Elsevier

Materials Letters

Volume 264, 1 April 2020, 127315
Materials Letters

Differences in surface mechanical properties of Zr-based bulk metallic glass related to stress condition

https://doi.org/10.1016/j.matlet.2020.127315Get rights and content

Highlights

  • Double V-notched defects to construct stress gradients.

  • Stress and Poisson effect caused the indentation morphological anisotropy.

  • Inhomogeneous stress induced significant differences in surface mechanical properties.

Abstract

Differences in surface mechanical properties of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) was revealed through indentation tests. Inhomogeneous stress condition was constructed through fabricating asymmetrical double V-notches. Indentation responses at various locations along with and perpendicular to the connection line between the notch tips indicated a significant difference in surface hardness with range from 1.012 GPa to 1.463 GPa, corresponding to a tensile stress ranging from 11.8 MPa to 246.3 MPa. The stress gradients induced by prefabricted notches cased the anisotropy of residual indentation morphologies. Furthermore, selected area electron diffraction analysis adjacent to the notch tip exhibited typical halo ring and amorphous state, which indicated that the morphological anisotropy and differences in indentation responses were mainly caused by inhomogeneous stress and Poisson effect rather than local crystallization behaviors.

Introduction

The surface mechanical properites of structural materials determine the reliability under service condition, including friction, abrasive wear, high pressure contact and surface fatigue. A series of modification processes to enhance the surface mechanical properties [1], [2], such as shot peening, laser strengthening, chemical or physical vapor deposition, have been used in a broad variety of fiels. Meanwhile, typical depth-sensing nanoindentation [3] and nano scratch testing are effective techniques to directly reveal the evolution behavior of microstructure and obtain parameters involving bonding strength, friction coefficient [4], wear rate, coating thickness, Young’s modulus and hardness [5]. Bulk metallic glasses (BMGs) do not exist crystal defects (dislocation or grain boundary) [6], [7], exhibit unique surface mechanical properties including excellent wear resistance and higher hardness than conventional crystalline alloys [8]. The actual service condition of BMGs frequently involve combined loading or inhomogeneous stress condition. Although qualitative changes in surface mechanical properties induced by inhomogeneous stress condition were investigated on magnesium alloy [9] and monocrystalline silicon [10], the quantitative corelation between the evolution of surface mechanical properties and stress field need to be taken into account. Meanwhile, considering the amorphous state determines the surface mechanical properties of BMGs, the crystallization behaviors induced by inhomogeneous stress field with stress gradient may also affect the surface properties. In this letter, taking typical Zr55Cu30Al10Ni5 BMG as target, a feasible construction method of inhomogeneous stress field was clarified, through the descriptions of morphological anisotropy, indentation responses and local potential crystallization behavior, the effect of inhomogeneous stress on differences in surface mechanical properties was studied.

Section snippets

Finite element analysis

In order to establish a controllable inhomogeneous stress field, asymmetrical double V-notches with included angles of 40°, axial spacing of 1 mm and transverse spacing of 0.2 mm were designed as shown in Fig. 1a. The notch radiuses were designed as 60 μm considering the subsequent wire cutting using molybdenum wire with diameter of 120 μm. Finite element analysis (using ANSYS workbench 15.0) was carried out to obtain the stress contours. As shown in Fig. 1a, the experimental stress–strain (σ-ε

Experiments and discussions

On basis of the established inhomogeneous stress field, flat plate amorphous state Zr55Cu30Al10Ni5 (at. %) [13], [14] specimens, with consistent size (gauge length section: l × w × t: 3.5 × 1.2 × 0.6 mm) as the FEA model shown in Fig. 1b, were fabricated through arc melting, casting, wire cutting and subsequent single side mechanical polishing. Meanwhile, according to the defined two node paths, successive indentation tests (using a Berkovich indenter, Nano Indenter G200, Agilent) were carried

Conclusions

Inhomogeneous stress induced significant differences in surface mechanical properties were experimentally revealed. Prefabrication of double V-notched defects were an feasible method to construct stress gradients along with and perpendicular to the connection line between the notch tips. The coupling effect of tensile stress and Poisson effect caused the indentation morphological anisotropy and inhomogeneity of local plastic flow behavior and contributed to the inhomogeneity of hardness

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

This work is funded by the National Natural Science Foundation of China (51875241, U1601203), National Key R&D Program of China (2018YFF010124, YS2018YFA070002) and Jilin Province Science and Technology Development Plan (20190302078GX, 20180201126GX).

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