JPH0332738B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for observing a metal structure using a high-temperature microscope, and particularly to a method for observing a metal structure over a long period of time under an ultra-high vacuum using a high-temperature metallurgical microscope. [Prior Art] A high-temperature metallurgical microscope is generally used to examine changes in metal structure due to temperature changes. This device consists of a vacuum furnace, a high temperature microscope, a vacuum means, a temperature measuring means, a temperature controlling means, etc. Conventionally, when observing metal structures using the high-temperature metallurgical microscope, a combination of an oil rotary pump and an oil diffusion pump has been used as a vacuum means for evacuating the vacuum furnace into which the sample is charged. A neoprene O-ring is used as a sealant. The ultimate vacuum in the vacuum furnace obtained through such vacuum means, without a liquid nitrogen trap, is
It is about 10 ^-6 Torr, and even when using a liquid nitrogen trap, the maximum is about 10 ^-7 Torr. In addition, as a vacuum gauge, Pirani vacuum gauge for 1 to 10 ^-3 Torr,
Equipped with an ionization vacuum gauge for 10 ^-3 to ^{10 -7} . That is, the conventional method was to observe the structure of the sample surface while heating the sample in a vacuum furnace with a vacuum level of 10 ^-7 Torr. [Problems to be Solved by the Invention] By the way, according to the above-mentioned method for observing metal structures at a low degree of vacuum, oxidation of the sample surface inevitably occurs during observation, which makes it difficult to see the true surface structure of the sample. However, there was a particular drawback in that continuous observation over a long period of time was impossible. The inventors investigated the reason why metal samples are oxidized at a vacuum level of about 10 ^-7 Torr, and found that it is due to the overwhelming amount of H ₂ O remaining in the vacuum furnace. I found out. this
It is thought that H ₂ O can be removed by creating a higher degree of vacuum in the vacuum furnace, but this has not yet been implemented for the following three reasons. The first reason is that it was extremely difficult to create a higher vacuum. For example, conventionally, vacuum pumps have been used in combination with oil diffusion pumps and oil rotary pumps, but with such a combination, the degree of vacuum that can be achieved is at most limited.
The temperature was 10 ^-7 Torr, and the inner walls of the vacuum furnace and the surfaces of the jigs had to be thoroughly cleaned. However, achieving such cleaning is not easy. The second reason is that although it is necessary to select the sealing material and container for the vacuum furnace depending on the temperature inside the vacuum furnace and the target degree of vacuum, it has been difficult to do so. FIG. 1 shows the change over time in the amount of gas released from each material. It is conventionally known that sealants and container materials emit gas in vacuum even at high temperatures. From Figure 1, for example, even if 18-8 stainless steel is used as the container material,
It can be seen that the ultra-high vacuum standard surface treated material, whose surface has been thoroughly cleaned, releases less gas than ordinary 18-8 stainless steel. Furthermore, it can be seen from FIG. 1 that the amount of gas released varies greatly depending on the material of the sealing material, such as neoprene, Viton, Teflon, and steel. In this way, it is necessary to select the sealing material and container material, and this also becomes a cause of hindering the achievement of a high vacuum itself. The third reason why high vacuum was not implemented in the vacuum furnace is how high the vacuum inside the vacuum furnace should be, and how high the partial pressure of H ₂ O (g) inside the vacuum furnace would be. As a result, there has been no scientific understanding of whether oxidation of the metal surface is prevented or suppressed. Therefore, even if high vacuum itself were possible, it would not be possible to effectively suppress oxidation. Furthermore, according to the structure observation method using a high-temperature microscope under the known state of the art, for example, when the sample is a metal, while the sample metal is heated to a high temperature and the sample surface structure is observed, Another problem was that metal was deposited, making microscopic observation difficult. Regarding this problem, a method has been proposed in which a close-contact sliding transparent quartz plate is used to slide a shutter that can be operated from the outside by a small amount to expose the transparent surface of the viewing window glass and allow observation to be continued. Furthermore, when the sample is likely to evaporate at high temperatures, a method is known in which an inert gas such as Ar is sealed in the vacuum furnace to make the sample less likely to evaporate. However, in this case as well, there was a problem in that as the temperature inside the furnace rose, the observation window glass became opaque due to metal deposition. Therefore, the present invention solves the problems described above, such as the problem that the surface of the sample is oxidized during observation, making it impossible to observe the surface, and the problem that metal is deposited on the glass of the observation window of the vacuum furnace, making observation impossible. With the aim of solving this problem and providing a new method that enables continuous observation of metal structures over long periods of time using a high-temperature microscope, they came up with the following technology. [Means for Solving the Problems] As a result of research to achieve the above-mentioned object, the present invention has developed an advantageous method having the following gist. That is, the present invention provides a high-temperature microscope that, after charging a metal sample into a vacuum furnace, sets the inside of the vacuum furnace to a predetermined degree of vacuum, and then observes the surface structure of the sample using an optical metallurgical microscope while heating the sample. In the method for observing metal structures, first, the inside of the vacuum furnace ^is
By creating a vacuum of 10 ^-8 Torr and then heating to a temperature of 60 to 250°C, the water present in the vacuum furnace is converted into H ₂ O (g) (water vapor) or the water present in the vacuum furnace. Oxygen produced by dissociation of H ₂ O
The inside of the vacuum furnace is made to have _a high vacuum with a pressure of 1×10 ^-8 Torr or less, and the partial pressure of H ₂ in the gas remaining inside the furnace is made higher than the partial pressure of H ₂ O (g). This is a method for observing a metal structure using a high-temperature microscope, which is characterized by holding and observing. More preferably, the residual gas in the vacuum furnace, that is, the partial pressure of each of H ₂ O, O ₂ , CO, and CO ₂ , is set to 1×10 ⁻⁹ Torr or less at room temperature and observed; High-purity H ₂ was introduced into the vacuum furnace from outside the furnace for observation, and after charging the sample into the vacuum furnace, the inside of the vacuum furnace was brought to a high vacuum with a pressure of 1 × 10 ^-8 Torr or less. ,
Next, the thick oxide film on the sample surface is peeled off using an inert gas and a sputtering electron gun for observation. [Function] According to the research conducted by the present inventors, the H ₂ O partial pressure inside the vacuum furnace cannot be sufficiently lowered even at the highest degree of vacuum, 10 ^-7 Torr, using conventional observation methods. It turned out that I couldn't do it. As a result, for example, when the sample is an iron alloy, the sample surface reacts with H ₂ O to produce iron oxide, and the thickness of this iron oxide increases rapidly over time, which in turn causes the sample to become thicker. It has become impossible to directly observe the true tissue. Therefore, the present inventors basically investigated the relationship between the oxidation process of metal, the degree of vacuum in the vacuum furnace, and the quality of the vacuum atmosphere. In other words, an alloy containing 1.0% P and the remainder substantially consisting of Fe (hereinafter this alloy will be referred to as Fe-
As a result of researching the surface oxidation process of a 1.0% P alloy under vacuum and measuring the amount of oxidation using Auger electron spectroscopy, the following phenomena were newly discovered. Figure 2 shows the energy of Augier electrons obtained by Auger electron spectroscopy of Fe-1.0%P alloy.
FIG. 3 is a diagram showing the relationship between Ev and the amount of change DN/DE in the number of Augier electrons per unit energy. The measurement conditions were to break the sample piece in a vacuum chamber at 7×10 ^-11 Torr, and then immerse it in 1× Ar containing saturated water vapor at 15°C.
Introduced to 10 ^-7 Torr and held for 10 minutes,
After the vacuum chamber was evacuated to 10 ^-10 Torr, points on the intragrain fracture surface were measured using an electron beam with a thickness of 5 μm at an acceleration voltage of 2 KV and a modulation voltage of 6 V. According to the figure, by the above operations and measurements,
It can be seen that oxygen exists on the intragrain fracture surface of the Fe-1.0%P alloy. That is, 7×
After the sample was fractured under a vacuum of 10 ^-11 Torr, Ar containing saturated water vapor at 15°C was introduced at 1 × 10 ^-7 Torr.
Even after contacting the specimen fracture surface for 10 minutes and reducing the pressure of the vacuum vessel again to 10 ^-10 Torr,
It was found that a large amount of oxygen was present on the fractured iron surface. Therefore, in order to confirm in which process the oxygen present on the surface of the fractured sample in the above experiment was adsorbed to the fracture surface and reacted with the sample on the fracture surface, the inventors further conducted the following experiment. I went there. In this experiment, Fe-1.0%P alloy was
The specimen was fractured in a vacuum chamber at 10 ^-11 Torr, and the quantitative change in oxygen present on the intragranular fracture surface was measured over time using Auger electron spectroscopy. The results are shown in FIG. This measurement was performed at the same point on the intragrain fracture surface using an electron beam with a thickness of 5 μm at a primary electron beam acceleration voltage of 3 KV and modulation voltage of 3 V after 1, 10, 20, and 30 hours had elapsed. be. According to the same figure, Fe-1.0%P after intragrain fracture
It was found that the amount of oxygen present on the surface of the alloy increased over time, but was much less than the experimental oxygen amount shown in FIG. Therefore, the oxygen present on the intragrain fracture surface of the sample in the experiment shown in Figure 2 is not the oxygen that is present as a result of adsorption or reaction at the time of sample fracture in the experiment shown in Figure 3, but at 15°C. By introducing Ar containing saturated water vapor amount at 1×10 ^-7 Torr, adsorption,
Alternatively, it was confirmed that it was oxygen in the reacted H ₂ O. In addition, as mentioned above, oxygen that is adsorbed or reacted to fracture surfaces within grains is
It can be seen that even if the vacuum chamber in which the sample is charged is placed in a high vacuum of 10 ^-10 Torr, it will not separate. In order to observe the high-temperature structure of a sample in such a state, it is assumed that iron oxides will be easily generated if the sample is heated. That is, as a result of mass spectrometry analysis of the residual gas in the vacuum container at 7×10 ^-11 Torr, the partial pressures of H ₂ and H ₂ O (g) are P _H2 = 6.0×10 ^-11 Torr and P _H2O(g) = respectively. 1
×10 ^-12 Torr, and the O ₂ partial pressure P _O2 was below the measurable limit of 10 ^-13 Torr. Then 15
Ar containing saturated water vapor at 1×10 ^-7 Torr
inside the vacuum container when introduced so that
H ₂ O (g) partial pressure P _{H2 (g)} is 1.6×10 ^-9 Torr,
When the vacuum container is evacuated to 10 ^-10 Torr after 10 minutes
The O ₂ partial pressure was below the measurement limit. In other words, after the sample is fractured in a reducing atmosphere of 7×10 ^-11 Torr and P _H2 /P _H2O(g) = 60, the intragranular fracture surface of the sample is 1.6×
1× by Ar containing H ₂ O (g) at 10 ^-9 Torr
By exposing the sample to an oxidizing atmosphere of 10 ^-7 Torr, P _H2 /P _H2O(g) = 0.04,
The grain boundary fracture surface of the P alloy appears as the large oxygen peak shown in Figure 2 by combining with oxygen in H ₂ O. From the above, the oxidation of the sample surface in the conventional vacuum furnace for high-temperature microscopes is caused by the O ₂ remaining in the vacuum furnace.
It was surmised that the reaction was mainly due to H ₂ O rather than due to oxidation. Therefore, in order to prevent or suppress the oxidation of the iron alloy sample in the vacuum furnace for high-temperature microscopes, it was found that the most important thing is to remove H ₂ O in the vacuum furnace. That is, according to the present invention, after a metal sample is charged into a vacuum furnace, the inside of the vacuum furnace is mainly heated to 1×10 ^-8 Torr.
We have newly discovered that by creating a vacuum at this high level, the true surface structure of a sample can be observed over a long period of time. In this regard, according to the conventional method, the degree of vacuum in the vacuum furnace is at most 10 ^-7 Torr, and it is necessary to elucidate the relationship between the degree of oxidation of the metal surface structure and temperature at a degree of vacuum higher than the above degree of vacuum. This has never been done in the past because, as mentioned above, the technique of increasing the degree of vacuum itself becomes increasingly difficult. Moreover, if one skilled in the art could simply create a higher vacuum at 10 ^-7 Torr, the oxidation of the metal surface would be slowed down at high temperatures; This led to the new finding that the tendency for metal surface oxidation to be suppressed or suppressed is not strengthened by the use of vacuum, but that metal oxidation is suppressed or suppressed only by specifying the quality of the vacuum. It is. [Example] According to the present invention, in the case of alloys that undergo oxidation relatively easily, such as iron alloys, H ₂ O, O ₂ , CO, and CO ₂ remaining in the vacuum furnace are removed at room temperature. By keeping the partial pressure below 10 ^-9 Torr, the sample surface can be observed for a long period of time. Metallic iron reacts with oxygen in O2 and _{H2O ,} CO, and _{CO2 to} become iron oxides such as wustite (FeO), magnetite ( _Fe3O4 ), and _{hematite (Fe2O3 )} . . The rates of these reactions depend on the _O2 partial pressure P _O2 and temperature. FIG. 4 is a diagram showing what kind of oxide metal iron changes to as P _O2 and temperature in an equilibrium state. For example, at 1000°C and 1 atmosphere of P _O2 (760 Torr), iron exists in equilibrium and stably as hematite. However, at 1000℃, P _O2 1×10 ^-10
At atmospheric pressure (7.6×10 ^-8 Torr), magnetite is the equilibrium stable phase. Further, at 1000° C. and _an O ₂ partial pressure P _O2 of 1×10 ⁻¹⁵ atmospheres (7.6×10 ⁻¹³ Torr) or less, γ iron in a metallic state becomes an equilibrium stable phase. In other words, at 1000℃, 1×10 ^-15 atm (7.6×
Iron oxides (hematite, magnetite, wustite) are expressed by the following formula (1) at an O ₂ partial pressure of less than 10 ^-13 Torr):
The figure shows that the reaction occurs as shown in (2) and (3), and that it ultimately exists stably as metallic iron Fe and oxygen gas O ₂ . 6Fe ₂ O ₃ 4Fe ₃ O ₄ +O ₂ ...(1) 2Fe ₃ O ₄ 6FeO+O ₂ ...(2) 2FeO2Fe+O ₂ ...(3) By the way, the O ₂ partial pressure P _O2 in the vacuum furnace is equal to that of the conventional exhaust gas. When exhausted by a device, the general formula is H ₂ O.
(g) The partial pressure P is known to be much lower than _{O2 (g)} , and the oxidation reaction of metal samples is determined by H ₂ O, especially when the sample is heated to high temperatures. At a certain temperature T (K), H ₂ + 1/2O ₂ H ₂ O (g) ...(4) is in equilibrium, and the standard free energy change of this reaction is ΔG°H ₂ O (g) ( cal/mol) is O.
According to Kubaschewski et al., ΔG° _H2O(g) = −57250 + 4.48TlogT−2.21T (5) is given by T = 298 to 2500K. In the equilibrium state of equation (4) above, P _O2
(Atmospheric pressure) is logP _O2 = 2ΔG° _H2(g) /2.303RT−2logP _H2 /P _H2O(g) ……
(6). Calculate the relationship between T(K) and logP _O2 based on the above equations (5) and (6), and calculate the value of P _H2 /P _H2O(g) as shown in Figure 5.
When changed to 100, 50, 10, 1, 0.1, 0.01
The relationship between logP _O2 and temperature T (K) was investigated. From this result, below each curve for a given value of P _H2 /P _{H2O (g),} the reaction proceeds with H2O (g) as the stable phase, but above the curve, H ₂ and O ₂ are in the stable phase. Therefore, dissociation of H ₂ O(g) occurs. on the curve
H ₂ , O ₂ and H ₂ O (g) exist in equilibrium. Further, from the same figure, the reaction between O ₂ dissociated from H ₂ O (g) and metal iron Fe is as shown in the following formula (7). Fe+1/2O ₂ FeO ……(7) The standard free energy change in this reaction ΔG° _FeO (cal/mol) is according to O.Kubaschewski et al. ΔG° _FeO = −62050+14.95T ……(8) T=298 ~1642K Given by the above formula (8). Assuming that the activities of Fe and FeO are equal, the equilibrium P _O2 (atmospheric pressure) in the above reaction formula (7) is calculated using the above formula (9) as logP _O2 = 2ΔG° _FeO /2.303RT...(9) . This calculation result is
Indicated by an x in the figure. Larger than the x mark line in the same figure.
In the region of the P _O2 value, that is, the region below the x-marked line, the reaction progresses in the direction of producing FeO, and FeO
exists as a stable phase. On the other hand, in the region above the x-marked line, the reaction proceeds in the direction in which FeO decomposes, and Fe and O ₂ become stable phases. Here, the partial pressure ratio of the residual gas H ₂ and H ₂ O (g) in the vacuum furnace obtained by an exhaust pump such as a turbo molecular pump, sputter ion pump, cryopump, or oil diffusion pump with cold trap. P _H2 /
Since the value of P _H2O(g) is 50 or more, in equilibrium theory, in the vacuum of the above state, FeO will be reduced, and as shown in Figure 5, 1000~
While it is clear that Fe and O ₂ exist stably at 1650K, in the residual gas atmosphere in the vacuum furnace as described in the explanation of Figure 2, P _H2 /
The value of P _H2O(g) is 0.004, and the value of P _H2 /P _H2O(g) is 0.01 in the residual gas atmosphere in the vacuum furnace when using an oil rotary pump and an oil diffusion pump. Figure 5 shows that in terms of equilibrium, iron exists stably as FeO in the temperature range of 1000 to 1650K. Next, we investigated in detail the equilibrium theory at which P _H2 /P _H2O(g) value oxidizes metallic iron. The reaction between FeO and H ₂ is shown in the above formula (4) and the above formula (7).
Therefore, FeO + H ₂ Fe + H ₂ O (g) ...(10) The above formula (10) is obtained, but the standard free energy change ΔG°Fe (cal/mol) is calculated from the above formula (5) and the above formula (8). Assuming that the activities of Fe and FeO are equal, ΔG° _Fe = ΔG° _H2O(g) −ΔG° _FeO = O ₂ = 2.303R
T logP _H2 /P _H2O(g) ...(11) It can be obtained from the above formula (11). The calculation results are shown in FIG. When the reaction of equation (10) above is in equilibrium, P _H2 /
The value of P _H2O(g) changes on a curve with temperature. When the value of P _H2 /P _H2O(g) is smaller than the curve (lower in the figure), the reaction of equation (10) above proceeds in the direction of producing FeO. On the other hand, when the value of P _H2 /P _H2O(g) is larger than the curve (upper part of the figure), it can be seen that the reaction of equation (10) above proceeds in the direction of decomposing FeO and producing Fe. . As described above, in the present invention newly discovered by the present inventors, metal iron can be heated to high temperature (1000 to 1600K) without oxidizing it.
Equilibrium theory also supports the fact that in order to maintain the P _H2 /P _H2O(g) at about 2 or higher, it is necessary to maintain an atmosphere of about 2 or more. However, just because it is stable according to equilibrium theory does not mean that FeO will not be produced at all. In other words, this only shows that FeO is more unstable than Fe from an equilibrium standpoint, but to discuss how unstable it is, we must consider the kinetics of the reaction. Must be. Figure 7 shows the time (h) of heating from 800° to 1100° in the atmosphere of the metallic iron H ₂ O (g) - H ₂ -Ar meter and the weight increase Δm/A (mg cm ^-2 ) of the sample. This is the result of a kinetic experiment by HJKock showing the relationship between In this iron oxidation experiment, the total pressure was 1 atm, Ar was 70%, and H ₂
and H ₂ O (g) is 30%, as shown in the figure.
It was carried out under the value of P _H2 /P _H2O(g) . All the above experiments at each temperature are _H2 partial pressure P _H2 and _H2O (g) partial pressure
It can be seen that the reaction occurs in the region below the curve in FIG. 6 where the ratio of P _H2O(g) is P _H2 /P _H2O(g) <1.00, that is, in an oxidizing atmosphere. In FIG. 7, the weight increase of the sample at each experimental temperature increases almost linearly with time. Therefore, when the oxide layer is extremely thin, the amount of weight increase (Δm/A) due to the oxidation reaction is expressed by the linear equation (12) of time (t) in a constant atmosphere and temperature. Δm/A=kt...(12) Now, as seen in the above equation (7), the oxidation reaction of Fe is as follows: 1 mol of FeO and 1/2 mol of O ₂
is generated. The reaction rate constant k shown in the above equation (12)
is determined from FIG. 7, and the time to required for an oxidized weight increase of 2×10 ^-7 g/cm ² is calculated as shown in Table 1. However, at 2×10 ^-7 g/cm ² , FeO is on the surface of Fe.
This is the amount that produces 0.1 μm. Next, the reaction rate constant k
is the partial pressure of H ₂ and H ₂ O (g) in the atmosphere and when the thickness of the oxide layer is approximately 10 μm or less, 1/k=α(P _H2 /P _H2O(g) ) ^2.6 ...(13) Since the relationship α: constant has been clarified, P _H2 /
When the value of P _H2O(g) is 50, 0.04, and 0.01, the time t ₅₀ , t _0.04 , and t _0.01 required for FeO to form a thickness of 0.1 μm on the iron surface are calculated and shown in Table 1. Ta.

〔Effect of the invention〕

As explained above, in the past, it was not possible to observe the structure of a metal material that is easily oxidized for a long time while heating it at a high temperature, but according to the present invention, the above-mentioned drawbacks of the conventional method can be completely eliminated. , the true high-temperature structure of a sample can be observed in detail over a long period of time. Further, the present invention is an industrially useful invention that can greatly contribute to the study of the temperature dependence of the structure of metal materials, and can greatly contribute to the development and manufacture of industrial materials.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the amount of gas released from vacuum sealing materials and vacuum container materials at room temperature and time, and Figure 2 is a diagram based on Auger electron spectroscopy.
Figure 3 shows the results of measuring the amount of oxygen reacting on the intragranular fracture surface of Fe-1.0%P alloy.
Figure 4 shows the results of Auger electron spectroscopy, which measured the amount of oxygen reacting on the intragranular fracture surfaces of the %P alloy over time. Figure 5, a diagram showing the relationship between pressure and temperature, shows the relationship between H ₂ , O ₂ , H ₂ O (g), and the temperature (K) of Fe, O ₂ , FeO and P _O2 in an equilibrium state. Figure 6 shows Fe in equilibrium state,
H ₂ O (g), FeO, H ₂ temperature (K) and P _H2 /P _{H2O (g)}
Figure 7 shows the relationship between
FIG. 2 is a diagram showing the relationship between the amount of weight increase due to oxidation of iron in H ₂ O-H ₂ -Ar and time.

Claims (1)

[Scope of Claims] 1. After charging a metal sample into a vacuum furnace, the inside of the vacuum furnace is brought to a predetermined degree of vacuum, and then, while heating the sample, the sample surface structure is observed using an optical metallurgical microscope. In the method for observing metal structures using a microscope, first, the inside of the vacuum furnace is heated at 1×10 ^-3 to 1×
By creating a vacuum of 10 ^-8 Torr and then heating to a temperature of 60 to 250°C, the water present in the vacuum furnace can be converted into H ₂ O (g) (water vapor) or the water present in the vacuum furnace. The oxygen produced by the dissociation of H ₂ O is discharged as O ₂ , and the inside of the vacuum furnace is
Using a high-temperature microscope, which is characterized by creating a high vacuum with a pressure of 1 × 10 ^-8 Torr or less and maintaining the H ₂ partial pressure in the gas remaining inside the furnace higher than the H ₂ O (g) partial pressure for observation. How to observe metal structure. 2 The residual gas in the vacuum furnace, namely H ₂ O,
The partial pressure of each of O ₂ , CO , and CO ₂ is 1 at room temperature.
The method according to claim 1, characterized in that the observation is carried out at a temperature of ×10 ^-9 Torr or less. 3. Claim 1, characterized in that high-purity H ₂ is introduced into the vacuum furnace from outside the furnace for observation.
The method according to any one of items 1 to 2. 4 After loading the sample into the vacuum furnace, make the inside of the vacuum furnace a high vacuum with a pressure of 1 × 10 ^-8 Torr or less, and then use an inert gas and a sputtering electron gun to remove the thick oxide film on the sample surface. Claims 1 to 3, characterized in that the metal structure is observed after peeling and removal.
The method described in any one of paragraphs. 5 After loading the sample into the vacuum furnace, the vacuum furnace is made into a high vacuum with a pressure of 1×10 ^-8 Torr or less, and then a small amount of high-purity O ₂ is introduced while heating the sample, and the sample is heated. By forming and maintaining an oxide film with a thickness of 1.0 μm or less on the surface, the metal structure under the oxide film can be observed for a long time through the oxide film. A method according to any one of ranges 1 to 4.