Chemisorptive electron emission and atomic force microscopy as probes of plastic deformation during fracture at a metal/
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We examine the use of chemisorptive emission (electron emission accompanying the adsorption of a reactive gas on a metal surface) and atomic force microscopy as measures of plastic deformation during fracture along a metallic Mg/glass interface. Localized ductile deformation in the metallic phase enhances the fracture energy, exposes metallic Mg to the reactive O2 atmosphere, and produces intense emissions. The number of electrons emitted following fracture in low-pressure oxygen atmospheres is strongly correlated with the total energy expended during failure (peel energy). The presence of localized ductile deformation is verified by atomic force microscopy (AFM): voids are observed on surfaces yielding significant emissions and enhanced fracture energies. These voids are not observed on samples yielding the lowest peel energies and emission intensities, i.e., where the contribution of deformation to the peel energy is negligible. Quantitative use of roughness data derived from the AFM images is, however, problematic. The potential for chemisorptive electron emission as a probe of deformation along interfaces involving Mg, Ti, Zr, and Al is promising.
I. INTRODUCTION Plastic deformation in the metallic phase of ceramicmetal composites often makes a dominant contribution to the energy required for fracture. In particular, the amount of deformation exhibited by the metallic phase is a strong function of the tortuosity or roughness of the interface,12 the thickness of the metallic phase,3'4 and the presence of phase segregated material (interphases) along the interface.3 At present, estimates of the contribution of deformation to the toughness of ceramic-metal composites involve careful fractographic examination and numerical modeling. More direct methods are desirable, and are especially important where ductile deformation of the metallic phase and brittle fracture at or near the interface occur simultaneously.5-6 In this work, we examine the phenomenon of chemisorptive emission (CSE—electron emission accompanying the adsorption of reactive gases with clean metal surfaces) as a possible probe of plastic behavior. In previous work,7 we showed that CSE is a sensitive measure of plasticity during tensile deformation of a number of metals. In principle, the method is similar to the use of photoelectron emission as a probe of tensile deformation8 and fatigue damage9 in metals such as Al and Mg. Dislocations intersecting the surface increase the surface area of the metallic phase and thus increase a) Permanent
address: Department of Mechanical Engineering, Kansai University, 3-3-35 Yamatecho, Suita, Osaka 564, Japan. J. Mater. Res., Vol. 10, No. 8, Aug 1995 http://journals.cambridge.org
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the photoelectron emission intensities. The chief advantage of CSE is its uniform sensitivity to deformation in a variety of sample geometries. Ambient gas molecules are adsorbed uniformly on complex surfaces, whereas in photoelectron measurements, portions of the surface are often shadowed from the UV source. Fu
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