JP2015184216A - Method for measuring strength of grain boundary - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring the strength of a grain boundary, capable of more accurately evaluating a stress even without using a cantilever large in size and even without performing finite element analysis, etc., in consideration of a bending moment.SOLUTION: The method for measuring the strength of a grain boundary comprises the steps of: manufacturing an ultra-microscopic tensile test piece having one grain boundary in an intermediate portion by focused ion beam processing; manufacturing a both-ends fixed beam having a microscopic size from a single crystal silicon wafer; fixing a lower end portion of the ultra-microscopic tensile test piece at the middle position of the both-ends fixed beam; fixing an upper end portion of the ultra-microscopic tensile test piece to a micro sampling probe; moving the micro sampling probe to apply a tensile load to the ultra-microscopic tensile test piece; and calculating a load applied to the ultra-microscopic tensile test piece from the flexure and spring constant of the both-ends fixed beam.

Description

  The present invention relates to a method for measuring grain boundary strength, which is useful for diagnosing the possibility of occurrence of stress corrosion cracking in reactor piping and the like, particularly stress corrosion cracking caused by grain boundary oxidation.

  When a material that forms piping, etc. is exposed to the reactor coolant for a long time, damage that breaks at grain boundaries called stress corrosion cracking occurs when the material, environment, and stress conditions overlap. .

  Conventionally, nuclear power plants have been regularly inspected with nuclear reactors shut down to ensure their soundness and reliability. The inspection of stress corrosion cracking occurring in the reactor piping and the like was also performed at the time of inspection, and mainly visual inspection was performed by an inspection means using an underwater video camera. Moreover, the crack detection by ultrasonic flaw detection was performed about the location which cannot be visually observed.

  However, it has been difficult for these inspection methods to detect signs of stress corrosion cracking.

  Stress corrosion cracking occurs after a long incubation period, and the crack growth rate increases with the progress of cracking after the occurrence. If a decrease in grain boundary strength during the latent period of stress corrosion cracking can be detected, the possibility of the occurrence of stress corrosion cracking in a reactor pipe or the like can be diagnosed by comparing with the stress generated by stress corrosion cracking. However, it is difficult to measure the strength of a specific grain boundary with the conventional macroscopic material testing technique.

  Accordingly, the present inventors have used ultra-fine processing using focused ion beam (FIB) as a method for examining the relationship between the microstructure of individual grain boundaries of polycrystalline materials and the fracture characteristics of the composition. A small tensile test method was developed (Non-Patent Documents 1 and 2). In this method, an ultra-fine tensile specimen including a single crystal grain boundary is prepared by FIB processing from a grain boundary whose microstructure and composition have been clarified using an electron microscope. As shown in FIG. A secondary ion microscope image observed in the FIB device with the bottom end of the ultrafine tensile test piece fixed to the tip of a cantilever made of single crystal silicon and the top end of the ultrafine test piece fixed to a microprobe. (Secondary ion microscope, SIM) A tensile test is performed in which the microprobe is pulled in the axial direction of an ultra-small tensile specimen while observing in situ. FIG. 5 shows an example of in-situ observation of the tensile test in the FIB apparatus. With the SIM image, the load can be calculated from the observation of the deformation of the ultra-small tensile specimen and the bending of the cantilever.

Katsuhiko Fujii, Koji Fukutani, “Development of FIB In-machine Tensile Test Method for Grain Boundary Strength Measurement”, INS JOURNAL, Vol. 17, p. 172 (2010) Terutsu Miura, Katsuhiko Fujii, Hiromasa Nishioka, Koji Fukuya, Hirotoshi Tachiuchi, “Evaluation of Intergranular Fracture Properties of Neutron Irradiated Stainless Steel by Ultra-Micro Tensile Test”, INS JOURNAL, Vol. 19, p. 155 (2012)

  In the above-described conventional method, in the above-described ultra-fine tensile test method, a greater bending moment acts on the ultra-fine tensile test piece as the deflection of the cantilever increases. When testing with a large load is required, it is necessary to reduce the effect of bending moment on the stress by increasing the cantilever size and reducing deflection, and for a more accurate evaluation of the bending moment Analysis such as finite element analysis that takes into account is necessary.

  Therefore, the present invention provides a crystal grain boundary strength measuring method capable of more accurate stress evaluation without using a cantilever having a large size or performing analysis such as a finite element analysis considering a bending moment. The purpose is to provide.

  In order to achieve the above object, a grain boundary strength measuring method according to the present invention includes a step of producing an ultra-fine tensile specimen having one grain boundary in the middle by focused ion beam processing, and single crystal silicon. A step of fabricating a micro-sized both-end fixed beam from a wafer, a step of fixing a lower end portion of the ultra-small tensile test piece at a center position of the both-end fixed beam, and a micro-end of the upper end portion of the ultra-micro tensile test piece. A step of fixing to the sampling probe; a step of moving the micro sampling probe to apply a tensile load to the ultra-small tensile test piece; And a step of calculating a load.

  According to the present invention, a bending moment is not applied to the ultra-small tensile test piece by fixing the ultra-fine tensile test piece at the center position of the both ends fixed beam of a minute size and performing a tensile test. A tensile load can be applied perpendicularly to a micrometer ultra-small tensile test piece, and a more accurate stress evaluation can be performed.

It is an example of the ultra micro tensile test piece used for the crystal grain boundary strength measuring method which concerns on this invention, Comprising: (a) is a front view, (b) shows a side view. It is a SIM image for demonstrating the crystal grain boundary intensity | strength measuring method which concerns on this invention, Comprising: (a) is a general view, (b) is an enlarged view of the square surrounding part of (a), (c) is (b). It is the enlarged view which expanded further. It is a SIM image for demonstrating the crystal grain boundary strength measuring method which concerns on this invention, Comprising: The change of a tensile test is shown in time series. It is a SIM image for demonstrating the conventional grain boundary strength measuring method. It is a SIM image which shows the time-sequential change by the conventional grain boundary strength measuring method shown in FIG.

  Embodiments of the grain boundary strength measuring method according to the present invention will be described below with reference to FIGS.

  The tensile test piece to be measured can be manufactured by collecting and processing from a reactor pipe or the like. Specifically, first, a minute piece is cut out from a liquid contact surface such as a reactor pipe. The reason for cutting out from the wetted surface is that the possibility of stress corrosion cracking can be diagnosed by examining the strength of the oxidized grain boundary. The dimension of the minute piece to be cut out can be, for example, about vertical to 10 mm, horizontal to 10 mm, and thickness to about 1 mm.

  The crystal grain boundaries to be measured are specified by examining the position and character of each grain boundary by electron backscatter diffraction (EBSD) on the cut out small piece. For example, OIM (Orientation imaging microscopy) manufactured by TLS can be used for electron backscatter diffraction.

  Next, a micrometer-sized ultra-fine tensile test piece including the specified grain boundary of the fine piece is manufactured by FIB processing. FIG. 1 shows a front view (a) and a side view (b) of an ultra-fine tensile test piece 1. The sample in the illustrated example has a prismatic shape as a whole. Grooves or notches 3 and 3 are formed along the grain boundary 2 from both sides, strengthening the plastic restraint around the grain boundary and increasing the stress on the grain boundary. Is to work. The FIB processing can be performed in the FIB apparatus by using micro sampling of the FIB system manufactured by Hitachi High-Technologies Corporation.

  In addition, a small fixed beam is manufactured from a single crystal silicon wafer. FIG. 2A shows a SIM image of the produced both-end fixed beam (micro beam). This both-end fixed beam (micro beam) can also be created by FIB processing. The micro beam in Fig. 2 (a) has a beam part size of 232 x 7 x 7 µm, and the spring constant is an ultra-micro hardness test. It was 460 N / m as measured by a machine.

  As shown in FIG. 2 (b), the lower end of the ultra-small tensile test piece is fixed by tungsten vapor deposition at the center position of the both ends fixed beam, and the upper end of the ultra-fine tensile test piece is within the FIB system. It is fixed by tungsten vapor deposition to a microsampling probe that can move in three axial directions.

  Referring to FIG. 1, the grain boundaries shown in FIG. 2C extend in a direction perpendicular to the tensile direction, that is, in the vertical direction.

  Next, the microsampling probe is moved upward at a moving speed of, for example, 0.1 mm / second, and a tensile test is performed until the ultra-fine tensile test piece breaks as shown in FIG. Record the SIM image during the test, and record the shape change of the ultra-small tensile test piece and the deflection of the both ends fixed beam (micro beam).

  The load applied to the ultra-small tensile test piece can be calculated by the following formula (1) from the deflection of the both-end fixed beam (micro beam) and the spring constant.

F = (16 Ewt ³ / l ³ ) · d (1)
In Formula (1), F is a load, E is a Young's modulus, w is a thickness dimension of a both-end fixed beam, w is a width dimension of the both-end fixed beam, l is a length dimension of the both-end fixed beam, and d is a both-end fixed beam. It is deflection. The ratio of deflection (d) to load (F) is the spring constant.

  The breaking stress of the specimen is obtained as the average stress obtained by dividing the breaking load by the cross-sectional area of the fracture surface.

  According to the above grain boundary strength measuring method according to the present invention, since a bending moment does not act on the ultra-small tensile test piece, a small gauge part (in FIG. 1, a central part having a width of 0.3 mm and a length of 0.1 mm). ) Can be measured even for ultra-fine tensile specimens, and a tensile load can be applied in the vertical direction to ultra-fine tensile specimens, enabling more accurate stress evaluation than conventional methods. It becomes.

  Stress generated by stress corrosion cracking is known for each material, and by comparing the measured grain boundary strength with the stress generated by stress corrosion cracking, the possibility of the occurrence of stress corrosion cracking in reactor piping, etc. is predicted to some extent It becomes possible.

1 Ultra-fine tensile specimen 2 Grain boundary 3 Groove

Claims (1)

A step of producing an ultra-micro tensile specimen having one grain boundary in the middle by focused ion beam processing;
A step of fabricating a micro-sized both-end fixed beam from a single crystal silicon wafer;
Fixing the lower end of the ultra-fine tensile test piece at the center position of the both-ends fixed beam;
Fixing the upper end of the ultra-fine tensile specimen to a microsampling probe;
Moving the micro-sampling probe to apply a tensile load to the ultra-fine tensile specimen;
Calculating the load applied to the ultra-small tensile test piece from the deflection of the both-ends fixed beam and the spring constant.