Characterization of microscopic strain and stress in polycrystalline material using white X-ray microbeam diffraction(T007_1)

摘要

White X-ray microbeam diffraction has emerged as a powerful tool for the study of internal stress andelastic strain variations within a single grain of polycrystalline materials. In simple terms, this analysistechnique uses microstructural information obtained mainly from Laue diffraction patterns and energyprofile of the comprising Laue spots. They are obtained by irradiating X-ray microbeam onto a localsampling volume and detecting resulting diffracted beam in transmission geometry with a flat paneldetector and solid state detector. Above mentioned parameters are sensitive to local deformation statesof sampling volume. For example, Laue spots reveal some shift, streak, or split after deformation due tolattice rotation and formation of subgrains. Critical in this analysis is the detection and use of clear Lauespots; accurate measurements could be achieved provided that clearly indexed high-intensity Lauespots are used. In the case of polycrystalline shape memory and superelastic materials, accuratestress/stain measurements may be more challenging because of involved complex deformationmechanisms, which are considered to be important information on the microscopic evolution ofstress/strain. The underlining deformation mechanisms in these types of materials are dislocation slipand martensitic transformation. Stress/strain in these polycrystalline materials are expected to be veryinhomogeneous because evolution behavior of martensitic transformation strongly depends oncrystallographic orientation of grains, and newly formed martensite phase is expected to affectstress/strain states of surrounding matrix phase. Moreover, interactions between neighboring grainscould additionally affect local stress/strain distributions within a grain, which makes the analysis morecomplicated. The orientation analysis method developed by JASRI at SPring-8, Japan, seems to beworthy of attention, which is, for example, found to be effective in visualizing grain images1). Thistechnique is capable of simultaneous strain measurement under load conditions, and thus providesinformation on deformation mechanism and internal strain variation in polycrystalline materials. Morerecently, a technique also allowing the analysis of principal stress has been developed. The purpose ofthis study is to analyze the stress/strain evolution in a superelastic Cu-Al-Mn alloy using thesetechniques with white X-ray micro-beam diffraction. We have performed in situ observation onmicrostructure and stress/strain evolution in many grains having different orientations during tensileloading and unloading. Present article briefly shows the analysis technique used for the stress/strainmeasurement and the obtained results are discussed mainly taking into account the effects ofmartensitic transformation.