Research on the Mg-based Metallic Glass Films with Functional Gradient and Phase Change Properties

鎂基金屬玻璃膜的梯度功能及相變特性研究

Student thesis: Doctoral Thesis

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Author(s)

  • Ge WU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date25 Aug 2015

Abstract

We successfully fabricated Mg-Cu-Y metallic glass films (MGFs) by using a magnetron sputtering method. The amorphous nature of these films resulted in an abnormally high hardness, and their relative physical properties were significantly changed after crystallization. This thesis begins from an engineering viewpoint by focusing on Mg-based MGF as phase change storage and structural coating applications for wear resistance and corrosion resistance, and then focus on a series of studies.

Mg-based MGFs with different chemical compositions were deposited on an unheated Si (0 0 1) substrate by direct current (dc)-magnetron sputtering at various Ar pressures. The resistivity and reflectivity were changed after crystallization of the films. In fact, the Mg58Cu29Y13 MGF fabricated by 0.5 Pa Ar pressure during sputtering has the largest resistivity contrast of 36.4% and reflectivity contrast of 10% between the amorphous and crystalline states. Therefore, Mg58Cu29Y13 MGF is well-suited for use in phase change storage applications.

The Mg58Cu29Y13 MGF also exhibited phase separation phenomena above the glass transition temperature Tg. The activation energy for nucleation decreased substantially in the Mg-enriched area, thereby resulting in an increase of the nucleation rate above the crystallization temperature TX. As such, the Mg58Cu29Y13 MGF has rapid phase change properties and hence significant potential for use in phase change storage.
The wear and corrosion resistances of the traditional Mg alloy can be improved by depositing Mg-based MGF on its surface. Furthermore, surface mechanical attrition treatment (SMAT) of the alloy results in the formation of a hardening layer on the surface of the alloy, which reduces the hardness mismatch between the film and the substrate. The adhesion between the film and the substrate also increases owing to SMAT-induced improvements of the diffusion properties of the sample surface. The reduced hardness mismatch and increased adhesion lead to excellent wear and impact resistance of the Mg-based MGF deposited SMATed Mg alloy under high load.

In addition, the Mg-based MGF on the SMATed Mg alloy effectively suppresses crack propagation during the tensile testing, which leads to a significant increase in the tensile plasticity of the Mg alloy.

The nanoscale heterogeneous structure of the Mg-based MGF was verified via several experimental methods. In order to reduce this heterogeneity, low rate deposition, appropriate substrate heating and substrate bias voltage were applied during sputtering, and then an ultra-hard Mg-based MGF was fabricated; the hardness of the MGF increases from 5.2 GPa to 6.3 GPa. This ultra-hard Mg-based MGF has significant potential for use in fabricating ultra-fast phase change storage.

A granular-structured Mg-based MGF fabricated on the Mg alloy substrate is attributed to the homogeneity of the deposition. Moreover, by increasing the negative bias voltage during sputtering, the size of the amorphous particle is reduced from several micrometers until the particles were no longer observed. Furthermore, a Mg-based metallic nano-grained glass with particle diameter of ~25 nm was fabricated by using the SMATed Mg alloy as a substrate. This nano-grained glass has significant potential for use in hydrophobic and catalytic applications.

    Research areas

  • Metallic Glass Film, Phase Change, Mg alloy, SMAT, Mechanical Properties