Microstructural evolution in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

https://doi.org/10.1016/j.matchar.2020.110423Get rights and content

Highlights

  • A nanocrystalline surface layer with average grain size of 98 nm is fabricated by SFT.

  • The recrystallized grain significantly weakens the texture intensity of the surface.

  • Shear bands and stacking faults can promote the microstructual evolution.

  • The second-phase particle is seriously broken, which accelarate the refining process due to the dislocation pinning.

Abstract

A nanocrystalline surface layer with average grain size of 98 nm of Mg-6Zn-0.2Y-0.4Ce-0.5Zr (wt%) alloy was fabricated successfully by sliding friction treatment (SFT). The abundant second-phase particles are heavily broken and homogeneously distributed in the matrix after SFT. Electron backscattered diffraction (EBSD) indicates that the intensity of basal texture shows a gradually decreasing trend with the decreasing distance from the treated surface due to the new recrystallized grains with random grain orientation. The process of grain refinement is found to involve the following stages: (i) the twins and dislocations are activated under the low strain area; (ii) the high density of dislocations accumulated near the second-phase particles promote the formation of substructures and the shear bands; (iii) the increasing strain arouses the transformation of substructures; (iv) multiple stacking faults will induce the nucleation of new strain-free grains under the high strain; (v) the nanosized grains are acquired. The abundant fine second-phase particles give large contribution for the refining process due to the pinning effect on the dislocations.

Introduction

Among the field of light alloys, the prime candidates for application in various structural components of automobiles and aerospace would be the magnesium (Mg) alloys because of their low density and high specific strength [1]. Although the significance of Mg alloys as a class of engineering materials has been acknowledged, the poor workability, low strength-ductility synergy, among other things, limit their wider applications [2,3]. As we know, the grain size usually has a positive influence on many material performances, such as strength, plasticity, hardness etc. [4]. Therefore, a lot of schemes have been tried to improve the disadvantages of Mg alloys by refining the grain size, such as alloying with rare-earth elements, deformation with extrusion or rolling [[5], [6], [7]]. However, the grain size of Mg alloys is hard to refine to nanometer scale by these common methods.

In order to fabricate the ultrafine grains, researches begun to explore some novel methods, such as friction stir processing (FSP), high pressure torsion (HPT), sliding friction treatment (SFT), which have outstanding grain refining effect [[8], [9], [10]]. Among these methods, SFT is a new process technology conducted on a specially designed device in a ball-on-disc contact configuration, which alters the microstructure of the near-surface layer by a quite high strain rate in a short period [11]. Many researchers successfully obtained a nanocrystalline layer with an average grain size of below 100 nm on crystalline Cu and pure tantalum sheets by SFT [12]. Meanwhile, they found that the nano-grained layer can improve some performance of the material surface, such as a significantly enhanced osteoblast response, a better electrochemical corrosion characteristics and biocompatibility [13,14]. Therefore, based on existing studies, some researchers try to conduct the SFT on Mg alloys for acquiring the nano-grains and hoping to improve the material performance. Huo et al. studied the AZ31 alloy sheets fabricated by SFT, which stated that the mechanical properties and corrosion resistance are simultaneously enhanced by the formation of nano-grained surface layer [11]. Further, the refining mechanism of AZ31 alloy was analyzed in Refs [15, 16], which illustrated the dynamic recrystallization (DRX) during the SFT and revealed a special banded structure aroused by the high strain rate. However, the existing researches are mainly aimed at pure metals and some conventional commercial magnesium alloys that almost have no second-phase particles. Many researches on wrought magnesium alloys show that the deformation process will alter the morphology, size and distribution of the second-phase particles [17,18]. The detailed microstructural evolution of second-phase containing Mg alloys fabricated by SFT is not clear. What's more, the plastic deformation will influence the grain orientation and result in some typical texture phenomena, such as fiber texture and bimodal texture [19,20]. But rare research reports the influence of severe plastic deformation on the texture modification. Solving these problems is helpful to provide a theoretical guidance for using SFT to improve the disadvantages of Mg alloys. Therefore, a Mg-Zn-Y-Ce-Zr alloy extrusion sheet which contains abundant second-phase particles is used to explore the microstructural evolution and the texture during the SFT.

In this work, we applied SFT on Mg-6Zn-0.2Y-0.4Ce-0.5Zr (wt%) alloy sheet to acquire the nano-grained layer. Detailed transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) characterizations have been performed with aims to reveal the evolution of microstructure from the interior to the treated surface, and corresponding texture modification is also analyzed. It is believed that this work may provide an insight on the plastic deformation behaviors of the nanocrystalline metals and lay a foundation for future researches.

Section snippets

Sliding friction treatment (SFT)

The SFT process was performed with a ball-on-disc contact configuration, in which a hot-extruded Mg-6Zn-0.2Y-0.4Ce-0.5Zr (wt%) alloy plate was pressed with a 10-mm-diameter WC-Co ball under high pressure. A plastically deformed layer on the sample surface was acquired by this kind of alternate sliding, with detailed processes being presented in Ref [21]. The sliding experiments were carried out under a normal load of 500 N with a sliding velocity of 0.2 m/s. In order to acquire effectively

Microstructure

Fig. 1 shows the microstructural observations of the SFTed surface and the cross section. Fig. 1(a) shows the macroscopic feature of the surface after treatment, indicated by the dotted box. Friction marks can be observed along the sliding direction (SD), and the treated surface is more lustrous than the untreated surface. The comparison of optical observations in two parts (yellow and red box in Fig. 1a) is shown in Fig. 1(b, c). A bimodal structure with fine equiaxed grains and un-dynamic

Discussion

A typical microstructure of as-extruded Mg-6Zn-0.2Y-0.4Ce-0.5Zr (wt%) alloy sheet subjected to SFT is the gradient-decreased grain size from the deep matrix to the treated surface, which is attributed to the gradient-increased strain and strain rate in the cross section. The following part of this section will discuss the modification of microstructure in different areas.

Conclusions

Based on the microstructural analyses and discussions in different areas of Mg-6Zn-0.2Y-0.4Ce-0.5Zr alloy subjected to SFT, the following conclusions can be obtained:

  • (1)

    A distribution of grain size across the depth of Mg-6Zn-0.2Y-0.4Ce-0.5Zr alloy sheet from bimodal coarse structure to nano-grains with average grain size of 98 nm is generated by SFT.

  • (2)

    Fine recrystallized grains aroused by the high strain own random grain orientation, which brings a gradually decreasing trend of texture intensity.

  • (3)

    The

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank the financial supports from the National Key Research and Development Program of China (No. 2016YFB0301100), the National Natural Science Foundation of China (No. 51571043) and Fundamental Research Funds for the Central Universities (Nos. 2018CDJDCL0019, cqu2018CDHB1A08 and 2018CDGFCL0005). The author would like to thank joint lab for electron microscopy of Chongqing University.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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