A single-crystal iron-based superelastic alloy with high strain recovery capability and its preparation method

Abstract

A single-crystal iron-based superelastic alloy with high strain recovery capability and its preparation method are disclosed, belonging to the technical field of iron-based superelastic alloys. Its chemical composition and mass percentage are: Ni: 30-40%, Al: 5-15%, Ti: 0.5-5%, Cr: 0.5-5%, Mo: 0.5-5%, Y: 0.01-0.1%, with the balance being Fe and unavoidable impurity elements. The preparation method includes vacuum smelting, protective atmosphere electroslag remelting secondary refining, homogenization heat treatment and hot rolling, directional annealing, cyclic heat treatment, quenching, and aging treatment. This invention, through the combination of composition and production process, significantly improves the single-crystal preparation efficiency while obtaining high strain recovery performance. The prepared iron-based superelastic alloy single crystal has a grain size ≥66mm and a strain recovery capability ≥7.1%.

Description

Technical Field

[0001] This invention belongs to the field of superelastic alloy technology, specifically relating to a single-crystal iron-based superelastic alloy with high strain recovery capability and its preparation method. Background Technology

[0002] Superelastic alloys can generate strains far exceeding their elastic limits under external forces and recover to their pre-deformation state after stress unloading. This unique shape memory effect makes them widely applicable in numerous fields such as automotive mechanics, aerospace, vibration damping, and smart sensors. Over the past few decades, researchers have focused heavily on the development of relatively inexpensive iron-based superelastic alloys. FeMnSi alloys, with their near-room-temperature martensitic transformation onset temperature and good shape memory effect, are considered highly promising shape memory alloys. However, due to their high martensitic transformation thermal hysteresis (>100℃), they are semi-thermoelastic, resulting in poor superelasticity. The research and application of FePt alloys are limited by the high price of Pt, hindering their large-scale application potential. The superelasticity of FeNiCoAl alloys is highly dependent on strong recrystallization texture, and good superelasticity can only be achieved in thin sheets cold-rolled with 98.5% reduction. FeMnAlNi-based alloys can only exhibit good superelasticity under single-crystal or columnar crystal microstructures. Currently, in existing FeMnAlNi-based alloys, improving the single-crystal preparation efficiency often leads to an increase in the alloy's intergranular fracture tendency, making it impossible to improve preparation efficiency and material properties simultaneously. This results in difficulties for such alloys in practical engineering production and applications.

[0003] The hyperelastic properties of hyperelastic alloys are closely related to their martensitic phase transformation mechanism. Understanding the martensitic phase transformation law is a prerequisite for the development and optimization of hyperelastic alloys. Studying the martensitic phase transformation in materials is of great significance for understanding and improving the mechanical properties of materials, especially hyperelasticity. Among these, controlling the composition and phase composition is an important direction for its development. This invention prepares a high-strain-recovery iron-based hyperelastic alloy by redesigning the alloy composition system and its production process. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a high-strain-recovery iron-based superelastic alloy and its preparation method. Based on extensive research, the alloy's composition ratio and key production control points are determined.

[0005] The objective of this invention can be achieved using the following technical solutions:

[0006] The chemical composition and mass percentage of a single-crystal iron-based superelastic alloy with high strain recovery capability involved in this invention are as follows: Ni: 30-40%, Al: 5-15%, Ti: 0.5-5%, Cr: 0.5-5%, Mo: 0.5-5%, Y: 0.01-0.1%, with the balance being Fe and unavoidable impurity elements.

[0007] The mass percentages of Ti, Cr, Mo, and Y elements satisfy the following relationship:

[0008] Ti+Cr+Mo+Y: 3~10%, and 1<Ti / Cr<1.5.

[0009] The functions and proportions of the above elements are based on the following:

[0010] Ti and Cr: Adding a small amount of Ti to FeNiAl alloys can significantly promote abnormal grain growth, while adding a small amount of Cr inhibits it. By adjusting and promoting abnormal grain growth, larger single crystal rods can be obtained, thereby improving the superelastic properties of the alloy. This places demands on the Ti / Cr ratio. Extensive experimental research has shown that when the mass ratio of Ti / Cr is less than 1.5, larger single crystals can be obtained, which has a positive impact on improving the superelasticity of FeNiAl alloys. Furthermore, FeNiAl alloys are very sensitive to quenching rates. If the cooling rate is too fast, intergranular cracks will appear; if the cooling rate is too slow, a γ phase detrimental to performance will form. Adding a certain amount of Ti can reduce the quenching sensitivity of FeNiAl alloys to some extent. Through multiple simulations and experimental verifications, controlling the weight percentage of Ti between 0.5% and 5% can significantly reduce the quenching sensitivity of the alloy. Therefore, controlling the weight percentage of Ti at 0.5%-5% and the weight percentage of Cr at 0.5%-5%, with 1 < Ti / Cr mass ratio < 1.5, can significantly improve the efficiency of single crystal preparation while reducing its quenching sensitivity to a certain extent, thereby improving its superelastic properties.

[0011] Mo: From the perspective of alloy composition, controlling the phase composition of FeNiAl alloys hinges on stabilizing the α phase and controlling the precipitation state of the γ phase. Studies have shown that increasing the amount of ferrite-stabilizing elements drastically inhibits the precipitation of the γ phase. Mo is a ferrite-stabilizing element and can also alter the precipitation state of the γ phase, resulting in finer, needle-like γ phases. This toughening γ phase can reduce the crack sensitivity of FeNiAl alloys and improve the tendency for intergranular fracture caused by quenching. Furthermore, in FeNiAl superelastic alloys, the coherent nanophase (B2) is closely related to its superelasticity. The B2 phase strengthens the matrix by inhibiting dislocation slip and provides elastic energy, improving the phase transformation thermoelasticity. In this invention, adding a certain amount of Mo can reduce the grain boundary migration rate of the B2 phase during heat treatment, allowing the direct precipitation of B2 phases of a certain size, increasing the coherence between the B2 phase and the collective α phase, thereby improving the superelastic properties of the alloy. Extensive experimental verification has shown that controlling the weight percentage of Mo at 0.5%-5% has a very beneficial effect on improving the strain recovery capability of the alloy.

[0012] Y (rare earth element): In FeNiAl alloys, Y acts as a deoxidizer, effectively reducing the oxygen content and significantly improving the mechanical properties of the alloy. Furthermore, adding a small amount of Y can purify the grain boundaries, resulting in fewer impurities and a smaller grain boundary influence zone. This reduces the thermoelastic martensitic transformation and its resistance, increases the alloy's order, and makes plastic deformation more difficult, thus improving the alloy's hyperelastic properties. However, when the Y content exceeds a certain level, it has a significant grain-refining effect, leading to a marked increase in finer grain boundaries. During the martensitic transformation, crossing these grain boundaries requires more irreversible heat, and the alloy's order is also reduced, negatively impacting its hyperelastic properties. Through multiple simulations and experimental verifications, controlling the weight percentage of Y at 0.01-0.1% has a positive effect on improving the alloy's hyperelastic properties.

[0013] Adding the aforementioned Ti, Cr, Mo, and Y alloying elements can adjust the phase transformation temperature and slip critical stress, making stress-induced martensitic phase transformation easier and thus improving the superelastic properties of the alloy. However, when the amount of alloying elements added is too high, it will reduce the order of the parent phase, causing the thermoelastic martensitic phase transformation to become a non-thermoelastic martensitic phase transformation, which severely reduces its superelastic properties. After multiple simulations and experimental verifications, controlling the total weight percentage of Ti+Cr+Mo+Y at 3-10% is more beneficial for improving the superelastic properties of the alloy.

[0014] The preparation method of the above-mentioned high strain recovery single-crystal iron-based superelastic alloy includes the following steps: vacuum smelting, protective atmosphere electroslag remelting secondary refining, homogenization heat treatment and hot rolling, directional annealing, cyclic heat treatment, quenching and aging treatment.

[0015] The homogenization heat treatment and rolling process includes three steps: heating, rolling, and cooling.

[0016] Heating: A two-stage heating method is adopted. The first stage involves heating to 800-900℃ and holding for 2-4 hours; the second stage involves heating to 1100-1300℃ and holding for 24-48 hours.

[0017] Rolling: Unidirectional rolling is adopted, and the rolling temperature is controlled at 950~1050℃;

[0018] Cooling: After rolling, air cooling or sand cooling is used, and the cooling end temperature is ≤200℃.

[0019] Furthermore, in the two-stage heating, the heating rate of both the first and second stages is 10–20 °C / h.

[0020] Furthermore, in the directional annealing process, the rolled billet is heated in a region with a width of 1 to 100 mm. At the same time, the temperature of the heated region is controlled at 1100 to 1300 °C and a temperature field with a temperature gradient of 1 to 500 °C / mm is set so that the rolled billet passes through the temperature field at a speed of 1 to 300 μm / s.

[0021] Furthermore, in the cyclic heat treatment process, the temperature is increased to 1200-1300℃ at a rate of 0.1-200℃ / min, held for 10-120min, and then cooled to ≤1200℃ at a rate of 0.1-200℃ / min, held for 10-120min.

[0022] Furthermore, in the quenching and aging process, the quenching temperature is 1200-1300℃; the aging temperature is 100-500℃; and the aging time is 60-600 min.

[0023] The beneficial effects of the above technical solution are as follows: Based on the FeNiAl alloy, the present invention redesigns the composition system, introduces Ti, Cr, Mo, and Y alloying elements, and precisely controls the content and ratio of these alloying elements. By performing homogenization heat treatment, hot rolling, directional annealing, cyclic heat treatment, quenching, and aging treatment on the ingot, the single crystal preparation efficiency of the alloy is controlled, promoting thermoelastic martensitic phase transformation and reducing its quenching sensitivity, thereby obtaining a single crystal iron-based superelastic alloy with high strain recovery capability. The single crystal grain size of the superelastic alloy prepared by the present invention is ≥65mm, and the strain recovery capability is ≥7%. Attached Figure Description

[0024] Figure 1 This is a diagram showing the relationship between single-crystal grain size and strain recovery capability in an embodiment of the present invention. Detailed Implementation

[0025] Based on the chemical composition range designed according to the present invention, the specific chemical composition and mass content of the product are shown in Table 1 when smelted in a 50kg vacuum induction furnace.

[0026] Examples 1-5 of this invention disclose a method for preparing a single-crystal iron-based superelastic alloy with high strain recovery capability, comprising the following steps: vacuum smelting, protective atmosphere electroslag remelting secondary refining, homogenization heat treatment and hot rolling, directional annealing, cyclic heat treatment, quenching and aging treatment; the specific steps are as follows:

[0027] (1) Vacuum smelting process: Select pure metal raw materials such as iron, manganese, aluminum, nickel, titanium, chromium, molybdenum and yttrium, and mix them according to the required weight percentage content. Then, fully melt them in a vacuum protective atmosphere to obtain alloy vacuum ingots.

[0028] (2) Protective atmosphere electroslag remelting secondary refining process: The obtained vacuum ingot is demolded and subjected to protective atmosphere electroslag remelting secondary refining.

[0029] (3) Homogenization heat treatment and rolling process: including three steps: heating, rolling and cooling;

[0030] Heating: A two-stage heating method is adopted. The first stage involves heating to 800-900℃ at a rate of 10-20℃ / h and holding for 2-4 hours. The second stage involves heating to 1100-1300℃ at a rate of 10-20℃ / h and holding for 24-48 hours.

[0031] Rolling: Unidirectional rolling is adopted, and the rolling temperature is controlled at 950~1050℃;

[0032] Cooling: After rolling, air cooling or sand cooling is used, and the cooling end temperature is ≤200℃.

[0033] (4) Directional annealing process: The rolled billet is heated in a zone, and a temperature field with a temperature gradient is set up near the heated zone, so that the rolled billet passes through the heated zone and the temperature field to start directional annealing. The width of the heated zone is 10 to 100 mm, and the temperature of the heated zone is controlled at 1100 to 1300 ℃, and a temperature field with a temperature gradient of 10 to 500 ℃ / mm is set up, so that the rolled billet passes through the temperature field at a speed of 10 to 300 μm / s.

[0034] (5) Cyclic heat treatment process: Heat to 1200-1300℃ at a rate of 10-200℃ / min, hold for 10-120min, then cool to ≤1200℃ at a rate of 10-200℃ / min, and hold for 10-120min.

[0035] (6) Quenching and aging process: Quenching temperature is 1200~1300℃; aging temperature is 100~500℃, and aging time is 60~600min.

[0036] Comparative example: The preparation was carried out using conventional methods, namely, vacuum smelting, electroslag remelting under protective atmosphere, secondary refining, homogenization heat treatment, hot rolling, quenching and aging treatment.

[0037] The chemical composition and mass content of the superelastic alloys in Examples 1-5 and the comparative examples are shown in Table 1;

[0038] The parameter control for each production process in Examples 1-5 and the comparative examples is shown in Tables 2-5;

[0039] The microstructure and strain recovery capabilities of the hyperelastic alloys obtained in Examples 1-5 and the comparative examples were characterized, and the results are shown in Table 6.

[0040] Table 1. Chemical composition (wt%) of the examples and comparative examples

[0041]

[0042] In Table 1, the balance is Fe and unavoidable impurity elements.

[0043] Table 2. Homogenization heat treatment and rolling process parameters for the examples and comparative examples

[0044]

[0045] Table 3. Parameters of Directional Annealing Process in Examples

[0046]

[0047] Table 4. Process parameters of the cyclic heat treatment in the example

[0048]

[0049] Table 5. Quenching and Aging Process Parameters for Examples and Comparative Examples

[0050]

[0051] Table 6. Comparison of microstructure and properties of the superelastic alloys in the examples and comparative examples.

[0052]

[0053] As shown in Table 6, compared with the comparative examples, Examples 1-5 of the present invention yielded single-crystal iron-based superelastic alloys with higher strain recovery capabilities. Therefore, under the composition system and processing technology of the present invention, single-crystal iron-based superelastic alloys with high strain recovery capabilities can be prepared.

Claims

1. A single-crystal iron-based superelastic alloy with high strain recovery capability, characterized in that, Its chemical composition and mass percentage are as follows: Ni: 30-40%, Al: 5-15%, Ti: 0.5-5%, Cr: 0.5-5%, Mo: 0.5-5%, Y: 0.01-0.1%, with the balance being Fe and unavoidable impurity elements; The total mass percentage of the elements Ti, Cr, Mo, and Y satisfies the following relationship: 3%≤Ti+Cr+Mo+Y≤10%; The mass percentage content of the Ti and Cr elements satisfies the following relationship: 1 < Ti / Cr < 1.5; The preparation method of the iron-based superelastic alloy includes vacuum smelting, protective atmosphere electroslag remelting secondary refining, homogenization heat treatment and hot rolling, directional annealing, cyclic heat treatment, quenching and aging treatment. The directional annealing process involves heating the rolled billet in a heated area with a width of 10–100 mm. The temperature of the heated area is controlled at 1100–1300 °C, and a temperature field with a temperature gradient of 10–500 °C / mm is set, so that the rolled billet passes through the temperature field at a speed of 10–300 μm / s. The cyclic heat treatment process involves heating to 1200-1300℃ at a rate of 10-200℃ / min, holding at that temperature for 10-120min, and then cooling to ≤1200℃ at a rate of 10-200℃ / min, holding at that temperature for 10-120min. The quenching and aging processes are performed with a quenching temperature of 1200–1300℃ and an aging temperature of 100–500℃ for 60–600 min.

2. The method for preparing a single-crystal iron-based superelastic alloy with high strain recovery capability according to claim 1, characterized in that, It includes vacuum smelting, protective atmosphere electroslag remelting secondary refining, homogenization heat treatment and hot rolling, directional annealing, cyclic heat treatment, quenching and aging treatment processes; The directional annealing process involves heating the rolled billet in a heated area with a width of 10–100 mm. The temperature of the heated area is controlled at 1100–1300 °C, and a temperature field with a temperature gradient of 10–500 °C / mm is set, so that the rolled billet passes through the temperature field at a speed of 10–300 μm / s.

3. The method for preparing a single-crystal iron-based superelastic alloy with high strain recovery capability according to claim 2, characterized in that, The homogenization heat treatment and rolling process includes three steps: heating, rolling, and cooling. Heating: A two-stage heating method is adopted. The first stage involves heating to 800-900℃ and holding for 2-4 hours; the second stage involves heating to 1100-1300℃ and holding for 24-48 hours. Rolling: Unidirectional rolling is adopted, and the rolling temperature is controlled at 950~1050℃; Cooling: After rolling, air cooling or sand cooling is used, and the cooling end temperature is ≤200℃.

4. The method for preparing a single-crystal iron-based superelastic alloy with high strain recovery capability according to claim 3, characterized in that, The two-stage heating system has a heating rate of 10–20 °C / h for both the first and second stages.

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