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