Fig. 5-2 In-situ observation techniques show materials loaded, deformed, and failed throughout their lives at high temperatures.

Fig. 5-3 Voids nucleated (top), grew (middle), and coalesced (bottom) along the grain boundary of Ni based alloy loaded at 800 degrees cent. Fine and coarse precipitates are visible in the grains and along the grain boundaries.

Materials loaded (stressed) continuously at high temperatures will deform until failure. Deformation increases with elapsed time. This phenomenon is called creep. As humans have their own lifetimes, so do materials in structures. We can observe the deformation of materials at high temperatures throughout their lifetimes by means of an electron microscope combined with a materials testing machine. This in-situ observation technique is illustrated in Fig. 5-2. The results are very useful for diagnosing the remaining life to failure for the materials of components in use. Metallic materials consist of grains, which are akin to cells in the human body. These grains have a key role in the components of mechanical structures made from inorganic materials. Grains will deform in the loading direction. However it is not always true that the grains will fail, particularly at high temperatures. In fact, it is the boundaries between adjacent grains that cause failures. A vacancy exists when an atom is missing from a normal lattice position. The fraction of vacant lattice sites increases rapidly with temperature. Vacancies easily move under loading and make voids on grain boundaries at high temperatures. Strain is supplied by the plating out of atoms from the voids while the voids are growing, as shown in Fig. 5-3. Coalescence of voids makes crevices along grain boundaries. Crevices will appear at different grain boundaries here and there. Grains bonded tightly in materials before loading will be separated by internal forces and structures will begin to sustain damage.

Reference K. Kikuchi, In-situ Observation Techniques and Micromechanics of Heat-Resistant Structural Alloy, Proc. 41st Jpn. Natl. Symp. on Strength, Fracture and Fatigue, 75 (1996).

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