JP6176021B2 - Electrochemical nanoindentation test method - Google Patents

Description

  The present invention relates to a nanoindentation test apparatus used for a nanometer-scale micro-indentation test, and more particularly, to an electricity that enables nanoindentation measurement while electrochemically introducing hydrogen into a sample (charging). The present invention relates to a chemical nanoindentation test apparatus and a test method thereof.

  In recent years, as further development of high-strength steel is being promoted, it has been regarded as important to take radical measures against the problem of hydrogen embrittlement. In steel, hydrogen is trapped by various lattice defects such as dislocations, grain boundaries, inclusions, and strain fields, causing destruction. For this reason, it can be said that it is important to investigate the influence of hydrogen on the mechanical properties (hardness, elastic modulus, etc.) and the dislocation motion in a minute region.

  In general, a nano-indentation (hereinafter, also referred to as “NI”) method is used to evaluate mechanical properties of a microstructure such as a grain boundary or a precipitate. In the NI method, a nanoscale indenter is pushed into the surface of a sample, and the indentation load and penetration depth (indentation depth) of the indenter at that time are sequentially acquired. From the relationship between the load and depth, the extreme surface layer (surface) And a microscopic hardness and elastic modulus at a depth of 1 μm or less).

  By the way, in the extreme surface layer of the sample where the indenter is pushed in, even if hydrogen is introduced up to the saturation amount, if the sample is placed in the atmosphere, the introduced hydrogen is diffused and released, and the hydrogen concentration decreases in a short time. Resulting in. For this reason, even if NI measurement is performed in a state where the surface of the sample is exposed to the air atmosphere, it cannot be said that the measurement result directly reflects the influence of hydrogen. In order to cope with such inconvenience, an NI test apparatus incorporating an electrochemical cell for cathodic hydrogen charging is used as a sample holder, and NI is introduced into the sample while electrochemically introducing hydrogen into the electrolyte. An electrochemical NI (hereinafter also referred to as “EC-NI”) method for measuring is proposed (for example, see Non-Patent Documents 1 and 2).

Afrooz Barnoush, Horst Vehoff, “Scripta Materialia”, Volume 55, Issue 2, July 2006, p. 195-198 Makoto Chiba, Masahiro Seo, “Corrosion Science”, Volume 44, Issue 10, October 2002, p. 2379-2391

  Hydrogen embrittlement occurs when a steel material containing hydrogen is deformed at a low strain rate and does not occur at a high strain rate. On the other hand, in the conventional EC-NI method, thermal deformation of the indenter or sample during measurement, so-called thermal drift cannot be ignored, and the indentation time for obtaining reproducible data is limited to several tens of seconds or less. Yes. For this reason, under the present circumstances, it is difficult to measure at a low strain rate where the indentation time becomes long, and it is difficult to say that the influence of hydrogen can be accurately measured.

An object of the present invention is to provide an electrochemical nanoindentation test apparatus and test method having the following characteristics:
Accurately measure the effect of hydrogen at a low strain rate where hydrogen embrittlement becomes apparent.

  The gist of the present invention is as follows.

(I) a test apparatus using an electrochemical nanoindentation method,
An electrochemical cell that holds the sample and electrochemically introduces hydrogen into the surface of the sample;
An indenter that is pushed into the surface of the sample in the course of hydrogen being introduced;
A controller for controlling indentation load and indentation time of the indenter;
A load generator and a transducer that is a detector of the indentation load and displacement of the indenter;
An arbitrary waveform generator for adding amplitude modulation to the indentation load;
A lock-in amplifier that detects contact rigidity;
An arithmetic device for calculating the nano hardness of the sample,
An electrochemical nanoindentation device that adds amplitude modulation to the indentation load when the indenter is indented into the sample.

(II) Electrochemical nanoindentation test method,
When introducing an indenter into the surface of the sample while electrochemically introducing hydrogen to the surface of the sample, an amplitude modulation is added to the indentation load of the indenter by an arbitrary waveform generator, and from the generated displacement of the indenter, An electrochemical nanoindentation test method in which contact hardness is detected by a lock-in amplifier, and nanohardness is calculated from indentation load and contact rigidity acquired sequentially.

  In the above test method, it is preferable that the indenter is pushed in for 60 minutes or more and the amplitude modulation frequency is in a range of 10 to 300 Hz.

The electrochemical nanoindentation test apparatus and test method of the present invention have the following remarkable effects:
The effect of hydrogen can be accurately measured at a low strain rate where hydrogen embrittlement becomes obvious.

FIG. 1 is a schematic diagram of a Ph curve. FIG. 2 is a diagram showing Ph curves in the atmosphere and in the electrolyte obtained by measurement by the RF method with an indentation time Δt of 60 min. FIG. 3 is a diagram showing the time change of the electrolyte measured in an environment where the temperature is 20 ° C. and the humidity is 20% to 30%. FIG. 3 (a) shows the time change of the water level. Indicates time variation of buoyancy. FIG. 4 is a diagram showing a Ph curve obtained by pressing an indenter into the surface of a 56Ni-43Cr alloy. FIG. 5 is a diagram showing a Ph curve obtained by pushing an indenter into the surface of SUS316L. FIG. 6 is a diagram showing the relationship between the change rate (increase rate) of nano hardness due to hydrogen introduction and the indentation time (strain rate), and FIG. 6 (a) shows the case of pure Ni, and FIG. 6 (b). 6 shows the case of 79Ni-20Cr, FIG. 6C shows the case of 69Ni-30Cr, FIG. 6D shows the case of 56Ni-43Cr, and FIG. 6E shows the case of SUS316L. FIG. 7 is a cross-sectional view schematically illustrating the configuration of the nanoindentation test apparatus according to the embodiment. FIG. 8 is an exploded perspective view showing an electrochemical cell incorporated in the nanoindentation test apparatus according to the embodiment. FIG. 9 is a perspective view showing a state in which the electrochemical cell of the embodiment is assembled and incorporated in the NI test apparatus.

  The electrochemical nanoindentation test apparatus and test method of the present invention have been completed through the following confirmation test.

1. Confirmation Test The EC-NI test apparatus currently in practical use cannot accurately measure the indentation depth unless the indenter pushing time is shortened to about several tens of seconds. This is because if the indentation time is long, the thermal drift of the indenter and the sample affects. Further, it is known that the presence or absence of hydrogen embrittlement greatly depends on the strain rate. For example, when a tensile test is performed on a pure iron sample into which hydrogen is introduced at room temperature, the fracture surface becomes a pseudo-cleavage surface peculiar to hydrogen embrittlement when the strain rate is as small as 10 ⁻⁴ / s or less. On the other hand, when the strain rate is large at 10 ⁻³ / s or more, the fracture surface becomes a dimple fracture surface and no influence of hydrogen is recognized.

Here, when a spherical indenter with a radius of R is pushed down to a depth of h _max with a pushing time of Δt, the strain rate ε _dot (means the time derivative of the strain ε. In the following equation, the symbol “ε” The dot symbol “·” is attached to the upper part.) Is given by the following equation (1).

By substituting R = 1 μm, h _max = 200 nm, and Δt = 5 s into the equation (1) as actual test conditions for EC-NI measurement, ε _dot = 2.4 × 10 ⁻² / s is obtained. . The value of the strain rate ε _dot is larger than a typical strain rate (10 ⁻⁴ / s or less) at which hydrogen embrittlement occurs in the above tensile test.

Therefore, in order to study the hydrogen embrittlement mechanism in more detail by the EC-NI method, it is essential to reduce the maximum indentation depth h _max or increase the indentation time Δt and reduce the strain rate ε _dot. It is. When the maximum indentation depth h _max is reduced, the noise / signal ratio increases with respect to the indentation depth, and as a result, accurate nano hardness cannot be obtained. Therefore, it is effective to increase the pushing time Δt. In order to obtain a strain rate of 10 ⁻⁴ / s from the equation (1), the indentation time must be Δt ≧ 60 min (minutes), and it can be said that measurement that corrects the influence of thermal drift is necessary.

  Therefore, as will be described later, an EC-NI method that uses the EC-NI test apparatus according to an embodiment of the present invention and can indent the indenter under a condition where the indentation time is long as Δt ≧ 60 min, that is, at a low strain rate. Thus, the influence of hydrogen on the nano hardness of the fcc alloy was investigated.

<Measurement principle of low strain rate EC-NI method>
In normal NI measurement, an indenter is pushed into the surface of the sample, the indentation depth h determined from the displacement of the indenter at that time, and the indentation load P are sequentially obtained, and the relationship between the indentation load P and the indentation depth h. That is, a Ph curve is obtained. FIG. 1 is a schematic diagram of a Ph curve. Then, based on the P-h curve obtained, analyzed by Oliver and Pharr proposed by analysis method (O-P method), we derive the nano hardness H _n and the elastic modulus E _r. That is, as shown in FIG. 1, the nanohardness H _n and the elastic modulus _Er use the maximum load P _max and the slope (hereinafter also referred to as “contact stiffness”) S of the unloading portion in the Ph curve. The following formulas (2) and (3) are used.

The sample is partially elastically deformed when the indenter is pushed in. For this reason, the maximum indentation depth h _max is not simply equal to the depth of the indentation (hereinafter also referred to as “contact depth”) h _c after the indenter is removed. The contact depth h _c and the maximum push-in depth h _max have the relationship of the following expression (4).

In the above formulas (2) and (3), A _c is the projected area of the indentation and is a function of the contact depth h _c defined by the shape of the indenter. Function of the form indentations projected area A _c may be determined experimentally by pushing an indenter in advance in the standard sample. Such O-P method analysis using from P-h curve, there are two characteristics simultaneously and conveniently the advantage of the elastic modulus E _r and the nano hardness H _n. However, since the displacement of the indenter changes gradually due to thermal drift, if the push-in time Δt is long, an accurate Ph curve cannot be obtained, and as a result, an accurate nano hardness H _n can be obtained. Can not.

Therefore, in order to overcome such problems of the conventional EC-NI method, the possibility of EC-NI measurement using a reference frequency method (RF method: Reference Frequency method) in which the indenter is microvibrated at a constant frequency. It was investigated. When the indenter is pushed into the sample, a minute amplitude modulation of several hundred Hz is added to the pushing load P by an arbitrary waveform generator. The contact stiffness S is sequentially detected and acquired by the lock-in amplifier from the resulting vibration of the indenter displacement. Further, by using the contact stiffness S and the indentation load P acquired sequentially and the known elastic modulus _Er , the indentation projected area A _c and the contact depth h _c at an arbitrary load during indenter indentation can be obtained by the above equation (3). Ask. This elastic modulus _Er is obtained by analyzing a normal Ph curve measured with a short indentation time Δt in which thermal drift is negligible by the OP method.

The above equation (2), the nano-hardness H _n at any load from indentation projected area A _c is obtained. Further, the indentation depth h and the Ph curve can be obtained from the contact depth h _c by the above equation (4). Thus, when the RF method is used, the indentation depth is obtained indirectly from the contact rigidity. The period of amplitude modulation of the load is several tens of milliseconds (milliseconds), and the thermal drift is extremely small during this period. Therefore, the influence of the thermal drift on the contact rigidity S detected by the lock-in amplifier may be ignored. If the RF method is used, there is a possibility that an accurate measurement without the influence of the thermal drift can be performed.

<Measurement procedure>
Three types of Ni—Cr alloys (79Ni-20Cr, 69Ni-30Cr, 56Ni-43Cr), pure Ni, and SUS316L having the compositions shown in Table 1 below were used as samples. Both are fcc alloys. Of these, pure Ni and SUS316L samples were annealed in an infrared vacuum furnace, then mechanically polished with emery paper and alumina abrasive grains, and the surface processed layer by mechanical polishing was removed by electrolytic polishing. The annealing temperature and time were set to 1270 K × 900 s (furnace cooling) for the pure Ni sample and 1270 K × 1800 s (furnace cooling) for the SUS316L sample. The sample of the Ni—Cr alloy was subjected to the same mechanical polishing and electrolytic polishing on the hot forged material (forging temperature 1470K, air cooling). The crystal grain size of the pure Ni sample was about 200 μm, the crystal grain size of the SUS316L sample was about 20 μm, and the crystal grain size of the Ni—Cr alloy sample was about 50 μm. Each sample size was 10 mm square and 2 mm thick.

As for hydrogen embrittlement resistance evaluated by a tensile test at a strain rate of 3 × 10 ⁻⁴ / s and evaluated from a decrease in elongation at break by introduction of hydrogen, SUS316L is the best, followed by pure Ni, 79Ni-20Cr, It was favorable in the order of 69Ni-30Cr and 56Ni-43Cr.

  A sample was incorporated into an EC-NI test apparatus (an electrochemical cell installed in TriboIndenter-950 manufactured by Hysitron), which is an embodiment of the present invention, and EC-NI measurement was performed at a low strain rate. The basic configuration of the EC-NI test apparatus and its electrochemical cell will be described later.

A borate buffer solution (0.15 M H ₃ BO ₄ +0.0375 M Na ₂ B ₄ O _7, pH 8.62) was used as an electrolyte for cathodic hydrogen charging, and the cathode current density was 2.5 A / m. ² was set and hydrogen was introduced under current control. In order to achieve the indentation only into the crystal grains of the sample, a diamond spherical indenter having a radius of curvature of about 1 μm was used as the indenter.

The strain rate ε _dot was adjusted by setting the maximum load so that the maximum indentation depth was about 200 nm and changing the indentation time. For this reason, the maximum (drive) load was set to 4000 μN for each sample of Ni—Cr alloy and the sample of SUS316L, and the maximum load was set to 3000 μN for the sample of pure Ni. At that time, in order to correct the thermal drift by using the R-F method, a sinusoidal amplitude modulation with a frequency of 200 Hz was added to the indentation load P by an arbitrary waveform generator. The load amplitude of this modulation was dynamically changed during the indenter pushing so that it became 3% of the value of the pushing load P.

The elastic modulus _Er required for correcting the thermal drift by the RF method was obtained by analyzing the Ph curve obtained by a normal method with an indentation time Δt of 5 s and the OP method. In all the samples, the value of the elastic modulus _Er was constant at about 200 GPa regardless of whether hydrogen was introduced.

Table 2 below shows the indentation time Δt at which the measurement was performed, the value of the strain rate ε _dot corresponding thereto (R = 1 μm, h _max = 200 nm, calculated from the above equation (1)), and the number of tests performed (N number) ). The strain rate ε _dot was changed within a wide range from 1.2 / s to 1.1 × 10 ⁻⁵ / s, and the N number was set to 2 or 3 at each indentation time Δt.

<Test results>
≪Effect of electrolyte on low strain rate EC-NI measurement≫
If the indenter is pushed in with a long indentation time Δt, the electrolyte may evaporate in the process, and the buoyancy may change, or the surface tension acting on the indenter may change, and accurate measurement may not be possible. is there. Therefore, first, for the 56Ni-43Cr alloy, the Ph curve measured in the atmosphere is compared with the Ph curve measured without introducing hydrogen in the electrolyte. The effect on the effect was examined.

  FIG. 2 is a graph showing a Ph curve obtained by measurement by the RF method with an indentation time Δt of 60 min. Here, the Ph curve in the figure is an average curve of three Ph curves obtained for the same crystal grain, and the curve of the unloading part is omitted. The Ph curve in the atmosphere and the Ph curve in the electrolyte were obtained by pressing an indenter into the same crystal grain. From the results shown in FIG. 2, it was found that the locus of the Ph curve is almost the same between the atmosphere and the electrolyte. Therefore, the influence of buoyancy and surface tension on the measurement result is small, and it can be determined that the correction of thermal drift using the RF method can be applied in the atmosphere and in the electrolyte.

  Furthermore, it was examined whether the influence of the electrolyte solution can be ignored even during the introduction of hydrogen. For this reason, it was investigated how the evaporation amount of the electrolytic solution changed with or without hydrogen introduction in the 56Ni-43Cr alloy. FIG. 3 is a diagram showing the time change of the electrolyte measured in an environment where the temperature is 20 ° C. and the humidity is 20% to 30%. FIG. 3 (a) shows the time change of the water level. Indicates time variation of buoyancy. The water level was calculated from the measured value, the density of the electrolytic solution, and the opening area of the electrochemical cell (the exposed area of the stored electrolytic solution) by measuring the change in the mass of the electrolytic solution with an electronic balance. On the other hand, the buoyancy was calculated from the water level and the actual shape of the indenter.

  As shown in FIG. 3, the water level and the buoyancy changed in almost the same manner with time regardless of the presence or absence of hydrogen introduction. In particular, the buoyancy was found to change only 2 μN even after 6 hours (360 min). Therefore, it can be said that the influence of buoyancy and surface tension can be ignored when hydrogen is introduced.

≪Strain rate dependence of hydrogen effect≫
In order to clarify the dependence of the strain rate ε _{dot that} is affected by hydrogen, the indenter is pushed in with the indentation time Δt, with or without hydrogen introduced in the electrolyte, and the Ph curve is measured. did. FIG. 4 is a diagram showing a Ph curve obtained by pressing an indenter into the surface of a 56Ni-43Cr alloy. FIG. 5 is a diagram showing a Ph curve obtained by pushing an indenter into the surface of SUS316L.

  In order to eliminate differences due to crystal grains, the data at the same indentation time Δt was measured by indenting an indenter into the same crystal grain in each sample. The Ph curves in FIGS. 4 and 5 are average curves of the three or two Ph curves obtained as in the case of FIG. Data with an indentation time of Δt = 1 s was measured by a conventional EC-NI method not applying the RF method, and data with an indentation time of Δt ≧ 5 s was measured by applying the RF method.

In the 56Ni-43Cr alloy, as shown in FIG. 4, under the conditions where the indentation time is Δt = 1 s and Δt = 5 s, when hydrogen is introduced, the maximum indentation depth h _max decreases, and as a result, the nano hardness H _n increased. On the other hand, under the condition where the indentation time is Δt ≧ 60 min, no change in the maximum indentation depth h _max due to hydrogen was observed.

On the other hand, in SUS316L, as shown in FIG. 5, at any indentation time Δt in which the measurement was performed, when hydrogen was introduced, the maximum indentation depth h _max slightly decreased. It was found that H _n increased slightly.

  FIG. 6 is a diagram showing the relationship between the change rate (increase rate) of nano hardness due to hydrogen introduction and the indentation time (strain rate) for each sample. 6 (a) shows the case of pure Ni, FIG. 6 (b) shows the case of 79Ni-20Cr, FIG. 6 (c) shows the case of 69Ni-30Cr, and FIG. 6 (d) shows the case of 56Ni-43Cr. FIG. 6E shows the case of SUS316L.

In FIG. 6, the change in nano hardness represents the rate of change of the value after hydrogen introduction relative to the value before hydrogen introduction, a positive value means hardening, and a negative value means softening. The value of the nano hardness H _{n when} the indentation time is Δt ≦ 1 s is obtained by analyzing the Ph curve measured by the conventional method by the OP method, and the nano hardness H _n when the indentation time is Δt ≧ 5 s. The value of was obtained from the value of the contact stiffness S at the maximum load (4000 μN for SUS316L and Ni—Cr alloy, 3000 μN for pure Ni) measured by the RF method.

In the case of Ni—Cr alloys, the degree of increase in nano hardness due to the introduction of hydrogen decreased as the strain rate ε _dot decreased. In particular, at a low strain rate of ε _dot ≦ 3.3 × 10 ⁻⁵ / s, 56Ni-43Cr has zero change in nanohardness due to hydrogen introduction, whereas 69Ni-30Cr has a high Ni content. The alloy and the 79Ni-20Cr alloy were softened by about 10% by introducing hydrogen. In the case of SUS316L, about 10% was cured regardless of the strain rate ε _dot . On the other hand, in the case of pure Ni, no change in nano hardness due to hydrogen introduction occurred at any strain rate ε _dot . Thus, samples with lower hydrogen embrittlement resistance tended to cause greater softening due to hydrogen introduction at a low strain rate of ε _dot ≦ 3.3 × 10 ⁻⁵ / s.

<Summary of confirmation test>
The following findings were obtained from the results of the above confirmation test.

In the case of 79Ni-20Cr, 69Ni-30Cr, and 56Ni-43Cr, the degree of increase in nano hardness due to hydrogen introduction decreases as the strain rate decreases. In particular, 79Ni-20Cr and 69Ni-30Cr exhibit softening when the strain rate is ε _dot ≦ 3.3 × 10 ⁻⁵ / s.
In the case of SUS316L, at any strain rate, it is cured by about 10% by introducing hydrogen.
In the case of pure Ni, no change in nanohardness occurs due to hydrogen introduction at any strain rate.
-Samples with lower hydrogen embrittlement resistance tend to cause greater softening due to hydrogen introduction at a low strain rate of ε _dot ≦ 3.3 × 10 ⁻⁵ / s.

Thus, by correcting the thermal drift using the RF method in the EC-NI measurement, a low strain rate EC-NI (hereinafter referred to as “SSR-EC”) capable of indentation at a low strain rate in a hydrogen environment. (Also referred to as “-NI”). According to the measurement using the SSR-EC-NI method, the strain rate ε _dot dependence of the hydrogen effect on the Ni—Cr alloy, SUS316L, and pure Ni was determined, and the indentation time Δt was 180 min (ε _dot = 1.1 × 10 ^-5 / s)), and the influence of hydrogen on the microstructure can be accurately measured at a low strain rate at which hydrogen embrittlement becomes apparent.

2. SSR-EC-NI Test Apparatus Hereinafter, an SSR-EC-NI test apparatus according to an embodiment of the present invention and an electrochemical cell as a holder incorporated in the NI test apparatus will be described in detail.

  FIG. 7 is a cross-sectional view schematically showing the configuration of the NI test apparatus of the embodiment. FIG. 8 is an exploded perspective view showing the electrochemical cell incorporated in the NI test apparatus, and FIG. 9 is a perspective view showing a state where the electrochemical cell is assembled and incorporated in the NI test apparatus. The electrochemical cell 1 includes a container part 2 and a pedestal part 3.

  The container part 2 is a composite of aluminum nitride (AlN) and boron nitride (BN) that has chemical resistance and insulation, has high rigidity, excellent machinability, and high thermal conductivity ( Hereinafter, it is also referred to as “AlN-BN composite”, and has a circular hole 2 a penetrating along the vertical direction in order to hold the electrolytic solution 11 at the center. An electrolytic solution storage recess 2b for storing the electrolyte solution 11 is formed at the center of the upper surface of the container portion 2, and the electrolytic solution storage recess 2b is greatly enlarged in connection with the circular hole 2a.

  In addition, a screw insertion hole 2c that penetrates along the vertical direction is formed on the edge of the container portion 2 in order to insert a plurality of screws 4 used for fastening with the pedestal portion 3. These screw insertion holes 2 c are formed with screw storage recesses 2 d for storing the heads of the screws 4 in the container part 2 so that the heads of the screws 4 do not protrude from the upper surface of the container part 2. Yes. Furthermore, the wiring 6a, 7a from the counter electrode 6 and the reference electrode 7 respectively is accommodated on the edge of the upper surface of the container part 2 so that the wirings 6a, 7a are not projected from the upper surface of the container part 2, respectively. Wiring housing grooves 2e and 2f are formed.

  A ring-shaped counter electrode 6 is disposed concentrically with the circular hole 2 a at the bottom of the electrolytic solution storage recess 2 b of the container portion 2. The wiring 6a of the counter electrode 6 is drawn out through the wiring housing groove 2e, and the terminal 6b at the tip thereof is fixed to the side surface of the container portion 2 with a screw or the like (not shown). A reference electrode 7 is disposed in the circular hole 2 a of the container part 2. The wiring 7a of the reference electrode 7 is drawn out through the wiring housing groove 2f, and the terminal 7b at the tip thereof is fixed to the side surface of the container portion 2 with a screw or the like (not shown). As the counter electrode 6, platinum, gold, or the like can be used. As the reference electrode 7, a platinum, gold, silver-silver chloride electrode or the like can be used.

  The pedestal portion 3 is made of an AlN-BN composite like the container portion 2, and an electrode plate storage recess 3 a for storing the electrode plate 5 is formed at the center thereof. In addition, screw holes 3 b that are paired with the screw insertion holes 2 c of the container portion 2 are formed at the edge of the upper surface of the pedestal portion 3.

  The electrode plate 5 is placed in the electrode plate storage recess 3 a of the pedestal 3. This electrode plate 5 functions as an electrode, and of course has conductivity, but also has thermal conductivity. As the electrode plate 5, copper is suitable. When the electrode plate 5 is made of copper, the hardness is preferably 1 / 2H or more. A load is applied to the electrode plate 5 as the indenter 14 is pushed during NI measurement. If the electrode plate 5 is made of copper having a hardness of 1/2 H or more, the electrode plate 5 itself has sufficient rigidity. This is because the electrode plate 5 is not deformed with the indentation load of the indenter 14 and the NI measurement can be performed more accurately.

  A sample 8 is stacked on the electrode plate 5 of the pedestal 3. A spacer 9 is placed on the electrode plate 5 so as to surround the sample 8. The spacer 9 has thermal conductivity. The spacer 9 is not particularly limited as long as it has thermal conductivity, and a metal, particularly copper, is preferable.

  Further, the container part 2 is stacked on the sample 8, and the pedestal part 3 and the container part 2 are fastened by the screw 4 in this state. Accordingly, the sample 8 is sandwiched between the electrode plate 5 of the pedestal portion 3 and the container portion 2 and is electrically integrated with the electrode plate 5, and a part of the surface thereof is inserted into the circular hole 2 a of the container portion 2. Opened. The screw 4 has thermal conductivity. The material of the screw 4 is not particularly limited as long as it has thermal conductivity, and a metal, particularly aluminum, is preferable. Actually, an O-ring 10 is disposed between the sample 8 and the container portion 2 concentrically with the circular hole 2a. This is because the O-ring 10 prevents the electrolyte solution 11 from leaking out.

  The electrochemical cell 1 assembled in this manner is incorporated into an NI test apparatus (eg, TriboIndenter-950 manufactured by Hysitron). More specifically, a temperature-variable heat transfer table 12 (see FIGS. 7 and 9) is disposed on a measurement table of an NI test apparatus (not shown), and a pedestal 3 is mounted on the heat transfer table 12. Fixed. The heat transfer table 12 is a device that generates heat (including cold). The heat transfer table 12 can be constituted by, for example, a Peltier element, and in this case, the temperature can be adjusted within a range from a low temperature range below -20 ° C. to + 80 ° C. to a high temperature range above room temperature. . When the temperature adjustment only in the high temperature region is sufficient, the heat transfer table 12 with a built-in coil heater can be applied. However, when measuring at room temperature, the heat transfer table 12 is unnecessary, and the electrochemical cell 1 is directly fixed on the measurement table.

  The pedestal 3 can be fixed to the heat transfer table 12 as follows. As shown in FIG. 9, a leaf spring clip 13 is installed on the heat transfer table 12. The pedestal portion 3 is detachably fixed to the heat transfer table 12 by the clip 13.

  Here, the terminal 5a of the electrode plate 5 protrudes to the outside, and the terminal 5a is connected to the cathode (-electrode) of the power supply device. On the other hand, the counter electrode 6 and the reference electrode 7 have their terminals 6b and 7b connected to the anode (positive electrode) of the power supply device.

  The indenter 14 of the NI device is, for example, a diamond spherical indenter having a tip radius of curvature of about 1 μm, and is held by a transducer 24 which is a load generator and a load and displacement detector. Here, the NI apparatus according to the present embodiment has an indentation load P applied to the indenter 14 by an arbitrary waveform generator 22 connected to the transducer 24 so that measurement by the RF method can be performed. Amplitude modulation can be added. That is, the indenter 14 is pushed into the sample 8 while being given a minute vibration along the pushing direction at the time of NI measurement using the RF method, and is not vibrated at the time of NI measurement not applying the RF method. In this configuration, the sample 8 is pushed. The indentation load P and the indentation time Δt of the indenter 14 are controlled by the controller 20 connected to the transducer 24. The indentation load P and the displacement of the indenter 14 are detected by the transducer 24. Further, the contact rigidity of the sample 8 is detected by a lock-in amplifier 23 connected to the transducer 24. Various measurement calculations are performed by the calculation device 21 connected to the lock-in amplifier 23 and the transducer 24.

  In this manner, the electrolytic solution 11 is filled in the circular hole 2a and the electrolytic solution storage recess 2b of the container portion 2 in a state where the electrochemical cell 1 is incorporated in the NI test apparatus. Thereby, the counter electrode 6 and the reference electrode 7 are immersed in the electrolyte solution 11. The electrolytic solution 11 is a solution in which the pH is not greatly changed even if the concentration is somewhat changed, that is, an electrolytic solution having a buffering effect on the hydrogen ion concentration. As the electrolyte solution 11, for example, a borate buffer solution is suitable.

  In the electrochemical cell 1, if the temperature of the heat transfer table 12 is adjusted to a low temperature or a high temperature, the heat (including cold heat) of the heat transfer table 12 is transferred to the sample 8 through the pedestal 3 having heat conductivity and the electrode plate 5. At the same time, it is efficiently transmitted to the electrolytic solution 11 through the pedestal 3, the screw 4 and the container 2 which also have thermal conductivity. As a result, the temperature of the sample 8 and the electrolytic solution 11 can be changed in a short time and maintained at a desired temperature.

  In particular, since the space around the sample 8 is filled with the spacer 9 having thermal conductivity, heat transfer to the sample 8 and the electrolyte solution 11 is made more effective.

If a cathode current is applied to the electrode plate 5 while maintaining the sample 8 and the electrolytic solution 11 at a desired temperature, the sample 8 functions as a working electrode with respect to the electrolytic solution 11, and the electrolytic solution undergoes an electrochemical reaction. 11 accumulates on the surface of the sample 8, and the sample 8 is charged with hydrogen. In the NI measurement, while maintaining the sample 8 and the electrolytic solution 11 at a desired temperature, the indenter 14 is pushed into the surface of the sample 8 while applying a cathode current to the electrode plate 5, thereby supplying hydrogen to the sample 8 at the desired temperature. While being introduced, the nano hardness H _n and elastic modulus _Er can be measured “in situ”.

Specifically, first, in order to grasp the elastic modulus _Er of the sample 8, EC-NI measurement is performed without applying the RF method. The pushing time Δt at this time is a short time in which the thermal drift can be ignored, and is within a range of 0.1 s to 30 s, for example. This is because an indentation time shorter than 0.1 s is difficult in terms of the capability of the apparatus, and conversely, an indentation time longer than 30 s affects the thermal drift and cannot provide highly reproducible data. More preferably, it is in the range of 1 s to 10 s, and 5 s is practically preferable. In the EC-NI measurement without applying the RF method, as described above, the indenter 14 is pushed into the surface of the sample 8 without vibrating, and the indentation depth h determined from the displacement of the indenter 14 at that time and the indentation load P Are acquired sequentially. From the relationship between the indentation load P and the indentation depth h, the elastic modulus _Er is derived from the analysis using the OP method.

Subsequently, regarding the nano hardness H _n of the sample 8, in order to measure the influence of hydrogen at a low strain rate at which hydrogen embrittlement becomes obvious, EC-NI measurement is performed by applying the RF method. The indentation time Δt at this time is a long time to realize a low strain rate, and can be 60 min or more. In the EC-NI measurement using the RF method, as described above, when the indenter 14 is pushed into the sample 8, a minute amplitude modulation of several hundred Hz is added to the pushing load P by the arbitrary waveform generator 22, and the contact rigidity is obtained. By detecting S by the lock-in amplifier 23, the indentation load P and the contact rigidity S are sequentially acquired. Further, the indentation projection area A _c and the contact depth h _c at an arbitrary load during the indenter indentation are calculated by the above equation (3) using the indentation load P and the contact stiffness S acquired sequentially and the known elastic modulus _Er. Ask. Then, the nano hardness H _n is obtained from the indentation projection area A _c by the above equation (2). In short, in EC-NI measurement using the R-F method, the thermal drift can be corrected, and thereby the nano hardness H _n including the influence of hydrogen can be accurately determined at a low strain rate at which hydrogen embrittlement becomes apparent. It becomes possible to measure.

  In the EC-NI measurement using the R-F method, the amplitude modulation added to the indentation load P is practically a sinusoidal amplitude modulation. However, it may be rectangular wave amplitude modulation or triangular wave amplitude modulation. The frequency of the modulation is in the range of 10 to 300 Hz. This is because if the frequency is too low, thermal drift affects and data with high reproducibility cannot be obtained. On the other hand, too high a frequency is difficult due to the capability of the apparatus. More preferably, it is in the range of 100 to 200 Hz, and practically 200 Hz is preferable.

  In addition, when performing EC-NI measurement using the R-F method, the load amplitude of the amplitude modulation added to the indentation load P is in the range of 1 to 10% of the value of the indentation load P. This is because the contact stiffness cannot be detected by the lock-in amplifier when the load amplitude is too small. On the other hand, if the load amplitude is too large, the accuracy of the indentation load deteriorates. Practically, the range of 1 to 3% is preferable.

  In the electrochemical cell 1 of the present embodiment, a load is applied to the pedestal portion 3 as the indenter 14 is pushed during the NI measurement. However, since the pedestal portion 3 is made of a highly rigid AlN-BN composite, The pedestal 3 is not deformed with the pushing load of 14, and the NI measurement can be performed accurately. Moreover, since the container part 2 and the pedestal part 3 are made of an AlN-BN composite having excellent machinability, they can be molded even in complicated shapes.

  Moreover, the electrochemical cell 1 of this embodiment can hold | maintain a lot of electrolyte solution 11 with the circular hole 2a of the container part 2, and the recessed part 2b for electrolyte solution storage. Thereby, the following effect is acquired. NI measurement while charging hydrogen at a low strain rate takes a long time. Then, the electrolyte solution 11 gradually evaporates and decreases during NI measurement. In this regard, in the electrochemical cell 1 of the present embodiment, since the initial amount of the electrolyte solution 11 is large, it is possible to allow a decrease in the electrolyte solution 11 during the NI measurement, and the NI measurement over a long time. Can also respond sufficiently.

  Moreover, since the electrode plate storage recess 3a is formed in the pedestal portion 3, even if the electrolyte 11 leaks from the O-ring 10, the electrolyte accumulates in the electrode plate storage recess 3a and leaks to the outside. There is no. Thereby, the following effect is acquired. Under the electrochemical cell 1, a high-voltage device such as a heat transfer table 12 is provided, and there is a danger of an electrical short circuit if the electrolyte solution 11 is applied here. In this regard, the electrochemical cell 1 of the present embodiment is excellent in safety because there is no leakage of the electrolyte solution 11 to the outside.

  Moreover, in the electrochemical cell 1 of this embodiment, since the head of the screw 4 for fastening the container part 2 and the base part 3 is accommodated in the screw accommodating recess 2d, the head of the screw 4 is the container. It does not protrude from the upper surface of the part 2. Similarly, since the wirings 6 a and 7 a from the counter electrode 6 and the reference electrode 7 are stored in the wiring storage grooves 2 e and 2 f, the wirings 6 a and 7 a do not protrude from the upper surface of the container portion 2. Thereby, the following effect is acquired. The length of the indenter 14 for NI measurement is short to ensure measurement accuracy. Then, as the indenter 14 is pushed during NI measurement, the head that holds the indenter 14 approaches the upper surface of the container unit 2. In this respect, in the electrochemical cell 1 of this embodiment, the heads of the screws 4 and the wires 6a and 7a from the electrodes 6 and 7 do not protrude from the upper surface of the container part 2, so that the head for gripping them and the indenter 14 is used. Can be prevented.

  In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, an electrolyte solution injection port and an electrolyte solution discharge port may be provided on the side surface of the container portion 2 so as to penetrate to the electrolyte solution storage recess 2b. As a result, the electrolyte solution 11 can be injected into the electrolyte storage recess 2b through the electrolyte injection port, or the electrolyte solution 11 can be discharged from the electrolyte storage recess 2b through the electrolyte discharge port. The electrolyte solution 11 can be injected and discharged in a timely manner, and is useful for dealing with NI measurement over a long period of time.

  However, the electrochemical cell incorporated in the NI test apparatus of the present embodiment may be one proposed in Non-Patent Documents 1 and 2 as long as the R-F method can be applied.

  The EC-NI test apparatus of the present invention holds a sample, electrochemical cells for electrochemically introducing hydrogen into the surface of the sample, an indenter that is pushed into the surface of the sample in the process of introducing hydrogen, Controller for controlling indenter indentation load and indentation time, load generator, transducer for detecting indentation load and displacement of indenter, arbitrary waveform generator for adding amplitude modulation to indentation load, and contact stiffness detection A lock-in amplifier and an arithmetic unit that calculates the nano hardness of the sample, as described above, accurately measures the nano hardness including the influence of hydrogen at a low strain rate at which hydrogen embrittlement becomes apparent. It becomes possible.

  In addition, the EC-NI test method of the present invention adds amplitude modulation to the indentation load by an arbitrary waveform generator when pushing the indenter into the sample surface while electrochemically introducing hydrogen into the sample surface. Since the configuration is such that the contact stiffness is detected by a lock-in amplifier from the amplitude modulation of the generated displacement of the indenter, the indentation load and the contact stiffness are sequentially obtained, and the nano hardness is calculated. It becomes possible to accurately measure the nanohardness including the influence of hydrogen at a low strain rate where the conversion becomes obvious.

  The EC-NI test apparatus and test method of the present invention can be effectively used to elucidate the mechanism of hydrogen embrittlement.

1: electrochemical cell, 2: container part, 2a: circular hole,
2b: concave portion for storing electrolyte, 2c: screw insertion hole, 2d: concave portion for storing screw,
2e, 2f: groove part for wiring storage,
3: pedestal, 3a: recess for storing electrode plate, 3b: screw hole,
4: Screw, 5: Electrode plate, 5a: Terminal of electrode plate,
6: counter electrode, 6a: wiring, 6b: terminal,
7: Reference electrode, 7a: Wiring, 7b: Terminal,
8: Sample, 9: Spacer, 10: O-ring, 11: Electrolytic solution,
12: Heat transfer table, 13: Clip, 14: Indenter,
20: Controller, 21: Arithmetic unit, 22: Arbitrary waveform generator,
23: Lock-in amplifier, 24: Transducer

Claims (1)

An electrochemical nanoindentation test method,
When introducing an indenter into the surface of the sample while electrochemically introducing hydrogen to the surface of the sample, an amplitude modulation is added to the indentation load of the indenter by an arbitrary waveform generator, and from the generated displacement of the indenter, The contact stiffness is detected by a lock-in amplifier, and the nano hardness is calculated from the indentation load and contact stiffness acquired sequentially.
An electrochemical nanoindentation test method in which the indentation time is 60 minutes or more and the frequency of the amplitude modulation is in a range of 10 to 300 Hz.