2018 Volume 59 Issue 4 Pages 669-673
The microstructure and room temperature tensile properties of Mg–12Gd–3Y–2Zn–0.5Zr (wt%) alloy processed by repetitive upsetting and extrusion (RUE) at decreasing temperature condition were investigated. The RUE was carried out up to cumulative strains of around 5.4 with decreasing temperature from 753 to 683 K pass-by-pass. With increasing RUE passes, average grain size was gradually decreased from 58 to 7.3 µm and microstructure became more homogeneous. Block-shaped long period stacking ordered (LPSO) phases at grain boundary were broken into small blocks or rods. Lamellar LPSO structures dissolved gradually and β-Mg5(Gd,Y) phase particles precipitated at grain boundaries. Both strength and ductility were improved simultaneously with increasing RUE passes. After 4 RUE passes, the ultimate tensile strength, yield strength and elongation to failure of the alloy reached to 351 MPa, 262 MPa and 10.3%, respectively. The significant improvement of mechanical properties could be ascribed to grain refinement, dispersion of β-Mg5(Gd,Y) phase particles and redistribution of fragmented block-shaped LPSO phases.
Rare earth (RE) containing Mg alloys are promising for the structural applications in automotive and aerospace industries due to their low density, high strength, good thermal stability and good castability.1) In recent years, Mg–Gd–Y–Zn–Zr series alloys have received more and more attention due to their unique long period stacking ordered (LPSO) structure and excellent mechanical properties. However, the relatively low ductility restricts their further applications.2) It is well known that microstructure refinement is an effective way to improve the ductility and strength of Mg alloys. There are recent reports on achieving grain refinement and improved mechanical properties of Mg alloys fabricated by severe plastic deformation (SPD) techniques such as high-pressure torsion (HPT), cyclic extrusion compression (CEC), equal channel angular pressing (ECAP) and multiple forging (MF).3–6) Among various SPD methods, repetitive upsetting and extrusion (RUE) proposed is an effective way to fabricate relative large bulk alloys without changing the shape of samples, which can be suitable for industrial applications.7) By now, RUE has been employed on some commercial wrought magnesium alloys, such as AZ61, AZ80 and AZ91.8–10) However, there have been few reports about the application of RUE process on Mg–Gd–Y–Zn–Zr alloys. In the present work, the RUE process with decreasing temperature was performed on Mg–12Gd–3Y–2Zn–0.5Zr (wt.%) alloys, the microstructures and mechanical properties after different RUE passes were studied systematically.
The Mg–12Gd–3Y–2Zn–0.5Zr (wt.%) alloy provided by Yinguang Magnesium Industry Co. Ltd. was used in this study. The cylinder billets for RUE with dimensions of 50 mm in diameter and 200 mm in height were cut from the central part of as-cast ingot, and then homogenized at 793 K for 57.6 ks, following quenched in water at 343 K.
The procedure of RUE and thermomechanical process employed in the RUE under decreasing temperature condition were schematically illustrated in Fig. 1. The upsetting and extrusion cavities were designed to be 70 mm and 50 mm in diameter, respectively. Thus, the accumulated strain for per RUE pass was calculated to be 1.35 approximately according to the following equation:
| \begin{equation} \varepsilon = 4n\ln(D/d) \end{equation} | (1) |
Schematic illustration of the repetitive upsetting and extrusion (RUE) (a) and thermomechanical process by RUE under decreasing temperature condition (b).
RUE was carried out on a 6300 KN hydraulic press with speed of 5 mm/s. During RUE, the temperature was decreased pass by pass from 753 to 683 K. Before each passes, the billets and the dies were heated to deformation temperature and kept isothermally for 15 min, and then lubricated with an oil-based graphite lubricant.
Specimens for microstructure analysis were cut from the central region of RUEed samples, and then mechanically polished and etched by a solution of 100 mL ethanol, 6 g picric acid, 5 mL acetic and 10 mL water. The microstructure morphologies on the longitudinal section were examined by optical microscope (OM, Zeiss Axio Imager A1m) and scanning electron microscopy (SEM, Hitachi SU5000) equipped with an energy dispersive X-ray spectrometer (EDS), respectively. The grain size was measured by the liner intercept method. Mechanical tensile specimens with 5 mm in diameter and 25 mm in gauge length were cut from the RUEed samples along extrusion direction (ED). The tensile tests were performed using Instron-3382 tension machine with a crosshead speed of 2 mm/min at room temperature (RT). The transversal fracture morphologies after tensile tests were also observed by SEM.
Figure 2 shows the OM and SEM images of the as-homogenized alloy. The average grain size was estimated to be about 58 µm. It can be seen from Fig. 2(a) that the block-shaped phases are discontinuously distributed along the grain boundaries, and a few needle-like phases distributed in grains. Moreover, some fine particles randomly scattered in the alloy. A further observation shows that some of the block-shaped phases grow into grain interior with specific orientation with α-Mg matrix, as shown in the magnified SEM image (Fig. 2(b)). The stoichiometric composition of the block-shaped phase measured by EDS is close to Mg12RE1Zn1, same as the composition reported for the 14H LPSO phase in the Mg–Y–Zn alloys.11) In addition to LPSO phases, fine cuboidal particles can also be observed in Fig. 2(b), which is identified to be RE-rich particles.
OM and SEM micrographs of the as-homogenized Mg–12Gd–3Y–2Zn–0.5Zr (wt.%) alloy.
Figure 3 shows the longitudinal microstructures of alloys subjected to RUE with various passes at decreasing temperature. After 1 pass RUE, as shown in Fig. 3(a), the coarse grains are elongated along the extrusion direction (ED). Moreover, many fine grains are formed along the coarse grain boundaries and adjacent to LPSO phase, which indicates the occurrence of dynamic recrystallization (DRX). Since LPSO phase exhibits higher elastic modulus than α-Mg matrix, the stress concentration is easily generated in the vicinities of LPSO/α-Mg interfaces, which could impede dislocation movements effectively and thereby promote DRX process via particle stimulated nucleation (PSN) mechanism.12) With increase of number of RUE passes, as seen from Fig. 3(b)–(c), the area covered by fine grains increased, leading to a homogeneous and fine microstructure. After 4 passes, as seen from Fig. 3(d), the whole matrix of the alloy is taken up by homogeneous and fine DRXed grains. It is noteworthy that newly fine DRX grains are free of the lamellar LPSO phases when the alloy after being 1 pass RUE, as shown in Fig. 3(a). With increasing RUE passes (Fig. 3(b)–Fig. 3(d)), the areas of DRX grains expand gradually and no obvious contrast of lamellar LPSO phases can be found, which indicated that these phases may dissolve during RUE process.
Optical micrographs of the longitudinal section of the alloys after RUE with different passes: (a) 1 pass; (b) 2 passes; (c) 3 passes and (d) 4 passes.
Figure 4 exhibits the SEM micrographs of the RUEed alloy. It can be seen from Fig. 4(a), the block-shaped LPSO phases are stretched along the extrusion direction after 1 pass. With increasing RUE passes, the block-shaped LPSO phases undergoes the severve plactic deformation due to induced large accumulative strain. After 4 passes, it can be seen that the block-shaped LPSO phases are broken into small pieces and distributed along the extrusion direction, as shown in Fig. 4(c). A further observation in Fig. 4(d) indicates that the some block-shaped LPSO phases are broken into small blocks or rods with several micrometers. In addition, fine particles can be observed in the alloy and majority of them distributed along the DRXed grain boundaries. According to the EDS analysis, the particles should be β-Mg5(Gd,Y) phase with the chemical composition of Mg–11.13Gd–4.28Y–1.43Zn–0.41Zr (at%), which are dynamically precipitated during RUE process.13) Moreover, the volume fraction of the particles seems to increase with increasing RUE passes.
SEM images of the longitudinal section of the alloys after RUE with different passes: (a) 1 pass; (b) 2 passes; (c) 4 passes; (d) is the partial magnified micrographs of secondary phases corresponding with (c).
Figure 5 shows the statistical evaluation for the variation of grain size of the alloy with RUE passes. It can be seen that a steady reduction in grain size with increasing RUE passes. After 1 pass of RUE, the average grain size decreases from 58 to 35 µm. With the further increase of RUE passes, the average grain size decreases gradually and reaches minimum of ∼7.3 µm as the alloy is RUEed for 4 passes. During the RUE process, the interior dislocation density increases gradually owing to the dislocation multiplication. Then the subgrains nucleate as a result of the accumulation, pile-up and rearrangement of dislocations, and evolve to fine DRX grains by absorbing mobile dislocations during the further RUE passes, leading to a homogeneous and fine microstructure. In addition, the lamellar LPSO phases distributed in the initial coarse grains are gradually dissolved into Mg-matrix due to the expansion of the area of DRX grains during the RUE process. Furthermore, the block-shaped LPSO phases at grain boundaries with discontinuous network distribution are stretched and gradually broken into the small blocks and particles which are scattered in the matrix uniformly and align along the extrusion direction. It can be indicated that besides grain refinement of the α-Mg matrix, the redistribution of fragmented block-shaped LPSO phases and the dissolution of lamellar LPSO structure are main feature of microstructure evolution of the alloy during RUE process.
Effect of number of RUE passes on average grain size.
Figure 6 shows the influence of number of RUE passes on tensile mechanical properties of the alloys at room temperature. It is apparent that the RUE process can effectively improve the mechanical properties of Mg–12Gd–3Y–2Zn–0.5Zr (wt%) alloy. After 1 RUE pass, the ultimate tensile strength (UTS), yield strength (YS) and elongation to fracture are 292 MPa, 215 MPa and 6.3%, repectively, which are obviously higher than that of as-homogenized alloy with 207 MPa (UTS), 174 MPa (YS) and 2.7%. With increase in the number of RUE passes, both the strength and ductility of the alloy are improved gradually. After 4 RUE passes, the UTS, YS and elongation to fracture of the alloy reaches to 351 MPa, 262 MPa and 10.3%, where are 1.7 times, 1.5 times and 4 times as the as-homogenized alloy, respectively. The significant improvement in mechanical properties could be attributed to the fact that microstructure change by RUE process, including grain refinement, dispersion of fine β-Mg5(Gd,Y) phase particles and redistribution of fragmented block-shaped LPSO phases. Grain refinement is proved to be the most effective way to improve the strength and ductility simultaneously for metallic materials. In this study, the grains are graudually refined with increasing RUE passes. As a result, the UTS and YS of the alloy increased with the same trend according to Hall–Petch relationship.14) Moreover, the refined grains could also release the induced stress concentration during room-temperature tensile deformation and improve the ductility through grain boundary sliding mechanism. In addition, both the scattered LPSO phases and β-Mg5(Gd,Y) phases distributed at grain boundaries could restrict slide of dislocations and motion of grain boundaries effectively, which contribute to the improvement of the strength.15)
Tensile properties of the RUE formed alloy with different numbers of RUE passes.
Figure 7(a) shows the tensile stress-strain curves of as-homogenized and 4-pass RUEed alloys at room temperature. Figure 7(b) and 7(c) present SEM images of fracture morphologies of the tensile specimens, respectively. As shown in Fig. 7(b), the cleavage planes and amount of tearing ridges can be observed, which indicated that the fracture mode of the as-homogenized alloy is brittle fracture. This due to the fact that stress concentration easily occurs at the interface between matrix α-Mg and LPSO phase during the tensile test at room temperature, and then micro cracks are prone to form and expand in this area leading to the fracture of the alloy.16) Up to 4 passes as shown in Fig. 7(c), it can be seen that profuse of dimples appear on the fracture surface with few tearing ridges. A further observation (Fig. 7(d)) indicates that some fine particles distributed at the bottom of dimples, which are confirmed be the precipitated β-Mg5(Gd,Y) phases via EDS analysis. This indicates that fracture mechanism of the alloy altered into the ductile fracture after 4 RUE passes, which corresponds to the great ductility improvement.
Tensile stress–strain curves at room temperature of as-homogenized and 4-pass RUEed alloys (a), SEM images of fracture surfaces of tensile specimens of as-homogenized (b) and 4-pass RUEed alloys (c and d).
This work was financially supported by the Basic Application research Foundation of Shanxi Province, China (project number: 201601D021094) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi.
