Wenjun Cai

Optimizing ductility and fracture of amorphous metal thin films on polyimide using multilayers

Aluminum–manganese (Al–Mn) thin films with manganese concentration up to 20.5 at.% were deposited on polyimide (PI) substrates. A variety of phases, including supersaturated fcc (5.2 at.% Mn), duplex fcc and amorphous (11.5 at.% Mn), and completely amorphous phase (20.5 at.% Mn) were obtained by adjusting alloying concentration in the film. Tensile deformation and subsequent fracture of strained Al–Mn films on PI were investigated experimentally and by finite element simulations. Compared with crystalline and dual phase counterparts, amorphous thin film exhibits the highest fracture stress and fracture toughness, but limited elongation. Based on the fracture mechanism model, a multilayer scheme was adopted to optimize the ductility and the fracture properties of the amorphous film/PI system. It was found that by sandwiching the amorphous film (20.5 at.% Mn) between two ductile Cu layers, the elongation can be improved by more than ten times, and the interfacial fracture toughness by more than twenty times. This design provides important guidelines to obtain optimized mechanical properties of future flexible electronics devices.

Anisotropic Mechanical and Giant Magneto-Impedance Properties of Cobalt-Rich Amorphous Ribbons

A comparative study was performed on the mechanical and giant magneto-impedance (GMI) properties in the longitudinal and transverse directions of Co69Fe4Ni1Mo2B12Si12 amorphous ribbons. Both mechanical and GMI properties were found to be anisotropic. Kerr microscopy shows the presence of a stripe-type domain structure with the magnetic easy axis parallel to the longitudinal direction. The fracture strength, elastic modulus, and fracture toughness in the transverse direction was higher than those in the longitudinal direction. A larger GMI response was achieved in the transverse direction at a frequency range where both the domain wall motion and spin rotation dominantly contributed to the effective permeability and hence the magneto-impedance. The current study paves the way for designing Co-rich amorphous ribbons as desirable components in electronics such as magnetic sensors.

Bayesian latent degradation performance modeling and quantification of corroding aluminum alloys

Abstract Corrosion-induced degradation of aluminum alloys is a key causing factor of structural failure for many safety-critical and mission-critical engineering components/systems, such as aircraft wing and nuclear battery. Successful modeling and quantification of their degradation performance is important yet challenging since the degradation performance is not often directly observed. Existing data-driven degradation models often assume the directly observable degradation performance. Although existing physical models allow evaluating latent degradation performance, they are not flexible enough to capture the degradation complexity and fail to quantify (i) the model parameters uncertainty due to the limited experimental data; and (ii) the degradation performance variability among multiple test units. In this paper, we propose a physical-statistical modeling approach to evaluating and quantifying the latent degradation performance of corroding aluminum alloys at both individual and population levels. Based on the non-destructive testing data, the fractional order system dynamics is first considered to capture the complex electrochemical process of corroding aluminum alloys. Bayesian hierarchical models are further established to take into account both the parameters uncertainty of individual test unit and the population variability of multiple units. Real case studies are also provided to illustrate the proposed work and demonstrate its effectiveness in the experimental testing applications of different corroding metallic units.

Determining Tribocorrosion Rate and Wear-Corrosion Synergy of Bulk and Thin Film Aluminum Alloys

The increasing complexity and severity of service conditions in areas, such as aerospace and marine industries, nuclear systems, microelectronics, batteries, and biomedical devices, etc., impose great challenges on the reliable performance of alloys exposed to extreme conditions where mechanical and electrochemical attack co-exist. Finding ways for alloys to mitigate the combined attack of wear and corrosion (i.e., tribocorrosion) under such extreme conditions is thus highly critical for improving their reliability and service lifetime when used in such conditions. The challenge lies in the fact that wear and corrosion are not independent of each other, but rather work synergistically to accelerate the total material loss. Thus, a reliable method to evaluate the tribocorrosion resistance of metals and alloys is needed. Here, a protocol for measuring the tribocorrosion rate and wear-corrosion synergy of Al-based bulk and thin film samples in a corrosive environment under room temperature is presented.