Elsevier

Materials Letters

Volume 264, 1 April 2020, 127352
Materials Letters

Effect of laser surface re-melting on the microstructure and properties of Zr alloy

https://doi.org/10.1016/j.matlet.2020.127352Get rights and content

Highlights

  • The surface hardness and corrosion resistance of the Zr-5Ti alloy are improved by LSR process.

  • Grain refinement reduces the pitting corrosion sensitivity of the alloy.

  • The oxide content of passivation films of Zr-5Ti increases after LSR, which increases the corrosion resistance of the alloy.

Abstract

The effects of laser surface re-melting (LSR) on the mechanical and corrosion properties of Zr-5Ti are investigated. After LSR, the microstructure of the alloy-surface has a finer phase-structure and higher density of grain boundaries. Furthermore, the surface hardness of the Zr-5Ti alloy improves significantly due to grain refinement. With increasing distance from the surface, the hardness decreases gradually because the laths coarsen gradually. Corrosion resistance after LSR also clearly improves due to grain refinement, which also reduces the pitting corrosion sensitivity of the alloy. The generated passivation film of the laser-treated alloy and substrate is also analyzed in this paper using X-photoelectron spectroscopy.

Introduction

Zirconium (Zr) and Zr alloys, which are commonly used structural materials, are widely used in the nuclear, chemical, and aerospace industry because of their small thermal-neutron absorption cross-section and good corrosion resistance [1]. Unfortunately, Zr has the highest pitting susceptibility compared with other valve metals such as titanium and niobium [2]. In addition, the hardness of Zr alloys (approximately 200 HV) is insufficient to meet the requirements of structural materials.

The microstructures of alloys, e.g. phase composition and grain size in the matrix, greatly affect both corrosion and mechanical properties [3], [4]. For instance, adding Be to pure Zr has a strong effect on grain refinement, i.e., it improves its mechanical properties [5]. Previous research demonstrated that grain refinement in passivation films can lead to superior corrosion-resistance [6]. Recently, laser surface re-melting (LSR) was used to alter the surface properties by improving microstructure and phase composition. For instance, the surface properties of Zr-2.5Nb alloy, with laser-induced microstructures (especially dense nanotwins), were improved after laser treatment [7]. This is why we chose to investigate LSR, with respect to its potential to improve surface hardness and corrosion resistance of Zr alloys.

In this paper, the surface hardness and corrosion properties of the alloy Zr-5Ti after LSR is investigated. Furthermore, the primary factors that determine the resulting hardness and corrosion resistance are also discussed.

Section snippets

Experimental

Zr-5Ti (wt%) alloy was melted in a ZHT-001 vacuum consumable electro-arc furnace using sponge Zr (Zr + Hf > 99.5 wt%) and sponge Ti (99.7 wt%). To ensure compositional homogeneity, Zr-5Ti alloy was re-melted three times and then hot forged in single β region. Electric-discharge machining was used to cut the alloy into 30 × 30 × 6 mm pieces. Prior to LSR, the alloy surface was polished, cleaned, and dried. Then, the alloy surface was treated using a continuous 3 kW CO2 laser with a wavelength of

Results and discussion

The XRD patterns of as-prepared (AP) and laser surface re-melted (LR) alloys are shown in Fig. 1(a). The experimental results reveal that the phase composition of AP and LR alloys consisted of single α phases, whereas other phases were not observed. Fig. 1(b) and (c) show the microstructure of AP and LR alloys, respectively. It is clear that the width (~560 nm) of α lath of LR alloy is significantly smaller than that of AP alloy (~4.43 μm), which is due to the rapid solidification and cooling

Conclusion

In summary, the surface hardness and corrosion behavior of Zr-5Ti alloy, following LSR, were investigated. The phase composition of LR alloy showed no change. However, the alloy microstructure was significantly more refined after LSR. Furthermore, the surface hardness of the Zr-5Ti alloy was significantly improved, which is due to the α lath refinement. With increasing distance from the surface, the laths gradually coarsened and the hardness gradually decreased. According to the Tafel and

Credit authorship contribution statement

P.F. Ji: Conceptualization, Data curation, Methodology, Software, Investigation, Writing - original draft. B. Li: Validation, Formal analysis, Visualization, Software. S.G. Liu: Validation, Formal analysis, Visualization, Writing - review & editing. B.H. Chen: Investigation, Resources, Supervision, Data curation. X.Y. Zhang: Writing - review & editing. M.Z. Ma: Writing - review & editing. R.P. Liu: Conceptualization, Writing - review & editing.

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.

Acknowledgment

This study was supported by the NSFC (Grant no. 51531005/51827801/51671166).

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