X.H. Du

New criterion in predicting glass forming ability of various glass-forming systems

It has been confirmed that glass-forming ability (GFA) of supercooled liquids is related to not only liquid phase stability but also the crystallization resistance. In this paper, it is found that the liquid region interval (Tl – Tg) characterized by the normalized parameter of Tg/Tl could reflect the stability of glass-forming liquids at the equilibrium state, whilst the normalization of supercooled liquid region ΔTx=(Tx – Tg), i.e. ΔTx/Tx (wherein Tl is the liquidus temperature, Tg the glass transition temperature, and Tx the onset crystallization temperature) could indicate the crystallization resistance during glass formation. Thus, a new parameter, defined as ξ = Tg/Tl+ΔTx/Tx is established to predict the GFA of supercooled liquids. In comparison with other commonly used criteria, this parameter demonstrates a better statistical correlation with the GFA for various glass-forming systems including metallic glasses, oxide glasses and cryoprotectants.

Microstructure and Mechanical Properties of Nano-ZrO2 and Nano-SiO2 Particulate Reinforced AZ31-Mg Based Composites Fabricated by Friction Stir Processing

Friction stir processing (FSP) has been applied to fabricate 10~20 vol% nano-sized ZrO2 and 5~10 vol% nano-sized SiO2 particles into an Mg-AZ31 alloy to form bulk composites under the FSP parameters of advancing speed of 800 rpm and pin rotation of 45 min/min. The microstructures and mechanical properties of the resulting composites were investigated. The clustering size of nano-ZrO2 and nano-SiO2 particles, measuring average ~200 nm was relatively uniformly dispersed, and the average grain size of the both Mg alloy of the composites varied within 1.0~2.0 μm after four FSP passes. No evident interfacial product between ZrO2 particles and Mg matrix was found during the FSP mixing in AZ31-Mg/ZrO2. However, significant chemical reactions at the AZ31-Mg/SiO2 interface occurred to form the Mg2Si phase. The mechanical responses of the nano-composites in terms of hardness and tensile properties are examined and compared.

A Modified Glass Formation Criterion for Various Glass Forming Liquids with Higher Reliability

A modified indicator of the glass forming ability (GFA) from the previous γ = Tx/(Tl+Tg) for various glass forming liquids is proposed based on a conceptual approach which combines more acceptable physical metallurgy views in terms of the time-temperature-transformation diagrams. It is found that the glass forming ability for glass forming liquids is closely associated mainly with two factors, i.e. (2Tx−Tg) and Tl (wherein Tx is the onset crystallization temperature, Tg the glass transition temperature, and Tl the liquidus temperature), and could be predicated by a unified parameter γm defined as (2Tx−Tg)/Tl. This approach is confirmed and validated by experimental data in various glass forming systems including oxide glasses, cryoprotectants and metallic glasses, which all shows a higher reliability when their glass forming ability is predicted by the modified parameter.

Designing a stronger interface through graded structures in amorphous/nanocrystalline ZrCu/Cu multilayered films.

Many multilayered nano-structures appear to fail due to brittle matter along the interfaces. In order to toughen them, in this study, the microstructure and interface strength of multilayered thin films consisting of amorphous ZrCu and nanocrystalline Cu (with sharp or graded interfaces) are examined and analyzed. The interface possesses a gradient nature in terms of composition, nanocrystalline phase size and volume fraction. The bending results extracted from the nano-scaled cantilever bending samples demonstrate that multilayered films with graded interfaces would have a much higher interface bending strength/strain/modulus, and an overall improvement upgrade of more than 50%. The simple graded interface design of multilayered thin films with improved mechanical properties can offer much more promising performance in structural and functional applications for MEMS or optical coating.

Nano-scaled diffusional or dislocation creep analysis of single-crystal ZnO

The nanoindentation time-dependent creep experiments with different peak loads are conducted on c-plane (0001), a-plane (112¯0) and m-plane (101¯0) of single-crystal ZnO. Under nano-scaled indentation, the creep behavior is crystalline orientation-dependent. For the creep on (0001), the stress exponent at low loads is ∼1 and at high loads ∼4. The stress exponents under all loads are within 3∼7 for the creep on (112¯0) and (101¯0). This means that diffusion mechanism and dislocation mechanism is operative for different planes and loads. The relative difficulty of dislocations activation is an additional factor leading to the occurring of diffusion creep on the c-plane of single-crystal ZnO.

Superplastic deformation behavior of a spray-deposited eutectic NiAl/Cr(Mo) alloy doped with Dy

Abstract The tensile deformation behavior at elevated temperatures and the associated mechanisms are investigated for hot isostatically pressed NiAl-28Cr-5.9Mo-0.1Dy fabricated through spray forming. Superplasticity is observed at above 1323 K under an initial strain rate lower than 10−3 s−1 with an m value of 0.5. The activation energies for superplastic flow are found to be half of that for self-diffusion in NiAl. Grain boundary sliding between NiAl and adjacent NiAl or Cr(Mo) grains with local range accommodation, i. e., dynamic recovery, is suggested to be responsible for the superplastic deformation of the alloy.