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Microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy processed by integrated extrusion and equal channel angular pressing
Journal of Alloys and Compounds 705 (2017) 118e125
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy processed by integrated extrusion and equal channel angular pressing Sicong Zhao, Erjun Guo*, Guojian Cao, Liping Wang, Yuchao Lun, Yicheng Feng School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2016 Received in revised form 21 January 2017 Accepted 11 February 2017 Available online 15 February 2017
The ultrafine-grained Mg-Nd-Zn-Zr alloy was obtained by an integrated extrusion and equal channel angular pressing at 350 C with one pass. Microstructural observations showed a significant refinement of grain structure after a single pass extrusion. The average grain size was reduced to around 500 nm. Numerous b1 phases and GP zones were observed in the extruded and subsequently aged alloy. A weak fiber texture with 〈1010〉 tilting angle of ~30 from normal direction towards extrusion direction was found in the extruded alloy. The extruded and subsequently aged alloy presented similar texture to that of the extruded alloy. The extruded alloy exhibited the yield strength (YS), ultimate tensile strength (UTS) and elongation of 248 MPa, 288 MPa and 14.4%, respectively. The extruded alloy after aging treatment achieved higher strength and slightly lower ductility with the YS, UTS and elongation of 264 MPa, 307 MPa and 12.9%, respectively. Compared to the as-cast alloy, the mechanical properties of the extruded and subsequently aged alloy increased 158%, 65% and 34% in YS, UTS and elongation, respectively. The increase in the strength of the extruded and subsequently aged alloy was attributed to both the grain refinement and the precipitation strengthening. The fracture surfaces of the extruded alloy were composed of a lot of small dimples and some cleavage planes. After the aging treatment, the cleavage planes were increased resulting in a decline in the ductility. © 2017 Elsevier B.V. All rights reserved.
Keywords: Mg alloys Extrusion Equal channel angular pressing Microstructure Mechanical properties
1. Introduction Mg alloys have attracted considerable attention in the past decade because of their high specific strength, good damping characteristics, and other advantages [1e3]. However, Mg alloys exhibit poor mechanical properties at room temperature, which is ascribed to the limited slip systems in the hexagonal close-packed structure [4,5]. Severe plastic deformation (SPD) is recognized as a promising technique to refine the microstructure and improve the mechanical properties [6]. Up to now, several different SPD processing techniques have been extensively studied, including highpressure torsion (HPT) [7], multi-axial forging (MAF) [8], accumulative roll-bonding (ARB) [9] and equal channel angular pressing (ECAP) [10]. Among these SPD techniques, ECAP is regarded as the most efficient SPD processing technique for the production of bulk ultrafine-grained Mg alloys with grain sizes below 1 mm [11]. However, ECAP is not widely used in industrial practices due to some limitations. For instance, the plunger of the press has a
* Corresponding author. E-mail address: [email protected] (E. Guo). http://dx.doi.org/10.1016/j.jallcom.2017.02.122 0925-8388/© 2017 Elsevier B.V. All rights reserved.
limited travel distance, which limits the length of workpiece [12]. Particularly, ECAP usually requires 4e10 passes to obtain a stable uniform grain size. This suggests that ECAP is not a continuous process, making it difficult for industrial applications. Furthermore, the texture induced by ECAP promotes basal slip in Mg alloys, which reduces the strength. To alleviate the above mentioned problems, the extrusion and equal channel angular pressing should be performed in one processing chain. Orlov [13e15] proposed a semi-continuous SPD technique that was characterized as an integrated process combining conventional extrusion and ECAP in a single processing step. This process was applied to the production of ZK60 Mg alloy workpiece. The strength, ductility and corrosion resistance of extruded alloy have been significantly improved. It was attributed to the reduced grain size of extruded alloy was around 1.6 mm. Nevertheless, the ultrafine-grained microstructure with grain size below 1 mm was not obtained in their experiments. In order to further refine the microstructure, a greater shear strain should be achieved by reducing the channel angle of the extrusion die. As an age-hardenable Mg alloy, the Mg-Nd-Zn-Zr alloy has great potential for industrial applications, especially in the field of biodegradable
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implant materials such as stents, bone fixtures, plates, and screws [16]. Up to now, the influences of the aging treatment on microstructure, textures and mechanical properties of the ultrafinegrained Mg-Nd-Zn-Zr alloy were not clear. In this work, the process of the integrated extrusion and equal channel angular pressing (IEECAP) is modified to produce the ultrafine-grained Mg-Nd-Zn-Zr alloy with grain size close to 500 nm. For this purpose, several ECAP passes are reduced to one pass and the angle F between the channels is reduced to 90 . The effect of IEECAP and the subsequent aging treatment on microstructure, textures and mechanical properties of the Mg-Nd-Zn-Zr alloy is investigated. A systematical understanding the relationship between the microstructure and mechanical properties is provided. 2. Experimental materials and methods 2.1. Experimental material and processing The alloy with target compositions of Mg-3.0Nd-0.4Zn-0.5Zr (wt.%) was prepared using commercially pure Mg (>99.95 wt.%), pure Zn (>99.9 wt.%), Mg-25 wt.%Nd and Mg-30 wt.%Zr master alloy ingots. The materials were melted at ~780 C in an electric resistance furnace under a mixed atmosphere of CO2 and SF6 with a volume ratio of 200:1. The liquid melt was stirred to ensure the homogeneity and then cast into a steel permanent mold with preheated temperature at 200 C. The as-cast alloy ingots were solution-treated at 530 C for 8 h covering with graphite powder. The solution-treated alloy ingots were cut into rectangular billets with dimensions of 30 mm 30 mm 100 mm. In order to minimize the effect of grain growth during IEECAP process and to minimize the load on the plunger, the solution-treated billets were extruded at 350 C. The rectangular billets were extruded to bars with dimensions of 10 mm 10 mm. Graphite powders were used as the lubricant. The plunger speed is 3 mm s1. The extruded specimens were aged at 200 C for 4 h. The solution-treated
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specimens without extrusion were also aged at 200 C for 4 h for comparisons. The schematics of IEECAP die and the corresponding workpiece extracted from the die after a single pass extrusion are shown in Fig. 1. The iso-view and side-view of IEECAP die present in Fig. 1(a) and (b), respectively. The extrusion, normal and transverse directions are abbreviated as ED, ND and TD, respectively. The angle F of the IEECAP die is 90 . As revealed in Fig. 1(c), the IEECAPed bar with smooth surfaces was achieved. 2.2. Characterization of microstructures, textures and mechanical properties The specimens for the microstructure characterization were cut along the ND-ED plane. To analyze the microstructures of the ascast, the solution-treated and the aged alloys, the specimens were polished and etched with a solution of 2.5 g picric acid, 2.5 ml acetic acid, 50 ml ethanol and 50 ml water, and observed by an optical microscope (OLYMPUS GX71). The TEM (JEM-2100, 200 kV) experiments were conducted to observe the precipitated morphologies. The TEM specimens were mechanically grounded to ~40 mm, and then prepared by ion milling operating at ~3.6 kV ion gun energy and ~4 milling angle. The fracture surface morphologies of specimens after tensile tests were observed by a scanning electron microscope (FEI Quanta 200). The average grain size was determined by the linear intercept method. For measurements of grain sizes, at least 10 images, including TEM and optical images, were repeated for each condition to improve precision. The texture measurements were conducted using a X-ray diffractometer (X'Pert-PRO) equipped with Cu Ka at 40 kV and 40 mA. The specimens for the texture characterization were cut along the ND-ED plane. The texture measurements were done over the specimens tilting from 0 to 70 and azimuthal rotating from 2.5 to 357.5 with 5 steps in both directions. The {0002} and {1010} pole figures were analyzed by X'Pert texture software. The mechanical properties were characterized by tensile tests, which were carried out at room temperature with a strain rate of
Fig. 1. Schematics of IEECAP die and the corresponding workpiece extracted from the die after a single pass extrusion, (a) iso-view, (b) side-view and (c) macrograph of the workpiece processed by IEECAP.
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Fig. 2. Optical micrographs of (a) as-cast alloy, (b) solution-treated alloy and (c) solution-treated and subsequently aged alloy.
1 103 s1. The tensile test specimens with dimensions of 5 mm in gauge width, 2.5 mm in gauge thickness and 25 mm in gauge length were machined by the electrical discharge machining from the bars parallel to ED, and then cleaned with ethanol. For tensile testing, at least 5 specimens were repeated for each condition to improve the precision.
3. Results and discussion 3.1. Microstructures of as-cast, solution-treated and aged Mg3.0Nd-0.4Zn-0.5Zr alloys Fig. 2 shows the optical microstructures of the as-cast, solutiontreated and aged Mg-3.0Nd-0.4Zn-0.5Zr alloys. The microstructure of the as-cast alloy is composed of equiaxed a-Mg grains and eutectic compounds at the grain boundaries, as shown in Fig. 2(a). As reported in Ref. [17], the eutectic compound is b phase (Mg12Nd). The average grain size of the as-cast alloy is ~30 mm. After a solution treatment at 530 C for 8 h, the b phase is fully dissolved into the matrix, as shown in Fig. 2(b). The grains also present equiaxed morphology. The average grain size of solution-treated alloy is ~73 mm. Clearly, the grain size significantly increases after the solution treatment. It is noted that some undissolved particles are visible in the matrix after the solution treatment. The undissolved particles are known to be Zn-Zr compounds [18,19], which are heterogeneous in the matrix. After a subsequent aging treatment at 200 C for 4 h, the optical micrograph of aged alloy is close to that of
the solution-treated alloy. Because the aging treatment leads to the formation of nano-sized precipitates, which cannot be observed by optical microscopy. In order to investigate the nano-sized precipitates, the aged alloy is further investigated by TEM. Fig. 3 shows the TEM bright field image and the corresponding selected-area diffraction (SAD) patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the solution treatment at 530 C for 8 h and the subsequent aging treatment at 200 C for 4 h. Numerous plate-shaped precipitates uniformly distribute in the matrix after the aging treatment. The plate-shaped precipitates are ~20 nm in length. Most plate-shaped precipitates are elongated with their longitudinal axis parallel to {1120} planes. Thus, the plate-shaped precipitates habit on {1120} prism planes. It was identified to be Guinier-Preston (GP) zones in the recent studies [20e23]. This is further confirmed by weak diffraction streaks observed at 1/ 2(0110) from [0001] zone axes, as shown in Fig. 3 (b).
3.2. Microstructures of extruded Mg-3.0Nd-0.4Zn-0.5Zr alloy Fig. 4 shows the TEM bright field images of Mg-3.0Nd-0.4Zn0.5Zr alloy after the extrusion at 350 C. Compared with the grain size of solution-treated alloy, the average grain size is significantly refined after one pass of IEECAP extrusion, reducing from ~73 mm to ~0.5 mm, as shown in Fig. 4(a). The grain refining is the consequence of the dynamic recrystallization during the extrusion process. Furthermore, the extruded alloy exhibits a few coarse secondphase particles, which are unevenly distributed in the matrix. The
Fig. 3. TEM bright field image (a) and corresponding SAD patterns (b) of Mg-3.0Nd-0.4Zn-0.5Zr alloy after a solution treatment at 530 C for 8 h and subsequent aging treatment at 200 C for 4 h. The beam is parallel to [0001]a direction.
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Fig. 4. TEM bright field images and corresponding SAD patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the extrusion at 350 C: (a) the distribution of coarse second-phase particles; (b) the magnified image of coarse second-phase particles; (c), (d) and (e) is the corresponding SAD patterns of particles A, B and C in (b), respectively. The beam is parallel to [110]b1 direction in (a) and (b). The beam is parallel to [211]b1 direction in (c).
magnified micrograph of the coarse second-phase particles is shown in Fig. 4(b). The size of the coarse second-phase particles ranges from ~20 nm to ~300 nm. In order to identify these particles, three types of coarse second-phase particles, marked as A, B and C in Fig. 4(b), are selected to conduct the SAD experiments. The SAD patterns of particles A, B and C are shown in Fig. 4 (c), (d) and (e), respectively. These SAD patterns agree with the b1 phase (Mg3Nd, fcc, a ¼ 0.744 nm) as reported in Ref. [21]. Although the coarse second-phase particles have a dramatic changes in size, all of them are b1 phases. 3.3. Microstructures of extruded and subsequently aged Mg-3.0Nd0.4Zn-0.5Zr alloy Fig. 5 shows the TEM images and SAD patterns of Mg-3.0Nd0.4Zn-0.5Zr alloy after one pass of IEECAP extrusion at 350 C and subsequent aging treatment at 200 C for 4 h. Fig. 5(a) reveals that the grain size of extruded and subsequently aged alloy is similar to that of the corresponding extruded alloy. Moreover, some coarse second-phase particles can be observed in the matrix. The morphology of the coarse second-phase particles is similar to that of b1 phase observed in the extruded alloy as shown in Fig. 5(b). The SAD patterns of a coarse second-phase particle in Fig. 5(b) are shown in Fig. 5(c). The SAD patterns demonstrate that the coarse second-phase particles are b1 phase. Apart from the coarse secondphase particles, numerous fine precipitates are visible in Fig. 5(b). The fine precipitates distribute evenly in the matrix. The magnified image of the fine precipitates in matrix is shown in Fig. 5(d). The average size of this precipitates is ~5 nm. The diameter of fine precipitates is significantly smaller as compared to that of b1 phase.
The morphology and size of the fine precipitates agree well with the that of GP zones in the aged Mg-Nd [22] and Mg-Nd-Zn-Zr alloys [23]. 3.4. Textures The {0002} and {1010} pole figures of the IEECAPed alloys are shown in Fig. 6. The IEECAP technique is based on combining a conventional direct extrusion and a single pass of ECAP. After IEECAP process, the extruded alloy exhibits a weak 〈1010〉 fiber texture as shown in Fig. 6(a) and (b). The maximum of the {0002} and {1010} pole density is 4.527 and 2.963, respectively. For this fiber, the 〈1010〉 direction tilts an angle of ~30 from ND towards ED. The severe shear deformation results in a rotation of basal planes during the single pass of ECAP process. The similar orientation has been observed in different Mg alloys [24]. Fig. 6(c) and (d) reveal that the texture intensity of the extruded and subsequently aged alloy is similar to that of the extruded alloy. Therefore, the orientation of the matrix is almost unchanged after the aging treatment. Unlike many of the SPD processed Mg alloys leading to the formation of relatively strong texture, the IEECAPed Mg-Nd-Zn-Zr alloy exhibits the weaker texture. Three factors are responsible for the texture weakening in this study. Firstly, the addition of RE, Zn and Zr elements in Mg alloys increases the tendency of soluteclustering drag effect, which promotes the forming of shear banding during IEECAP processing. Numerous shear banding as nucleation sites enhances the texture randomization [25,26]. Secondly, the deformation twinning formed during SPD processing is proposed as nucleation sites for recrystallization. In this case, the recrystallized grains can be given rise to form the new orientations,
Fig. 5. TEM images and corresponding SAD patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the extrusion at 350 C and subsequent aging treatment at 200 C for 4 h. (a) the TEM bright field image of the alloy; (b) the fine precipitates and coarse second-phase particles in matrix; (c) the SAD patterns of a b1 particle in (b), B//[211]b1; (d) the magnified image of the fine precipitates in matrix.
Fig. 6. The {0002} and {1010} pole figures of the Mg-3.0Nd-0.4Zn-0.5Zr alloy. (a) and (b) extruded alloy; (c) and (d) extruded and subsequently aged alloy.
S. Zhao et al. / Journal of Alloys and Compounds 705 (2017) 118e125 Table 1 Mechanical properties of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions. Condition
YS (MPa)
UTS (MPa)
As-cast Solution-treated aged Extruded Extruded þ aged
102 ± 4 81 ± 3 137 ± 5 248 ± 6 264 ± 6
186 205 254 288 307
± ± ± ± ±
5 6 6 7 8
Elongation (%) 9.6 ± 0.4 20.1 ± 1.1 7.4 ± 0.4 14.4 ± 0.8 12.9 ± 0.7
Fig. 7. Engineering stress-strain curves of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions.
which are distinct from the deformation orientation [26]. Finally, the second phase particles can provide more randomly oriented nuclei, which gives rise to weaker recrystallization texture [27]. 3.5. Mechanical properties Mechanical properties of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions are summarized in Table 1. The engineering
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stress-strain curves are shown in Fig. 7. The alloy in the as-cast condition shows the YS, UTS and elongation to failure of 102 MPa, 186 MPa and 9.6%, respectively. The microstructure of the as-cast alloy is composed of equiaxed a-Mg grains and b phase at the grain boundaries. The b phase can readily induce the stress concentration at the grain boundaries during the tensile process. Therefore, the as-cast sample exhibits poor elongation. After the solution treatment at 530 C for 8 h, the YS, UTS and elongation are 81 MPa, 205 MPa and 20.1%, respectively. As mentioned above, the solution treatment leads to the grain coarsening and the b phase dissolution. According to the Hall-Petch relationship [28], the decreased YS is mainly attributed to the grain coarsening. The b phase dissolution significantly reduces the stress concentration at the grain boundaries, which improves the elongation. Compared to the solution-treated alloy, both YS and UTS of the aged alloy are further improved, but the elongation decreases dramatically. As mentioned previously, numerous GP zones form in the matrix during aging process. Most plate-shaped GP zones are elongated with their longitudinal axis parallel to {1120} prism planes. The GP zones formed on the prism planes provide the most effective barrier to basal slipping in the matrix. Moreover, some deformation studies on the Mg alloy polycrystal at room temperature have shown that the critical resolved shear stress for basal slip is significantly lower than for non-basal (prismatic and pyramidal) slip [29]. As a consequence, the most important available slip system in Mg alloys is the basal slip at room temperature. Therefore, numerous GP zones bring about the most effective barriers to basal slipping resulting in the enhancement of the tensile strength [30]. The precipitation strengthening mechanism is responsible for the strength improvement under the aged condition. However, due to the dislocation gliding hindered by GP zones, the dislocation pileup may act as the crack sources. It leads to reduce the elongation. The tensile curves of extruded alloy show huge differences in tensile properties comparing with as-cast, solution-treated and aged alloys. The extruded alloy exhibits the YS, UTS and elongation to failure of 248 MPa, 288 MPa and 14.4%, respectively. Compare to the as-cast alloy, the YS and UTS increase dramatically. It is known
Fig. 8. The tensile fracture microstructures of Mg-3.0Nd-0.4Zn-0.5Zr alloy in (a,f) as-cast, (b) solution-treated, (c) aged, (d,g) extruded and (e,h) extruded and subsequently aged conditions.
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that the dynamic recrystallization occurs during the extrusion process. The grain size of ~500 nm is achieved after extrusion. The extruded Mg alloy with ultrafine grains has achieved a high strength according to previous studies [31,32]. Meanwhile, numerous b1 particles form in matrix after the extrusion. The strength can be effectively improved by the precipitate strengthening [33]. Therefore, the improved mechanical properties are mainly attributed to the grain refinement and precipitation strengthening. The extruded alloy after aging treatment has achieved the YS, UTS and elongation to failure of 264 MPa, 307 MPa and 12.9%, respectively. The YS and UTS are further improved after the aging treatment. The texture of the extruded and subsequently aged alloy is similar to that of extruded alloy. It indicates that the influences of texture evolution on tensile properties can be neglected. Nevertheless, the presence of fine precipitates further enhance precipitation strengthening for the extruded and subsequently aged alloy. Compared to the as-cast alloy, the mechanical properties of the extruded and subsequently aged alloy are improved 158%, 65% and 34% in YS, UTS and elongation, respectively. 3.6. Fracture surface morphology Fig. 8 shows the typical fracture microstructures of Mg-3.0Nd0.4Zn-0.5Zr alloy in different conditions. Fig. 8(a) and (f) reveals that the as-cast alloy exhibits brittle rupture, which is evident to the corresponding tensile curve in Fig. 7. Fig. 8(b) shows the fracture surfaces of the solution-treated alloy consist of cleavage planes and some tear ridges. After aging treatment, the alloy exhibits the trans-granular fracture as shown in Fig. 8(c). Nevertheless, the size of cleavage plane of the aged alloy is smaller than that of the solution-treated alloy. After the IEECAP process, the fracture surfaces of the extruded alloy are obviously different from that of ascast, solution-treated and aged alloys. The fracture surfaces of the extruded alloy are composed of some small dimples and cleavage planes as shown in Fig. 8(d) and (g). The fracture surfaces of extruded alloy are classified as quasi-cleavage fracture. The fracture microstructure of the extruded and subsequently aged alloy is shown in Fig. 8(e) and (h). The observations indicate that the dimples decrease. Moreover, the cleavage planes further increase, which leads to the ductility declining. 4. Conclusions (1) The grain size of the Mg-3.0Nd-0.4Zn-0.5Zr alloy is significantly refined after IEECAP processing. Compared to the solution-treated alloy, the average grain size of IEECAPed alloy was reduced from ~73 mm to ~0.5 mm. Numerous b1 phases and GP zones were observed in the extruded and subsequently aged alloy. (2) The weak fiber texture with 〈1010〉 tilting angle of ~30 from ND towards ED was found in the extruded alloy. The texture after the aging treatment was similar to that of the extruded alloy. (3) The extruded alloy exhibited the YS, UTS and elongation to failure of 248 MPa, 288 MPa and 14.4%, respectively. The extruded alloy after the aging treatment has achieved the YS, UTS and elongation to failure of 264 MPa, 307 MPa and 12.9%, respectively. The aging treatment further improved mechanical properties of the alloy fabricated by IEECAP. The superior strength of the extruded and subsequently aged alloy was mainly attributed to the combination of grain refinement and precipitation strengthening. (4) The fracture surfaces of the extruded alloy were composed of a lot of small dimples and some cleavage planes. After the
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Microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy processed by integrated extrusion and equal channel angular pressing Sicong Zhao, Erjun Guo*, Guojian Cao, Liping Wang, Yuchao Lun, Yicheng Feng School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2016 Received in revised form 21 January 2017 Accepted 11 February 2017 Available online 15 February 2017
The ultrafine-grained Mg-Nd-Zn-Zr alloy was obtained by an integrated extrusion and equal channel angular pressing at 350 C with one pass. Microstructural observations showed a significant refinement of grain structure after a single pass extrusion. The average grain size was reduced to around 500 nm. Numerous b1 phases and GP zones were observed in the extruded and subsequently aged alloy. A weak fiber texture with 〈1010〉 tilting angle of ~30 from normal direction towards extrusion direction was found in the extruded alloy. The extruded and subsequently aged alloy presented similar texture to that of the extruded alloy. The extruded alloy exhibited the yield strength (YS), ultimate tensile strength (UTS) and elongation of 248 MPa, 288 MPa and 14.4%, respectively. The extruded alloy after aging treatment achieved higher strength and slightly lower ductility with the YS, UTS and elongation of 264 MPa, 307 MPa and 12.9%, respectively. Compared to the as-cast alloy, the mechanical properties of the extruded and subsequently aged alloy increased 158%, 65% and 34% in YS, UTS and elongation, respectively. The increase in the strength of the extruded and subsequently aged alloy was attributed to both the grain refinement and the precipitation strengthening. The fracture surfaces of the extruded alloy were composed of a lot of small dimples and some cleavage planes. After the aging treatment, the cleavage planes were increased resulting in a decline in the ductility. © 2017 Elsevier B.V. All rights reserved.
Keywords: Mg alloys Extrusion Equal channel angular pressing Microstructure Mechanical properties
1. Introduction Mg alloys have attracted considerable attention in the past decade because of their high specific strength, good damping characteristics, and other advantages [1e3]. However, Mg alloys exhibit poor mechanical properties at room temperature, which is ascribed to the limited slip systems in the hexagonal close-packed structure [4,5]. Severe plastic deformation (SPD) is recognized as a promising technique to refine the microstructure and improve the mechanical properties [6]. Up to now, several different SPD processing techniques have been extensively studied, including highpressure torsion (HPT) [7], multi-axial forging (MAF) [8], accumulative roll-bonding (ARB) [9] and equal channel angular pressing (ECAP) [10]. Among these SPD techniques, ECAP is regarded as the most efficient SPD processing technique for the production of bulk ultrafine-grained Mg alloys with grain sizes below 1 mm [11]. However, ECAP is not widely used in industrial practices due to some limitations. For instance, the plunger of the press has a
* Corresponding author. E-mail address: [email protected] (E. Guo). http://dx.doi.org/10.1016/j.jallcom.2017.02.122 0925-8388/© 2017 Elsevier B.V. All rights reserved.
limited travel distance, which limits the length of workpiece [12]. Particularly, ECAP usually requires 4e10 passes to obtain a stable uniform grain size. This suggests that ECAP is not a continuous process, making it difficult for industrial applications. Furthermore, the texture induced by ECAP promotes basal slip in Mg alloys, which reduces the strength. To alleviate the above mentioned problems, the extrusion and equal channel angular pressing should be performed in one processing chain. Orlov [13e15] proposed a semi-continuous SPD technique that was characterized as an integrated process combining conventional extrusion and ECAP in a single processing step. This process was applied to the production of ZK60 Mg alloy workpiece. The strength, ductility and corrosion resistance of extruded alloy have been significantly improved. It was attributed to the reduced grain size of extruded alloy was around 1.6 mm. Nevertheless, the ultrafine-grained microstructure with grain size below 1 mm was not obtained in their experiments. In order to further refine the microstructure, a greater shear strain should be achieved by reducing the channel angle of the extrusion die. As an age-hardenable Mg alloy, the Mg-Nd-Zn-Zr alloy has great potential for industrial applications, especially in the field of biodegradable
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implant materials such as stents, bone fixtures, plates, and screws [16]. Up to now, the influences of the aging treatment on microstructure, textures and mechanical properties of the ultrafinegrained Mg-Nd-Zn-Zr alloy were not clear. In this work, the process of the integrated extrusion and equal channel angular pressing (IEECAP) is modified to produce the ultrafine-grained Mg-Nd-Zn-Zr alloy with grain size close to 500 nm. For this purpose, several ECAP passes are reduced to one pass and the angle F between the channels is reduced to 90 . The effect of IEECAP and the subsequent aging treatment on microstructure, textures and mechanical properties of the Mg-Nd-Zn-Zr alloy is investigated. A systematical understanding the relationship between the microstructure and mechanical properties is provided. 2. Experimental materials and methods 2.1. Experimental material and processing The alloy with target compositions of Mg-3.0Nd-0.4Zn-0.5Zr (wt.%) was prepared using commercially pure Mg (>99.95 wt.%), pure Zn (>99.9 wt.%), Mg-25 wt.%Nd and Mg-30 wt.%Zr master alloy ingots. The materials were melted at ~780 C in an electric resistance furnace under a mixed atmosphere of CO2 and SF6 with a volume ratio of 200:1. The liquid melt was stirred to ensure the homogeneity and then cast into a steel permanent mold with preheated temperature at 200 C. The as-cast alloy ingots were solution-treated at 530 C for 8 h covering with graphite powder. The solution-treated alloy ingots were cut into rectangular billets with dimensions of 30 mm 30 mm 100 mm. In order to minimize the effect of grain growth during IEECAP process and to minimize the load on the plunger, the solution-treated billets were extruded at 350 C. The rectangular billets were extruded to bars with dimensions of 10 mm 10 mm. Graphite powders were used as the lubricant. The plunger speed is 3 mm s1. The extruded specimens were aged at 200 C for 4 h. The solution-treated
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specimens without extrusion were also aged at 200 C for 4 h for comparisons. The schematics of IEECAP die and the corresponding workpiece extracted from the die after a single pass extrusion are shown in Fig. 1. The iso-view and side-view of IEECAP die present in Fig. 1(a) and (b), respectively. The extrusion, normal and transverse directions are abbreviated as ED, ND and TD, respectively. The angle F of the IEECAP die is 90 . As revealed in Fig. 1(c), the IEECAPed bar with smooth surfaces was achieved. 2.2. Characterization of microstructures, textures and mechanical properties The specimens for the microstructure characterization were cut along the ND-ED plane. To analyze the microstructures of the ascast, the solution-treated and the aged alloys, the specimens were polished and etched with a solution of 2.5 g picric acid, 2.5 ml acetic acid, 50 ml ethanol and 50 ml water, and observed by an optical microscope (OLYMPUS GX71). The TEM (JEM-2100, 200 kV) experiments were conducted to observe the precipitated morphologies. The TEM specimens were mechanically grounded to ~40 mm, and then prepared by ion milling operating at ~3.6 kV ion gun energy and ~4 milling angle. The fracture surface morphologies of specimens after tensile tests were observed by a scanning electron microscope (FEI Quanta 200). The average grain size was determined by the linear intercept method. For measurements of grain sizes, at least 10 images, including TEM and optical images, were repeated for each condition to improve precision. The texture measurements were conducted using a X-ray diffractometer (X'Pert-PRO) equipped with Cu Ka at 40 kV and 40 mA. The specimens for the texture characterization were cut along the ND-ED plane. The texture measurements were done over the specimens tilting from 0 to 70 and azimuthal rotating from 2.5 to 357.5 with 5 steps in both directions. The {0002} and {1010} pole figures were analyzed by X'Pert texture software. The mechanical properties were characterized by tensile tests, which were carried out at room temperature with a strain rate of
Fig. 1. Schematics of IEECAP die and the corresponding workpiece extracted from the die after a single pass extrusion, (a) iso-view, (b) side-view and (c) macrograph of the workpiece processed by IEECAP.
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Fig. 2. Optical micrographs of (a) as-cast alloy, (b) solution-treated alloy and (c) solution-treated and subsequently aged alloy.
1 103 s1. The tensile test specimens with dimensions of 5 mm in gauge width, 2.5 mm in gauge thickness and 25 mm in gauge length were machined by the electrical discharge machining from the bars parallel to ED, and then cleaned with ethanol. For tensile testing, at least 5 specimens were repeated for each condition to improve the precision.
3. Results and discussion 3.1. Microstructures of as-cast, solution-treated and aged Mg3.0Nd-0.4Zn-0.5Zr alloys Fig. 2 shows the optical microstructures of the as-cast, solutiontreated and aged Mg-3.0Nd-0.4Zn-0.5Zr alloys. The microstructure of the as-cast alloy is composed of equiaxed a-Mg grains and eutectic compounds at the grain boundaries, as shown in Fig. 2(a). As reported in Ref. [17], the eutectic compound is b phase (Mg12Nd). The average grain size of the as-cast alloy is ~30 mm. After a solution treatment at 530 C for 8 h, the b phase is fully dissolved into the matrix, as shown in Fig. 2(b). The grains also present equiaxed morphology. The average grain size of solution-treated alloy is ~73 mm. Clearly, the grain size significantly increases after the solution treatment. It is noted that some undissolved particles are visible in the matrix after the solution treatment. The undissolved particles are known to be Zn-Zr compounds [18,19], which are heterogeneous in the matrix. After a subsequent aging treatment at 200 C for 4 h, the optical micrograph of aged alloy is close to that of
the solution-treated alloy. Because the aging treatment leads to the formation of nano-sized precipitates, which cannot be observed by optical microscopy. In order to investigate the nano-sized precipitates, the aged alloy is further investigated by TEM. Fig. 3 shows the TEM bright field image and the corresponding selected-area diffraction (SAD) patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the solution treatment at 530 C for 8 h and the subsequent aging treatment at 200 C for 4 h. Numerous plate-shaped precipitates uniformly distribute in the matrix after the aging treatment. The plate-shaped precipitates are ~20 nm in length. Most plate-shaped precipitates are elongated with their longitudinal axis parallel to {1120} planes. Thus, the plate-shaped precipitates habit on {1120} prism planes. It was identified to be Guinier-Preston (GP) zones in the recent studies [20e23]. This is further confirmed by weak diffraction streaks observed at 1/ 2(0110) from [0001] zone axes, as shown in Fig. 3 (b).
3.2. Microstructures of extruded Mg-3.0Nd-0.4Zn-0.5Zr alloy Fig. 4 shows the TEM bright field images of Mg-3.0Nd-0.4Zn0.5Zr alloy after the extrusion at 350 C. Compared with the grain size of solution-treated alloy, the average grain size is significantly refined after one pass of IEECAP extrusion, reducing from ~73 mm to ~0.5 mm, as shown in Fig. 4(a). The grain refining is the consequence of the dynamic recrystallization during the extrusion process. Furthermore, the extruded alloy exhibits a few coarse secondphase particles, which are unevenly distributed in the matrix. The
Fig. 3. TEM bright field image (a) and corresponding SAD patterns (b) of Mg-3.0Nd-0.4Zn-0.5Zr alloy after a solution treatment at 530 C for 8 h and subsequent aging treatment at 200 C for 4 h. The beam is parallel to [0001]a direction.
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Fig. 4. TEM bright field images and corresponding SAD patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the extrusion at 350 C: (a) the distribution of coarse second-phase particles; (b) the magnified image of coarse second-phase particles; (c), (d) and (e) is the corresponding SAD patterns of particles A, B and C in (b), respectively. The beam is parallel to [110]b1 direction in (a) and (b). The beam is parallel to [211]b1 direction in (c).
magnified micrograph of the coarse second-phase particles is shown in Fig. 4(b). The size of the coarse second-phase particles ranges from ~20 nm to ~300 nm. In order to identify these particles, three types of coarse second-phase particles, marked as A, B and C in Fig. 4(b), are selected to conduct the SAD experiments. The SAD patterns of particles A, B and C are shown in Fig. 4 (c), (d) and (e), respectively. These SAD patterns agree with the b1 phase (Mg3Nd, fcc, a ¼ 0.744 nm) as reported in Ref. [21]. Although the coarse second-phase particles have a dramatic changes in size, all of them are b1 phases. 3.3. Microstructures of extruded and subsequently aged Mg-3.0Nd0.4Zn-0.5Zr alloy Fig. 5 shows the TEM images and SAD patterns of Mg-3.0Nd0.4Zn-0.5Zr alloy after one pass of IEECAP extrusion at 350 C and subsequent aging treatment at 200 C for 4 h. Fig. 5(a) reveals that the grain size of extruded and subsequently aged alloy is similar to that of the corresponding extruded alloy. Moreover, some coarse second-phase particles can be observed in the matrix. The morphology of the coarse second-phase particles is similar to that of b1 phase observed in the extruded alloy as shown in Fig. 5(b). The SAD patterns of a coarse second-phase particle in Fig. 5(b) are shown in Fig. 5(c). The SAD patterns demonstrate that the coarse second-phase particles are b1 phase. Apart from the coarse secondphase particles, numerous fine precipitates are visible in Fig. 5(b). The fine precipitates distribute evenly in the matrix. The magnified image of the fine precipitates in matrix is shown in Fig. 5(d). The average size of this precipitates is ~5 nm. The diameter of fine precipitates is significantly smaller as compared to that of b1 phase.
The morphology and size of the fine precipitates agree well with the that of GP zones in the aged Mg-Nd [22] and Mg-Nd-Zn-Zr alloys [23]. 3.4. Textures The {0002} and {1010} pole figures of the IEECAPed alloys are shown in Fig. 6. The IEECAP technique is based on combining a conventional direct extrusion and a single pass of ECAP. After IEECAP process, the extruded alloy exhibits a weak 〈1010〉 fiber texture as shown in Fig. 6(a) and (b). The maximum of the {0002} and {1010} pole density is 4.527 and 2.963, respectively. For this fiber, the 〈1010〉 direction tilts an angle of ~30 from ND towards ED. The severe shear deformation results in a rotation of basal planes during the single pass of ECAP process. The similar orientation has been observed in different Mg alloys [24]. Fig. 6(c) and (d) reveal that the texture intensity of the extruded and subsequently aged alloy is similar to that of the extruded alloy. Therefore, the orientation of the matrix is almost unchanged after the aging treatment. Unlike many of the SPD processed Mg alloys leading to the formation of relatively strong texture, the IEECAPed Mg-Nd-Zn-Zr alloy exhibits the weaker texture. Three factors are responsible for the texture weakening in this study. Firstly, the addition of RE, Zn and Zr elements in Mg alloys increases the tendency of soluteclustering drag effect, which promotes the forming of shear banding during IEECAP processing. Numerous shear banding as nucleation sites enhances the texture randomization [25,26]. Secondly, the deformation twinning formed during SPD processing is proposed as nucleation sites for recrystallization. In this case, the recrystallized grains can be given rise to form the new orientations,
Fig. 5. TEM images and corresponding SAD patterns of Mg-3.0Nd-0.4Zn-0.5Zr alloy after the extrusion at 350 C and subsequent aging treatment at 200 C for 4 h. (a) the TEM bright field image of the alloy; (b) the fine precipitates and coarse second-phase particles in matrix; (c) the SAD patterns of a b1 particle in (b), B//[211]b1; (d) the magnified image of the fine precipitates in matrix.
Fig. 6. The {0002} and {1010} pole figures of the Mg-3.0Nd-0.4Zn-0.5Zr alloy. (a) and (b) extruded alloy; (c) and (d) extruded and subsequently aged alloy.
S. Zhao et al. / Journal of Alloys and Compounds 705 (2017) 118e125 Table 1 Mechanical properties of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions. Condition
YS (MPa)
UTS (MPa)
As-cast Solution-treated aged Extruded Extruded þ aged
102 ± 4 81 ± 3 137 ± 5 248 ± 6 264 ± 6
186 205 254 288 307
± ± ± ± ±
5 6 6 7 8
Elongation (%) 9.6 ± 0.4 20.1 ± 1.1 7.4 ± 0.4 14.4 ± 0.8 12.9 ± 0.7
Fig. 7. Engineering stress-strain curves of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions.
which are distinct from the deformation orientation [26]. Finally, the second phase particles can provide more randomly oriented nuclei, which gives rise to weaker recrystallization texture [27]. 3.5. Mechanical properties Mechanical properties of Mg-3.0Nd-0.4Zn-0.5Zr alloy in different conditions are summarized in Table 1. The engineering
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stress-strain curves are shown in Fig. 7. The alloy in the as-cast condition shows the YS, UTS and elongation to failure of 102 MPa, 186 MPa and 9.6%, respectively. The microstructure of the as-cast alloy is composed of equiaxed a-Mg grains and b phase at the grain boundaries. The b phase can readily induce the stress concentration at the grain boundaries during the tensile process. Therefore, the as-cast sample exhibits poor elongation. After the solution treatment at 530 C for 8 h, the YS, UTS and elongation are 81 MPa, 205 MPa and 20.1%, respectively. As mentioned above, the solution treatment leads to the grain coarsening and the b phase dissolution. According to the Hall-Petch relationship [28], the decreased YS is mainly attributed to the grain coarsening. The b phase dissolution significantly reduces the stress concentration at the grain boundaries, which improves the elongation. Compared to the solution-treated alloy, both YS and UTS of the aged alloy are further improved, but the elongation decreases dramatically. As mentioned previously, numerous GP zones form in the matrix during aging process. Most plate-shaped GP zones are elongated with their longitudinal axis parallel to {1120} prism planes. The GP zones formed on the prism planes provide the most effective barrier to basal slipping in the matrix. Moreover, some deformation studies on the Mg alloy polycrystal at room temperature have shown that the critical resolved shear stress for basal slip is significantly lower than for non-basal (prismatic and pyramidal) slip [29]. As a consequence, the most important available slip system in Mg alloys is the basal slip at room temperature. Therefore, numerous GP zones bring about the most effective barriers to basal slipping resulting in the enhancement of the tensile strength [30]. The precipitation strengthening mechanism is responsible for the strength improvement under the aged condition. However, due to the dislocation gliding hindered by GP zones, the dislocation pileup may act as the crack sources. It leads to reduce the elongation. The tensile curves of extruded alloy show huge differences in tensile properties comparing with as-cast, solution-treated and aged alloys. The extruded alloy exhibits the YS, UTS and elongation to failure of 248 MPa, 288 MPa and 14.4%, respectively. Compare to the as-cast alloy, the YS and UTS increase dramatically. It is known
Fig. 8. The tensile fracture microstructures of Mg-3.0Nd-0.4Zn-0.5Zr alloy in (a,f) as-cast, (b) solution-treated, (c) aged, (d,g) extruded and (e,h) extruded and subsequently aged conditions.
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that the dynamic recrystallization occurs during the extrusion process. The grain size of ~500 nm is achieved after extrusion. The extruded Mg alloy with ultrafine grains has achieved a high strength according to previous studies [31,32]. Meanwhile, numerous b1 particles form in matrix after the extrusion. The strength can be effectively improved by the precipitate strengthening [33]. Therefore, the improved mechanical properties are mainly attributed to the grain refinement and precipitation strengthening. The extruded alloy after aging treatment has achieved the YS, UTS and elongation to failure of 264 MPa, 307 MPa and 12.9%, respectively. The YS and UTS are further improved after the aging treatment. The texture of the extruded and subsequently aged alloy is similar to that of extruded alloy. It indicates that the influences of texture evolution on tensile properties can be neglected. Nevertheless, the presence of fine precipitates further enhance precipitation strengthening for the extruded and subsequently aged alloy. Compared to the as-cast alloy, the mechanical properties of the extruded and subsequently aged alloy are improved 158%, 65% and 34% in YS, UTS and elongation, respectively. 3.6. Fracture surface morphology Fig. 8 shows the typical fracture microstructures of Mg-3.0Nd0.4Zn-0.5Zr alloy in different conditions. Fig. 8(a) and (f) reveals that the as-cast alloy exhibits brittle rupture, which is evident to the corresponding tensile curve in Fig. 7. Fig. 8(b) shows the fracture surfaces of the solution-treated alloy consist of cleavage planes and some tear ridges. After aging treatment, the alloy exhibits the trans-granular fracture as shown in Fig. 8(c). Nevertheless, the size of cleavage plane of the aged alloy is smaller than that of the solution-treated alloy. After the IEECAP process, the fracture surfaces of the extruded alloy are obviously different from that of ascast, solution-treated and aged alloys. The fracture surfaces of the extruded alloy are composed of some small dimples and cleavage planes as shown in Fig. 8(d) and (g). The fracture surfaces of extruded alloy are classified as quasi-cleavage fracture. The fracture microstructure of the extruded and subsequently aged alloy is shown in Fig. 8(e) and (h). The observations indicate that the dimples decrease. Moreover, the cleavage planes further increase, which leads to the ductility declining. 4. Conclusions (1) The grain size of the Mg-3.0Nd-0.4Zn-0.5Zr alloy is significantly refined after IEECAP processing. Compared to the solution-treated alloy, the average grain size of IEECAPed alloy was reduced from ~73 mm to ~0.5 mm. Numerous b1 phases and GP zones were observed in the extruded and subsequently aged alloy. (2) The weak fiber texture with 〈1010〉 tilting angle of ~30 from ND towards ED was found in the extruded alloy. The texture after the aging treatment was similar to that of the extruded alloy. (3) The extruded alloy exhibited the YS, UTS and elongation to failure of 248 MPa, 288 MPa and 14.4%, respectively. The extruded alloy after the aging treatment has achieved the YS, UTS and elongation to failure of 264 MPa, 307 MPa and 12.9%, respectively. The aging treatment further improved mechanical properties of the alloy fabricated by IEECAP. The superior strength of the extruded and subsequently aged alloy was mainly attributed to the combination of grain refinement and precipitation strengthening. (4) The fracture surfaces of the extruded alloy were composed of a lot of small dimples and some cleavage planes. After the
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