Elsevier

Scripta Materialia

Volume 131, 1 April 2017, Pages 84-88
Scripta Materialia

Regular Article
Novel micro-flat springs using the superior elastic properties of metallic glass foils

https://doi.org/10.1016/j.scriptamat.2017.01.012Get rights and content

Abstract

A thin metallic glass foil of 100 mg mass forming a sinusoidal arc behaves as non-conventional flat micro-spring withstanding loads 105 times higher than its load. Upon a normal load applied on the top of the arc, the foil deforms elastically leading to sinusoidal wavy patterns of higher order. The lifespan of the novel spring is higher than conventional low cycle springs and can potentially be further improved by eliminating surface and edge preparation induced defects. This unique behavior of metallic glass foils has the potential to revolutionize the field of springs and can be exploited for numerous applications.

Introduction

Springs are critical components in almost all modern technologies at various scales, from airplanes and trains to micro-electromechanical systems (MEMS) devices, with a primary role to store elastic energy and/or absorb mechanical shocks. The principal function of springs is based on the elastic deformation of the spring material (commonly steel, copper, or nickel alloys) under an applied load and the recovery of its initial shape after unloading [1]. In this respect, an ideal spring material would exhibit very high mechanical strength and elastic limit, much like the extraordinary properties of metallic glasses.

Unlike crystalline metals, metallic glasses lack a periodic lattice with slip planes on which mobile dislocations can cause plastic flow. Consequently, they show exceptionally high mechanical strengths up to 5 GPa and a wide elastic deformation range on the order of 2% before the onset of plastic deformation, about ten times higher than conventional crystalline metals [2], [3]. In addition, due to their amorphous structure and glassy nature, metallic glasses exhibit near net shape casting ability and excellent super-plastic formability in the super cooled liquid region, properties that make them attractive for various applications in different technological fields including in the biomedical, MEMS and NEMS (Micro- and Nano-Electro Mechanical Systems) sectors [4], [5], [6], [7]. However, the bottleneck for their wider application is related to their limited ductility in tension. Nevertheless, this limitation does not impede the exploitation of their elastic properties in spring applications.

Recently, Aljerf et al. [8] have shown that metallic glass foils can take complex shapes and wavy forms without thermal embrittlement through a rapid thermal annealing treatment (thermo-elastic processing) by controlling the structural relaxation kinetics. More recently, we have reported on a reversible elastic undulatory response of an arc-shaped metallic glass foil under normal load that can be utilized as an electromechanical switch [9].

Here, we explore the elastic deformation of metallic glass foils and the formation of sinusoidal wavy patterns for the design of a novel type of micro-flat spring with enhanced properties and functionality. The relation between load and displacement for the novel type of springs is overall exponential with discrete discontinuities at the position of multiplications of the sinusoidal arcs. The fatigue life of the novel springs was found to be better that of low cycle conventional springs.

Section snippets

Experimental details

Commercially available Fe-based metallic glass foils (Metglass, Fe90.65B3.9Cr2.75Si2.7 at.%) with 19 μm thickness and 25 mm width and various lengths within the range 20 to 55 mm were elastically deformed to form sinusoidal arcs and fixed on a support.

The amorphous structure of the foils was verified by X-ray diffraction (XRD) using a Rigaku Geigerflex diffractometer with Cu Ka radiation.

A normal load was applied on top of the arc surface using a Tinius Olsen H10kS compression machine. An Allied

Elastic wavy response of metallic glass spring-foils

When a normal load F is applied on the top of a metallic glass foil that has been elastically shaped to form a sinusoidal arc, Fig. 1a, the foil exhibits an extraordinary elastic/buckling behavior that can be utilized for developing a novel type of non-linear flat springs. Upon loading the foil deforms elastically; at specific values of load or displacement, the foil changes shape increasing the number of sinusoidal arcs from one initial arc successively to 2, 3, 4, 5 (and more) arcs, Fig. 1

Conclusion

The exceptional mechanical properties of metallic glasses allow a thin foil elastically shaped to a sinusoidal arc form to exhibit a unique reversible elastic wavy response, which can be exploited for designing of micro-flat springs of a novel type. Under a normal load applied on the top of the sinusoidal arc, the foil deforms elastically leading to the successive formation of sinusoidal wavy patterns of higher order. The non-linear load versus displacement response allows the foil to act as a

Acknowledgments

This work is dedicated to the memory of Professor Alain Reza Yavari (one of the authors) who had been an inspirational mentor and a beloved friend to the co-authors. The work was funded by the ANR Emergence project New Vitrified Springs (Grant #ANR-12-EMMA-0054) coordinated by Pr. A.R. Yavari.

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