Orientation imaging: The emergence of a new microscopy
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INTRODUCTION
THIS article describes a new microscopy for probing microstructure in polycrystalline materials. Polycrystalline microstructures are extremely rich in diversity, and no single microscopy can resolve the entire spectrum o f structures present. New forms o f microscopy inevitably expose new features o f microstrncture and new insights into structure-properties relationships. The microscopy described here is called "orientation imaging," owing to the fact that contrast is formed by gradients o f local lattice orientation. More specifically, backscattered Kikuchipatterns are analyzed to determine lattice orientation in small localized regions. In its current configuration, w h i c h is described in Section II, orientation imaging microscopy utilizes a standard scanning electron microscope in conjunction with high-gain t e l e vision technology to interrogate the flux o f backscattered electrons emanating from crystalline regions of ~ 0 . 2 / ~ m in size. Recently developed algorithms for rapidly indexing patterns, coupled with stage (or beam) motion automation, enable scanning o f the polycrystalline surface with a probe which is extremely sensitive to the precise lattice orientation at each scan point. The contrast obtained in such scanning results from precisely defined spatial gradients in lattice orientation present in the scanned surface. At its current state o f development, it is evident that orientation imaging offers a very attractive coupling o f several disparate methods o f microstructural inquiry. Standard optical or scanning electron microscopy relies heavily upon various mechanisms o f contrast formation which are related to lattice orientation gradients, but in rather imprecise ways. T o cite an example, the effect o f chemical etching o f metallographic surfaces is the formation o f a surface topography w h i c h is dependent upon crystal orientation. The disturbed l a y e r between grains may be attacked at a different rate from the grain interiors. Detailed knowledge o f lattice orientation or disorientation effects on etching is rarely known, but conventional microscopy often relies upon such topographic variation to form contrast. Thus, the size and shapes o f grains or crystallites and their chemical phase B R E N T L. A D A M S , Professor, and K A R S T E N K U N Z E , Postdoctoral Associate, are with the Department of Manufacturing Engineering and Engineering of Technology, Brigham Y o u n g University, P r o v o , UT 84602. STUART I. WRIGHT, Postdoctoral Associate, is with Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted June 4 , 1992. METALLURGICAL TRANSACTIONS A
are revealed, but it is not generally possible to discern the lattice orientation o f individual components. In contrast, the standard methods o f modern texture analysis, w h i c h rely upon X-ray o r neutron diffraction, provide information about volume fractions o f grains o f specified lattice orientation but no information about grain size, shape morphology, or spatial distribution. O
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