A high-titanium solid-solution-strengthened cobalt-free face-centered cubic multi-principal element alloy and a preparation method thereof

Abstract

The application discloses a high-content titanium solid solution strengthening cobalt-free face-centered cubic multi-component alloy and a preparation method thereof. a Ni b Cr c Mn d ) 100‑e Ti e (at.%), wherein 30<=a<=45, 30<=b<=45, 0<=c<=25, 0<=d<=25, 7.5<=e<=9.5, and a+b+c+d=100, and at least one of c and d is greater than or equal to 5. The preparation process of the application adopts a two-step method, namely, I weighing and melting, and II solid solution heat treatment of the ingot. The microstructure of the multi-component alloy is single FCC solid solution phase equiaxed crystal organization, and the unique high-content titanium solid solution phase structure can simultaneously improve the strength and plasticity and toughness of the material. The short process is suitable for industrial production, the raw material cost is low, and the obtained alloy can be directly used as a structural material or is further plastic processed.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, specifically relating to a high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy and its preparation method. Background Technology

[0002] Multi-principal alloys are a class of alloys composed of three or more elements whose configuration entropy change satisfies ΔS. conf A novel solid solution alloy with a phase density of ≥1R. This alloy is characterized by a "high entropy effect" caused by its multi-principal element properties, which inhibits the formation of complex intermetallic compounds, resulting in simple solid solution phases: FCC (face-centered cubic), BCC (body-centered cubic), or HCP (hexagonal close-packed). Among these, single-phase FCC multi-principal element alloys exhibit excellent plasticity, low-temperature mechanical properties, and corrosion resistance, making them potential structural-functional integrated materials for extreme service environments.

[0003] The low yield strength of single-phase FCC multi-principal element alloys is one of the key factors restricting their large-scale application in engineering. Researchers have adopted various methods to improve the strength of FCC high-entropy alloys, including twinning, phase transformation-induced strengthening, introduction of interstitial atoms, and grain refinement. Among these, the first three strengthening methods are only applicable to specific FCC high-entropy alloy systems; while grain refinement can effectively improve the strength of the alloy, it will cause a significant loss of plasticity, especially when the grains are uniformly refined to nanocrystals or ultrafine grains, the plasticity decreases sharply.

[0004] Introducing drastic solid solution strengthening without altering the microstructure of a single-phase FCC matrix is ​​an effective way to enhance its strength. Literature [MPAgustianingrum, S. Yoshida, N. Tsuji et al. “Effect of aluminum addition on solid solution strengthening in CoCrNi medium-entropy alloy”, Journal of Alloys and Compounds, 2019, 781: 866-872] shows that aluminum substitution in solid solution can improve the strength of CoNiCr medium-entropy alloys, and while hindering dislocation slip, it does not cause severe stress concentration, thus largely avoiding the strength / plasticity inversion problem. However, the solid solution strengthening effect of aluminum is not significant, and excessive aluminum doping can lead to a transformation of the alloy's phase structure to BCC (body-centered cubic). Furthermore, the high Co content in CoNiCr alloys results in high alloy prices. Therefore, the bottleneck in the design and application of single-phase FCC multi-principal elements lies in how to reduce the cost of the alloy and obtain a significant solid solution strengthening effect (maintaining the single-phase FCC structure of the alloy), while simultaneously improving the plasticity and toughness of the alloy, thereby solving the problem of insufficient room temperature mechanical properties of single-phase FCC alloys. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention designs and develops a high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy and its preparation method. The aim is to achieve rapid preparation of a single-phase FCC multi-principal element alloy with simultaneous improvement in strength and toughness by using a short process (melting + solid solution heat treatment) to completely dissolve high-content titanium elements into the alloy matrix.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy, wherein the atomic percentage expression of the multi-principal element alloy composition is (Fe a Ni b Cr c Mn d ) 100-e Ti e , where 30≤a≤45, 30≤b≤45, 0≤c≤25, 0≤d≤25, 7.5≤e≤9.5, and a+b+c+d=100, and at least one of c and d is greater than or equal to 5.

[0008] Preferably, the atomic percentage expression for the multi-principal element alloy composition is (Fe a Ni b Cr c Mnd ) 100-e Ti e , where 30≤a≤45, 30≤b≤45, 0≤c≤25, 10≤d≤25, 7.5≤e≤9.5, and a+b+c+d=100.

[0009] Preferably, the microstructure of the multi-principal element alloy is a single FCC solid solution phase equiaxed crystal structure.

[0010] The preparation method of the above-mentioned high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy includes the following steps:

[0011] The raw materials of the multi-principal alloy are mixed and smelted to obtain an alloy ingot. The alloy ingot is then subjected to solid solution heat treatment to obtain a high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal alloy.

[0012] Preferably, the preparation method specifically includes the following steps:

[0013] (1) Polish the surface of pure metal elements Fe, Ni, Cr, Mn, Ti or intermediate alloys containing these elements to remove oxide scale, and weigh the raw materials according to atomic percentage.

[0014] (2) Under a protective atmosphere, the raw materials taken symmetrically are subjected to vacuum induction melting or vacuum arc melting. During the melting process, an electromagnetic field is added for stirring. The melting is repeated 4 to 8 times to obtain an alloy ingot.

[0015] (3) The obtained alloy ingot is subjected to solution heat treatment and cooled to obtain a high-content titanium solution-strengthened cobalt-free face-centered cubic multi-principal element alloy.

[0016] More preferably, the protective atmosphere is argon.

[0017] More preferably, the pressure of the protective atmosphere is 0.01-0.1 MPa.

[0018] More preferably, the protective atmosphere is first evacuated to a vacuum of 1×10⁻⁶. -2 The solution is obtained by filling with a protective gas at a pressure below 1 Pa.

[0019] More preferably, the solution heat treatment temperature is 1100-1200℃, and the holding time is 4-12h.

[0020] More preferably, the cooling is water quenching to room temperature.

[0021] The concept behind the above technical solution is based on a combination of "maximizing lattice volume mismatch" and "high-stability FCC multi-principal element alloy matrix." Titanium is doped into a high-stability FCC multi-principal element alloy matrix to achieve a microstructure where titanium with high volume mismatch is completely dissolved within the FCC matrix. "Maximizing lattice volume mismatch" utilizes titanium doping to significantly increase the lattice constant of the FCC matrix, maximizing the FCC lattice distortion. "High-stability FCC multi-principal element alloy matrix" refers to increasing the Fe and Ni content and decreasing the Cr and Mn content in the system, allowing for greater adjustability of the valence electron concentration (VEC) of the matrix FCC, ensuring that a single-phase FCC structure is maintained even after a large amount of titanium is dissolved. Considering that Co is a stabilizing element in the high-temperature phase of the L12 structure Co3Ti, this invention removes Co from the alloy system, achieving a perfect single-phase FCC solid solution phase structure. Ultimately, high concentrations of titanium with high volume mismatch were dissolved in the FCC matrix, which effectively improved the work hardening ability of the alloy, thereby delaying the occurrence of plastic instability and improving the material's macroscopic uniform plastic deformation ability.

[0022] This innovative design concept resulted in the prepared alloy having higher tensile yield strength, tensile strength, and elongation after fracture than the comparative alloy prepared under the same conditions without titanium solution.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] (1) This invention solves the problem that traditional processes can only dissolve a small amount of alloying elements or other impurity phases (intermetallic compound phases) that cannot be dissolved. The high-stability FCC multi-principal alloy matrix design achieves that even after dissolving a high content of titanium, it is still a single FCC solid solution phase.

[0025] (2) Traditional strengthening methods usually cannot improve strength and toughness at the same time. This invention adopts the design method of "maximizing lattice volume mismatch" and "high stability FCC multi-principal alloy matrix" to achieve high concentration of high volume mismatch titanium elements completely dissolved in the matrix. Furthermore, the solid solution effectively improves the work hardening ability of the alloy and delays the occurrence of plastic instability, so that the alloy can obtain high strength while improving its toughness.

[0026] (3) This invention only requires one solution treatment and heat treatment of the cast alloy to obtain a strong and tough FCC alloy that can be directly used as a structural component material, or to perform plastic processing to further obtain a material with ultra-high strength / toughness. Compared with traditional multi-principal element alloy design and preparation processes, this invention removes the expensive element cobalt, and has a short process and low cost, and has a very good development prospect. Attached Figure Description

[0027] Figure 1 BSE images of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 1, as well as its titanium-free and cobalt-containing comparative alloys.

[0028] Figure 2 Room temperature tensile stress-strain curves of the high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 1, and its titanium-free and cobalt-containing comparative alloys.

[0029] Figure 3 BSE images of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 3 and its titanium-free comparative alloy.

[0030] Figure 4 Room temperature tensile stress-strain curves of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 3 and its titanium-free comparative alloy. Detailed Implementation

[0031] To make the present invention easier to understand, the invention will now be described in further detail with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0032] A high-content titanium solid solution strengthened low-cost cobalt-free face-centered cubic multi-principal element alloy, wherein the atomic percentage expression of the alloy composition is (Fe a Ni b Cr c Mn d ) 100-e Ti e (at.%), where 30≤a≤45, 30≤b≤45, 0≤c≤25, 0≤d≤25, 7.5≤e≤9.5, and a+b+c+d=100, and at least one of c and d is greater than or equal to 5.

[0033] The high-titanium solid solution strengthened, low-cost, cobalt-free face-centered cubic multi-principal element alloy exhibits a single FCC solid solution phase equiaxed grain structure. Its tensile yield strength, tensile strength, and elongation after fracture are all higher than those of the titanium-free solid solution-free comparative alloy prepared under the same conditions.

[0034] The aforementioned titanium solid solution strengthened low-cost cobalt-free face-centered cubic multi-principal element alloy is prepared according to the following process steps:

[0035] (1) Polish the surface of pure metal elements Fe, Ni, Cr, Mn, Ti or intermediate alloys of these elements to remove oxide scale, and weigh the pure metal elements or intermediate alloys according to the above atomic percentages.

[0036] (2) Under the protection of argon, a good amount of pure metal element or intermediate alloy is subjected to vacuum induction melting or vacuum arc melting. During the melting process, an electromagnetic field is added for stirring, and the melting is repeated 4 to 8 times.

[0037] (3) The obtained alloy ingot is subjected to solution heat treatment at a temperature of 1100-1200℃ for 4-12 hours, and then quenched in water to room temperature to obtain the alloy described in this invention.

[0038] The above-mentioned method for preparing low-cost, cobalt-free face-centered cubic multi-principal element alloys with titanium solid solution strengthening includes:

[0039] In step (1), a certain mass is added to the easily burnable elements, specifically 1 wt.% (mass fraction) for Cr and 5 wt.% for Mn.

[0040] In step (2), the polished raw materials are placed into a water-cooled copper crucible in order of their melting point, with the lowest melting point at the bottom and the highest melting point at the top. This prevents the alloy composition from deviating from the theoretical design due to incomplete melting of high-melting-point metals or volatilization of low-melting-point metals at high temperatures. Before starting smelting, the furnace is evacuated to a vacuum level below 7 × 10⁻⁶. -3 The pressure is increased to 0.05 MPa, then high-purity argon gas is introduced to bring the furnace pressure to 0.05 MPa. Between each melting operation, the alloy ingot is flipped over before the next melting operation begins.

[0041] In step (3), in order to avoid the alloy ingot being oxidized during the high-temperature solution treatment, the alloy ingot needs to be vacuum sealed or heat-treated in a flowing argon atmosphere.

[0042] Example 1:

[0043] The titanium-content solid solution-strengthened, low-cost, cobalt-free face-centered cubic high-entropy alloy and its short-process preparation method described in this embodiment specifically include the following steps:

[0044] (1) Remove the oxide scale from the surface of pure elemental metals Fe, Ni, Cr, Mn, and Ti by polishing, and weigh them according to the following atomic percentage (at.%): (Fe 32 Ni 32 Cr 12 Mn 24 ) 91 Ti9, i.e., Fe: 29.12%, Ni: 29.12%, Cr: 10.92%, Mn: 21.84%, Ti: 9%, with Cr supplemented by 1 wt.% and Mn supplemented by 5 wt.%, and the purity of each pure metal element ≥99.7 wt.%.

[0045] (2) Place the weighed raw materials into the water-cooled copper crucible in order of lower melting point at the bottom and higher melting point at the top; evacuate the furnace to achieve a vacuum degree of 6×10⁻⁶. -3The pressure inside the furnace was increased to 0.05 MPa by introducing high-purity argon gas. Vacuum induction melting was then performed, with electromagnetic stirring added during the process. The melting was repeated four times. Between each melting operation, the alloy ingot was flipped over before starting the next melting cycle.

[0046] (3) The obtained alloy ingot is placed in an atmosphere heat treatment furnace, and after flowing argon gas is introduced, solid solution heat treatment is started at a temperature of 1100℃ and a holding time of 12h. Then it is water quenched to room temperature.

[0047] (4) Fe was prepared under the same conditions as described above. 32 Ni 32 Cr 12 Mn 24 The alloy is the titanium-free contrast alloy of this embodiment.

[0048] (5) Prepare (Fe) under the same conditions as described above. 32 Ni 32 Cr 12 Co 24 ) 91 Ti9 alloy (Fe: 29.12%, Ni: 29.12%, Cr: 10.92%, Co: 21.84%, Ti: 9%), is the cobalt-containing comparative alloy of this embodiment.

[0049] BSE images of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 1, and its titanium-free and cobalt-containing comparative alloys are shown below. Figure 1 The room temperature tensile stress-strain curves of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 1, and its titanium-free and cobalt-containing comparative alloys are shown below. Figure 2 .

[0050] As can be seen, the high-titanium solid solution strengthened low-cost cobalt-free face-centered cubic high-entropy alloy prepared in this embodiment has a single-phase FCC solid solution equiaxed grain structure with an average grain size of 190 μm; its room temperature tensile yield strength is 372 MPa, tensile strength is 846 MPa, and elongation after fracture is 38.9%, exhibiting excellent mechanical properties. Its titanium-free comparative alloy has an average grain size of 212 μm; its room temperature tensile yield strength is 253 MPa, tensile strength is 599 MPa, and elongation after fracture is 37.8%. Its cobalt-containing comparative alloy has a microstructure of FCC matrix + lath-like precipitates with an average grain size of 198 μm; its room temperature tensile yield strength is 543 MPa, tensile strength is 680 MPa, and elongation after fracture is only 8.2%.

[0051] Example 2:

[0052] The high-titanium solid solution-strengthened, low-cost, cobalt-free face-centered cubic high-entropy alloy and its short-process preparation method described in this embodiment specifically include the following steps:

[0053] (1) Remove the oxide scale from the surface of pure elemental metals Fe, Ni, Mn, and Ti by polishing, and weigh them according to the following atomic percentage (at.%): (Fe 38 Ni 38 Mn 24 ) 92.5 Ti 7.5 The composition is Fe: 35.15%, Ni: 35.15%, Mn: 22.2%, Ti: 7.5%, with Mn supplemented at 5 wt.%, and the purity of each pure metal element ≥ 99.7 wt.%.

[0054] (2) Place the weighed raw materials into the water-cooled copper crucible in order of lower melting point at the bottom and higher melting point at the top; evacuate the furnace to achieve a vacuum degree of 5×10⁻⁶. -3 The pressure inside the furnace was increased to 0.05 MPa by introducing high-purity argon gas. Vacuum arc melting was then performed, with electromagnetic stirring added during the process. The melting was repeated four times. Between each melting operation, the alloy ingot was flipped over before starting the next melting cycle.

[0055] (3) The obtained alloy ingot is placed in an atmosphere heat treatment furnace, and after flowing argon gas is introduced, solid solution heat treatment is started at a temperature of 1100℃ and a holding time of 8h. Then it is water quenched to room temperature.

[0056] (4) Fe was prepared under the same conditions as described above. 38 Ni 38 Mn 24 The alloy is the titanium-free contrast alloy of this embodiment.

[0057] The high-titanium solid solution-strengthened, low-cost, cobalt-free face-centered cubic high-entropy alloy prepared in this embodiment exhibits a single-phase FCC solid solution equiaxed grain structure with an average grain size of 212 μm. It has a room-temperature tensile yield strength of 369 MPa, a tensile strength of 779 MPa, and an elongation at fracture of 39.9%, demonstrating excellent mechanical properties. In contrast, the comparative alloy has an average grain size of 201 μm, a room-temperature tensile yield strength of 256 MPa, a tensile strength of 539 MPa, and an elongation at fracture of 36.3%.

[0058] Example 3:

[0059] The high-content titanium solid solution strengthened low-cost cobalt-free face-centered cubic medium-entropy alloy and its short-process preparation method described in this embodiment specifically include the following steps:

[0060] (1) Remove the oxide scale from the surface of pure elemental metals Fe, Ni, Mn, and Ti by polishing, and weigh them according to the following atomic percentage (at.%): (Fe38 Ni 38 Mn 24 ) 90.5 Ti 9.5 The composition is Fe: 34.39%, Ni: 34.39%, Mn: 21.72%, Ti: 9.5%, with Mn supplemented at 5 wt.%, and the purity of each pure metal element ≥ 99.7 wt.%.

[0061] (2) Place the weighed raw materials into the water-cooled copper crucible in order of lower melting point at the bottom and higher melting point at the top; evacuate the furnace to achieve a vacuum degree of 5×10⁻⁶. -3 The pressure inside the furnace was increased to 0.05 MPa by introducing high-purity argon gas. Vacuum arc melting was then performed, with electromagnetic stirring added during the process. This melting was repeated five times. Between each melting operation, the alloy ingot was flipped over before starting the next melting cycle.

[0062] (3) The obtained alloy ingot is placed in an atmosphere heat treatment furnace, and after flowing argon gas is introduced, solid solution heat treatment is started at a temperature of 1100℃ and a holding time of 4h. Then it is water quenched to room temperature.

[0063] (4) Fe was prepared under the same conditions as described above. 38 Ni 38 Mn 24 The alloy is the titanium-free contrast alloy of this embodiment.

[0064] BSE images of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 3 and its titanium-free comparative alloy are shown below. Figure 3 The room temperature tensile stress-strain curves of the high-titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy prepared in Example 3 and its titanium-free comparative alloy are shown below. Figure 4 .

[0065] As can be seen, the titanium solid solution strengthened low-cost cobalt-free face-centered cubic medium-entropy alloy prepared in this embodiment has a single-phase FCC solid solution equiaxed grain structure with an average grain size of 218 μm; its room temperature tensile yield strength is 392 MPa, its tensile strength is 836 MPa, and its elongation after fracture is 36.2%, exhibiting excellent mechanical properties. In contrast, the comparative alloy has an average grain size of 241 μm; its room temperature tensile yield strength is 213 MPa, its tensile strength is 516 MPa, and its elongation after fracture is 35.7%.

[0066] Example 4:

[0067] The titanium solid solution strengthened low-cost cobalt-free face-centered cubic high-entropy alloy and its short-process preparation method described in this embodiment specifically include the following steps:

[0068] (1) Remove the oxide scale from the surface of pure elemental metals Fe, Ni, Cr, and Ti by polishing, and weigh them according to the following atomic percentage (at.%): (Fe 38 Ni 38 Cr 24 ) 92.5 Ti 7.5 The composition is Fe: 35.15%, Ni: 35.15%, Cr: 22.2%, Ti: 7.5%, with Cr supplemented at 1 wt.%, and the purity of each pure metal element is ≥99.7 wt.%.

[0069] (2) Place the weighed raw materials into the water-cooled copper crucible in order of lower melting point at the bottom and higher melting point at the top; evacuate the furnace to achieve a vacuum degree of 6×10⁻⁶. -3 The pressure inside the furnace was increased to 0.05 MPa by introducing high-purity argon gas. Vacuum induction melting was then performed, with electromagnetic stirring added during the process. The melting was repeated four times. Between each melting operation, the alloy ingot was flipped over before starting the next melting cycle.

[0070] (3) The obtained alloy ingot is placed in an atmosphere heat treatment furnace, and after flowing argon gas is introduced, solid solution heat treatment is started at a temperature of 1200℃ and a holding time of 10h. Then it is water quenched to room temperature.

[0071] (4) Fe was prepared under the same conditions as described above. 38 Ni 38 Cr 24 The alloy is the titanium-free contrast alloy of this embodiment.

[0072] The titanium solid solution strengthened low-cost cobalt-free face-centered cubic high-entropy alloy prepared in this embodiment has a single-phase FCC solid solution equiaxed grain structure with an average grain size of 312 μm. Its room temperature tensile yield strength is 348 MPa, tensile strength is 766 MPa, and elongation after fracture is 41.1%, exhibiting excellent mechanical properties. In contrast, the comparative alloy has an average grain size of 332 μm, a room temperature tensile yield strength of 220 MPa, tensile strength of 591 MPa, and elongation after fracture of 40.1%.

[0073] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy, characterized in that: The atomic percentage expression for the composition of the multi-principal element alloy is (Fe a Ni b Cr c Mn d ) 100-e Ti e Where 30≤a≤45, 30≤b≤45, 0≤c≤25, 0≤d≤25, 7.5≤e≤9.5, and a+b+c+d=100, at least one of c and d is greater than or equal to 5; the microstructure of the multi-principal element alloy is a single FCC solid solution phase equiaxed crystal structure.

2. The high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy according to claim 1, characterized in that: The atomic percentage expression for the composition of the multi-principal element alloy is (Fe a Ni b Cr c Mn d ) 100-e Ti e , where 30≤a≤45, 30≤b≤45, 0≤c≤25, 10≤d≤25, 7.5≤e≤9.5, and a+b+c+d=100.

3. The method for preparing the high-content titanium solid solution strengthened cobalt-free face-centered cubic multi-principal element alloy according to claim 1 or 2, characterized in that, Includes the following steps: The raw materials of the multi-principal alloy are mixed and smelted to obtain an alloy ingot. The alloy ingot is subjected to solution heat treatment at a temperature of 1100-1200℃ for a holding time of 4-12h. Then, it is water quenched to room temperature to obtain a high-content titanium solution-strengthened cobalt-free face-centered cubic multi-principal alloy.

4. The preparation method according to claim 3, characterized in that, Includes the following steps: (1) Polish the surface of pure metal elements Fe, Ni, Cr, Mn, Ti or intermediate alloys containing these elements to remove oxide scale, and weigh the raw materials according to atomic percentage; (2) Under a protective atmosphere, the raw materials taken symmetrically are subjected to vacuum induction melting or vacuum arc melting. During the melting process, an electromagnetic field is added for stirring, and the melting is repeated 4 to 8 times to obtain an alloy ingot. (3) The obtained alloy ingot is subjected to solution heat treatment and cooled to obtain a high-content titanium solution-strengthened cobalt-free face-centered cubic multi-principal element alloy.

5. The preparation method according to claim 4, characterized in that, The protective atmosphere is argon.

6. The preparation method according to claim 4, characterized in that, The pressure of the protective atmosphere is 0.01-0.1 MPa.

7. The preparation method according to claim 4, characterized in that, The protective atmosphere is first evacuated to a vacuum of 1×10⁻⁶. -2 The solution is obtained by filling with a protective gas at a pressure below 1 Pa.

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