Evaluation method for low-temperature ductile-brittle transition of body-centered cubic metal
The relative motion model of screw dislocation and blade dislocation is established through transmission electron microscopy and selective electron diffraction technology, which solves the error problem of tough plasticity evaluation of low-temperature steel, and realizes efficient production and reliable evaluation of toughness of low-temperature steel.
Patent Information
- Application Number
- CN202510556547.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-29
- Publication Date
- 2025-07-18
AI Technical Summary
The prior art has errors and uncertainties in judging the tough plasticity transition of low-temperature steels, especially in the low temperature environments in polar and high-altitude areas. Traditional computer simulations are difficult to accurately describe the microstructure and mechanical properties of materials, resulting in high production costs of low-temperature steels.
Transmission electron microscope is used to observe dislocations and combine selective electron diffraction technology to obtain dislocation orientation information, establish a physical model of relative motion of screw dislocations and blade dislocations, and judge the transformation of tough plasticity by calculating the relative velocity ratio α of screw dislocations and blade dislocations.
A simplified evaluation standard is provided, which accurately judges the toughness and brittleness transformation of low-temperature steel through dislocation motion model, reduces the experimental workload, and improves the production efficiency of low-temperature steel and the reliability of material toughness.
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Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of material property evaluation, and specifically to a method for evaluating the low-temperature ductile-brittle transition of body-centered cubic metals. Background Art
[0002] The construction of a polar power, as an important part of the construction of a maritime power, is inseparable from polar navigation, scientific research, and development equipment and facilities such as polar icebreakers, polar scientific research vessels, polar submersibles, and polar scientific research stations. The construction of the above-mentioned equipment and facilities requires the use of a large amount of low-alloy-content low-temperature steel. Low-temperature steel is a material with a design temperature lower than or equal to -20°C, good low-temperature toughness, anti-brittle fracture performance, and a relatively low ductile-brittle transition temperature, mainly used for manufacturing low-temperature equipment and containers.
[0003] Low-temperature steel is generally divided into austenitic low-temperature steel and ferritic low-temperature steel. Austenitic low-temperature steel has high low-temperature toughness and generally no ductile-brittle transition temperature. According to different alloy compositions, it can be divided into nickel-based and nickel-free systems, and its service temperature is above -60°C; such as nickel-containing Fe-Cr-Ni systems, Fe-Cr-Ni-Mn systems, and low-cost austenitic low-temperature steels of the Fe-Mn-Al system with "Mn substituting for Ni". Ferritic low-temperature steel generally has an obvious ductile-brittle transition temperature, and when the temperature drops to a certain critical value (or range), a sudden drop in toughness will occur.
[0004] With the vigorous development of natural resource exploitation in polar and alpine regions, the lowest temperature reaches -70°C and below, which puts higher requirements on the performance and output of low-temperature steel. China is a "Ni-poor" country. To produce low-temperature steel with strong low-temperature toughness, Ni elements are usually added during the production of low-temperature steel. It is found that for every 1% increase in Ni content in steel, the alloy cost can be increased by 5%, which will greatly increase the alloy cost. In order to produce low-temperature steel with better quality and reduce costs, it is necessary to study the fundamental ductility-plasticity transition mechanism of low-temperature steel and establish a judgment standard and model for low-temperature ductility-plasticity transition to escort the production of low-temperature steel.
[0005] Although traditional computer simulation can predict the ductility and plasticity of materials by establishing a microscopic structure model and a mechanical property model of the material and using the computer to simulate the deformation and fracture process of the material at low temperatures. It can simulate microscopic mechanisms such as dislocation movement, grain boundary behavior, crack initiation and propagation inside the material. The accuracy of the model depends on the accurate description of the microscopic structure and mechanical properties of the material, and there are still certain errors and uncertainties. For some complex practical problems, such as the influence of multi-phase structures and impurity elements of materials, the simulation is difficult and the model needs to be further improved. Summary of the Invention
[0006] The object of the present invention is to provide a method for judging the low-temperature brittle-ductile transition of body-centered cubic metals, aiming to improve the problem that the traditional judgment criteria for the ductile-brittle transition of low-temperature steels need to be improved.
[0007] A method for judging the low-temperature brittle-ductile transition of body-centered cubic metals includes observing the dislocations in a sample using a transmission electron microscope, then using the selected area electron diffraction technique to perform crystallographic characterization on the grains where the dislocations are located to obtain their orientation information; after obtaining the dislocation orientation information, establishing a physical model of the relative movement of screw dislocations and edge dislocations, establishing a calculation method for the relative velocity ratio α of screw dislocations and edge dislocations, and judging the ductile-brittle transition based on the magnitude of the α value.
[0008] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, the ideal model includes a straight Frank-Read dislocation source and an inner semi-circular ring. The inner semi-circular ring is formed by the movement of an edge dislocation within a unit time, and the distance moved by the edge dislocation is r; if the screw dislocation and the edge dislocation have the same velocity, a semi-circular ring with a dotted line radius of 2r can be formed at the outermost periphery.
[0009] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, since the core of the screw dislocation is not a planar core and the velocity of the screw dislocation is less than that of the edge dislocation, a physical model can be established based on the ideal model.
[0010] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, when the screw dislocation part does not move and the edge dislocation part continues to move forward, a semi-elliptical dislocation shape is formed; the edge dislocation moves forward a distance of x, and the area slipped by the edge dislocation is A edge 。
[0011] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, the dislocation line can only move forward and cannot loop back to form a renewable Frank-Read dislocation source. At this time, the dislocation source is a one-time one.
[0012] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, when the screw dislocation expands to both sides, an outer semi-elliptical dislocation shape is formed; the screw dislocation moves a distance of y, and the light green area slipped by the screw dislocation is A screw 。
[0013] As a preferred embodiment of the method for judging the low-temperature brittle-ductile transition of body-centered cubic metals according to the present invention, when A screw >A edge When, the dislocation can bow backward to form an effective Frank-Read dislocation source.
[0014] As a preferred embodiment of the method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to the present invention, wherein: record the major axis length and minor axis length of the semi-ellipse where the dislocation finally bows out, and record the ratio of the major axis to the minor axis as k, and the geometric relationship formed between the ratio α and the ratio k is
[0015]
[0016] As a preferred embodiment of the method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to the present invention, when α is greater than 0.5, the body-centered cubic metal undergoes a ductile-brittle transition, and dislocations continuously proliferate in the form of Frank-Read dislocation sources.
[0017] As a preferred embodiment of the method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to the present invention, the sample needs to be pre-stretched by 3% and 13% respectively at -100 °C, then sampled along the axial direction of the pre-stretched specimen, and prepared by electrolytic twin-jet method after grinding and thinning.
[0018] The beneficial effects of the present invention are as follows: Use a transmission electron microscope to observe the dislocations in the sample, and then use the selected area electron diffraction technique to perform crystallographic characterization on the grains where the dislocations are located to obtain their orientation information; after obtaining the dislocation orientation information, establish a physical model of the relative movement of screw dislocations and edge dislocations, establish a calculation method for the relative velocity ratio α of screw dislocations and edge dislocations, and judge the ductile-plastic transition by the magnitude of the α value. Brief Description of the Drawings
[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on the structures shown in these drawings without creative efforts.
[0020] Figure 1 is the relative movement model of screw dislocations and edge dislocations;
[0021] Figure 2 is a schematic diagram of the effective dislocation model of rare-earth-containing low-temperature steel;
[0022] Figure 3 is a schematic diagram of the effective dislocation model of rare-earth-free low-temperature steel.
[0023] The realization, functional characteristics and advantages of the object of the present invention will be further described in conjunction with the embodiments with reference to the drawings. Detailed Embodiments
[0024] The technical solutions in the embodiments will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.
[0025] The present invention is implemented as follows: A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal includes observing dislocations in a sample using a transmission electron microscope, then using the selected area electron diffraction technique to perform crystallographic characterization on the grains where the dislocations are located to obtain their orientation information; after obtaining the dislocation orientation information, a physical model of the relative movement of screw dislocations and edge dislocations is established, and a calculation method for the relative velocity ratio α of screw dislocations and edge dislocations is established, and the ductile-brittle transition is judged by the magnitude of the α value.
[0026] Preferably, the ideal model includes a straight Frank-Read dislocation source and an inner semi-circular ring. The inner semi-circular ring is formed by the movement of edge dislocations in unit time, and the distance moved by the edge dislocations is r; if the velocities of screw dislocations and edge dislocations are the same, a semi-circular ring with an outermost dotted line radius of 2r can be formed.
[0027] Preferably, since the core of a screw dislocation is not a planar core and the velocity of a screw dislocation is less than that of an edge dislocation, a physical model can be established based on the ideal model.
[0028] Preferably, when the screw dislocation part does not move and the edge dislocation part continues to move forward, a semi-elliptical dislocation shape is formed; the edge dislocation moves forward a distance of x, and the area slipped by the edge dislocation is A edge .
[0029] Preferably, the dislocation line can only move forward and cannot loop back to form a renewable Frank-Read dislocation source. At this time, the dislocation source is a one-time one.
[0030] Preferably, when the screw dislocation expands to both sides, an outer semi-elliptical dislocation shape is formed; the screw dislocation moves a distance of y, and the area slipped by the screw dislocation is A screw .
[0031] Preferably, when A screw >A edge the dislocation can bow backward to form an effective Frank-Read dislocation source.
[0032] Preferably, record the major axis length and minor axis length of the semi-ellipse when the dislocation finally bows out, and record the ratio of the major axis to the minor axis as k. The geometric relationship formed between the ratio α and the ratio k is
[0033]
[0034] Preferably, when α is greater than 0.5, the body-centered cubic metal undergoes a ductile-brittle transition, and dislocations continuously proliferate in the form of Frank-Read dislocation sources.
[0035] Preferably, the samples were pre-stretched at -100 °C by 3% and 13% respectively, and then samples were taken along the axial direction of the pre-stretched specimens. After grinding and thinning, the electrolytic twin-jet method was used for preparation.
[0036] The technical solution of the present invention will be further described below in conjunction with specific embodiments.
[0037] Example 1
[0038] The following specifically introduces the evaluation method for the ductile-brittle transition of low-temperature steel.
[0039] The rod-shaped tensile specimens were pre-stretched at -100 °C by 3% and 13% respectively, and then samples were taken along the axial direction of the pre-stretched specimens. After grinding and thinning, TEM specimens were prepared by the electrolytic twin-jet method.
[0040] The electrolytic twin-jet method includes the following steps: cutting a 0.5 mm sample with a wire cutting machine, grinding it with 400# sandpaper to 200 μm, and then gradually changing to 600#, 1000#, 1500#, 2000# and grinding it to 60 - 70 μm. Subsequently, electrolytic twin-jet thinning was carried out (the solution was 5% perchloric acid in acetic acid), and the parameters were as follows:
[0041] Electrolytic voltage - 40 V;
[0042] Electrolytic current - 43 s;
[0043] Electrolytic time - 76 mA.
[0044] During the electrolysis process, by applying an external potential, the ions in the solution undergo oxidation-reduction reactions on the electrode surface, thereby processing the sample.
[0045] On this basis, TEM detection was carried out on this sample. The transmission electron microscope (TEM) was used to observe the dislocations in the steel at the nanometer level, and then the selected area electron diffraction technique (SAED) was used to perform crystallographic characterization on the grains where the dislocations were located to obtain their orientation information, as Figure 2 shown.
[0046] After obtaining the dislocation orientation information of this sample, it is necessary to establish an effective physical model for the relative movement of screw dislocations and edge dislocations, as shown in (a) of Figure 1 . The black straight line is a Frank-Read dislocation source. Only with the movement of the edge dislocation, it can move to the blue solid line within unit time and form a semi-circular ring with a radius of r. If the screw dislocation and the edge dislocation have the same speed at this time and the dislocations continue to move, a semi-circular ring with a radius of 2r of the outermost blue dotted line can be formed.
[0047] However, for BCC metals, since the core of the screw dislocation is a non-planar core, the velocity of the screw dislocation is less than that of the edge dislocation.
[0048] Considering extreme conditions, when the velocity of the screw dislocation is 0 (i.e., Vs = 0), the situation is as Figure 1 shown in (b) below. The edge dislocation part continues to move forward, while the screw dislocation part does not move, forming the semi-elliptical dislocation shape shown in the figure. Among them, the forward movement distance of the edge dislocation is x, and the light pink area slipped by the edge dislocation is A edge . At this time, the dislocation line can only move forward and cannot loop back to form a renewable Frank-Read dislocation source. Therefore, the dislocation source at this time is disposable.
[0049] When considering the actual situation, the screw dislocation expands to both sides, but its velocity is slower than that of the edge dislocation. The result is as Figure 1 shown in (c) below, and a peripheral semi-elliptical dislocation shape will be formed. Among them, the movement distance of the screw dislocation is y, and the light green area slipped by the screw dislocation is A screw . At this time, only when the area where the screw dislocation expands to the side is larger than the area where the edge dislocation slips forward (i.e., A screw > A edge ), can the dislocation bow backward to form an effective Frank-Read dislocation source.
[0050] To sum up, to form an effective Frank-Read dislocation source, A screw > A edge is required. When the slipping areas of the screw dislocation and the edge dislocation are equal, we can get:
[0051] A screw = A edge
[0052] In addition, to form an effective dislocation source, the condition:
[0053] x < r
[0054] should also be satisfied. When recording the expansion of the screw dislocation to both sides, since its velocity is slower than that of the edge dislocation, a peripheral semi-elliptical dislocation shape will be formed. Record the major axis length and minor axis length of the semi-ellipse when the dislocation finally bows out, and record the ratio of the major axis to the minor axis as k. The geometric relationship between the ratio α of the relative velocities of the screw dislocation and the edge dislocation and the ratio k of the major axis to the minor axis is According to the calculation method of the ratio of the relative velocities of the dislocations, for body-centered cubic metals to undergo ductile-brittle transition, at least the α value should be greater than 0.5. Only when the area where the screw dislocation expands to the side is larger than the area where the edge dislocation slips forward, can the dislocation bow backward to form an effective Frank-Read dislocation source.
[0055] To form an effective dislocation source, the following conditions must also be met: the forward movement distance x of the edge dislocation is less than the radius r of the semi-circle formed by the movement of the edge dislocation per unit time. Only when the relative velocity ratio of the screw dislocation and the edge dislocation is greater than 0.5 can an effective dislocation source be formed, and the dislocations multiply continuously in the form of Frank-Read dislocation sources, making the material have better toughness.
[0056] The above method can be used to judge the transition of low-temperature ductility and brittleness of body-centered cubic metals by referring to the following steps:
[0057] 1) Based on the bowed dislocation line after movement, establish an elliptical geometric model. Define the movement direction of the edge dislocation as the X-axis and the movement direction of the screw dislocation as the Y-axis.
[0058] 2) Define the <222> crystallographic direction family direction as the long axis direction of the ellipse - the X-axis, and the perpendicular direction as the short axis direction of the ellipse - the Y-axis. Select the ellipse models that meet the long and short axis directions as effective physical models for calculation and statistics.
[0059] 3) Record the length of the major axis and the minor axis of the semi-ellipse where the dislocation finally bows out, and record the ratio of the major axis to the minor axis as k.
[0060] 4) The geometric relationship between the ratio α of the relative velocities of the screw dislocation and the edge dislocation and the ratio k of the major axis to the minor axis is
[0061] Example 2
[0062] In this example, low-temperature steels with and without rare earths are used as experimental specimens to verify the evaluation method of Example 1.
[0063] The mass fractions of the components of the experimental specimen containing rare earths are as follows:
[0064] Carbon 0.06%; Silicon 0.19%; Manganese 1.52%; Aluminum 0.05%; Niobium 0.032%; Chromium 0.19%; Nickel 0.01%; Copper 0.01%; Zirconium ≤0.002%; Phosphorus 0.009%; Sulfur 0.001%; Cerium 0.0009%; Lanthanum 0.0006%; The balance is iron.
[0065] The mass fractions of the components of the experimental specimen without rare earths are as follows:
[0066] Carbon 0.06%; Silicon 0.16%; Manganese 1.30%; Aluminum 0.032%; Niobium 0.028%; Chromium 0.12%; Nickel 0.03%; Copper 0.02%; Zirconium 0.012%; Phosphorus 0.010%; Sulfur 0.001%; The balance is iron.
[0067] The experimental steps of the experimental specimen containing rare earths are as follows:
[0068] S1, AsFigure 1 When recording the expansion of a screw dislocation to both sides, since its velocity is slower than that of an edge dislocation, a peripheral semi-elliptical dislocation shape will be formed; thus, an effective physical model is established.
[0069] S2. As Figure 2 Based on this, an effective physical model is established. Record the major axis length and minor axis length of the semi-ellipse finally bowed out by the dislocation, and record the ratio of the major axis to the minor axis as k; the geometric relationship between the ratio α of the relative velocities of the screw dislocation and the edge dislocation and the ratio k of the major axis to the minor axis is
[0070] S3. According to the calculation method of the ratio of the relative velocities of the dislocations, calculate and statistically analyze the effective physical models with α value greater than 0.5.
[0071] The experimental steps for the experimental specimens without rare earth are as follows:
[0072] S1. As Figure 1 When recording the expansion of a screw dislocation to both sides, since its velocity is slower than that of an edge dislocation, a peripheral semi-elliptical dislocation shape will be formed; thus, an effective physical model is established.
[0073] S2. As Figure 3 Based on this, an effective physical model is established. Record the major axis length and minor axis length of the semi-ellipse finally bowed out by the dislocation, and record the ratio of the major axis to the minor axis as k; the geometric relationship between the ratio α of the relative velocities of the screw dislocation and the edge dislocation and the ratio k of the major axis to the minor axis is
[0074] S3. According to the calculation method of the ratio of the relative velocities of the dislocations, calculate and statistically analyze the effective physical models with α value greater than 0.5.
[0075] Table 1 shows the calculation and statistical results of the physical models in Example 1 and Example 2 of the present invention. It can be found that the experimental results of rare earth improving the low-temperature toughness of the alloy can be correlated with the calculation and statistical results α of the model. When the effective physical models with α value greater than 0.5 dominate, the alloy material exhibits better toughness.
[0076] Table 1 Calculation and Statistical Results of the Models in Examples 1 and 2
[0077] α＞0.5 α＜0.5 rare earth-containing alloy 27 42 rare earth-free alloy 35 20
[0078] From the comparison results of the embodiments of the present invention, it can be seen that the present invention adopts a simple evaluation criterion and establishes a visual physical model based on experimental results, providing a reliable basis for the low-temperature ductile-brittle transition of body-centered cubic metals from the perspective of dislocation movement. At the same time, the present invention makes a well-founded distinction of the low-temperature toughness of alloys with different compositions through the calculation results of the statistical model. This method is different from traditional computer simulation and the ductile-brittle transition temperature obtained from a large number of low-temperature impact experiments. It ingeniously uses the dislocation movement model as a starting point, greatly simplifying the workload of experimental personnel. It also provides a multi-dimensional reference for the research and development of new types of low-temperature steel.
[0079] Through the analysis of the dislocation models shown by TEM of two alloys with different compositions, it can be seen that the influence of rare earth elements on dislocation density and the resulting toughness is significant.
[0080] The respective components of the above experimental specimens containing rare earths have the following functions:
[0081] Carbon (C) 0.06%: Although the content is very low, carbon is usually used in alloys to increase strength and hardness. Especially when combined with elements such as chromium and molybdenum, it can form carbides to enhance wear resistance.
[0082] Silicon (Si) 0.19%: Silicon can increase the strength of the alloy, improve local corrosion resistance, increase hardenability and temper resistance in quenched and tempered steels, and is one of the main alloying elements in multi-element alloy structures.
[0083] Manganese (Mn) 1.52%: Manganese is a good deoxidizer and desulfurizer, which can increase the strength, hardness, elastic limit, wear resistance and corrosion resistance of steel.
[0084] Aluminum (Al) 0.05%: Aluminum is mainly used for deoxidation and grain refinement. In nitriding steels, it promotes the formation of a hard and corrosion-resistant nitrided layer, can inhibit the aging of low-carbon steel, and improve the toughness of steel at low temperatures.
[0085] Niobium (Nb) 0.032%: As a microalloying element, niobium can significantly increase the yield strength of steel, improve the toughness, high-temperature oxidation resistance and corrosion resistance of steel, and reduce the brittle transition temperature of steel.
[0086] Chromium (Cr) 0.19%: Chromium can increase the hardenability of steel and has a secondary hardening effect, improving the hardness and wear resistance of steel. When the content exceeds 12%, it gives the steel good high-temperature oxidation resistance and oxidation corrosion resistance.
[0087] Nickel (Ni) 0.01%: Nickel strengthens ferrite and refines pearlite in steel, increasing strength with little effect on plasticity.
[0088] Copper (Cu) 0.01%: Copper in steel can improve the atmospheric corrosion resistance of ordinary low-alloy steel. Especially when used in combination with phosphorus, adding copper can also increase the strength and yield ratio of the steel.
[0089] Zirconium (Zr) ≤ 0.002%: Zirconium in the alloy mainly acts as a grain refiner, improving the strength and toughness of the alloy, and at the same time having good corrosion resistance.
[0090] Phosphorus (P) 0.009%: Phosphorus usually exists as an impurity element in the alloy. However, in some cases, it can increase the hardness and brittleness of the steel, which is beneficial to obtaining good machining surface quality.
[0091] Sulfur (S) 0.001%: Sulfur is usually regarded as a harmful element in the alloy because it reduces the plasticity and toughness of the alloy. However, in free-cutting steel, an appropriate amount of sulfur can improve the cutting performance.
[0092] Cerium (Ce) 0.0009%: As a rare earth element, cerium can improve the high-temperature performance of the alloy, enhancing its oxidation resistance and creep resistance.
[0093] Lanthanum (La) 0.0006%: Lanthanum is also a rare earth element. It can increase the high-temperature strength and oxidation resistance of the alloy in the alloy, and at the same time helps to refine the grains and improve the mechanical properties of the alloy.
[0094] The combined action of these elements determines the comprehensive properties of the alloy, including strength, hardness, toughness, wear resistance, corrosion resistance, and machinability, etc. These elements can also jointly determine the microstructure and macroscopic properties of the alloy by affecting the movement and distribution of dislocations.
[0095] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent structural transformation made using the content of the specification of the present invention under the inventive concept of the present invention, or direct / indirect application in other related technical fields, is included in the patent protection scope of the present invention.
Claims
1. A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal, characterized in that, It includes observing dislocations in a sample using a transmission electron microscope, and then using selected area electron diffraction technology to perform crystallographic characterization on the grains where the dislocations are located to obtain their orientation information; After obtaining the dislocation orientation information, a physical model of the relative motion of screw dislocations and edge dislocations is established, and a calculation method for the relative velocity ratio α of screw dislocations and edge dislocations is established. The ductile-brittle transition is judged by the magnitude of the α value.
2. The method for judging the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 1, characterized in that The ideal model includes a straight Frank-Read dislocation source and an inner semi-circular ring. The inner semi-circular ring is formed by the movement of edge dislocations in unit time, and the distance moved by the edge dislocations is r; if the velocities of screw dislocations and edge dislocations are the same, a semi-circular ring with a dotted line radius of 2r can be formed at the outermost periphery.
3. The evaluation method for the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 2, characterized in that, Since the core of a screw dislocation is not a planar core and the velocity of a screw dislocation is less than that of an edge dislocation, a physical model can be established on the basis of the ideal model.
4. A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 3, characterized in that When the screw dislocation part does not move and the edge dislocation part continues to move forward, a semi-elliptical dislocation shape is formed; the forward movement distance of the edge dislocation is x, and the area slipped by the edge dislocation is A edge .
5. A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 4, characterized in that The dislocation line can only move forward and cannot loop back to form a renewable Frank-Read dislocation source. At this time, the dislocation source is a one-time one.
6. The evaluation method for the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 4, characterized in that, When the screw dislocation expands to both sides, a peripheral semi-elliptical dislocation shape is formed; the movement distance of the screw dislocation is y, and the area where the screw dislocation slips through is A screw .
7. A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 6, characterized in that, When A screw > A edge can the dislocation bow out backward to form an effective Frank-Read dislocation source.
8. A method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 7, wherein Record the major axis length and minor axis length of the semi-ellipse finally bowed out by the dislocations, and record that the ratio of the major axis to the minor axis is k. The geometric relationship formed between the ratio α and the ratio k is 9. The method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 8, wherein When α is greater than 0.5, ductile-brittle transition occurs in body-centered cubic metals, and dislocations continuously proliferate in the form of Frank-Read dislocation sources.
10. The method for evaluating the low-temperature ductile-brittle transition of a body-centered cubic metal according to claim 1, wherein, The samples were pre-stretched at -100 °C with deformations of 3% and 13% respectively, and then samples were taken along the axial direction of the pre-stretched specimens. After grinding and thinning, they were prepared by the electrolytic twin-jet method.