Showing papers by "Mingwei Chen published in 2003"
TL;DR: Transmission electron microscope observations that provide evidence of deformation twinning in plastically deformed nanocrystalline aluminum underscore a transition from deformation mechanisms controlled by normal slip to those controlled by partial dislocation activity when grain size decreases to tens of nanometers, and they have implications for interpreting the unusual mechanical behavior of nanocrystaline materials.
Abstract: We report transmission electron microscope observations that provide evidence of deformation twinning in plastically deformed nanocrystalline aluminum The presence of these twins is directly related to the nanocrystalline structure, because they are not observed in coarse-grained pure aluminum We propose a dislocation-based model to explain the preference for deformation twins and stacking faults in nanocrystalline materials These results underscore a transition from deformation mechanisms controlled by normal slip to those controlled by partial dislocation activity when grain size decreases to tens of nanometers, and they have implications for interpreting the unusual mechanical behavior of nanocrystalline materials
1,039 citations
TL;DR: High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces, an example of how shock loading can result in the synthesis of novel structures and materials with substantially altered properties.
Abstract: High-resolution electron microscope observations of shock-loaded boron carbide have revealed the formation of nanoscale intragranular amorphous bands that occur parallel to specific crystallographic planes and contiguously with apparent cleaved fracture surfaces. This damage mechanism explains the measured, but not previously understood, decrease in the ballistic performance of boron carbide at high impact rates and pressures. The formation of these amorphous bands is also an example of how shock loading can result in the synthesis of novel structures and materials with substantially altered properties.
481 citations
TL;DR: In this paper, a high temperature microsample tensile testing technique has been employed to characterize the mechanical behavior of a platinum modified nickel aluminide bond coat at 0% and 28% of cyclic life in the temperature range of 25 to 1150 °C.
191 citations
TL;DR: In this paper, phase transformations in a platinum modified nickel aluminide bond coat were investigated by in situ high temperature X-ray diffraction analysis, and the effect of the martensite on TBC life is sensitive to the transformation temperatures relative to the creep strength of the bond coat.
129 citations
TL;DR: In this paper, the structure of the as-received bond coat was confirmed to be an ordered B2 phase, but significant lattice strains were found which were associated with the formation of a modulated structure.
Abstract: Microstructural evolution induced by thermal cycling in a platinum modified diffusion aluminide bond coat was investigated with transmission electron microscopy and elevated temperature X-ray diffraction (XRD). Before thermal cycling, the structure of the as-received bond coat was confirmed to be an ordered B2 phase, but significant lattice strains were found which were associated with the formation of a modulated structure. Thermal cycling resulted in significant changes in the microstructure of the bond coat. The compositional development assisted by chemical diffusion during thermal cycling has been related to the transformation of the bond coat from its original B2 structure to a Ni-rich L1 0 martensite. The L1 0 martensite was found to be stable at temperatures below approximately 600 °C and the B2 parent phase stable at elevated temperatures. Quantitative XRD measurements indicated that the volume of the B2 phase is approximately 2% larger than that of the martensite, which produces a ∼0.7% linear transformation strain.
99 citations
TL;DR: In this article, the kinetics and mechanism of diffusionally accommodated interfacial sliding under far-field shear and normal stresses was studied, based on diffusion-bonded Al-Si-Al sandwich specimens.
30 citations
TL;DR: In this paper, a platinum-modified diffusion aluminide bond coat was investigated with transmission electron microscopy (TEM), X-ray diffraction, and electron microprobe analysis.
Abstract: Microstructural and chemical evolution induced by thermal cycling of a platinum-modified diffusion aluminide bond coat was investigated with transmission electron microscopy (TEM), X-ray diffraction, (XRD) and electron microprobe analysis. As-fabricated, the bond coat was confirmed to be an ordered B2 structure, but the underlying microstructure was found to be modulated. Thermal cycling resulted in a primarily outward diffusion of Ni and the formation of a Ni-rich bond coat containing secondary L12 precipitates. Closer inspection of the bond coat revealed a transformation from its original B2 structure to a L10 martensite. In-situ TEM observations indicated that the martensite is stable at lower temperatures and that the parent B2 structure reappears at higher temperatures. These observations can be used to explain the variations in strength that have recently been measured in thermally cycled bond coats. The resulting transformation strain is also argued to play an important role in determining the accumulation of stress and strain in thermally cycled thermal barrier coatings (TBCs).
22 citations