Microstructural evolution and superplasticity in an Mg–Gd–Y–Zr alloy after processing by different SPD techniques
Introduction
Low density, good castability, high specific strength and stiffness, and reasonable cost make magnesium alloys attractive for aerospace and automotive applications. Despite these advantages, poor formability at low temperatures is one of the most important limitations of Mg alloys. This disadvantage arises from the limited slip systems in the hexagonal close-packed (hcp) structure [1]. In order to overcome this limitation, attempts have been made to enhance their formability through grain refinement and the use of superplastic forming which will permit the fabrication of lightweight structural components having complex shapes [2], [3], [4], [5]. Severe plastic deformation processes (SPD), with the capability of imposing large amounts of strain, have been used extensively for grain refinement and thus improving the formability of Mg alloys. For SPD processing, equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are among the most conventional and effective methods used to achieve grain refinement and superplasticity t in different Mg alloys.
Concerning the grain refinement efficiency of different SPD methods, although there have been many investigations in the literature on the superplasticity of different Mg alloys after processing by ECAP or HPT, there are onlylimited comparisons between the grain refinement efficiency of the different methods. While such a comparison was made previously [6] between HPT, ECAP, accumulative roll bonding (ARB) and ball milling (BM) based on data for Ni, Fe, and Al materials, there has been no comparison for Mg or Mg alloys. In addition, the resultant effects of grain refinement on the superplastic behavior of these materials are not available. A comparison of the grain sizes obtained by ECAP and HPT is given in Table 1 [7], [8], [9] for some Cu and Al alloys and it is readily apparent that the grain sizes produced by HPT are much smaller than for ECAP. This difference is due primarily to the much higher strains attained in HPT processing where the ability to achieve smaller grain sizes in HPT was demonstrated in early experiments [10], [11] and, in addition, it was shown more recently that, by comparison with ECAP, HPT produces a higher fraction of grain boundaries having high angles of misorientation [12].
The most important problem of fine-grained Mg alloys produced by SPD methods is the occurrence of microstructural instability at high temperatures. Accordingly, attempts have been made to improve their thermal stability through the addition of different alloying elements. It was reported that the addition of gadolinium (Gd) and other rare earth (RE) elements can lead to a remarkable improvement in the thermal stability of microstructure and mechanical properties at high temperatures due to solution hardening and precipitation strengthening [13], [14]. Accordingly, there are many investigations reporting superplasticity in fine grained Mg–Gd alloys with average grain sizes in the range of 1–10 µm processed by extrusion [15], [16], [17], [18], friction stir processing (FSP) [19], [20] and rolling [21], [22]. However, only limited reports are available to date documenting superplasticity in these alloys after processing by ECAP [23], [24] or HPT [25].
The strain rate sensitivity (SRS) of materials is a characteristic property of superplastic materials and it can be obtained through localized methods or by conducting conventional tensile testing. Shear punch testing (SPT) is an example of a localized method that was introduced recently as an appropriate technique for measuring the SRS in different materials processed by SPD and a summary of the results available to date was given in an earlier report [25]. It should be noted that the validity of the SPT-tensile correlation was demonstrated several years ago [26]. The advantages of studying superplasticity by SPT was described earlier [27] and it includes the requirement for using only very small amounts of material as is readily produced using HPT processing. Thus, SPT is used exclusively in this work to investigate superplasticity.
Although the main features and effects of ECAP and HPT on the microstructure and superplasticity of Mg–Gd–Y–Zr alloys was discussed in earlier publications, a clear comparison between ECAP and HPT cannot be made based on these published data since they were obtained for different alloys having different concentrations of Gd and Y (for example, the ECAP and HPT were performed on GW50 [24] and GW94 [25] alloys, respectively). This is because the Gd and Y elements can affect the grain size of Mg alloys and this is beyond the scope of the present paper. Therefore, the overall aim of this research was to investigate and compare the microstructural evolution and superplasticity in an Mg–Gd–Y–Zr alloy after processing by extrusion or by extrusion followed by either ECAP or HPT. It has to be mentioned that the optimum ECAP and HPT temperatures and strains were chosen so as to achieve the highest degree of grain refinement which can be obtained by each processing method, based on our earlier experiments on Mg–Gd–Zr [24] and Mg–Gd–Y–Zr alloys [25].
Section snippets
Experimental material and procedures
An alloy of Mg–5 wt% Gd–4 wt%Y–0.4 wt% Zr was prepared from high purity Mg, Mg–30Gd, Mg–30Y and Mg–30Zr master alloys by melting in an electric furnace under a covering flux. Tilt-casting was used to minimize casting defects and any melt turbulence. The molten material was poured into a steel die preheated to 573 K and then extrusion was conducted at 673 K using two extrusion ratios of 19:1 to a diameter of 10 mm for HPT samples or 8:1 to a square cross-section of 13×13 mm2 for ECAP samples.
Thin
Microstructural evolution
SEM micrographs, an EBSD orientation map and the grain size distribution of the alloy after extrusion with an extrusion ratio of 19:1 are shown in Fig. 1. It is apparent from inspection of Fig. 1a and b that the microstructure of the alloy consists of fine equiaxed grains indicating the occurrence of dynamic recrystallization (DRX) during extrusion at 673 K. The EBSD orientation map is shown in Fig. 1c and it is clear that fine equiaxed grains are formed by DRX during deformation. The grain size
Grain refinement by extrusion, ECAP and HPT
Severe plastic deformation processes have been used traditionally to achieve extensive grain refinement of different materials by exerting large amounts of strain. However, different SPD methods produce different efficiencies in the level of grain refinement. In the present research on a GW54 alloy, nano-sized grains with an average size of ~72 nm were obtained after 8 turns of HPT, a fine-grained microstructure with average grain size of ~2.2 µm was obtained after 4 passes of ECAP and after hot
Conclusions
Comparison of the microstructure and superplasticity of an Mg–5Gd–4Y–0.4Zr alloy processed by equal-channel angular pressing and high-pressure torsion yielded the following conclusions:
- 1.
An Mg–5Gd–4Y–0.4Zr alloy was investigated to determine the effect on grain refinement and superplastic properties when using the three different processing procedures of extrusion, ECAP or HPT. HPT was conducted at room temperature to the relatively large strain of ~72.5 but it was not possible to process the
Acknowledgments
The authors thank the Iran National Science Foundation (INSF) for support of this work under Grant no. 94013486. Three of the authors were supported by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS (PHRP, YH, TGL).
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