JP2019190886A - Method for evaluating polycrystal metal - Google Patents

Abstract

To estimate the temperature where polycrystal metal deforms.SOLUTION: The method for evaluating polycrystal metal includes: a crystal orientation measuring step of determining the crystal orientation based on the characteristics of back scattering of an electron beam applied to a sample of polycrystal metal; an orientation difference parameter calculation step of calculating the orientation difference parameter for regions around the crystal grain boundary based on the orientation difference of the crystal orientation; a standardization step of calculating reference data obtained by standardizing the orientation difference parameter; and a temperature estimation step of estimating the temperature at which the sample deforms from the correlation between the reference data calculated in advance and the temperature.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a method for evaluating polycrystalline metal.

  As an evaluation method for fatigue fracture of parts made of polycrystalline metal alloy, there is an EBSD method (Electron BackScatter Diffraction Pattern) using a scanning electron microscope. The EBSD method is a method for obtaining a crystal orientation in the vicinity of a fracture surface of a sample by stopping electron beams on the sample and backscattering electrons. For example, Patent Document 1 discloses an evaluation method in which the crystal orientation is measured using the EBSD method, the crack growth rate is calculated from the orientation difference of each crystal orientation, and the remaining life is evaluated.

JP2012-73126A

  Such a fatigue fracture evaluation method may be applied to components used at high temperatures such as a gas turbine. However, the orientation difference parameter used for analysis in the EBSD method has almost no temperature dependence, and it is difficult to estimate the deformation temperature using the EBSD method.

  The present invention has been made in view of the above-described problems, and an object thereof is to estimate a deformation temperature in a polycrystalline metal.

  The present invention adopts the following configuration as means for solving the above-described problems.

  As a first means, a crystal orientation measurement step for obtaining a crystal orientation based on a backscattering characteristic of an electron beam irradiated on a polycrystalline metal sample, and an orientation near a grain boundary based on the orientation difference of the crystal orientation An azimuth difference parameter calculating step for calculating a difference parameter, a normalization step for calculating reference data obtained by normalizing the azimuth difference parameter, and a deformation temperature of the sample based on a correlation between the reference data and temperature calculated in advance A temperature estimation step of estimating is employed.

  As the second means, in the first means, the means that the KAM (Kernel Average Misorientation) is used as the orientation difference parameter in the orientation difference parameter calculation step.

  As a third means, in the first or second means, a means is adopted in which the orientation difference parameter calculation step calculates an orientation difference parameter at a position in contact with the crystal grain boundary.

  As a fourth means, in any one of the above first to third means, in the normalization step, the crystal grain boundary orientation difference parameter calculated in the orientation difference parameter calculation step is used to calculate the orientation difference in a plurality of crystal grains. A method of normalizing by an average value of parameters is adopted.

  As a fifth means, in any one of the first to third means, in the normalization step, the orientation difference parameter of the crystal grain boundary calculated in the orientation difference parameter calculation step is changed to an orientation difference parameter in the crystal grain. The method of normalizing by the average value of is adopted.

  As a sixth means, in any one of the first to fifth means, a strain level estimation step of calculating a strain level of the sample based on a correlation between the orientation difference parameter and strain calculated in advance is provided. Adopt means.

  Polycrystalline metals have a characteristic that dislocations are more likely to occur at higher grain boundaries. According to the present invention, the orientation difference parameter near the crystal grain boundary is calculated from the crystal orientation measured by the EBSD method. Then, by standardizing the misorientation parameter near the grain boundary, the temperature dependence of the misorientation parameter can be clarified. Therefore, the deformation temperature can be estimated from the orientation difference parameter near the grain boundary.

It is a schematic diagram which shows the structure of the apparatus used for the evaluation method of the polycrystalline metal which concerns on one Embodiment of this invention. It is a figure which shows the measuring method by the EBSD method with respect to the sample of the polycrystalline metal in one Embodiment of this invention. It is a figure which shows the calculation method of the orientation difference parameter of the polycrystalline metal in one Embodiment of this invention. It is a flowchart which shows the procedure of the database preparation of the polycrystalline metal in one Embodiment of this invention. (A) is a graph which shows the value of the azimuth | direction difference parameter for every deformation | transformation temperature, (b) is a KAM-temperature correlation graph actually used for deformation | transformation temperature estimation. It is a flowchart which shows the metal evaluation method in one Embodiment of this invention.

  Hereinafter, an embodiment of a method for evaluating a polycrystalline metal according to the present invention will be described with reference to the drawings.

First, a measuring apparatus used in the polycrystalline metal evaluation method according to the present embodiment will be described.
FIG. 1 is a schematic diagram showing the configuration of an apparatus used in the polycrystalline metal evaluation method according to the present embodiment. As shown in this figure, the polycrystalline metal evaluation method according to the present embodiment is performed by a scanning electron microscope including a measuring device 1 and a control device 2. The measuring device 1 moves the electron beam, an electron emitting device 1a that irradiates the sample A with an electron beam, a sample storage unit 1b that stores the sample A, a detector 1c that detects reflected electrons reflected from the sample A, and the electron beam. A scanning coil (not shown). Such a measuring device 1 is controlled by the control device 2, moves the electron beam emitted from the electron emitting device 1 a by the scanning coil, irradiates the sample A, and detects an electron signal reflected by the sample A as a detector. It is an apparatus that detects the fine shape of the sample A by detecting it with 1c. The control device 2 is, for example, a computer, and controls the electron emission device 1a, the scanning coil, and the detector 1c of the measurement device 1.

  The analysis device 3 is a device that performs crystal orientation analysis based on a predetermined program, and includes a signal processing unit 3a, a storage unit 3b, an operation unit 3c, and a display unit 3d. The analysis device 3 stores a predetermined program in the storage unit 3b, and the signal processing unit 3a performs analysis processing on data detected by the detector 1c based on the predetermined program by an EBSD method described later. Such an analysis apparatus 3 is, for example, a computer, and can execute an analysis process based on an operation input to the operation unit 3c by an operator and display an analysis result on the display unit 3d.

  Here, the EBSD method (Electron BackScatter Diffraction Pattern) implemented in the present embodiment will be described. FIG. 2 is a diagram showing a measurement method by EBSD method for a polycrystalline metal sample A in the present embodiment.

The sample A to be measured in the present embodiment is, for example, a part made of SUS316 (stainless steel) deformed by receiving a load in the tensile direction.
The EBSD method is a method of analyzing the crystal orientation of a polycrystalline metal using the measuring device 1. In the EBSD method, as shown in FIG. 2, the sample A is tilted so that the electron beam forms an angle of about 20 degrees with respect to the surface on which the sample A is observed (the fracture surface in this embodiment). By irradiating with an electron beam, back scattering occurs in the sample A. As shown in FIG. 1, the detector 1 c is arranged in a direction of about 90 degrees with respect to the electron beam irradiation direction, and detects a diffraction pattern (Kikuchi pattern) due to electrons diffracted in the sample A. By analyzing this diffraction pattern, the crystal orientation at one point (pixel) of the sample A irradiated with the electron beam is measured. By repeating such electron beam irradiation, a plurality of pixels are irradiated with electrons in the crystal grains as shown in FIG. The crystal orientation mapping of the entire sample A can be created by performing such measurement by the EBSD method on the fracture surface of the sample A at regular intervals.

In the polycrystalline metal evaluation method of the present embodiment, an orientation difference parameter based on an orientation difference with respect to a crystal orientation reference in a pixel is used for evaluation. In the present embodiment, KAM (Kernel Average Misorientation) is used as the orientation difference parameter. KAM is a method of calculating the average of the azimuth difference θ between the target pixel (displayed in light color in FIG. 3) and the surrounding pixels (displayed in dark color in FIG. 3) that touch the boundary, as shown in Equation 1 below. is there. Note that θ _n (n: natural number) in Equation 1 represents the azimuth difference between pixels around the target pixel.
In addition, in the pixel in contact with the crystal grain boundary, the average of only the orientation difference (three in FIG. 3) of the pixel in contact with the pixel is calculated in the same crystal as the pixel.

Next, the polycrystalline metal evaluation method of this embodiment will be described.
First, the database creation process will be described with reference to FIG.

  First, a load test is performed on a test piece by an operator (step S1). In step S1, the worker applies a known load to the test piece under a plurality of temperature conditions, and applies strain to the test piece. Then, the crystal orientation of the test piece in which the strain is formed is measured by the above-described EBSD method (step S2).

  Next, an orientation difference parameter is calculated from the measured crystal orientation (step S3). In step S3, the analysis device 3 calculates KAM values for all pixels in the crystal grains in the measured crystal orientation.

Then, temperature reference data is created from the calculated orientation difference parameter (step S4). In step S4, the analysis apparatus 3 extracts KAM _gb that is a KAM value in a pixel near the crystal grain boundary from among the calculated KAM values in the crystal grain. Further, the analysis device 3 calculates the average value of the KAM values in the field of view of the detector 1c, and calculates the reference value by dividing KAM _gb by the average value (normalization). And the analyzer 3 memorize | stores the temperature reference data which linked | related the temperature conditions at the time of a test, and the reference value in the memory | storage part 3b.
FIG. 5A is a graph in which the normalized reference value is the vertical axis, and the distance (number of pixels) from the crystal grain boundary is the horizontal axis. As shown in this graph, the reference value tends to increase as the temperature increases. Furthermore, this tendency is considered to become more prominent as the distance from the crystal grain boundary is shorter. Therefore, it is possible to estimate the temperature tendency more accurately by extracting pixels having a short distance from the crystal grain boundary and calculating the orientation difference parameter. FIG. 5B is a graph of strain reference data with the reference value on the vertical axis and the temperature on the horizontal axis. The analysis device 3 stores, for example, strain reference data as illustrated in FIG. 5B in the storage unit 3b.

  Further, strain reference data is created from the calculated orientation difference parameter (step S5). In step S5, the analysis device 3 stores strain reference data in which the calculated KAM value is associated with the strain level derived from the load condition at the time of the test in the storage unit 3b.

Next, the metal evaluation method will be described with reference to FIG.
First, a database creation step is performed by the above-described method (step S11).
Then, a crystal orientation measurement step is performed (step S12). In step S12, the operator measures the crystal orientation of the sample A to be evaluated by the EBSD method.

Next, an azimuth difference parameter calculation step is performed (step S13). In step S <b> 13, the analysis device 3 calculates a KAM value at a pixel in the visual field in the crystal orientation measured for the sample A. Then, a normalization process is performed (step S14). In step S <b> 14, the analysis device 3 extracts KAM _gb that is a KAM value in a pixel near the crystal grain boundary from among the KAM values in the visual field calculated for the sample A. Further, the analysis device 3 calculates the average value of the KAM values in the visual field, and calculates the reference value by dividing KAM _gb by the average value (normalization).

Then, a strain level estimation step is performed (step S15). In step S16, the analysis device 3 estimates the strain level of the sample A based on the average value of the KAM values in the crystal grains and the strain reference data.
Furthermore, a temperature estimation process is performed (step S16). In step S15, the analysis apparatus 3 estimates the deformation temperature of the sample A based on the reference value of the sample A, the strain level, and the temperature reference data.

  According to this embodiment, the orientation difference parameter is calculated based on the crystal orientation of the sample A, and the deformation temperature is estimated from the orientation difference parameter based on the temperature reference data obtained by standardizing the orientation difference parameter. Thus, the deformation temperature can be estimated by calculating the orientation difference parameter. Such an evaluation method can be applied to a polycrystalline metal having a single-layer body-centered cubic lattice (Bcc) or face-centered cubic lattice (Fcc) structure.

  Further, according to the present embodiment, the orientation difference parameter is calculated based on the crystal orientation of the sample A, and the strain level is estimated from the orientation difference parameter. Therefore, the strain level can be estimated together with the deformation temperature, and the state of the sample A can be evaluated in more detail.

Further, according to the present embodiment, KAM _gb is calculated for a pixel in contact with the crystal grain boundary. As shown in FIG. 5A, the KAM value in the pixel in contact with the crystal grain boundary has a large temperature dependency, and therefore, the deformation temperature can be estimated more accurately.

  As mentioned above, although preferred embodiment of this invention was described referring drawings, this invention is not limited to the said embodiment. Various shapes, combinations, and the like of the constituent members shown in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the spirit of the present invention.

(1) In the above embodiment, KAM is used as the orientation difference parameter, but the present invention is not limited to this. For example, in austenitic stainless steel, it is also possible to use DMGB (Distribution of misorientation near grain boundary) as an orientation difference parameter. However, in this case, it is necessary to distinguish between the corresponding grain boundary (CSL grain boundary) and the random grain boundary. Also, any orientation difference parameter that can extract a pixel near the grain boundary and calculate the orientation difference distribution is applicable.

(2) In the above embodiment, as a standardization method, a method of dividing by the average value of the KAM values in the field of view of the detector 1c is adopted, but the present invention is not limited to this. It is good also as what remove | divides by the average value of the KAM value in the pixel in a crystal grain.

(3) In the above embodiment, the strain level is estimated. However, the present invention is not limited to this, and only the deformation temperature may be estimated.

(4) Further, as shown in FIG. 5A, the deformation temperature can be estimated by normalizing the KAM value of a pixel that is several pixels away from the grain boundary.

(5) In addition, the database creation step can be used for temperature estimation of various samples by performing a plurality of types of polycrystalline metals in advance.

DESCRIPTION OF SYMBOLS 1 Measurement apparatus 1c Detector 2 Control apparatus 3 Analysis apparatus

Claims (6)

A crystal orientation measuring step for obtaining a crystal orientation based on the characteristics of backscattering of an electron beam irradiated to a polycrystalline metal sample;
An orientation difference parameter calculation step for calculating an orientation difference parameter in the vicinity of a grain boundary based on the orientation difference of the crystal orientation,
A normalization step of calculating reference data obtained by normalizing the orientation difference parameter;
And a temperature estimation step of estimating a deformation temperature of the sample based on a correlation between the reference data calculated in advance and the temperature.   The polycrystalline metal evaluation method according to claim 1, wherein in the misorientation parameter calculation step, KAM (Kernel Average Misorientation) is used as the misorientation parameter.   3. The method for evaluating a polycrystalline metal according to claim 1, wherein in the misorientation parameter calculating step, an misorientation parameter at a position in contact with the grain boundary is calculated.   In the normalization step, the crystal grain boundary misorientation parameter calculated in the misorientation parameter calculation step is standardized by an average value of misorientation parameters in a plurality of crystal grains. The evaluation method of the polycrystalline metal as described in any one of these.   The normalization step is characterized in that the orientation difference parameter of the crystal grain boundary calculated in the orientation difference parameter calculation step is normalized by an average value of the orientation difference parameters in the crystal grain. The evaluation method of the polycrystalline metal as described in any one of Claims.   The polycrystal according to any one of claims 1 to 5, further comprising a strain level estimation step of calculating a strain level of the sample based on a correlation between the orientation difference parameter and strain calculated in advance. Metal evaluation method.

Images