 mm at room temperature. The RPW machine has two kinds of punches. One is called flat punch; the other is named taper punch [12]. One complete cycle of RPW process is illustrated in reference [12]. In the present paper, the compact was obtained via treated by 400 cycles of RPW process at room temperature. The loads of the punch will be adjusted automatically based on the properties of materials. In our investigation, the load of the flat punch is less than 40 t while the load of the taper punch is less than 10 t. At last, the compact was hot extruded into a rod with a diameter of 9 mm indirectly at 340 1C. The extruded rod was machined into dog-bone geometry with a gauge length of 25 mm and a diameter of 5 mm. The tensile axis was parallel to the extrusion direction (ED) at a stretching rate of 1 mm/min at room temperature. The microstructure of the extruded alloy was conducted by using an optical microscope (OM, Zeiss-Imager.A2m), transmission electron microscope (TEM, JEM-2100, JEOL) at 200 kV equipped with selected area electron diffraction (SAED) and X-ray energy dispersive (EDS). The specimens for OM observation were mechanically polished and etched in a solution of 4 ml nitric acid and 96 ml ethanol. The specimens for TEM observation were prepared by electro-polishing and ion beam milling. Macrotexture measurement was carried out on the X-ray diffractometer using reflection geometry and CuKa radiation. The orientation distribution function (ODF) was obtained from experimental pole figures data by using the series expansion method.
3.2. Microstructure Fig. 1 shows OM images (transverse section) of the Mg–11.8Gd–1.9Er–0.4Zr alloy fabricated by 400 cycles of RPW process followed by hot extrusion. It is indicated that significant strain contrasts are observed, which are introduced by RPW process. Some parts with white contrast are the areas suffered from straining lightly, as shown in Fig. 1a. Lots of tiny particles are observed, as shown in Fig. 1b. It is not found the clear grain boundaries, and the grain is not observed by the OM method. In order to investigate the microstructure of the alloy, the TEM observation has been carried out. TEM observation, as shown in Fig. 2, was carried out to investigate the strengthening mechanism of the present alloy. Evidently, the alloy is composed of fine grains as well as tiny precipitates. The value of the average grain size is 500 nm, and it is considered as ultrafine grains (UGs) which are further confirmed by SAED pattern. It is well known that, indicated by the Hall–Petch relationship, such fine grains in the case will lead to a high proof strength [16]. Additionally, it is obviously found that lots of tiny precipitates with the same morphology of ellipsoid are mainly located at the triple grain boundaries, and the size of the precipitates is in the range 50–100 nm. The same precipitates in morphology has been observed in the Mg–10Gd–5.7Y–1.62Zn– 0.65Zr alloy produced by HE [15], Mg–12Gd–3Y–0.5Zr alloy produced by ECAP [17], Mg–8.57Gd–3.72Y–0.54Zr alloy fabricated by multiaxial forging (MAF) [18]. The presence of the uniform and dense nanoscale precipitates played an important role in improving tensile strength significantly [15,18,19]. Fig. 3 shows the TEM images of the grains and grain boundaries at a higher magnification. It finds predominantly high-angle grain boundaries (HAGBs, grain boundary misorientations Z151), which has an excellent relatively thermal stability and results in further high ductility greatly via enhancing the mobility of grain boundaries [20,21]. Generally, a well combination of UGs and HAGBs is difficult to be obtained due to high processing temperature (4250 1C) and DRX [17,22,23]. Especially, the abundant metastable subgrains unfortunately produced by DRX and left in the wrought Mg alloy are harmful to mechanical properties [24,25]. However, from the scientific point of view, our present work displayed a meaningful result that the excellent combination of UGs and HAGBs could be achieved via a special method, such as RPW process followed by subsequent HE referred in the present investigation, which played an important role in improving mechanical properties.
3. Results and discussion 3.1. Mechanical properties Table 1 shows the tensile properties of the Mg–11.8Gd–1.9Er– 0.4Zr alloy (400 cycles of RPW process þ extruded). The values of the YTS and E are 455 MPa and 12%, respectively, which is substantially higher than those reported before in literatures [7,11,13–15]. As reported, Rokhlin [13] produced the Mg– 25.6Gd (wt%) alloy (castþ T6) with a YTS of 320 MPa and an elongation of 0.4%. Anyanwu et al. [14] developed an Mg– 17Gd–0.51Zr alloy (RþT5) companying with YTS of 360 MPa and an E of 4%. Recently, Li [7] produced a Mg–14Gd–0.5Zr (wt%) alloy (HE þR þT5) which had a high YTS of 445 MPa with an E of 2%. In reference [11], the Mg–10Gd–2Y–0.5Zr (wt%) alloy (castþT6) was produced and the YTS and E were 239 MPa Table 1 Comparative tensile properties and states in Mg alloys with similar compositions. Alloys (wt%)
States
Grain size (mm)
YTS (MPa)
UTS (MPa)
E (%)
Mg–14Gd–0.5Zr [7] Mg–14Gd–0.5Zr [7] Mg–4.2Y–3.4RE–0.45Zr [10] Mg–10Gd–2Y–0.5Zr [11] Mg–25.6Gd [13] Mg–12Gd–1.9Y–0.69Zr [14] Mg–9.3Gd–4.1Y–0.7Zr [14] Mg–10Gd–5.66Y–1.62Zn–0.65Zr [15] Mg–10Gd–5.66Y–1.62Zn–0.65Zr [15] Mg–11.3Gd–3.8Y–0.7Zr [27] Mg–11.8Gd–1.9Er–0.4Zr (present work)
HEþ R HEþ Rþ T5 HEþ Rþ T5 Cast þT6 HE RþT5 RþT5 HE HEþ T5 Cast þT6 RPWþ HE
20 20 25 43 – 100 100 0.9 1.1 – 0.5
305 445 141 239 320 360 290 419 473 300 455
375 482 240 362 353 400 310 461 542 330 500
3.5 2.0 22 4.7 0.4 5.0 16.0 3.6 8.0 2.0 12.0
K. Liu et al. / Materials Science & Engineering A 573 (2013) 127–131
The achievements of UGs and HAGBs may have a close relationship with the presence of the tiny precipitates at the triple grain boundaries. Therefore, a further investigation was carried out in order to investigate the tiny precipitates, as shown in Fig. 4. On the basis of the SAED patterns, it is found that the lattice constant of the precipitate is a ¼2.24 nm, a ¼ b ¼ g ¼901,
Fig. 1. OM images of the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion.
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and it belongs to the F4¯3m space group. Besides, the average composition of the precipitate is Mg86.18(Gd, Er)13.82, which is close to the equilibrium b phase (Mg5Gd, F_4_43m, a¼2.23 nm [15,26]). Similarly, as reported by Homma et al. [15], the equilibrium b phase has been observed in his investigation, and the b phase has a great effect on suppressing grain growths significantly during hot extrusion. Compared with the equilibrium b phase in literature [15], the equilibrium b phase in our
Fig. 3. TEM images of the grain boundaries obtained from the Mg–11.8Gd–1.9Er– 0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion: (a) Triple grain boundary and (b) high angle grain boundary between grain 1 and grain 2.
Fig. 2. TEM image obtained from the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion: (a) corresponding SAED pattern (b) and simulated scheme of the SAED pattern (c).
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investigation is the same in terms of morphology and distribution as those of the b phase referred in literature [15] besides the size, and the size in our investigation is relatively smaller. Frankly, the presence of the equilibrium b phase was unexpected in our investigation, which was also rarely reported previously. However, the present investigation indicated that it seems to be
Fig. 4. TEM images of the b phase observed from the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion and corresponding SAED pattern (B//[001]) (a) and interface between the matrix and the b phase (b).
more meaningful to control of the precipitation of the equilibrium b phase than that of the b00 phase, b0 phase and b1 phase observed in contribution [27–31] due to their low thermal stability. 3.3. Texture It is well known that a weak texture as well as UGs leads to a delay failure, which is attributed to an enhancement of activity of /c þaS slip [8,9,32]. Therefore, much work has been done on a combination of fine grain size and weak texture [33]. However, it is likely that acquisition of UGs and dispersed texture in an Mg alloy at the same time is difficult. For example, a strong (Max¼5.4) basal fiber texture in the extruded Mg–10Gd–5.7Y– 1.62Zn–0.65Zr (wt%) alloy (average grain size of 0.9 mm) [15], a sharp basal texture (Max ¼9.2) in the rolled Mg–14Gd–0.5Zr (wt%) alloy (average grain size of 20 mm) [7] and an intense basal texture (Max¼7.4) in the rolled Mg–0.73Y(wt%) alloy [6]. The strong texture directly resulted in a high plastic anisotropy in the alloys as stated above, in which therefore the elongation was inferior, such as 3.6% [15] and 2.0% [7]. In addition, a rolled Mg alloy after annealed treatment displayed a high elongation ( 22%) due to randomization of basal texture (Max¼2.4), while the YTS ( 141 MPa) decreased significantly due to grain growth ( 25 mm) [34]. Meanwhile, in our investigation, the texture intensity of the present alloy is weakened compared with those of alloys reported previously, and the max value is no more than 2.5. Therefore, it is interesting that the alloy produced by RPW process followed by hot extrusion seems to obtain ultrafine grainstructure as well as weak texture intensity value simultaneously, which accounts for the high ductility of the present alloy. In order to investigate the transformation of the macrotexture under different RPW cycles, the texture data obtained from the alloys treated by 0, 100, 200, 300 and 400 cycles of RPW processing followed by hot extrusion are present in the form of an threedimensional orientation distribution function (ODF), as shown in Fig. 5. It is indicated that the orientation distribution of the alloy without RPW process before hot extrusion is mainly at (301, 601, 901), while the orientation distribution of the alloy under 100 cycles of RPW process prior to hot extrusion is predominantly present at (01, 601, 451) and (01, 601, 1051). Furthermore, the distribution of orientation is inclined to random with further increment of cycles of RPW process. The orientation distribution of the alloy treated by 200 cycles of RPW process is mostly at (01, 601, 151), (01, 601, 451),
Fig. 5. ODF obtained from the Mg–11.8Gd–1.9Er–0.4Zr alloy treated by different cycles of RPW processing followed by hot extrusion.
K. Liu et al. / Materials Science & Engineering A 573 (2013) 127–131
(01, 601, 751) and (01, 601, 1051), respectively. As the RPW process cycle increases to 300 and 400, the orientation distribution is more dispersed. Weakening textures of the Mg alloys will lead to intrinsic plastic isotropy and high ductility [33], which is beneficial to improve the formability of the alloys [35,36]. The activation of the non-basal slip may responsible for the increment of the elongation [6,8,9]. The weak and dispersed texture, which is ascribed to the DRX, of the present alloy may be partly responsible for the high ductility. However, the mechanism responsible for refinement of grains and DRX is so far not clear, and the further work is now being conducted. Besides, as stated above, as well as modification of the texture, the ultrafine grain-structure of the alloy is one of the important factors for improving ductility via introducing more grain boundaries, dispersing stress concentration and activating non-basal slip [23,33,37,38]. Eventually, combining weak-dispersed texture and UGs, a high-strengthtoughness Mg–11.8Gd–1.9Er–0.4Zr alloy, with relatively lower content of RE but high tensile properties compared with the alloys in literature [7,15], was produced by RPW processing followed by subsequent hot extrusion, with a high UTS, YTS and E of 500 MPa, 455 MPa and 12%, respectively.
4. Conclusions In summary, the high mechanical properties of Mg–Gd based alloys have been achieved by means of RPW processing. According to the comparatively investigations about the strength and texture, the following conclusions can be drawn
(1) The high strength is mostly related to the ultrafine grains and the presence of the dense and uniform tiny stable b phase. (2) The ultrafine grains with high-angle grain boundaries as well as weak-dispersed texture result in a high elongation via activating the non-basal slip. (3) The RPW process has been identified as an effective approach to prepare outstanding high strength-toughness Mg alloys.
Acknowledgments This research is financially supported by National Natural Science Fund of China (51071004) and National Science and Technology Supporting Plan of the Twelve Five-year (2011BAE22B04-2).
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Development of extraordinary high-strength-toughness Mg alloy via combined processes of repeated plastic working and hot extrusion Ke Liu a, Xudong Wang a,b, Wenbo Du a,n a b
School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Beijing Institute of Aeronautical Materials, Beijing 10085, China
a r t i c l e i n f o
abstract
Article history: Received 17 October 2012 Received in revised form 12 February 2013 Accepted 15 February 2013 Available online 24 February 2013
An extraordinary high-strength-toughness Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy has been successfully fabricated by 400 cycles of repeated plastic working process followed by hot extrusion. The sample exhibited an ultimate tensile strength of 500 MPa, tensile yield strength of 455 MPa, companying with a failure elongation of 12.0% at room temperature. The significant enhancement in mechanical properties was attributed to the grain refinement, homogeneous b phase and modified texture. & 2013 Elsevier B.V. All rights reserved.
Keywords: Mg–Gd–Er–Zr alloy Repeated plastic working Severe plastic deformation Ultrafine grain Dynamic precipitate
1. Introduction Magnesium alloys have recently received a great deal of attention because of their low density, high specific strength and recycle. However, compared with traditional steel and Al alloys, the Mg alloys often display poor yield tensile strength (YTS) as well as relatively low elongation (E) at room temperature. Therefore, much work has been done to improve YTS and E simultaneously. Grain refinement is one of the most effective ways to improve YTS and E. For example, the highest-strength magnesium alloy of the Mg97Zn1Y2 alloy produced by a rapidly solidified powder metallurgy (RS P/M) technique is partly attributed to nanometer-scale grains [1,2]. Certainly, the presence of the highest-strength Mg alloy in the wide world displayed a higher specific strength than those of the conventional Ti-6Al-4V alloy and 7075-T6 aluminum alloy, which also promotes our enthusiasm to develop high-performance Mg-RE based alloys by means of other new and effective approach that improves the combination of tensile strength and elongation. Comparatively, these thermal mechanical process, such as hot extrusion (HE), rolling (R) and equal channel angular pressing (ECAP) often conducted to refine grain size, are more attractive in the fields of industry application comparing with the RS P/M processing. In general, the thermal–mechanical processing often leads to a great promotion in YTS via introducing refinement of
n
Corresponding author. Tel./fax: þ 86 10 67392917. E-mail address: [email protected] (W. Du).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.030
grains mainly attributed to the dynamic recrystallization (DRX) [3], relative strong texture [4] and precipitation [5]. Though the presence of the strong texture is beneficial to improve the YTS effectively, the unexpected plastic anisotropy is also induced [6]. For example, a high-strength Mg–14Gd–0.5Zr (wt%) alloy (HEþR) has been reported recently in literature [7], and it indicates that the YTS increased but the E conversely decreases as the intensity of the texture increases. Previously, the modification of texture has been pointed out by Mukai et al. [8], Hantzsche et al. [6], Cottam et al. [9] and Huang et al. [10], which is of importance to improve the E by eliminating plastic anisotropy. Thus, an effective method which can combine ultrafine grain size with modification of texture is a critical issue to improve YTS and E simultaneously [11]. Therefore, an investigation on how to produce a high-strengthtoughness alloy via combining the ultrafine grain size and modification of texture is essential at present. In the present investigation, we report an extraordinary high-strength-toughness Mg alloy of Mg–11.8Gd–1.9Er–0.4Zr (wt%) produced by repeated plastic working (RPW) processing followed by subsequent hot extrusion. The RPW process as one of severe plastic deformation (SPD) methods provided more attractive and meaningful results. The YTS and E of the alloy are 455 MPa and 12%, respectively. The significant enhancement in mechanical properties is attributed to the refinement of grains (the microstructure is dominated by high-angle grain boundaries and average grain size is 0.5 mm), uniform distribution of b phase and modified texture (max intensity of texture 2.5).
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K. Liu et al. / Materials Science & Engineering A 573 (2013) 127–131
and 4.7%, respectively. Moreover, Homma et al. [15] developed a new attractive extruded alloy of Mg–10Gd–5.7Y–1.62Zn–0.65Zr (wt%) with YTS and E of 419 MPa and 3.6%, respectively. Obviously, as far as the tensile properties (YTS and E) are concerned, the values of our present work are tremendously higher than those of the alloys referred above (see Table 1).
2. Experimental procedures The nominal composition of the Mg–11.8Gd–1.9Er–0.4Zr alloy was first produced by gravity casting, and then homogenized at 460 1C for 6 h. Then, the oxidized surface of the alloy was removed by a sander machine. The clean block alloy was machined into chips with a size of 1–5 mm by a storage tank crusher. The chips were compacted and mixed via the RPW machine in order to produce a compact with a diameter of 35 mm at room temperature. The RPW machine has two kinds of punches. One is called flat punch; the other is named taper punch [12]. One complete cycle of RPW process is illustrated in reference [12]. In the present paper, the compact was obtained via treated by 400 cycles of RPW process at room temperature. The loads of the punch will be adjusted automatically based on the properties of materials. In our investigation, the load of the flat punch is less than 40 t while the load of the taper punch is less than 10 t. At last, the compact was hot extruded into a rod with a diameter of 9 mm indirectly at 340 1C. The extruded rod was machined into dog-bone geometry with a gauge length of 25 mm and a diameter of 5 mm. The tensile axis was parallel to the extrusion direction (ED) at a stretching rate of 1 mm/min at room temperature. The microstructure of the extruded alloy was conducted by using an optical microscope (OM, Zeiss-Imager.A2m), transmission electron microscope (TEM, JEM-2100, JEOL) at 200 kV equipped with selected area electron diffraction (SAED) and X-ray energy dispersive (EDS). The specimens for OM observation were mechanically polished and etched in a solution of 4 ml nitric acid and 96 ml ethanol. The specimens for TEM observation were prepared by electro-polishing and ion beam milling. Macrotexture measurement was carried out on the X-ray diffractometer using reflection geometry and CuKa radiation. The orientation distribution function (ODF) was obtained from experimental pole figures data by using the series expansion method.
3.2. Microstructure Fig. 1 shows OM images (transverse section) of the Mg–11.8Gd–1.9Er–0.4Zr alloy fabricated by 400 cycles of RPW process followed by hot extrusion. It is indicated that significant strain contrasts are observed, which are introduced by RPW process. Some parts with white contrast are the areas suffered from straining lightly, as shown in Fig. 1a. Lots of tiny particles are observed, as shown in Fig. 1b. It is not found the clear grain boundaries, and the grain is not observed by the OM method. In order to investigate the microstructure of the alloy, the TEM observation has been carried out. TEM observation, as shown in Fig. 2, was carried out to investigate the strengthening mechanism of the present alloy. Evidently, the alloy is composed of fine grains as well as tiny precipitates. The value of the average grain size is 500 nm, and it is considered as ultrafine grains (UGs) which are further confirmed by SAED pattern. It is well known that, indicated by the Hall–Petch relationship, such fine grains in the case will lead to a high proof strength [16]. Additionally, it is obviously found that lots of tiny precipitates with the same morphology of ellipsoid are mainly located at the triple grain boundaries, and the size of the precipitates is in the range 50–100 nm. The same precipitates in morphology has been observed in the Mg–10Gd–5.7Y–1.62Zn– 0.65Zr alloy produced by HE [15], Mg–12Gd–3Y–0.5Zr alloy produced by ECAP [17], Mg–8.57Gd–3.72Y–0.54Zr alloy fabricated by multiaxial forging (MAF) [18]. The presence of the uniform and dense nanoscale precipitates played an important role in improving tensile strength significantly [15,18,19]. Fig. 3 shows the TEM images of the grains and grain boundaries at a higher magnification. It finds predominantly high-angle grain boundaries (HAGBs, grain boundary misorientations Z151), which has an excellent relatively thermal stability and results in further high ductility greatly via enhancing the mobility of grain boundaries [20,21]. Generally, a well combination of UGs and HAGBs is difficult to be obtained due to high processing temperature (4250 1C) and DRX [17,22,23]. Especially, the abundant metastable subgrains unfortunately produced by DRX and left in the wrought Mg alloy are harmful to mechanical properties [24,25]. However, from the scientific point of view, our present work displayed a meaningful result that the excellent combination of UGs and HAGBs could be achieved via a special method, such as RPW process followed by subsequent HE referred in the present investigation, which played an important role in improving mechanical properties.
3. Results and discussion 3.1. Mechanical properties Table 1 shows the tensile properties of the Mg–11.8Gd–1.9Er– 0.4Zr alloy (400 cycles of RPW process þ extruded). The values of the YTS and E are 455 MPa and 12%, respectively, which is substantially higher than those reported before in literatures [7,11,13–15]. As reported, Rokhlin [13] produced the Mg– 25.6Gd (wt%) alloy (castþ T6) with a YTS of 320 MPa and an elongation of 0.4%. Anyanwu et al. [14] developed an Mg– 17Gd–0.51Zr alloy (RþT5) companying with YTS of 360 MPa and an E of 4%. Recently, Li [7] produced a Mg–14Gd–0.5Zr (wt%) alloy (HE þR þT5) which had a high YTS of 445 MPa with an E of 2%. In reference [11], the Mg–10Gd–2Y–0.5Zr (wt%) alloy (castþT6) was produced and the YTS and E were 239 MPa Table 1 Comparative tensile properties and states in Mg alloys with similar compositions. Alloys (wt%)
States
Grain size (mm)
YTS (MPa)
UTS (MPa)
E (%)
Mg–14Gd–0.5Zr [7] Mg–14Gd–0.5Zr [7] Mg–4.2Y–3.4RE–0.45Zr [10] Mg–10Gd–2Y–0.5Zr [11] Mg–25.6Gd [13] Mg–12Gd–1.9Y–0.69Zr [14] Mg–9.3Gd–4.1Y–0.7Zr [14] Mg–10Gd–5.66Y–1.62Zn–0.65Zr [15] Mg–10Gd–5.66Y–1.62Zn–0.65Zr [15] Mg–11.3Gd–3.8Y–0.7Zr [27] Mg–11.8Gd–1.9Er–0.4Zr (present work)
HEþ R HEþ Rþ T5 HEþ Rþ T5 Cast þT6 HE RþT5 RþT5 HE HEþ T5 Cast þT6 RPWþ HE
20 20 25 43 – 100 100 0.9 1.1 – 0.5
305 445 141 239 320 360 290 419 473 300 455
375 482 240 362 353 400 310 461 542 330 500
3.5 2.0 22 4.7 0.4 5.0 16.0 3.6 8.0 2.0 12.0
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The achievements of UGs and HAGBs may have a close relationship with the presence of the tiny precipitates at the triple grain boundaries. Therefore, a further investigation was carried out in order to investigate the tiny precipitates, as shown in Fig. 4. On the basis of the SAED patterns, it is found that the lattice constant of the precipitate is a ¼2.24 nm, a ¼ b ¼ g ¼901,
Fig. 1. OM images of the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion.
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and it belongs to the F4¯3m space group. Besides, the average composition of the precipitate is Mg86.18(Gd, Er)13.82, which is close to the equilibrium b phase (Mg5Gd, F_4_43m, a¼2.23 nm [15,26]). Similarly, as reported by Homma et al. [15], the equilibrium b phase has been observed in his investigation, and the b phase has a great effect on suppressing grain growths significantly during hot extrusion. Compared with the equilibrium b phase in literature [15], the equilibrium b phase in our
Fig. 3. TEM images of the grain boundaries obtained from the Mg–11.8Gd–1.9Er– 0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion: (a) Triple grain boundary and (b) high angle grain boundary between grain 1 and grain 2.
Fig. 2. TEM image obtained from the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion: (a) corresponding SAED pattern (b) and simulated scheme of the SAED pattern (c).
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investigation is the same in terms of morphology and distribution as those of the b phase referred in literature [15] besides the size, and the size in our investigation is relatively smaller. Frankly, the presence of the equilibrium b phase was unexpected in our investigation, which was also rarely reported previously. However, the present investigation indicated that it seems to be
Fig. 4. TEM images of the b phase observed from the Mg–11.8Gd–1.9Er–0.4Zr (wt%) alloy fabricated by 400 cycles of RPW process followed by hot extrusion and corresponding SAED pattern (B//[001]) (a) and interface between the matrix and the b phase (b).
more meaningful to control of the precipitation of the equilibrium b phase than that of the b00 phase, b0 phase and b1 phase observed in contribution [27–31] due to their low thermal stability. 3.3. Texture It is well known that a weak texture as well as UGs leads to a delay failure, which is attributed to an enhancement of activity of /c þaS slip [8,9,32]. Therefore, much work has been done on a combination of fine grain size and weak texture [33]. However, it is likely that acquisition of UGs and dispersed texture in an Mg alloy at the same time is difficult. For example, a strong (Max¼5.4) basal fiber texture in the extruded Mg–10Gd–5.7Y– 1.62Zn–0.65Zr (wt%) alloy (average grain size of 0.9 mm) [15], a sharp basal texture (Max ¼9.2) in the rolled Mg–14Gd–0.5Zr (wt%) alloy (average grain size of 20 mm) [7] and an intense basal texture (Max¼7.4) in the rolled Mg–0.73Y(wt%) alloy [6]. The strong texture directly resulted in a high plastic anisotropy in the alloys as stated above, in which therefore the elongation was inferior, such as 3.6% [15] and 2.0% [7]. In addition, a rolled Mg alloy after annealed treatment displayed a high elongation ( 22%) due to randomization of basal texture (Max¼2.4), while the YTS ( 141 MPa) decreased significantly due to grain growth ( 25 mm) [34]. Meanwhile, in our investigation, the texture intensity of the present alloy is weakened compared with those of alloys reported previously, and the max value is no more than 2.5. Therefore, it is interesting that the alloy produced by RPW process followed by hot extrusion seems to obtain ultrafine grainstructure as well as weak texture intensity value simultaneously, which accounts for the high ductility of the present alloy. In order to investigate the transformation of the macrotexture under different RPW cycles, the texture data obtained from the alloys treated by 0, 100, 200, 300 and 400 cycles of RPW processing followed by hot extrusion are present in the form of an threedimensional orientation distribution function (ODF), as shown in Fig. 5. It is indicated that the orientation distribution of the alloy without RPW process before hot extrusion is mainly at (301, 601, 901), while the orientation distribution of the alloy under 100 cycles of RPW process prior to hot extrusion is predominantly present at (01, 601, 451) and (01, 601, 1051). Furthermore, the distribution of orientation is inclined to random with further increment of cycles of RPW process. The orientation distribution of the alloy treated by 200 cycles of RPW process is mostly at (01, 601, 151), (01, 601, 451),
Fig. 5. ODF obtained from the Mg–11.8Gd–1.9Er–0.4Zr alloy treated by different cycles of RPW processing followed by hot extrusion.
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(01, 601, 751) and (01, 601, 1051), respectively. As the RPW process cycle increases to 300 and 400, the orientation distribution is more dispersed. Weakening textures of the Mg alloys will lead to intrinsic plastic isotropy and high ductility [33], which is beneficial to improve the formability of the alloys [35,36]. The activation of the non-basal slip may responsible for the increment of the elongation [6,8,9]. The weak and dispersed texture, which is ascribed to the DRX, of the present alloy may be partly responsible for the high ductility. However, the mechanism responsible for refinement of grains and DRX is so far not clear, and the further work is now being conducted. Besides, as stated above, as well as modification of the texture, the ultrafine grain-structure of the alloy is one of the important factors for improving ductility via introducing more grain boundaries, dispersing stress concentration and activating non-basal slip [23,33,37,38]. Eventually, combining weak-dispersed texture and UGs, a high-strengthtoughness Mg–11.8Gd–1.9Er–0.4Zr alloy, with relatively lower content of RE but high tensile properties compared with the alloys in literature [7,15], was produced by RPW processing followed by subsequent hot extrusion, with a high UTS, YTS and E of 500 MPa, 455 MPa and 12%, respectively.
4. Conclusions In summary, the high mechanical properties of Mg–Gd based alloys have been achieved by means of RPW processing. According to the comparatively investigations about the strength and texture, the following conclusions can be drawn
(1) The high strength is mostly related to the ultrafine grains and the presence of the dense and uniform tiny stable b phase. (2) The ultrafine grains with high-angle grain boundaries as well as weak-dispersed texture result in a high elongation via activating the non-basal slip. (3) The RPW process has been identified as an effective approach to prepare outstanding high strength-toughness Mg alloys.
Acknowledgments This research is financially supported by National Natural Science Fund of China (51071004) and National Science and Technology Supporting Plan of the Twelve Five-year (2011BAE22B04-2).
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