Method for predicting the amount of solid solution of additive elements in aluminum alloys, method for preparing test material, and strength evaluation method using the same

Abstract

To provide a method for predicting a solid solution amount of an additive element, capable of enhancing prediction accuracy, a method for preparing a supply material, and a strength evaluation method using the same.SOLUTION: A method for predicting a solid solution amount of Mg, which is an example of an additive element in an aluminum alloy, predicts temporal change of the solid solution amount of Mg. The method for predicting the solid solution amount of Mg comprises a step 10 of identifying precipitates in an aluminum alloy from an equilibrium phase diagram created by a simulation based on the CALPHAD method. The method for predicting the solid solution amount of Mg further comprises a step 30 of predicting the temporal change of the solid solution amount of Mg from the identified precipitates by a simulation based on the Langer-Schwartz theory and the numerical solution method using the Kampmann-Wagner method.SELECTED DRAWING: Figure 25

Description

[Technical Field]

[0001] The present invention relates to a method for predicting the amount of added elements dissolved in an aluminum alloy, a method for preparing a test material, and a strength evaluation method using the test material. [Background technology]

[0002] Generally, when metallic materials are heated, changes in their properties (metallic structure) occur, and depending on the heating conditions, their strength may decrease. When metallic components are used under high temperature and long-term heating conditions, strength evaluation is carried out taking into account the state of the metallic structure after use.

[0003] An example of a product that is used under high-temperature, long-term heating conditions is a container called a "metal cask" that transports and stores spent fuel generated at nuclear power plants. The basket is a component of the metal cask that holds the spent fuel, and is heated to approximately 100-200°C due to the decay heat of the spent fuel. The period during which the basket is subjected to heat (design storage period) is a maximum of 60 years, including transportation before and after storage. The materials used for the basket are required to maintain their safety functions during the design storage period, and strength evaluations are carried out using test materials that simulate the metal structure after use.

[0004] For example, Non-Patent Document 1 describes that a test sample of a basket material (e.g., an aluminum alloy) is subjected to aging treatment at a higher temperature for a shorter time than the thermal history of the design storage period, and the strength is evaluated. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Japan Society of Mechanical Engineers, "Spent Fuel Storage Facility Standards: Metal Cask Structure Standards (2007 Edition)," published in February 2008 Summary of the Invention [Problem to be solved by the invention]

[0006] One possible method for simulating the metallurgical structure of an actual product that has been used under high-temperature, long-term heating conditions is to prepare a test material using the same heat treatment conditions as the actual product. However, for products such as metal casks, which have a design storage period of up to 60 years, it is difficult to experimentally determine the metallurgical structure after use.

[0007] On the other hand, as described in Non-Patent Document 1, it is conceivable to prepare test specimens by subjecting the basket material to aging treatment at a higher temperature for a shorter time than the thermal history of the design storage period, and then to perform strength evaluation.

[0008] Some elements added to materials increase their solid solubility limit in the base material as the temperature rises. For example, when magnesium in an aluminum alloy is aged at a temperature higher than the temperature at which the actual product will be used, the amount of magnesium dissolved in the base phase increases, increasing the material's strength. The method for preparing test materials described in Non-Patent Document 1 does not take into account changes in the solid solubility limit of the added elements, and there is a concern that simply aging at a temperature higher than the design storage period will result in the material having a higher strength than the material that has been stored for the design period.

[0009] Therefore, an object of the present invention is to provide a method for predicting the amount of added elements dissolved in an aluminum alloy, a method for preparing a test material, and a strength evaluation method using the test material, which can appropriately evaluate the material change that occurs when a metallic material is heated. [Means for solving the problem]

[0010] In order to solve the above problems, a method for predicting the amount of solid solution of an additive element according to a first aspect of the present invention is a method for predicting a change over time in the amount of solid solution of an element added to an aluminum alloy, the method comprising: a step of identifying precipitates of the aluminum alloy from an equilibrium phase diagram created based on the CALPHAD method; a step of predicting a change in the amount of solid solution of the additive element over time based on the identified precipitates using a numerical solution method based on the Langer-Schwartz theory and the Kampmann-Wagner method; This is a method for providing the above.

[0011] A second aspect of the present invention provides a method for predicting the amount of solid solution of an additive element in the method for predicting the amount of solid solution of an additive element in the first aspect of the present invention, further comprising the steps of: predicting a change over time in the amount of solid solution of each element in each precipitate based on the identified precipitates by a numerical solution using the Langer-Schwartz theory and the Kampmann-Wagner method; a step of adjusting the interfacial energy of each of the precipitates to be input into a simulation based on a numerical solution method using the Langer-Schwartz theory and the Kampmann-Wagner method so that a predicted change in electrical conductivity based on a change in the amount of each element dissolved in each of the precipitates and the matrix over time approaches a change in electrical conductivity obtained through a heat treatment experiment; inputting the adjusted interfacial energy into a simulation based on a numerical solution method using the Langer-Schwartz theory and the Kampmann-Wagner method to predict the change in the amount of Mg solid solution over time; This is a method for providing the above.

[0012] Furthermore, a third aspect of the present invention provides a method for predicting the amount of solid solution of an additional element, wherein the aluminum alloy in the method for predicting the amount of solid solution of an additional element according to the first or second aspect of the present invention is an aluminum alloy for a basket used in a metal cask, The period during which the change in the amount of the added element in solid solution over time is predicted is the designed storage period of the aluminum alloy for the basket used in the metal cask.

[0013] In addition, a fourth aspect of the present invention provides a method for producing a test material that simulates the change over time in the metal structure of an aluminum alloy for a basket used in a metal cask during a design storage period, the method comprising: calculating the overaging heat treatment conditions corresponding to the thermal history of the aluminum alloy for baskets during the design storage period by the Larson-Miller equation using constants obtained in a creep rupture test; a step of subjecting an aluminum alloy serving as a base material of the test material to the overaging heat treatment based on the calculated conditions of the overaging heat treatment; This is a method for providing the above.

[0014] A fifth aspect of the present invention provides a method for preparing a test material according to the fourth aspect of the present invention, further comprising the steps of: predicting the amount of dissolved Mg in the aluminum alloy for a basket at the end of a design storage period by the method for predicting the amount of dissolved added elements according to the first or second aspect of the present invention; a step of predicting the amount of dissolved Mg at a time corresponding to the end of the design storage period of the test material by the method for predicting the amount of dissolved added element according to the first or second invention; calculating a difference in the amount of soluble Mg obtained by subtracting the predicted amount of soluble Mg of the aluminum alloy for a basket from the predicted amount of soluble Mg of the test material; reducing the amount of Mg added to the aluminum alloy that is the base material of the test material by an amount equal to or greater than the calculated difference in the amount of dissolved Mg; This is a method for providing the above.

[0015] In addition, the sixth invention relates to a method for evaluating the strength of a test material, which is a method for evaluating the change over time in the material strength of an aluminum alloy for a basket used in the metal cask, based on a test material prepared by the test material preparation method of the fourth or fifth invention. [Effects of the Invention]

[0016] According to the method for predicting the amount of solid solution of an additional element, the accuracy of prediction can be improved. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a graph showing the effect of added elements on the recrystallization temperature of aluminum. [Figure 2] Al-Mn binary phase diagram. [Figure 3] Al-Mg binary phase diagram. [Figure 4] FIG. 1 is a schematic diagram of an overaging heat treatment. [Figure 5] FIG. 1 is a schematic diagram illustrating a method for determining the amount of Mg added to a specimen for mechanical testing. [Figure 6] 1 is a temperature-time graph showing a method for preparing specimens for mechanical testing. [Figure 7] 1 is a flowchart showing the heat treatment conditions for test materials for mechanical testing. [Figure 8] This is an equilibrium diagram for the test material (A5083). [Figure 9] 1 is a graph showing the change in volume fraction of precipitates accompanying heat treatment of test materials. [Figure 10] 1 is a graph showing the change in the amount of dissolved Mg during heat treatment of test materials. [Figure 11] This is an equilibrium diagram for the test material (HZ-A3004-H112). [Figure 12] 1 is a graph showing the change in volume fraction of precipitates with heat treatment of HZ-A3004-H112. [Figure 13] 1 is a graph showing the change in the amount of dissolved elements in HZ-A3004-H112 during heat treatment. [Figure 14] 1 is a graph showing the change in electrical conductivity of HZ-A3004-H112 due to heat treatment. [Figure 15] 1 is a temperature-time graph showing the design storage period and heating conditions of the heat treatment applied in the simulation. [Figure 16] This is an equilibrium diagram for HZ-A3004-H112. [Figure 17] 1 is a graph showing the change in the amount of dissolved Mg during the designed storage period and during heat treatment of HZ-A3004-H112. [Figure 18] 1 is a graph showing the change in the amount of dissolved Mg accompanying the heat treatment of a test material for mechanical testing. [Figure 19] 1 is a flowchart showing the conditions for preparing specimens for mechanical testing that simulate the material strength of HZ-A3004-H112 during the design storage period. [Figure 20] 1 is a graph showing the change in the amount of dissolved Mg relative to the mill sheet components of the test material. [Figure 21]1 is a graph showing the change in volume fraction of precipitates over the design storage period (200°C x 60 years) for the mill sheet components of HZ-A3004-H112. [Figure 22] 1 is a graph showing the change in volume fraction of precipitates with heat treatment for the mill sheet components of a test material for mechanical testing. [Figure 23] 1 is a graph showing the relationship between test temperature and 0.2% yield strength for HZ-A3004-H112 (initial material) and specimens for mechanical testing (heat-treated materials). [Figure 24] 1 is a graph showing the relationship between test temperature and tensile strength for HZ-A3004-H112 (initial material) and mechanical test specimens (heat-treated materials). [Figure 25] 1 is a flowchart showing a method for predicting the amount of dissolved Mg according to an embodiment of the present invention. [Figure 26] 10 is a flowchart illustrating the method for predicting the amount of dissolved Mg, which includes a step of adjusting and inputting interfacial energy. [Figure 27] 1 is a flowchart of a method for producing a test material according to an embodiment of the present invention. [Figure 28] 10 is a flowchart showing a manufacturing method of the same test material including a step of adding reduced Mg. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, a method for predicting the amount of dissolved additive elements, a method for preparing a test material, and a strength evaluation method using the test material according to an embodiment of the present invention will be described. [Method for predicting the amount of added elements dissolved in solid solution]

[0019] First, as an example of the method for predicting the amount of solid solution of an additional element, a method for predicting the amount of solid solution of Mg will be described. Of course, the additional element whose amount of solid solution is predicted by the method for predicting the amount of solid solution of an additional element is not limited to Mg. For example, the predicted additional element is at least one of Mg, Mn, Fe, Si, Cu, and Zn.

[0020] The method for predicting the amount of dissolved Mg is a method for predicting a change in the amount of dissolved Mg over time in an aluminum alloy. The aluminum alloy used in the method for predicting the amount of dissolved Mg may be an aluminum alloy for a basket used in a metal cask, or may be an aluminum alloy used as a test material. When the aluminum alloy used in the method for predicting the amount of dissolved Mg is an aluminum alloy for a basket used in a metal cask, the period over which the change in the amount of dissolved Mg over time is predicted is the designed storage period (e.g., 60 years) of the aluminum alloy for the basket.

[0021] As shown in FIG. 25, the method for predicting the amount of soluble Mg includes a step 10 of identifying precipitates in the aluminum alloy from an equilibrium phase diagram created by a simulation based on the CALPHAD method, and a step 30 of predicting the change in the amount of soluble Mg over time based on the identified precipitates by a numerical simulation using the Langer-Schwartz theory and the Kampmann-Wagner method.

[0022] That is, in step 10, precipitates in the aluminum alloy are identified from the equilibrium phase diagram created based on the CALPHAD method. Then, in step 30, the change in the amount of Mg in solid solution over time is predicted based on the identified precipitates by a numerical solution using the Langer-Schwartz theory and the Kampmann-Wagner method.

[0023] Thus, the method for predicting the amount of Mg in solid solution combines the CALPHAD method with numerical solutions based on the Langer-Schwartz theory and the Kampmann-Wagner method to predict the change in the amount of Mg in solid solution in an aluminum alloy over time, thereby improving the accuracy of prediction compared to classical nucleation theory.

[0024] As shown in FIG. 26, the method for predicting the amount of dissolved Mg may further include, between steps 10 and 30, step 20 of adjusting and inputting interfacial energy.

[0025] The process 20 includes steps 21, 22, and 23. In step 21, based on the identified precipitates, a time-dependent change in the amount of each element dissolved in each precipitate is predicted using a numerical solution based on the Langer-Schwartz theory and the Kampmann-Wagner method. In step 22, the interfacial energy of each precipitate is adjusted to be input into a simulation based on the numerical solution based on the Langer-Schwartz theory and the Kampmann-Wagner method so that a predicted change in electrical conductivity based on a time-dependent change in the amount of each element dissolved in each precipitate and the matrix approaches the change in electrical conductivity in a heat treatment experiment. In step 23, the adjusted interfacial energy is input into a simulation based on the numerical solution based on the Langer-Schwartz theory and the Kampmann-Wagner method to predict a time-dependent change in the amount of Mg dissolved in each precipitate.

[0026] In this way, according to the method for predicting the amount of Mg in solid solution that includes step 20, the change in the amount of Mg in solid solution in an aluminum alloy over time can be predicted through a simulation in which more appropriate interfacial energy is input, thereby further improving the accuracy of the prediction. [Method of preparing test material]

[0027] Next, a method for preparing the test material will be described.

[0028] The method for preparing the test material is a method for preparing a test material that conservatively simulates the change over time in the metal structure of an aluminum alloy for a basket used in a metal cask during the design storage period.

[0029] As shown in Figure 27, the method for preparing the test material includes a step 60 of calculating the over-aging heat treatment conditions corresponding to the thermal history of the aluminum alloy for the basket during the design storage period using the Larson-Miller equation with constants obtained from a creep rupture test, and a step 70 of subjecting the aluminum alloy that will be the base material of the test material to the over-aging heat treatment based on the calculated over-aging heat treatment conditions.

[0030] That is, in step 60, the overaging heat treatment conditions corresponding to the thermal history of the aluminum alloy for a basket during the design storage period are calculated using the Larson-Miller equation. The Larson-Miller equation is as follows:

[0031] LMP = T × (logt + C) LMP is the Larson-Miller parameter. T is the temperature [°C]. t is time [h]. C is a material constant, specifically 14. C=14 is obtained from creep rupture tests.

[0032] Then, in step 70, the aluminum alloy that will be the base material of the test material is subjected to the over-aging heat treatment based on the calculated conditions for the over-aging heat treatment.

[0033] In this way, the method for preparing the test material makes it possible to simulate the change over time in the metal structure of the aluminum alloy for the basket used in the metal cask during the designed storage period.

[0034] As shown in FIG. 28, the method for producing the test material may further include a step 50 of adding reduced Mg before the step 60.

[0035] The process 50 includes steps 51, 52, 53, and 54. Step 51 predicts the amount of soluble Mg in the aluminum alloy for the basket at the end of its design storage period using the method for predicting the amount of soluble Mg. Step 52 predicts the amount of soluble Mg in the test material at a time corresponding to the end of its design storage period using the method for predicting the amount of soluble Mg. Step 53 calculates a difference in the amount of soluble Mg obtained by subtracting the predicted amount of soluble Mg in the aluminum alloy for the basket from the predicted amount of soluble Mg in the test material. Step 54 reduces the amount of Mg added to the aluminum alloy that serves as the base metal for the test material by an amount equal to or greater than the calculated difference in the amount of soluble Mg.

[0036] In this way, the method for preparing a test material including step 50 reduces the amount of Mg added to the test material for solid solution strengthening, making it possible to simulate the metal structure of an aluminum alloy for a basket used in a metal cask after the design storage period. [Strength evaluation method]

[0037] Next, the strength evaluation method will be described.

[0038] The strength evaluation method evaluates the change over time in material strength of an aluminum alloy for a basket used in the metal cask, based on a test material prepared by the test material preparation method.

[0039] Thus, according to the strength evaluation method, since it is based on the test material prepared by the test material preparation method, it is possible to conservatively evaluate the change over time in the material strength of the aluminum alloy for the basket used in the metal cask. [basis]

[0040] Below, we will explain the concepts underlying the above content, as well as examples of simulations and experiments. For convenience, the aluminum alloy used for the basket of the metal cask will be referred to as HZ-A3004-H112. In tables and drawings, HZ-A3004-H112 may be abbreviated as HZ-A3004. "Strength changes of HZ-A3004-H112 during the design storage period"

[0041] It is known that the calorific value of spent fuel loaded into a cask decreases due to decay heat decay, and the basket temperature drops from approximately 200°C to approximately 100°C from the beginning to the end of the design storage period. It is believed that the heating during the design storage period causes material changes (metallic structure changes) in HZ-A3004-H112, resulting in a decrease in strength. Therefore, in order to perform an appropriate strength evaluation, a method was investigated for producing test specimens that simulate the material strength of HZ-3004 after the design storage period. "Reinforcement mechanism"

[0042] The four main strengthening mechanisms in HZ-A3004-H112 are as follows:

[0043] (1) Dislocation strengthening (work hardening): Working increases the dislocation density, hindering dislocation movement. (2) Grain refinement: Refining the grains hinders the movement of dislocations. (3) Dispersion strengthening: The movement of dislocations is hindered by the finely precipitated dispersed phase in the matrix and the elastic crystal lattice distortion caused by the precipitation of the dispersed phase. In HZ-A3004-H112, the Mn-based dispersed phase (Al6Mn) is mainly responsible for dispersion strengthening. (4) Solid solution strengthening: When atoms of different sizes than the parent metal atoms dissolve in solid solution, the surrounding crystal lattice is distorted, hindering the movement of dislocations. In HZ-A3004-H112, Mg is mainly responsible for solid solution strengthening.

[0044] In order to take into account the change in material strength during the design storage period and to perform an appropriate strength evaluation, it is necessary to consider a method for simulating material changes for each strengthening mechanism and prepare test materials. "Material changes occurring during the designed storage period"

[0045] An overview of the strengthening mechanism of HZ-A3004-H112 is shown in Table 1. We investigated methods to simulate the changes in material properties that occur when metal materials are kept at high temperatures for long periods of time, as well as the changes in material properties of HZ-A3004-H112 after the design storage period (60 years).

[0046] [Table 1] (1) Dislocation strengthening (work hardening), (2) Grain refinement

[0047] Figure 1 shows the effect of added elements on the recrystallization temperature of aluminum. The source is reference 1, "The Structure and Properties of Aluminum, (1991), pp. 160, 218, 222, 256, by the Japan Institute of Light Metals." When a metal material is heated at high temperatures for a long period of time, the strain energy stored within the crystals due to processing is released (a phenomenon known as recovery). This reduces dislocation density and material strength. Furthermore, new, strain-free crystals are generated and grow (recrystallization) in the structure disrupted by processing, resulting in coarsening of the crystal grains and a decrease in material strength. On the other hand, the presence of solute atoms hinders interface movement through interactions with dislocations and subgrain boundaries, making recrystallization difficult. In the case of HZ-A3004-H112, which contains approximately 1% Mg by mass, the recrystallization temperature exceeds 200°C, and recrystallization is unlikely to occur during the designed storage period.

[0048] The reduction in strength of HZ-A3004-H112 due to a decrease in dislocation density and coarsening of crystal grains is more likely to occur the longer it is held at high temperatures, so it is thought that this can be simulated by applying overaging heat treatment equivalent to the thermal history of the design storage period (60 years). (3) Dispersion strengthening by Mn-based dispersed phase

[0049] Figure 2 shows the Al-Mn binary equilibrium phase diagram. Mn hardly dissolves in Al below 300°C, but exists as a second phase (such as Al6Mn). The higher the temperature maintained, the more likely it is that strength will decrease due to a decrease in the number density of the Mn-based dispersed phase. However, at temperatures above 300°C, Mn dissolves in Al, making it impossible to simulate the strength decrease. Therefore, it is thought that simulation is possible by performing overaging heat treatment at temperatures below 300°C that is equivalent to the thermal history of the design storage period (60 years). (4)Solid solution strengthening by Mg

[0050] Figure 3 shows the Al-Mg binary equilibrium phase diagram (see Reference 1). Below the eutectic temperature (450°C), the solubility limit of Mg in Al decreases with decreasing temperature.

[0051] In the environment in which the basket is used, the drop in temperature during the design storage period is likely to lower the solid solubility limit of Mg, resulting in a decrease in material strength.

[0052] On the other hand, overaging heat treatment is performed at a temperature higher than the storage temperature, increasing the amount of Mg dissolved in the matrix and increasing the material strength. Therefore, overaging heat treatment alone increases the strength due to the effect of solution strengthening by Mg, and it is not possible to simulate the amount of Mg dissolved in HZ-A3004-H112 after the design storage period. To solve this problem, it is possible to perform overaging heat treatment on a material (mechanical test specimen) with a reduced amount of Mg added compared to HZ-A3004-H112. "Chemical composition of HZ-A3004-H112"

[0053] The specified values ​​for the chemical composition of HZ-A3004-H112 are shown in Table 2. HZ-A3004-H112 is based on the A3004 alloy specified in JIS H 4000, and is a material with a narrower range of specified components based on the following concepts.

[0054] · Impurity elements: Taking into consideration the effects of impurity elements on solid solution strengthening and precipitation strengthening, the allowable amount of addition is set low within the manufacturable range. · Mn: The lower limit of the specified range of components was set high in anticipation of strengthening the dispersion by the Mn-based dispersed phase. · Mg: The lower limit of the specified range of elements was set high in anticipation of solid solution strengthening by Mg.

[0055] [Table 2] "Simulation of material strength of HZ-A3004-H112 after the design storage period"

[0056] By reducing the amount of Mg added to HZ-A3004-H112 and subjecting it to over-aging heat treatment, we created test materials (mechanical test materials) that simulated the material strength of HZ-A3004-H112 after the design storage period, and used them in material property evaluation tests. An overview of the over-aging heat treatment and reduction in the amount of Mg added is provided below. (1) Overaging heat treatment

[0057] A schematic diagram of the overaging heat treatment is shown in Figure 4. The decrease in the number density of the Mn-based dispersed phase, the decrease in dislocation density, and the decrease in strength due to grain coarsening in HZ-A3004-H112 after the designed storage period were simulated by the overaging heat treatment.

[0058] The overaging heat treatment conditions were determined using the Larson-Miller parameter (LMP), which is one of the time-temperature parameter methods, to determine the overaging heat treatment conditions equivalent to the thermal history of the design storage period.

[0059] Furthermore, O material treatment according to JIS H 0001:1998 was performed. (2) Reduction of Mg addition

[0060] Figure 5 shows an overview of the method for determining the amount of Mg added to test materials for mechanical testing. The temperature of the overaging heat treatment is higher than that during the design storage period, and the amount of Mg in solid solution increases in the overheated HZ-A3004-H112. Because the increased amount of Mg in solid solution contributes to solid solution strengthening, strength cannot be evaluated conservatively. However, because the design storage period is 60 years, it is difficult to experimentally determine the amount of Mg in solid solution after use.

[0061] Therefore, a metallographic simulation was used to calculate the change in the amount of dissolved magnesium over a 60-year period.The procedure involved calculating the change in the amount of dissolved magnesium in HZ-A3004-H112 during the design storage period, and determining the amount of magnesium to be added to the specimen for mechanical testing so as to conservatively simulate the amount of dissolved magnesium in HZ-A3004-H112 after the design storage period following heat treatment.

[0062] Figure 6 shows the method for preparing the mechanical test specimens, which combines the above (1) and (2). The details of the method for simulating material changes are described in the next section, "Heat Treatment Conditions." "Heat treatment conditions"

[0063] (1) O material treatment The basket material was HZ-A3004-H112, but in order to conservatively evaluate the strength, O material treatment was carried out (the cooling condition after holding was air cooling).

[0064] (2) Overaging heat treatment The overaging heat treatment conditions were conservatively considered to be equivalent to the thermal history during the design storage period using the Larson-Miller parameter (LMP). The LMP is given by: LMP = T × (logt + C)

[0065] Here, t is time [h], T is temperature [°C], and C is the material constant (14). The value of the material constant C was set to 14 based on the value of the test conducted by the Japan Nuclear Energy Safety Organization.

[0066] The thermal history of the design storage period was set at 200°C for 60 years, and the influence of diffusion occurring in the constituent elements in the aluminum matrix was conservatively taken into consideration.

[0067] Furthermore, Mn hardly dissolves in Al at temperatures below 300°C. The overaging heat treatment temperature was selected from the range of 200°C to 300°C so that the state of Mn solid solution and precipitation would be equivalent to that during the designed storage period (approximately 200°C to 100°C).

[0068] Table 3 shows the results of a study of equivalent holding times at various heat treatment temperatures for the thermal history of the design storage period. For example, for the thermal history of the design storage period (200°C x 60 years), the equivalent holding time for overaging heat treatment at 275°C is 1054 hours. Therefore, the overaging heat treatment temperature was set to 275°C, and the overaging heat treatment time was set to 1500 hours, taking into account a safety factor for 1054 hours. The overaging heat treatment conditions are shown below. Example of overaging heat treatment conditions: 275°C x 1500 hours

[0069] [Table 3]

[0070] For reference, Table 3 also lists the results of a study assuming a temperature drop from 200°C to 100°C for 60 years (60 years for an isothermal drop from 200°C to 100°C) during the design storage period. In this case of 200°C to 100°C for 60 years, the equivalent holding time at 275°C for the design storage period is 172 hours. The overaging heat treatment time of 1,500 hours is a sufficiently conservative figure.

[0071] Sufficiently conservative overaging heat treatment conditions are not limited to the above-mentioned 275°C x 1500 hours, but may be 201°C x 55 years to 300°C x 191 hours. Figure 7 shows the heat treatment conditions for the mechanical test specimens.

[0072] "Consideration of the amount of Mg added" <1. Simulation Overview>

[0073] To calculate the metal structure of HZ-A3004-H112 after the designed storage period, particularly the amount of dissolved Mg, we used thermodynamic equilibrium calculation software (Thermo-Calc) and a precipitation calculation module (TC-Prisma). An overview of the simulation software is provided below.

[0074] Thermo-Calc is an integrated thermodynamic calculation software based on the CALPHAD method, which creates equilibrium phase diagrams using a thermodynamic database of experimental results and thermodynamic theory. It can calculate thermodynamic quantities, such as equilibrium phase diagrams, for multicomponent systems. While Thermo-Calc alone can only analyze equilibrium theory, combining it with the subroutine TC-Prisma enables dynamic simulations that take kinetics (time course) into account. The Thermo-Calc software described in this specification is developed by Thermo-Calc Software AB, uses the software version "Thermo-Calc 2021b," uses the database "TCAL6," and has the URL "https: / / thermocalc.com / products / thermo-calc / " (accessed September 8, 2021).

[0075] TC-Prisma is a precipitation calculation module included with Thermo-Calc that can calculate the nucleation, growth, and coarsening of precipitates under any heat treatment conditions for multi-component, multi-phase alloy systems. TC-Prisma performs the above calculations using numerical solutions based on the Langer-Schwartz (LS) theory and the Kampmann-Wagner (KWN) method. The KWN method solves time evolution equations using nucleation theory, growth rate models, and particle size distributions from the LS theory to calculate the time evolution of phase diagrams that take into account the nucleation, growth, and coarsening of precipitates. By inputting the alloy composition, holding temperature, time, etc., it is possible to output the time evolution of the amount of solute elements dissolved in the matrix and the volume fraction of precipitates. The TC-Prisma described in this specification was developed by Thermo-Calc Software AB, uses the MOBAL5 database, and has the URL https: / / thermocalc.com / products / add-on-modules / precipitation-module-tc-prisma / (accessed September 8, 2021). Note that because TC-Prisma is an add-on module for Thermo-Calc, there is no version number.

[0076] The source of the Langer-Schwartz (LS) theory is J.S. Langer, et al., "Kinetics of nucleation in near-critical fluids," Phys. Rev., A21 (1980), p. 948. The source of the numerical solution using the Kampmann-Wagner (KWN) method is R. Wagner, et al., "Homogeneous second-phase precipitation," G. Kostorz (Ed.), Phase transformations in materials, Wiley-VCH, New York (NY) (2001), p. 309.

[0077] To determine the amount of Mg to be added to the test material for mechanical testing, it is necessary to consider the change in the amount of dissolved Mg over time during the design storage period. In this section, Thermo-Calc and TC-prisma were used. The amount of dissolved Mg in HZ-A3004-H112 after the design storage period was calculated using the procedure shown below.

[0078] (1) An equilibrium phase diagram was created by simulating equilibrium theory using Thermo-Calc, and the precipitates formed in HZ-A3004-H112 were estimated. (2) The change in the amount of dissolved Mg in HZ-A3004-H112 during the designed storage period was estimated by kinetic simulation using TC-Prisma. (3) The amount of Mg added to the specimen for mechanical testing was estimated so that the amount of Mg dissolved after heat treatment would be lower than that of HZ-A3004-H112 after the design storage period. <2. Simulation validation>

[0079] The validity of the simulation software Thermo-Calc and TC-Prisma was verified. The study involved conducting simulations of the same alloy composition and thermal history in relation to literature that investigated the change in the amount of solid solution elements associated with heating of aluminum alloys, and confirming the consistency of the results. Furthermore, heat treatment tests and simulations were also conducted on HZ-A3004-H112, confirming the consistency. (1) Validation by literature

[0080] Simulations were performed using the same chemical components and heat treatment conditions as those in Reference 2 (Nakayama et al., Light Metals, Vol. 60, No. 2, (1996), pp. 135-140), which targets Al-Mg A5083 material, and the consistency of the results was verified. (1-a) Verification method for validity

[0081] (i) Table 4 shows the chemical composition of the test material used in the study in Reference 2. The equilibrium diagram of the test material (A5083) was calculated using Thermo-Calc, and the precipitates that formed in the test material were estimated.

[0082] [Table 4]

[0083] (ii) Using TC-Prisma, the changes in the volume fraction of precipitates and the amount of dissolved Mg in the test material under heat treatment conditions (180°C x 3000 hours) were calculated.

[0084] (iii) Using the measured values ​​in Reference 2, the simulation parameters (interfacial energy γ between the aluminum substrate and precipitates [J / m 2 ]) was fitted.

[0085] Here, the interfacial energy γ [J / m 2] is the energy that acts as a deterrent to phase decomposition when a supersaturated solid solution with an average composition C0 separates into compositions C1 and C2. The interfacial energy depends on the interface structure of different phases (such as the parent metal and precipitates) and determines the energy required for the nucleation of a second phase in the parent phase. By adjusting the interfacial energy of each precipitate, it is possible to obtain simulation results that are consistent with actual measurements regarding the material change phenomenon that accompanies the generation of precipitates. (1-b) Verification results of validity

[0086] The equilibrium phase diagram for the test material (A5083) calculated using Thermo-Calc is shown in Figure 8. In the equilibrium state, precipitates of Al6(Fe,Mn), Mg2Si, β-phase AlMg, and T-phase AlCuMgZn are formed.

[0087] The conditions for the interfacial energy γ used in the simulation are shown in Table 5. By adjusting the interfacial energies of Al6(Fe,Mn) and Mg2Si, the simulation results were consistent with the experimental results.

[0088] [Table 5]

[0089] Figure 9 shows the results of calculating the change in volume fraction of precipitates with heat treatment of the test material using TC-Prisma, and Figure 10 shows the results of calculating the change in the amount of dissolved Mg. When the test material was heat treated at 180°C after solution treatment, 6 After holding for 10 seconds, the volume fraction of the β phase increased rapidly and the amount of dissolved Mg decreased. The simulation results showed that the holding time at which Mg precipitation occurred and the absolute value of the amount of dissolved Mg were generally consistent with those reported in Reference 2. (2) Validation by HZ-A3004-H112

[0090] HZ-A3004-H112 was heat treated and a simulation was performed under the same conditions to verify the consistency of the results. (2-a) Experimental method

[0091] The chemical composition of the test materials is shown in Table 6. The test materials were solution-treated at 500°C for 2 hours and then quenched in water. They were then aged at 200°C for up to 3000 hours, and the change in electrical conductivity was measured. Electrical conductivity was measured using a Fischer Sigmascope SMP350 conductivity meter as IACS% (the ratio of electrical conductivity to pure copper).

[0092] [Table 6] (2-b) Verification method for validity

[0093] (i) The equilibrium phase diagram of the test material (HZ-A3004-H112) was calculated using Thermo-Calc, and the precipitates that would form were predicted.

[0094] (ii) Using TC-Prisma, we calculated the change in the volume fraction of precipitates and the change in the amount of added elements dissolved under heat treatment conditions (solution treatment → 200°C × 3000 hours).

[0095] (iii) Table 7 shows the effect of added elements on the electrical conductivity of Al alloys (see Reference 1). The electrical conductivity of an Al alloy depends on the amount of added elements dissolved or precipitated in the alloy. Based on the simulation results, the electrical conductivity was calculated using Table 7.

[0096] [Table 7]

[0097] (iv) The interfacial energy of the precipitates, γ [J / m 2 ]) was fitted. (2-c) Validity verification results

[0098] The equilibrium phase diagram for the test material (HZ-A3004-H112) calculated using Thermo-Calc is shown in Figure 11. At equilibrium, Al6(Fe,Mn), α-AlFeMnSi, Mg2Si, and T-phase_AlCuMgZn are formed.

[0099] The conditions for the interfacial energy γ used in the simulation are shown in Table 8. When the interfacial energies of Al6(Fe,Mn) and Mg2Si were fitted, simulation results consistent with the experimental results were obtained.

[0100] [Table 8]

[0101] For Al6(Fe,Mn), the default values ​​in Thermo-Calc were plotted on a graph of interfacial energy vs. temperature, and the plot was approximated to a quadratic function of temperature. In this way, the interfacial energy was approximated (simplified) to a quadratic function. For Mg2Si, the default values ​​in Thermo-Calc were plotted on a graph of interfacial energy vs. temperature, and the plot was approximated to a linear function of temperature, and the intercept was changed in the direction of increasing interfacial energy. By approximating (simplifying) the interfacial energy and changing the intercept in this way, the simulation results were able to match the experimental results.

[0102] Figure 12 shows the change in volume fraction of precipitates, and Figure 13 shows the calculated change in the amount of dissolved Mg. When the test material was heat treated at 200°C after solution treatment, 6 After the second holding, the volume fraction of the Mg2Si phase increased, and the amount of Mg in solid solution decreased.

[0103] The conductivity was calculated from the simulation results and compared with the experimental results in Figure 14. The retention time and amount of change in electrical conductivity generally agreed with the experimental results.

[0104] Based on the above considerations (1) and (2), it was determined that it is possible to calculate the amount of Mg in solid solution in HZ-A3004-H112 after the design storage period by applying simulations using Thermo-Calc and TC-Prisma. <3. Examination of the amount of Mg added to test materials for mechanical testing> (1) Overview

[0105] The amount of dissolved magnesium in HZ-A3004-H112 after the designed storage period was calculated by simulation, and the amount of magnesium to be added to the specimen for mechanical testing was examined. (2) Simulation method

[0106] (i) Table 9 shows the chemical compositions used to simulate changes in the amount of Mg in solid solution in HZ-A3004-H112. The equilibrium phase diagram for HZ-A3004-H112 was calculated, and the precipitates that formed were predicted.

[0107] [Table 9]

[0108] (ii) Table 10 shows the interfacial energy γ of the precipitates used in the simulation. The values ​​in Table 8, which were used to verify the validity of the precipitate interfacial energy γ for HZ-A3004-H112 in the above section "(2) Validation using HZ-A3004-H112," were used for the interfacial energy γ of the precipitates. Therefore, Table 10 is the same as Table 8. Figure 15 shows the design storage period and the heating conditions for the heat treatment. A simulation was performed under these conditions to determine the change in the amount of Mg solid solution in HZ-A3004-H112 over the design storage period.

[0109] [Table 10]

[0110] (iii) The amount of dissolved magnesium after heat treatment (275°C x 1500 hours) was determined by simulation, and the amount of magnesium to be added to the test material for mechanical testing was considered so that the amount would be below the value for HZ-A3004-H112 after the designed storage period. (3) Simulation results

[0111] The estimated equilibrium phase diagram for HZ-A3004-H112 is shown in Figure 16. At equilibrium, Al6(Fe,Mn), α-AlFeMnSi, Mg2Si, and T-phase_AlCuMgZn are formed.

[0112] Figure 17 shows the results of calculating the change in the amount of dissolved magnesium in HZ-A3004-H112 due to the design storage period and heat treatment.

[0113] When HZ-A3004-H112 was heated for the designed storage period, the amount of dissolved Mg added at 1.00 mass% decreased to approximately 0.962 mass% after storage. On the other hand, the amount of dissolved Mg in HZ-A3004-H112 after heat treatment at 275°C for 1500 hours was approximately 0.995 mass%, which was approximately 0.03 mass% higher than the value for HZ-A3004-H112 after the designed storage period.

[0114] Therefore, a simulation was performed in which the amount of Mg added to the specimen for mechanical testing was set to 0.95 [mass%], which is 0.05 [mass%] lower than that of HZ-A3004-H112.

[0115] Figure 18 and Table 11 show the results of calculations of the change in the amount of dissolved magnesium due to heat treatment of the mechanical test specimen. When the mechanical test specimen was heat treated, the amount of dissolved magnesium after holding was approximately 0.945 [mass%], which was approximately 0.02 [mass%] lower than the simulation result for HZ-A3004-H112 after the design storage period.

[0116] [Table 11]

[0117] From the above, it is believed that by adding 0.95 mass% of Mg to the specimen for mechanical testing and subjecting it to O material treatment and overaging heat treatment, it is possible to conservatively simulate the material strength of HZ-A3004-H112 after the design storage period.

[0118] The conditions for preparing specimens for mechanical testing are shown in Figure 19. The specimens prepared under the conditions shown in the figure were used to simulate the material strength of HZ-A3004-H112 after the design storage period and were subjected to material property evaluation tests. "Material property evaluation test" "Sample material"

[0119] Table 12 shows the test materials prepared. Based on the results of the aforementioned study on "Strength changes in HZ-A3004-H112 during the design storage period," the amount of magnesium added to the mechanical test material was set at 0.95 mass%. The chemical compositions of HZ-A3004-H112 and the mechanical test material were within the specified range. The magnesium content of all three mechanical test material specimens was near the target value of 0.95 mass%. Note that the magnesium content of the mechanical test material is not limited to the value shown in Table 12; it is sufficient if it is at least 0.05 mass% lower than the magnesium content of HZ-A3004-H112. For example, if the Mg content of HZ-A3004-H112 is 1.00-1.30 [mass%], the upper limit of the Mg content of the mechanical test specimen is 0.95-1.25 [mass%], and the lower limit of the Mg content of the mechanical test specimen is 0.80 [mass%].

[0120] [Table 12]

[0121] Simulations of heating and overaging treatment for the designed storage period were carried out on the prepared test material to confirm the appropriateness of the amount of Mg added. (1) Change in the amount of Mg dissolved in the test material (1-a) Study method

[0122] (i) Table 13 shows the composition of the simulation. For both HZ-A3004-H112 and the mechanical test specimens, the composition of the simulation was based on the mill sheet components of the specimens. Table 14 shows the interfacial energy γ of the precipitates used in the simulation. The value in Table 8, whose validity was verified, was used for the interfacial energy γ of the precipitates. Therefore, Table 14 is the same as Table 8. The values ​​in Figure 15 were used for the design storage period and heating conditions for heat treatment.

[0123] [Table 13]

[0124] [Table 14]

[0125] (ii) The amount of solute Mg in the heat-treated specimens for mechanical testing was determined and compared with the amount of solute Mg in the specimens in (i) above. (1-b) Results of the study

[0126] Figure 20 shows the calculation results for the change in the amount of dissolved Mg relative to the mill sheet composition of the test material. By heating and holding HZ-A3004-H112 for the designed storage period, the amount of dissolved Mg after holding is approximately 1.107 [mass%]. On the other hand, the amount of dissolved Mg in the heat-treated mechanical test material is approximately 0.934 [mass%], which is thought to be a conservative simulation of the Mg solid solution strengthening of HZ-A3004-H112 after the designed storage period.

[0127] The results of calculating the change in precipitate volume fraction are shown in Figures 21 and 22. No difference was observed in the types of precipitates formed in HZ-A3004-H112 and the specimens used for mechanical testing, and the amounts of precipitates after storage were roughly consistent. This suggests that the dispersion strengthening due to the Mn-based dispersed phase in HZ-A3004-H112 after the designed storage period has also been simulated.

[0128] From the above, it is believed that by subjecting the prepared specimens for mechanical testing to heat treatment, it was possible to simulate the material strength of HZ-A3004-H112 after the design storage period. "Material property evaluation test results"

[0129] The strength characteristics were confirmed using HZ-A3004-H112 (initial material) and specimens for mechanical testing (heat-treated material).

[0130] Figures 23 and 24 show the results of tensile tests at each test temperature for HZ-A3004-H112 (initial material) and the specimen for mechanical testing (heat-treated material).

[0131] Tables 15 and 16 summarize the tensile test results, and Tables 17 through 20 present the test data.

[0132] [Table 15]

[0133] [Table 16]

[0134] [Table 17]

[0135] [Table 18]

[0136] [Table 19]

[0137] [Table 20] "summary"

[0138] For HZ-A3004-H112, we calculated the material changes assuming the design storage period (60 years). Furthermore, we prepared specimens for mechanical testing simulating the state after the design storage period had elapsed, and conducted material property evaluation tests. The results are shown below.

[0139] (1) Using the Larson-Miller parameter (LMP), the overaging heat treatment conditions that are conservatively equivalent to the thermal history of the design storage period were examined, and the heat treatment conditions were determined as follows. Heat treatment conditions: O material treatment → overaging heat treatment (e.g., 275°C x 1500 hours)

[0140] (2) The amount of Mg to be added to the test material for mechanical testing was investigated through simulations using Thermo-Calc and TC-Prisma, and the amount of Mg to be added to the test material for mechanical testing was determined as follows. Mg addition amount: 0.95[mass%]

[0141] (3) Material property evaluation tests were conducted using mechanical test specimens manufactured and heat-treated under the above conditions, and material strength equivalent to that of HZ-A3004-H112 after the design storage period was obtained. [Explanation of symbols]

[0142] 10. Identifying precipitates in aluminum alloys 20. Adjusting and inputting interfacial energy 30. Process for predicting the change in the amount of dissolved Mg over time 50. Adding reduced Mg 60. Process for calculating the conditions for overaging heat treatment 70 Overaging heat treatment process

Claims

1. A method for predicting a change over time in the amount of solid solution of an element added to an aluminum alloy, comprising: Identifying precipitates of the aluminum alloy from an equilibrium phase diagram created based on the CALPHAD method; a step of predicting a change in the amount of solid solution of the additive element over time based on the identified precipitates using a numerical solution method based on the Langer-Schwartz theory and the Kampmann-Wagner method; A method for predicting the amount of solid solution of an added element, comprising:

2. a step of predicting a change over time in the amount of each element dissolved in each of the precipitates by a numerical solution using the Langer-Schwartz theory and the Kampmann-Wagner method based on the identified precipitates; adjusting the interfacial energy of each of the precipitates to be input into a simulation based on a numerical solution method using the Langer-Schwartz theory and the Kampmann-Wagner method so that a predicted change in electrical conductivity based on a change in the amount of each element dissolved in each of the precipitates and the matrix over time approaches a change in electrical conductivity obtained through a heat treatment experiment; inputting the adjusted interfacial energy into a simulation based on a numerical solution method using the Langer-Schwartz theory and the Kampmann-Wagner method to predict the change in the amount of Mg solid solution over time; The method for predicting the amount of solid solution of an additional element according to claim 1 , comprising:

3. The aluminum alloy is an aluminum alloy for a basket used in a metal cask, 3. A method for predicting the amount of solid solution of an additional element according to claim 1 or claim 2, wherein the period for which the change in the amount of solid solution of the additional element over time is predicted is the designed storage period of the aluminum alloy for the basket used in the metal cask.

4. A method for producing a test material that simulates the change over time in the metal structure of an aluminum alloy basket used in a metal cask during a design storage period, comprising: calculating the overaging heat treatment conditions corresponding to the thermal history of the aluminum alloy for baskets during the design storage period by the Larson-Miller equation using constants obtained in a creep rupture test; a step of subjecting an aluminum alloy serving as a base material of the test material to the overaging heat treatment based on the calculated conditions of the overaging heat treatment; Equipped with a step of predicting the amount of dissolved Mg in the aluminum alloy for a basket at the end of a design storage period by the method for predicting the amount of dissolved added elements according to claim 1 or 2; a step of predicting the amount of dissolved Mg at a time corresponding to the end of the design storage period of the test material by the method for predicting the amount of dissolved added element according to claim 1 or 2; calculating a difference in the amount of dissolved Mg obtained by subtracting the predicted amount of dissolved Mg of the aluminum alloy for a basket from the predicted amount of dissolved Mg of the test material; a step of reducing the amount of Mg added to the aluminum alloy that is the base material of the test material by an amount equal to or greater than the calculated difference in the amount of dissolved Mg; A method for preparing a test material, comprising:

5. A strength evaluation method for evaluating the change over time in material strength of an aluminum alloy for a basket used in the metal cask, based on a test material prepared by the test material preparation method according to claim 4.

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