Unveiling oxidation-induced superelasticity in metallic glass nanotubes

Oxidation can degrade the properties and functionality of metals. However, a research team co-led by scientists from the City University of Hong Kong (CityU) recently discovered that severely oxidized metallic glass nanotubes can achieve ultra-high recoverable elastic strain, surpassing most conventional super-elastic metals. . They also discovered the physical mechanisms underlying this super-elasticity.

Their discovery implies that oxidation of low-dimensional metallic glass can result in unique properties for applications in sensors, medical devices, and other nanodevices. The results were published in Natural materials under the title “Oxidation-induced superelasticity in metallic glass nanotubes”.

In recent years, the functional and mechanical properties of low-dimensional metals, including nanoparticles, nanotubes and nanosheets, have attracted attention due to their potential applications in small-scale devices, such as sensors, nanorobots and metamaterials. However, most metals are electrochemically active and susceptible to oxidization in ambient environments, often degrading their properties and functionality.

“Metallic nanomaterials have a high surface-to-volume ratio, which can be up to 10⁸ m^-1. So, in principle, they should be particularly prone to oxidation,” said Professor Yang Yong, from CityU’s Department of Mechanical Engineering, who led the research team with his collaborators.

“To use low-dimensional metals to develop next-generation devices and metamaterials, we need to fully understand the detrimental effects of oxidation on the properties of these nanometals and then find a way to overcome them.”

Therefore, Professor Yang and his team studied the oxidation of nanometals and, contrary to their expectations, they found that severely oxidized metallic glass nanotubes and nanosheets can achieve ultra-high recoverable elastic strain of up to approximately 14% at room temperature, which exceeds the mass. metallic glasses, metallic glass nanowires and many other super-elastic metals.

They fabricated metallic glass nanotubes with an average wall thickness of only 20 nm and nanosheets from different substrates, such as sodium chloride, polyvinyl alcohol and conventional photoresist substrates, with different concentration levels in oxygen.

They then performed 3D atom probe tomography (APT) and electron energy loss spectroscopy measurements. In the results, the oxides were dispersed in the metallic glass nanotubes and nanosheets, unlike conventional bulk metals, in which a solid oxide layer forms on the surface. As the oxygen concentration in the samples increased due to metal-substrate reactions, connected and percolating oxide networks formed inside the nanotubes and nanosheets.

In situ microcompression measurements also revealed that severely oxidized metallic glass nanotubes and nanosheets exhibited a recoverable strain of 10–20%, several times higher than that of most conventional superelastic metals, such as memory alloys. shape and metal erasers. The nanotubes also had an ultra-low elastic modulus of around 20 to 30 GPa.

To understand the mechanism behind this, the team conducted atomistic simulations, which indicated that the superelasticity originated from severe oxidation in the nanotubes and could be attributed to the formation of a percolation network of nano-oxides tolerant to damage in the amorphous structure. These oxide networks not only constrain atomic-scale plastic events during loading, but also lead to the recovery of elastic stiffness during unloading in the metallic glass nanotubes.

“Our research introduces a nano-oxide engineering approach for low-dimensional metallic glasses. The morphology of nano-oxides in metallic glass nanotubes and nanosheets can be manipulated by adjusting the oxide concentration, ranging from isolated dispersions to a connected network,” said Professor Yang.

“Through this approach, we can develop a class of heterogeneous nanostructured ceramic-metal composites by mixing metals with oxides at the nanoscale. Such composites have great potential for various future commercial applications and nanodevices operating in environments difficult, such as sensors, medical devices, micro and nano-robots, springs and actuators,” he added.