Chih-Pin Chuang

Microstructural Characteristics and Mechanical Behaviors of AlCoCrFeNi High-Entropy Alloys at Ambient and Cryogenic Temperatures

The phase-formation rule of high-entropy alloys (HEAs) with different microstructures is discussed, based on the atom-size difference in multicomponent alloys. For the single-phase HEA with the composition of AlCoCrFeNi, the yielding strengths and fracture strengths at cryogenic temperatures increase distinguishingly, compared to the corresponding mechanical properties at ambient temperature. However, the plasticity at 298 and 77 K changes very gently, while the fracture modes are intergranular and transgranular, respectively.

Microyielding of Core-Shell Crystal Dendrites in a Bulk-metallic-glass Matrix Composite

In-situ synchrotron x-ray experiments have been used to follow the evolution of the diffraction peaks for crystalline dendrites embedded in a bulk metallic glass matrix subjected to a compressive loading-unloading cycle. We observe irreversible diffraction-peak splitting even though the load does not go beyond half of the bulk yield strength. The chemical analysis coupled with the transmission electron microscopy mapping suggests that the observed peak splitting originates from the chemical heterogeneity between the core (major peak) and the stiffer shell (minor peak) of the dendrites. A molecular dynamics model has been developed to compare the hkl-dependent microyielding of the bulk metallic-glass matrix composite. The complementary diffraction measurements and the simulation results suggest that the interface, as Maxwell damper, between the amorphous matrix and the (211) crystalline planes relax under prolonged load that causes a delay in the reload curve which ultimately catches up with the original path.

Fatigue-Induced Damage in Zr-Based Bulk Metallic Glasses

In the present work, we investigate the effect of “fatigue” on the fatigue behavior and atomic structure of Zr-based BMGs. Fatigue experiments on the failed-by-fatigue samples indicate that the remnants generally have similar or longer fatigue life than the as-cast samples. Meanwhile, the pair-distribution-function (PDF) analysis of the as-cast and post-fatigue samples showed very small changes of local atomic structures. These observations suggest that the fatigue life of the 6-mm in-diameter Zr-based BMG is dominated by the number of pre-existing crack-initiation sites in the sample. Once the crack initiates in the specimen, the fatigue-induced damage is accumulated locally on these initiated sites, while the rest of the region deforms elastically. The results suggest that the fatigue failure of BMGs under compression-compression fatigue experiments is a defect-controlled process. The present work indicates the significance of the improved fatigue resistance with decreasing the sample size.

The effect of hydrogen charging on Ln-based amorphous materials

In present work, the effects of hydrogen charging on Ln-based (Ln=La,Ce) bulk-metallic glasses (BMG) are studied. The (La0.5Ce0.5)65Al10Co25 were charged with hydrogen by an electrochemical method in an alkali solution. The hydrogen concentration in the sample after a 36 h charge can reach as high as 1286 w-ppm. With the presence of hydrogen atoms, the hardness of specimen increased by 80% compared to the as-cast samples. The structural evolution of the amorphous matrix due to the hydrogen-uptake process was investigated by the high-energy x-ray scattering technique. The sample surface was crystallized after hydrogen charging. X-ray diffraction measurements revealed broad crystalline peaks superimposed on an amorphous-scattering pattern. The crystalline phase grew from the surface to at least one hundred microns deep into the amorphous matrix. The atomic arrangements of both amorphous and crystalline phases were characterized by the atomic pair-distribution function.