Multipoint indentation for material identification in three-dimensional observation based on serial sectioning
Introduction
The material properties depend on the microscopic structures and combinations of components. Internal structure observations are important for evaluating the quality of materials and the reliability of industrial products. Several types of structural observation techniques have been proposed and used for studies in the material science and processing fields. Nondestructive measurements, for example, as represented by X-ray computed tomography (CT), provide significant benefits not only for clinical diagnoses but also for the quality management of materials in industrial fields. In particular, three-dimensional (3D) microscopic structural observations provide an effective approach for evaluating the strength of materials. Macroscopic fractures of materials arise from the stress concentration area, which becomes the origin of crack propagation. Recently, high-energy X-rays (such as synchrotron X-rays) have been used for high-resolution observations of the internal structures of metal materials [1] with defects and crack propagations [2,3]. Micro CT techniques have led to the development of microscopic-scale structural analyses. The resolution can reach the submicron scale and allow for discussion of the scales of crystal characteristics. By contrast, serial sectioning is traditionally used for internal structure observation as a destructive measurement method. This method is required for analytical studies to identify the functional and mechanical behaviors in actual surfaces. Serial sectioning observation systems have been developed for conducting 3D internal structural observations of various materials [4]. For example, high-resolution observations have been conducted using scanning electron microscopy with a microtome [5] or focused ion beam [6]. X-ray spectrometry for elemental mapping [7] and electron backscatter diffraction techniques for grain characterization [8] can be combined with observations to identify specific areas in metal materials. Machining can be used to create observation surfaces in industrial hard materials, as a convenient method for serial sectioning. An automatic polishing system has been proposed for creating surfaces for optical microscopic observations [9,10]. An observation system based on a precision milling device has also been used for the 3D observation of composite materials [11]. Usually, mirror-like fine surfaces are required for optical microscopic observations; therefore, polishing is required for the surface creation process. Our previous studies focused on observations of the microstructures of hard tissues (including biological samples and metals) by means of serial precision cutting without a polishing process [12,13]. Recent studies have realized geometric observations of inclusions and cracks in steel materials without requiring manual interventions [14,15]. Although these methods are based on destructive sectioning processes, various observations and analytical methods can be combined with such sectioning processes. Three-dimensional elemental mapping is an application of serial sectioning observations [16]. X-ray fluorescence measurements in a sectioning system provide an identification process for the material properties, focusing on elemental analysis on a microscopic scale. Analyzing the mechanical properties on a micro scale is required in the engineering field. One study proposed hardness mapping based on the scratch force scanning method [17]. Another serial sectioning device was developed based on atomic force microscopy, combined with ultramicrotome sectioning [18].
Numerous material studies have focused on conducting mechanical tests in a microscopic region on a micro–nano scale. Indentation tests have been used not only for hardness evaluations but also as direct measurement methods for the evaluation of elastic characteristics in the microscopic region. These indentation tests must be operated with high precision and repeatable positioning techniques for the measurement of the 2D distribution of the elastic modulus. The high-quality surface processing of samples constructed using several materials (including hard and soft tissue) is important for optical observations and mechanical tests. This study proposes a 3D observation system based on a serial sectioning method constructed from a precision milling machine with a microscope and an indentation device. An advanced technique for precision cutting of materials is required to create highly finished surfaces, as observed in optical microscope imaging. Therefore, this study discusses whether precision cutting surfaces are sufficient for the observation of microstructures in materials. An optimal cutting condition for precision machining for the observation of composite elements is considered in this experiment. An indentation in the micro-Vickers hardness range is combined with a 3D observation to identify the components that are recognized by optical image analysis in terms of their mechanical properties. To evaluate the scale effects of the elastic modulus as measured in this indentation condition, the values were analyzed as related to the indentation depth. The effects of interactions with adjacent indentations on the measurement were confirmed in the context of evaluating the spatial resolution.
Section snippets
Three-dimensional micro slicer system
A series of serial sectioning observation systems (Riken Micro Slicer System (RMSS) −001, 002) have been developed for 3D observations of biological tissue [19,20]. An observation system (RMSS-003) based on a high-precision milling machine (UVM-350(J), Toshiba Machine Co., Ltd.) was also constructed for the observation of hard materials, such as biological mineralized structures and industrial metals. Precision cutting is a novel machining technique that can create flat mirrored surfaces (e.g.,
Specimen
Three experiments were planned for this study. The first two were conducted to confirm the system performance for future applications concerning serial sectioning observations. An aluminum alloy block of 10 mm × 10 mm × 10 mm, as shown in Fig. 4(a), was used for the various indentation tests. The sample for the serial sectioning observation experiment was constructed with metal wires, as shown in Fig. 4(b). Three wires (diameter: 1 mm) made of copper, aluminum, and brass were molded in a 10 mm
Indentation characteristics
Indentations on the aluminum alloy specimens were made under various indentation depth conditions. The results for the indentation modulus Es (obtained from the force–displacement curves) are shown in Fig. 5. There are indentation depth-dependent effects on the indentation modulus. The error bars indicate the standard deviations of the six indentation tests (n = 6) on the same surface. Although the values were high for small depth conditions of approximately 5 μm, the values showed a very
Discussion
The 3D internal structure comprising three metal wires was clearly obtained in serial sectioning observation, as shown in Fig. 7. The observations in this system were limited by the positioning resolution of the milling machine. Because the positioning system is controlled on a 0.1 μm feedback scale, there is no difference in X–Y positions of every microscopic imaging with 0.7 μm resolution on the surfaces during the serial sectioning process. The indentation range was limited by both the
Conclusion
Multipoint indentation tests were performed for the 3D internal structural observation using a serial sectioning technique. The indentation size effect on the indentation modulus measurements was observed in a precision displacement control system. Thus, the creation of flat surfaces is important for measuring with the indentation process employed in this system. An internal structure model including three metal wires of 1 mm diameter each was constructed for the observation system. The
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research: No. 15H02207 and Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials Integration for revolutionary design system of structural materials” (Funding agency: JST).
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