Thermal stability of surface nano-crystallization layer in AZ91D magnesium alloy induced by laser shock peening

doi:10.1016/j.surfcoat.2017.09.037

Surface and Coatings Technology

Volume 334, 25 January 2018, Pages 182-188

https://doi.org/10.1016/j.surfcoat.2017.09.037 Get rights and content

Highlights

•
Surface nano-crystalline of AZ91D induced by LSP exhibits excellent thermal stability below 300 °C.
•
High density dislocations induced by LSP prevent the growth of grain directly and provide conditions for recrystallization.
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Recrystallization occurred in the process of annealing leads to a further refinement of nano-crystalline.
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Precipitates hinder dislocation movement, which indirectly impede the migration of grain boundaries.
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Grain size evolution of surface nano-crystalline in AZ91D and the corresponding mechanism were illustrated.

Abstract

Thermal stability of surface nano-crystallization layer in AZ91D magnesium alloy induced by laser shock peening (LSP) was investigated in this paper. Transmission electron microscopy (TEM) and X-ray diffractometer (XRD) were employed to characterize the variation of microstructure and phase composition of AZ91D magnesium alloy after LSP with and without annealing. The enthalpy of the amorphous magnesium alloy after LSP was examined by differential scanning calorimeter (DSC). Results demonstrated that the average size of nano-crystalline on the surface of AZ91D alloy produced by LSP was 90–100 nm. A mass of high density dislocations was observed in LSPed layer and gradually disappeared during the following annealing treatment. Recrystallization phenomenon, which was confirmed by analysis of TEM and XRD results, leads to a further reduction of crystal dimension. There is not enough time for recrystallized grain to grow up when annealing at 200 °C–300 °C. Meanwhile, amounts of precipitates exist in grain boundaries hindered the movement of dislocations, which indirectly impeded the growth of gains. When the annealing temperature exceeded 300 °C, nano-grains became metastable and began to grow up dramatically in 20 h, which can be attributed to the coarsening of precipitates and the disappearance of dislocation. Therefore, thermal stability of surface nano-crystallization layer mainly depends on the combined effect of high density dislocations, recrystallization, precipitates, and the uneven distribution of grains.

Graphical abstract

TEM images of (a) dislocation structure and (b) recrystallized grains in LSPed AZ91D magnesium alloys after annealing treatment.

Variation of the average grain size of the top layer of LSPed sample with different annealing temperatures.

Introduction

Magnesium alloys, especially magnesium-aluminium alloys, are extensively used in the electronics, automotive and aerospace industries due to their low density and high strength-to-weight ratio. Among the various magnesium alloys, AZ91D used here is the most common commercial Mg alloy because of its best combination of mechanical properties and castability. However, magnesium alloys usually exhibit inferior high temperature properties and poor corrosion resistance, which seriously limit their potential use in industrial practice [1]. Since that the surface nano-crystallization significantly strengthens the physical and mechanical properties of metallic materials over coarse grained metallic materials due to the large volume fraction of grain boundaries in the matrix [2], some techniques have been developed to induce nano-structures. However, the nano-structures will lose its advantages with grain boundaries decreasing under elevated temperatures, which directly restrict its application in high temperature environment [3], [4]. Investigation on the thermal stability of nanocrystalline is meaningful for extensive use of nanocrystallized metal material. Therefore, extensive researches on the thermal stability of nano-crystalline materials have been conducted [5], [6], [7].

W.B. Liu et al. [5] investigated the nano-crystallization and microstructure evolution of quenched RAFM steel produced by means of surface mechanical attrition treatment (SMAT) during annealing heat treatment. In their study, rapid grain growth was observed after annealing for 5 min at 823 K on account of the release of residual stress. A. Bachmaier and Motz [6] found that nanostructure in alloyed Co materials showed a better stability compared with single-phase high purity Co. The introduced nanometer sized pores could significantly improve the thermal stability of nanostructures by circumventing abnormal grain growth. Moreover, grain boundaries pinning by solute segregation hinder grain boundary migration to certain extent. M. Saber et al. [7] determined the influence of 1–4% Zr additions on the thermal stability of mechanically alloyed nano-crystallization Fe-Cr alloys containing 10% and 18% Cr. The results obtained in this investigation showed that additions could bring effective grain size stability in the nano-crystalline up to a temperature of 900 °C. Meanwhile, the α → γ transformation did not influence the grain size stability.

Laser shock peening (LSP), as a novel surface strengthening technology which utilizes high level shock wave induced by short pulse laser to significantly improve the resistance of materials to surface-related failures such as fatigue fracture resistance, is adopted to induce nano-structures in this study [8], [9]. During LSP process, a high level of residual compressive stress is induced on top surface of material in several nano seconds and grains on material surface are refined into nanoscale with repeated impacts [10]. Surface nano-crystallization induced by LSP has been demonstrated to greatly enhance the surface properties without changing the chemical composition and shape of materials [11], [12].

Based on the above mentioned, LSP is an effective method to enhance mechanical property by inducing nano-crystallization layer on top surface of AZ91D magnesium alloy. However, the indepth and systematic investigates on thermal stability of nano-crystallines in AZ91D magnesium are very few. Therefore, extensive researches of LSP induced nano-crystallization layer in AZ91D magnesium alloy are needed, which will help to extend industry application of nano-crystallized AZ91D under high temperature conditions such as engines and missile shell. In the present work, mechanism of how elevated temperature affects the stability of nano-crystalline induced by LSP on the surface of AZ91D was revealed, which supports theoretical basis for further industrial application of nano-crystallized AZ91D in high temperature environment.

Section snippets

Experiments

Initial AZ91D magnesium alloy plate was cut into pieces with dimensions of 20 mm × 20 mm × 5 mm. As shown in Table 1 is the normal chemical composition of the material used in the present study. Prior to LSP, the cut specimens were polished by SiC sand paper with different grades of roughness (from 400# to 1600#). An ultrasonic cleaner was utilized to clean the polished samples in ethanol. In the LSP process, the confining layer was undertaken by a 2 mm-thick water layer and the absorbing layer was a

Microstructure characterization of surface layer after LSP

Fig. 1 (a) and (b) exhibit the microstructure of AZ91D alloy before and after LSP respectively. It indicates that the main phases of the original and LSP treated samples are Mg matrix (α-Mg) and the second phase (β-Al₁₂Mg₁₇) which are mainly distributed at the grain boundaries as lamellar [13]. The result of X-ray diffraction (XRD) taken from the LSPed sample (with and without annealing) also confirms the presence of two phases of α-Mg and β-Al₁₂Mg₁₇ (as shown in Fig. 2). As shown in Fig. 1

Conclusions

A nano-structured surface layer was successfully obtained on the surface of AZ91D magnesium alloy by means of LSP. The average grain size of surface nano-crystalline in AZ91D magnesium alloy after LSP is about 90–100 nm. The evolution of nano-crystalline and the corresponding mechanism were studied in detail. The result shows that high density dislocations induced by LSP directly prevent the growth of grains and provide conditions for recrystallization. Therefore, recrystallization occurred at

Acknowledgements

The authors are grateful to the projects supported by the National Natural Science Foundation of China (Grant Nos. 51479082, 51611130207), Jiangsu Outstanding Youth Fund (Grant No. BK20160014), the “333 Project” of Jiangsu Province (Grant No. BRA2015367), the project is supported by the Cooperation Project of Jiangsu Province (Grant No. BY2015064-05) and the 2015Iinnovation & Entrepreneurship Project of Jiangsu Province.

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Thermal stability of surface nano-crystallization layer in AZ91D magnesium alloy induced by laser shock peening

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experiments

Microstructure characterization of surface layer after LSP

Conclusions

Acknowledgements

Vacuum

Acta Mater.

Mater. Sci. Eng. A

Acta Mater.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Mater. Des.

Mater. Sci. Eng. A

Mater. Sci. Eng. A

J. Alloys Compd.

Tribol. Int.