Here is a translation of the Japanese text in the Jung paper. This was done by translate.google.come and by me. I made many changes to some paragraphs, but hardly any to other paragraphs. There may still be AI errors in the text.
The rest of the paper, including the abstract and captions are in English. The paper is: (J. Soc. Mat. Sci., Japan), Vol.49, No. 11, pp. 1242-1248, Nov. 2000 Deformation and Aging of Pd by Hydrogen Absorption-Desorption Cycles — Deformation of Pd at a Hydrogen Absorption-Desorption Cycle — You can download it here: https://www.jstage.jst.go.jp/article/jsms1963/49/11/49_11_1242/_article TRANSLATION: (J. Soc. Mat. Sci., Japan), Vol.49, No. 11, pp. 1242-1248, Nov. 2000 Deformation and Aging of Pd by Hydrogen Absorption-Desorption Cycles — Deformation of Pd at a Hydrogen Absorption-Desorption Cycle — by Young-guan JUNG* , Hideki SUEHIRO * and Yuzuru SAKAI ** * Yokohama National University Graduate School of Engineering Graduate Student, Yokohama National Univ., Hodogaya-gu, Yokohama, 240-8501 ** Regular Member Yokohama National University Faculty of Engineering 〒 240 - 8501 Yokohama City Hodogaya Ward Joban Tai, Dept. Of Faculty of Eng., Yokohama National Univ., Hodogaya-gu, Yokohama, 240-8501 Received the manuscript that "Effect of Cold Working on Hydrogen Storage Characteristic of Palladium (1st Report)" (Effects of Cold Work on Hydrogen Absorption in Pd, I), October 28, 1999 [This document includes a translation of the Japanese text only, not the English Abstract or English figure captions.] 1 Introduction Regarding the behavior of hydrogen in metallic structures, many studies 1) - 3) have been done mainly concerning the hydrogen embrittlement problem. Solid dissolved hydrogen is trapped in dislocations, voids and the like in a steel material structure such as carbon steel and stainless steel, and is thought to be a factor that promotes destruction, so research to elucidate the material embrittlement mechanism by hydrogen is being conducted. Then too, recently, from the viewpoint of global environmental problems, development of a hydrogen storage material as a clean hydrogen energy carrier has been actively underway, that is, development of some metals including rare earth metals that easily form hydrides which have the ability to absorb and release about 1000 times as much hydrogen as their own volume. As already seen in nickel-metal hydride batteries 5), etc., this product has been commercialized and the demand is rapidly increasing year by year. Furthermore, hybrid cars, which are attracting attention the car of the future, are also equipped with hydrogen batteries using misch metal, enabling environmentally friendly and fuel-efficient systems where metal materials absorb hydrogen, metal crystals As hydrogen enters the lattice, lattice expansion occurs, resulting in bulk expansion near 10 to 25% 7). When there is further hydrogen release, the bulk material shrinks in a relatively short time. When this expansion / contraction cycle is repeated, the hydride generates microcracks and is pulverized (micronized). This micronization leads to deterioration of the hydrogen absorbing metal and it becomes a problem because it reduces durability. Several studies on this micronization mechanism have been conducted at the practical material level, and internal strain accompanying volume expansion generates micro cracks, which promotes micronization, has been proposed 9). However, there are many unsolved problems such as the relationship between bulk deformation (volume expansion) at the macro-level and lattice expansion at the micro-level; the internal strain distribution caused by the lattice expansion; the occurrence of microcracks and their progression. These problems are obstacles to improving the durability of the hydrogen storage material. The authors conducted single-cycle and multiple-cycle hydrogen charge and discharge experiments by electrolysis using a Pd sample, which is a typical hydrogen storage material, and the basic relationship between material deformation, and the degradation accompanying hydrogen absorption. Our investigation shows that the Pd material exhibits gradually deformed fracture behavior as the hydrogenation cycle is repeated. This behavior is based on the occurrence of plastic strain and microcrack in one cycle, and the number of hydrogen cycles. The damage accumulates as the cycles are repeated. Therefore, in this paper, we report on the deformation / deterioration behavior in a single cycle hydrogen absorption-release process. Using the rolled material (as-received) and the strain-removed heat treated material, we investigated the macroscopic deformation behavior, lattice expansion behavior, residual strain, microcrack generation etc. after a single cycle of loading. 2 Test sample and experimental method 2.1 Sample Pd plate material (manufactured by Nilaco Inc., thickness 1 mm), of purity 99.9 mass%, heat-treated (annealed) at 600°C for 2 hours, was cut into a size of 20 mm in length and 10 mm in width and used as a test sample. The as-received material has undergone cold rolling processing and it has been shown that it has about 9% of compressive residual strain as measured by x-ray diffraction (detailed in part 3). Hereinafter, this material will be referred to as rolled material or heat-treated material after residual strain removal by heat treatment. Figure 1 (a) shows the microstructural photographs of plate surface A and plate surface B of rolled test specimens. Here, the A, B and C planes are the rolled face, the side face, and the cross section of the test piece respectively. The crystal grains found on the side of the rolled material undergo rolling processing and are elongated and flattened in the rolled direction. Figure 2 (a), (b) shows photographs of the structure of the A and B sides of the heat-treated test piece. In the specimen subjected to the heat treatment at 600°C, the structure recrystallized and the crystal grain recovered to a shape with the longitudinal and lateral ratio close to 1. In addition, almost no change in the thickness due to heat treatment was observed. Table I shows the crystal grain size calculated using the JIS H 0501 crystal grain size with a linear analysis and areal analysis. The results of the grain size calculation by the linear analysis method show that the heat treated material crystal grains are almost isotropic in shape in layers 66 μm on each surface. On the other hand, in the case of the rolled material, the crystal grain size is 126 μm on plate surface B, compared to 174 μm on plate surface A (a factor of 1.4 times larger) from the effects of compression, which can clearly be seen in a cross section of the plate. The average crystal grain size of the heat treated material is about 1/2 of that of the rolled material, and according to the lineral analysis, the average crystal grain size (GL) in the rolled direction and the value (GW, GT) in the direction perpendicular to the roll (in the case of the heat-treated material) the values are almost the same, at 56~57 μm, with the crystal grains growing isotropically. Whereas in the rolled material, the GL is almost the same on the 2 surfaces but GT (63 μm) And GW (135 μm), there is a difference of about 5 times between GT and GL, that is, the size of the crystal grain is flattened with the ratio of the roll direction: width direction: and thickness direction at about 5: 2: 1. 2.2 Experimental method Hydrogen absorption into Pd was carried out by the following electrochemical method 10). The electrolytic experiment cell is shown in Fig. 3. A Pd test piece was used as the cathode and a platinum wire (1.0 mm in diameter) was used as the anode. In order to make the current density uniform, a spiral shape was loosely wound around the cathode as shown in the figure. The electrolyte was a 0.1 M H2SO4 aqueous solution. Hydrogen was generated on the surface of Pd cathode with steady-state current electrolysis. Following the experiment result reported by the authors of Ref. [11], the current density was 100 mA/cm2, the electrolysis temperature was 25°C and the test was done at atmospheric pressure. Electrolysis was performed in 6 stages lasting 0 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, and 72 hours. After completion of electrolysis, the hydrogen absorbed in the sample is determined, but first, hydrogen absorbed in defects, cracks and the like in the vicinity of the surface of the test piece is vigorously released as a foamy gas due to chemical potential change due to halting electrolysis. The sample has to be left long enough for this hydrogen release to cease completely. 11) After leaving the sample for a while, the weight of the sample was measured with a semi-micro balance to determine the weight of hydrogen left inside the Pd plate. After weight measurement, the dimensions in the length, width and plate thickness direction were measured using a micrometer in order to examine macroscopic deformation of the test piece. In order to measure the lattice constant in the hydrogen absorbed state, an X-ray diffraction experiment was conducted with a CuKα ray and an Ni filter attached, with a voltage of 50 kV - 200 mA. After that, mirror finish with alumina powder, aqua regia treatment 12) was applied and a structure photograph was taken. The hydrogen storage capacity determined by gravimetric measurement is defined as the number ratio of hydrogen atoms absorbed per Pd atom in Pd bulk, expressed as n(H)/n(Pd) (hereinafter referred to as "hydrogen absorption ratio (H/Pd)0.6, etc. In the case of Pd, the hydrogen absorption ratio 1.0 is the maximum value of the hydrogen packing ratio). 3 Experiment Results 3.1 Deformation Behavior of Specimen Figure 4 shows the relation between the hydrogen absorption ratio to electrolysis time for both rolled and heat treated materials. The hydrogen absorption ratio rapidly increases until the electrolysis time reaches approximately 4 hours, and then settles at a constant value. The final hydrogen absorption ratio is 0.71 for heat treated material, and 0.70 for cold rolled worked material, and the absorption ratio of the heat treated material is higher than that of the heat treated material. Figure 5 shows deformation behavior of the Pd specimen in the length, width and thickness, over the duration of electrolysis. As in the previous figure, the material deformation also rapidly increases until the electrolysis time reaches 4 hours, but it becomes constant to a certain extent afterwards. This shows that as soon as hydrogen absorption is complete, macroscopic deformation of the material is completed. For the macroscopic deformation ratio, the difference between the rolled material and the heat treated material is not much in any dimension: for the length, the width, or the thickness direction. As is clear from the figure, the deformation ratio in the thickness direction is much larger than the deformation ratio in the length and width. Deformation of specimen due to hydrogen absorption is influenced by distribution of α phase and β phase states generated in the material. When hydrogen starts to enter the material surface, the α phase is formed first. The α phase has a relatively low hydrogen concentration which appears at the beginning of hydrogen diffusion, and its absorption ratio (H/Pd) is reported as 0.008 at room temperature and d = 0.3894 nm at atmospheric pressure. As the electrolysis continues, the gaps between lattice constituent atoms of Pd are filled with hydrogen atoms, and the density of hydrogen atoms reaches the high β phase. In this phase, the (H/Pd) ratio is 0.607 or more and the lattice constant is d = 0.4025 nm. 13) Figures 6, 7 show the distribution of β phase in one hour at the electrolysis time of the rolled material and the heat treated material for 30 minutes. After 30 minutes of electrolysis, the β phase grew in layers from the surface to about 2/5 of the plate thickness of both materials. At 1 hour after the electrolysis, β phase grows up to around ⅔ of the plate thickness. After 2 hours or more of electrolysis, the whole cross section becomes β phase. Macroscopic deformation of the material is thought to be due to the development of this β phase. That is, since the lattice constant of Pd crystal is d = 0.3890 nm, lattice expansion of about 3.47% occurs in one direction when the structure is converted into β-phase, and if this occurs in three directions, the volume increases by 10.8% As a result, remarkable bulk deformation occurs. Figure 8 shows a schematic view of the deformation of the cross section (sections B and C) of the Pd plate material over time (due to the increase in the hydrogen absorption ratio). Over time, the β phase grows from the surface of all the plates to the inside at substantially the same depth At first, at the corner of the cathode, the beta phase begins with the lattice expansion from two directions, so at one hour after electrolysis begins both ends of the specimen swell and become pillow-shaped. At this time, a thin α phase remains inside the plate thickness, and β phases having large lattice constants exist above and below the plate thickness in sandwich form, so that a tensile stress state occurs in the plate thickness direction in the center part. Therefore, after that, the specimen bulges in the thickness direction, finally becoming the thickest drum-shape with the center at the center (after 2 hours electrolysis). On the other hand, when viewed in the length and width direction, the α phase is horizontally distributed at the center part, and this acts as a constraint against elongation deformation, so deformation is suppressed. In Fig. 5, the reason why the deformation ratios in the thickness direction of the rolled material and the heat treated material are about 5 times larger than the deformation ratio in the length and width direction thought that it is caused by the above described β phase development process. Figure 9 shows the change in the volume expansion rate over the electrolysis time, but both rolled and heat treated materials have a final volume expansion of about 15.5%. This is larger than the estimated value of volume expansion due to β phase conversion of 10.8%. The figure also shows the deformation rate of the material after releasing hydrogen and shrunk, but the residual volumetric expansion of about 5% remains for both materials. The difference from the volume before hydrogen release is 10.5%, which corresponds to the expansion amount (10.8%) due to β phase conversion. 3.2 Deformation Behavior of Crystal Lattice The deformation of the specimen measured in the previous section is due to the fact that the crystal lattice deforms as hydrogen enters between the lattices of Pd atoms in a microscopic view. Knowing how the lattice expansion in this microfield relates to expansion of macroscopic material seems to be a necessary finding to discuss the micronization problem of hydrogen storage material. Therefore, dimensional change of crystal lattice by hydrogen absorption was obtained by performing x-ray diffraction using CuKα ray. Figure 10 shows the x-ray diffraction graph for the electrolysis time (the hydrogen absorption ratio) for 0, 30 minutes (0, 0.263). As the hydrogen absorption ratio (or electrolysis time) increases, the diffraction peak moves to the left. That is, according to Bragg's Law 14) 15) the movement of the diffraction peak to the left side means an increase in the lattice spacing, and as the hydrogen absorption ratio increases, the amount of movement also increases. Figure 11 shows how to calculate the lattice constant from the x-ray diffraction result after electrolysis time of 30 minutes. The lattice constant can be obtained by indexing the diffraction angle obtained from the diffraction peak using the least squares method.14) 15) Figure 12 shows the lattice constant change with respect to the hydrogen absorption ratio of the rolled material and the heat treated material obtained in this way. Although the lattice constant rapidly increases until the absorption ratio becomes approximately 0.2, the final value of the rolled material was approximately 0.4024 nm, and the final value of the heat treated material was 0.4026 nm, which is almost the same. Considering the accuracy of measurement, it can be said that they are almost equal. These values are almost the same as the Lewis result of β lattice constant 13) 0.4025 nm. The lattice constant rapidly increases to nearly equal the hydrogen absorption ratio of 0.2, and then becomes constant, the cause of which was determined by x-ray diffraction measurement of the average interstitial distance on the surface of the material. As shown in Fig. 8, the phase change of the material due to the penetration of hydrogen atoms occurs on the surface first and then grows inside the bulk. Since x-ray diffraction measures the lattice spacing in the vertical direction from the surface of the material, the result corresponds to the demand for the development of β phase in the material surface part. The reason is thought to be shown in Fig. 12: it appears the change of the (0~0.2) crystal lattice interval ends in the range where the absorption ratio is relatively low. Residual strain was determined from the rolled material and the heat treated material with electrolysis time of 0 hour and 72 hours according to the method of holes given by equation (1). (Eq. 1) Here, η is inhomogeneous strain, ε is crystallite size, λ is x-ray wavelength, β is integral width of diffraction peak and θis the diffraction angle. First, to calculate the diffraction angle and integral width for diffraction peaks of the series reflection, calculate the sin θ/λ value of the right term of equation (1) as the x axis, and the graph sin theta/lambda value of the left term of equation (1) as the Y axis beta-cos theta/lambda. Draw the crystallite size E from the reciprocal of the intercept value obtained by extrapolating the nonuniform strain 11 and sin theta/lambda = 0 from 1/2 of the slope of the straight line obtained by the method of least squares. 14) 15) For the test piece with rolling material and heat treated material for 0 hour electrolysis time, x-ray diffraction analysis on each A plane was performed. Thereafter, the same sample was electrolyzed for 72 hours, and then x-ray diffraction analysis was performed again. Results of the 111 series reflection are shown in Fig. 13, 14. In the rolled material, compressive strain is 0.090 at the electrolysis time of 0 hour, which is the initial residual stress. After 72 hours of electrolysis, if the entire structure is converted into β phase, the initial strain considerably decreases to 0.032. In thin plate type Pd, since the tensile deformation occurs in the thickness direction in the course of lattice expansion in β phase conversion, the initial strain of the compression type tends to be relaxed. However, since this strain is plastic strain, it is presumed that it is not completely relaxed. On the other hand, the initial strain of the heat-treated material was found to be almost 0, the amount of strain after electrolysis for 72 hours was 0.016, and the tensile strain slightly increased appeared. 4 Discussion >From the x-ray diffraction result, the expansion coefficient in the uniaxial direction of the crystal lattice due to β phase conversion was about 3.47%. If the bulk expansion was considered simply as a result of accumulation of crystal lattice expansion, the entire bulk was in the β phase. The volumetric expansion of the specimen is estimated to be 10.8%, but the measured value is about 15.5% according to Fig. 9, indicating bulk expansion larger than the theoretical speculated value. Further release of hydrogen will cause the bulk to shrink and stay at 5% of the volume before absorption. Expansion due to β phase formation is elasto-plastic. By release of hydrogen, 10% of volume expansion 15.5% is released and 5.5% is left as residual strain, but 10% released corresponds to volume increase of 10.8% due to lattice expansion and the rest is in micro crack, and plastic defects such as voids and dislocations, Figure 15 (a) through (c) show the expansion coefficient and the hydrogen storage ratio in the L, W and T directions of the test piece, respectively. In the L direction, the expansion of the bulk is proportional to the amount of hydrogen absorbed. The difference between the rolled material and the heat treated material is not seen between H / Pd (0 ~ 0.5). The expansion coefficient of the heat treated material was somewhat larger in the final state, after dehydrogenation. The dimensions are smaller than the pre-hydrogen storage size (20 mm), it decreases slightly, that is, it extends in the L direction together with hydrogen absorption, but it shrinks further than the size before hydrogen absorption with hydrogen release. At the time of maximum absorption rate (0.7), the coefficient of expansion of the heat treated material is 1.015. The hydrogen release causes the expansion coefficient to be 0.990, and if this difference is regarded as the elastic strain, the value is 2.5% The expansion-contraction behavior is almost the same, the maximum expansion coefficient in the case of the heat treated material is 1.017, the minimum shrinkage rate is 0.992, and as a result the elastic strain component is 2.5%; a value is slightly lower than the crystal lattice expansion coefficient of 3.47%. We assume that the deformation was suppressed due to deformation restraint in the W and L directions described in Figure 8. On the other hand, in the T direction, the behaviors clearly differ from those in the L and W directions, as shown in the same figure (c) the expansion coefficient of the heat treated material at the maximum absorbed amount is 1.110, and even after contraction accompanying hydrogen release plus strain remained, the expansion ratio was 1.075, the strain released by hydrogen release was 3.5%, and the residual strain was 7.5%. From the results, the bulk expansion in the plate thickness direction is due to lattice expansion. It is obvious that not only the elastic phenomenon but also the considerable plastic strain are accompanied by this accumulation of dislocations due to the lattice strain between α and β phases at the micro level, the occurrence of micro cracks, at the meso level, it is conceivable that deformation of crystal grains caused by deformation restraint, grain boundary cracking, etc. can occur. Figure 16 shows the surface microphotograph (2500 times) of the specimen before hydrogen charging. Linear cracks, micro cracks, voids and the like which are thought to be caused by processing are observed. Figure 17(a), (b) shows the surface microphotograph (2,500 times) of the rolled material and the heat treated material observed with a metallurgical microscope after carrying out 1 cycle of absorption / release process in Fig. 17 (a). In any case, the surface of the material is plastically deformed by hydrogen absorption. In the rolled material of Fig. 17 (c), mesh micro cracks are densely shown. In the heat treated material of (b), many sliding lines due to dislocation or twinning are observed. As described above, in the one-cycle storage / release process, the occurrence of internal distortion due to β phase formation and micro cracks and slip and blister occur, resulting in large destruction and damage, which is thought to be due to multi-cycle accumulation. The crystal grain size after hydrogen absorption was measured by lineral analysis, and it was confirmed that GT of the heat-treated material was 72 μm, GL was 6 μm, and the crystal grain was more deformed in the plate thickness direction. In the rolled workpieces, GT was obtained as 71 μm and GL as 298 μm on the B side, and deformation in the thickness direction is slightly larger. >From these results, the following can be said about the relationship between crystal lattice expansion and bulk deformation: The origin of bulk deformation is the expansion of the crystal lattice, which is inherently elastic behavior. However, the distortion of the α - β phase generated during the penetration of hydrogen atoms generates a large plastic strain due to the process of suppressing deformation caused by the shape of the material or the like. In Pd, the residual plastic strain after release is 7.5% in thickness T direction and 5% in residual volume strain, but in hydrogen absorbing alloy which is multi-element crystalline body, lattice expansion is uniform. In the bulk, strain accumulation should also be larger. Therefore, the deformation of the crystal grain is enlarged, the grain boundary cracking is promoted, and innumerable fine cracks will be generated. 5 Conclusion We studied the bulk deformation behavior and the micro deformation behavior in crystal lattice by hydrogen absorption by electrolysis in palladium rolled material and heat treated material. As a result, the following knowledge was obtained: (1) Bulk deformation is caused by lattice expansion due to β phase conversion based on hydrogen absorption, and plastic strain is superposed in addition to this elastic expansion component. Plastic strain is thought to be caused by deformation restraint in the process of bulk accumulation of strain between α and β phases. (2) The deformation ratio in the length, width and thickness direction of the bulk with respect to the hydrogen absorption ratio increases almost linearly proportionally. The deformation ratio in the thickness direction was about 5 times larger than the deformation ratio in length and width direction. (3) The lattice constant abruptly increases when the absorption amount H/Pd approaches about 0.2, but it is kept constant if the absorbed amount H/Pd exceeds about 0.2. This is because the absorption ratio is around 0.2 and the formation of β phase on the bulk surface is completed. 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