Browsing by Author "Chang, H. T."

Now showing 1 - 2 of 2

A transition of three to two dimensional Si growth on Ge (100) substrate
(2012-12-15)
Tu, W. -H.
;
Lee, C. -H.
;
Chang, H. T.
;
Lin, B. -H.
;
Hsu, C. -H.
For the initial growth of Si on Ge, three-dimensional Si quantum dots grown on the Ge surface were observed. With increasing Si thickness, the Si growth changes from three-dimensional to two-dimensional growth mode and the dots disappear gradually. Finally, the surface is smooth with the roughness of 0.26 nm, similar to the original Ge substrate, when 15 nm Si is deposited. More Ge segregation on the wetting layer leads to more open sites to increase the subsequent Si growth rate on the wetting layer than on the Si dots. The in-plane x-ray diffraction by synchrotron radiation is used to observe the evolution of tensile strain in the Si layer grown on Ge (100) substrate. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770408]
Dual heterogeneous structures lead to ultrahigh strength and uniform ductility in a Co-Cr-Ni medium-entropy alloy
(2020-05-13)
Du, X. H.
;
Li, W. P.
;
Chang, H. T.
;
Yang, T.
;
Duan, G. S.
Alloys with ultra-high strength and sufficient ductility are highly desired for modern engineering applications but difficult to develop. Here we report that, by a careful controlling alloy composition, thermomechanical process, and microstructural feature, a Co-Cr-Ni-based medium-entropy alloy (MEA) with a dual heterogeneous structure of both matrix and precipitates can be designed to provide an ultra-high tensile strength of 2.2GPa and uniform elongation of 13% at ambient temperature, properties that are much improved over their counterparts without the heterogeneous structure. Electron microscopy characterizations reveal that the dual heterogeneous structures are composed of a heterogeneous matrix with both coarse grains (10 similar to 30 mu m) and ultra-fine grains (0.5 similar to 2 mu m), together with heterogeneous L1(2)-structured nanoprecipitates ranging from several to hundreds of nanometers. The heterogeneous L1(2) nanoprecipitates are fully coherent with the matrix, minimizing the elastic misfit strain of interfaces, relieving the stress concentration during deformation, and playing an active role in enhanced ductility. Improving both strength and ductility simultaneously in structural metals and alloys remains a challenge. Here, the authors design a heterogeneous structure in a Co-Cr-Ni alloy that results in ultrahigh strength and significant uniform elongation.