EP3592875B1 - Aluminium alloy vacuum chamber elements which are stable at high temperature - Google Patents

Description

Field of the invention

The invention relates to aluminum alloy products intended to be used as elements of vacuum chambers in particular for the manufacture of integrated electronic circuits based on semiconductors, flat display screens as well as photovoltaic panels and their manufacturing process.

State of the art

Vacuum chamber elements for the manufacture of integrated electronic circuits based on semiconductors, flat display screens as well as photovoltaic panels, can typically be obtained from aluminum alloy sheets.

Vacuum chamber elements are elements intended for the manufacture of vacuum chamber structures and vacuum chamber internal components including vacuum chamber bodies, valve bodies, flanges, connection elements, elements sealing, passages, diffusers, electrodes. They are notably obtained by machining and surface treatment of aluminum alloy sheets.

To obtain satisfactory vacuum chamber elements, aluminum alloy sheets must have certain properties.

Indeed, the sheets must first of all have satisfactory mechanical characteristics to produce by machining parts having the desired dimensions and rigidity so as to be able to reach, without deformation, a vacuum generally of the level at least of the average vacuum (10 ^{- 3} - 10 ^-5 Torr ). Thus the desired breaking strength (R _m ) is generally at least 260 MPa and even more if possible. Furthermore, to be suitable for machining, sheets intended to be machined in mass must have homogeneous properties in thickness and present a low density of stored elastic energy coming from residual stresses. Furthermore, in certain applications the vacuum chamber elements are subjected to high temperatures and it is important that their resistance to deformation by creep at high temperatures is significant.

The level of porosity of the sheets must also be low enough to reach high vacuum if necessary (10 ^-6 - 10 ^-8 Torr). In addition, the gases used in vacuum chambers are frequently very corrosive and in order to avoid the risk of pollution of silicon wafers or liquid crystal devices by particles or substances coming from the elements of vacuum chambers and/or frequent replacement of these elements, it It is important to protect the surfaces of the vacuum chamber elements. Aluminum turns out to be an advantageous material from this point of view because it is possible to carry out a surface treatment generating an oxide layer resistant to reactive gases. This surface treatment includes an anodization step and the oxide layer obtained is generally called an anodic layer. In the context of the invention, the term “corrosion resistance” is more particularly understood to mean the resistance of anodized aluminum to corrosive gases used in vacuum chambers and to the corresponding tests. However, the protection provided by the anodic layer is affected by numerous factors linked in particular to the microstructure of the sheet (grain size and shape, phase precipitation, porosity) and it is always desirable to improve this parameter. Corrosion resistance is notably assessed by the so-called “bubble test” which consists of measuring the duration of the appearance of hydrogen bubbles on the surface of the anodized product upon contact with a diluted hydrochloric acid solution. The durations known in the state of the art are of the order of tens of minutes to a few hours.

To improve the vacuum chamber elements, the aluminum sheets and/or the surface treatment carried out can be improved.

The patent US 6,713,188 (Applied Materials Inc.) describes an alloy suitable for the manufacture of chambers for manufacturing semiconductors of composition (in % by weight) Si: 0.4 - 0.8; Cu: 0.15-0.30; Fe: 0.001 - 0.20; Mn 0.001 - 0.14; Zn 0.001 - 0.15; Cr: 0.04 - 0.28; Ti 0.001 - 0.06; Mg: 0.8 - 1.2. The parts are obtained by extrusion or machining to the desired shape. The composition allows control of the size of the impurity particles which improves the performance of the anodic layer.

The patent US 7,033,447 (Applied Materials Inc.) claims an alloy suitable for the manufacture of chambers for manufacturing semiconductors of composition (in % by weight) Mg: 3.5 - 4.0; Cu: 0.02 - 0.07; Mn: 0.005 - 0.015; Zn 0.08 - 0.16; Cr 0.02 - 0.07; Ti: 0 - 0.02; If <0.03; Fe < 0.03. The parts are anodized in a solution comprising 10% to 20% by weight of sulfuric acid, 0.5 to 3% by weight of oxalic acid at a temperature of 7 to 21°C. The best bubble test result is 20 hours.

The patent US 6,686,053 (Kobe ) claims an alloy having improved corrosion resistance, in which the anodic oxide comprises a barrier layer and a porous layer and in which at least part of the layer is altered to boehmite and/or pseudo-boehmite. The best result obtained in the bubble test is around 10 hours.

The patent application US 2009/0050485 (Kobe Steel, Ltd.) describes an alloy of composition (in % by weight) Mg: 0.1 - 2.0; If: 0.1 - 2.0; Mn: 0.1 - 2.0; Fe, Cr, and Cu ≤ 0.03, anodized so that the hardness of the anodic oxide layer varies in thickness. The very low content of iron, chromium and copper leads to a significant additional cost for the metal used.

The patent application US 2010/0018617 (Kobe Steel, Ltd.) describes an alloy of composition (in % by weight) Mg: 0.1 - 2.0; If: 0.1 - 2.0; Mn: 0.1 - 2.0; Fe, Cr, and Cu ≤ 0.03, the alloy being homogenized at a temperature of more than 550 °C down to 600 °C or less.

Patent applications US 2001/019777 And JP2001 220637 (Kobe Steel ) describe an alloy for chambers comprising (in % by weight) Si: 0.1 - 2.0, Mg: 0.1 - 3.5, Cu: 0.02 - 4.0 and impurities, the Cr content being less than 0.04%. These documents disclose in particular products obtained by carrying out a hot rolling step before being dissolved.

International demand WO2011/89337 (Constellium) describes a process for manufacturing unrolled cast products suitable for manufacturing vacuum chamber elements of composition, in weight percent, Si: 0.5 - 1.5; Mg: 0.5 - 1.5; Fe <0.3; Cu <0.2; Mn <0.8; Cr <0.10; Ti < 0.15.

The patent US 6,066,392 (Kobe Steel ) describes an aluminum material having an anodic oxidation film with improved corrosion resistance, in which cracks are not generated even in high temperature thermal cycles and corrosive environments.

The patent US 6,027,629 (Kobe Steel ) describes an improved surface treatment process for vacuum chamber elements in which the diameter of the pores of the anodic layer is variable in the thickness thereof.

The patent US 7,005,194 (Kobe Steel ) describes an improved surface treatment process for vacuum chamber elements in which the anodized film is composed of a porous layer and a non-porous layer whose structure is at least partly boehmite or pseudo -boehmite.

The patent application WO2014/060660 (Constellium France) relates to a vacuum chamber element obtained by machining and surface treatment of a sheet of thickness at least equal to 10 mm in aluminum alloy of composition, in % by weight, Si: 0.4 - 0.7; Mg: 0.4 - 0.7; Ti 0.01 - < 0.15, Fe <0.25; Cu <0.04; Mn <0.4; Cr 0.01 - <0.1; Zn <0.04; other elements < 0.05 each and < 0.15 in total, remains aluminum.

These documents do not mention the problem of improving resistance to creep deformation at high temperatures.

There is a need for further improved vacuum chamber elements, particularly in terms of resistance to creep deformation at high temperatures, while maintaining high properties of corrosion resistance, consistency of properties through thickness and suitability for machining.

Object of the invention

A first object of the invention is a vacuum chamber element obtained by machining and surface treatment of a sheet of thickness at least equal to 10 mm in aluminum alloy of composition, in % by weight, Si: 0 .4 - 0.7; Mg: 0.4 - 1.0; the ratio in % by weight Mg/Si being less than 1.8; Ti: 0.01 - 0.15, Fe 0.08 - 0.25; Cu <0.35; Mn <0.4; Cr: <0.25; Zn <0.04; other elements < 0.05 each and < 0.15 in total, remains aluminum, characterized in that the grain size of said sheet is such that the average linear intercept length ℓ , measured in the L/TC plane measured according to the ASTM E112 standard, is at least equal to 350 µm between surface and ½ thickness.

1. a. on coule une plaque de laminage en alliage d'aluminium de composition en % en poids, Si : 0,4 - 0,7 ; Mg : 0,4 - 1,0 ; le rapport en % en poids Mg/Si étant inférieur à 1,8 ; Ti : 0,01 - 0,15, Fe 0,08 - 0,25 ; Cu < 0,35 ; Mn < 0,4 ; Cr : < 0,25 ; Zn < 0,04 ; autres éléments < 0,05 chacun et < 0,15 au total, reste aluminium,
2. b. optionnellement, on homogénéise ladite plaque de laminage,
3. c. on lamine ladite plaque de laminage à une température supérieure à 400 °C pour obtenir une tôle d'épaisseur au moins égale à 10 mm,
4. d. on réalise un traitement de mise en solution de ladite tôle, optionnellement précédé par une opération d'écrouissage à froid, et on la trempe,
5. e. on détensionne ladite tôle ainsi mise en solution et trempée par traction contrôlée avec un allongement permanent de 1 à 5%,
6. f. on réalise un revenu de la tôle ainsi tractionnée,
7. g. optionnellement on réalise une déformation à froid supplémentaire d'au moins 3% et un traitement de recuit à une température d'au moins 500 °C, le traitement de recuit pouvant être réalisé avant ou après les étapes h ou i d'usinage et de traitement de surface,
8. h. on usine la tôle ainsi revenue en élément de chambre à vide,
9. i. on réalise un traitement de surface de l'élément de chambre à vide ainsi obtenu comprenant de préférence une anodisation réalisée à une température comprise entre 10 et 30 °C avec une solution comprenant 100 à 300 g/l d'acide sulfurique et 10 à 30 g/l d'acide oxalique et 5 à 30 g/l d'au moins un polyol,

ledit procédé comprenant des étapes de laminage et/ou de mise en solution et/ou de déformation à froid supplémentaire et recuit adaptées pour obtenir une taille de grain telle que longueur moyenne d'interception linéaire ℓ, mesurée dans le plan L/TC selon la norme ASTM E112,soit au moins égale à 350 µm entre surface et ½ épaisseur.A second object of the invention is a method of manufacturing a vacuum chamber element in which successively

1. has. an aluminum alloy rolling plate of composition in % by weight is cast, Si: 0.4 - 0.7; Mg: 0.4 - 1.0; the ratio in % by weight Mg/Si being less than 1.8; Ti: 0.01 - 0.15, Fe 0.08 - 0.25; Cu <0.35; Mn <0.4; Cr: <0.25; Zn <0.04; other elements < 0.05 each and < 0.15 in total, remaining aluminum,
2. b. optionally, said rolling plate is homogenized,
3. vs. said rolling plate is rolled at a temperature above 400°C to obtain a sheet with a thickness of at least 10 mm,
4. d. a solution treatment is carried out on said sheet, optionally preceded by a cold work hardening operation, and it is quenched,
5. e. said sheet thus put into solution and quenched is relieved by controlled traction with a permanent elongation of 1 to 5%,
6. f. an income is produced from the sheet thus pulled,
7. g. optionally, an additional cold deformation of at least 3% is carried out and an annealing treatment at a temperature of at least 500 °C, the annealing treatment being able to be carried out before or after steps h or i of machining and surface finishing,
8. h. the sheet thus returned is machined into a vacuum chamber element,
9. i. a surface treatment is carried out on the vacuum chamber element thus obtained preferably comprising an anodization carried out at a temperature between 10 and 30 ° C with a solution comprising 100 to 300 g/l of sulfuric acid and 10 to 30 g/l of oxalic acid and 5 to 30 g/l of at least one polyol,

said method comprising steps of rolling and/or solution and/or additional cold deformation and annealing adapted to obtain a grain size such as average linear intercept length ℓ , measured in the L/TC plane according to the ASTM E112 standard, i.e. at least equal to 350 µm between surface and ½ thickness.

Description of figures

• There figure 1 shows the granular structure of product A obtained in Example 1 on L/TC sections after Barker attack.
• There figure 2 shows the geometry of the specimen used for the hot creep deformation tests.
• There Figure 3 shows the granular structure of product F-1 ( Figure 3A ) and F-2 ( Figure 3B ) obtained in Example 2 on L/TC cuts after Barker attack.
• There figure 4 shows the granular structure of the products G and H obtained in Example 3 on L/TC sections after Barker attack, on the surface at ¼ thickness and at ½ thickness.
• There Figure 5 shows the stress profile in the thickness for direction L of the products obtained in Example 3.

Detailed description of the invention

The designation of alloys is done in accordance with the regulations of The Aluminum Association (AA), known to those skilled in the art. Definitions of metallurgical conditions are given in European standard EN 515. Unless otherwise stated, the definitions in standard EN12258-1 apply.

Unless otherwise stated, the static mechanical characteristics, in other words the breaking strength Rm, the conventional yield strength at 0.2% elongation Rp0.2 and the elongation at break A%, are determined by a tensile test according to standard ISO 6892-1, the sampling and direction of the test being defined by standard EN 485-1. Hardness is measured according to EN ISO 6506.

dans laquelle S : signifie Surface, ½ Ep signifie mi-épaisseur et ¼ Ep signifie quart-épaisseur.Grain sizes are measured according to ASTM E112. The average grain sizes are measured in the L/TC plane according to the intercept method of the standard (ASTM E112-96 §16.3). The average linear intercept length is measured in the longitudinal direction ℓ _{ℓ (0°)} and the transverse direction ℓ _{ℓ (90°)} . An average value in the L/TC plan ℓ , named mean linear intercept length in the L/TC plane, is calculated according to ℓ = ( ℓ _{ℓ (0°)} . ℓ _{ℓ (90°)} ) ^1/2 . The anisotropy index AI _ℓ is calculated according to AI _ℓ = ℓ _{ℓ (0°)} / ℓ _{ℓ (90°)} . We also calculate the variation in the thickness of ℓ _{ℓ (90°)} , Δ ℓ _{ℓ (90°)} according to the formula: $$\mathrm{\Delta }{\bar{\mathcal{L}}}_{\mathcal{L}\left(90°\right)}=\left(\max \left({\bar{\mathcal{L}}}_{\mathcal{L}\left(90°\right)}\left(\mathrm{S},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\mathrm{EP},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\mathrm{EP}\right)\right)-\min \left({\bar{\mathcal{L}}}_{\mathcal{L}\left(90°\right)}\left(\mathrm{S},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\mathrm{EP},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\mathrm{EP}\right)\right)\right)/\mathrm{mev}\left({\bar{\mathcal{L}}}_{\mathcal{L}\left(90°\right)}\left(\mathrm{S},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.\mathrm{EP},\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.\mathrm{EP}\right)\right)$$

in which S: means Surface, ½ Ep means mid-thickness and ¼ Ep means quarter-thickness.

By surface grain size, we mean in the context of the present invention the grain size measured after machining making it possible to remove 2 mm in the direction of the thickness. The breakdown voltage is measured according to EN ISO 2376:2010.

The present inventors have found that vacuum chamber elements have very advantageous properties in terms of resistance to creep deformation at high temperatures, while also having advantageous properties in terms of corrosion resistance, uniformity of properties and of machinability, are obtained for a specific 6xxx series aluminum alloy whose grain size is high and homogeneous in thickness compared to products known according to the state of the art. A method of manufacturing a vacuum chamber element comprising steps making it possible to obtain the grain size according to the invention has also been invented.

The composition of the aluminum alloy sheets making it possible to obtain the vacuum chamber elements according to the invention is in % by weight, Si: 0.4 - 0.7; Mg: 0.4 - 1.0; the ratio in % by weight Mg/Si being less than 1.8; Ti: 0.01 - 0.15, Fe 0.08 - 0.25; Cu < 0.35; Mn < 0.4; Cr: <0.25; Zn < 0.04; other elements < 0.05 each and < 0.15 in total, remains aluminum.

The contents of these elements make it possible in particular to obtain, in combination with the grain size according to the invention, high resistance to creep deformation at high temperature.

Magnesium and silicon are the major addition elements in the alloy products according to the invention. Their content has been precisely chosen so as to achieve sufficient mechanical properties, in particular a breaking strength in the TL direction of at least 260 MPa and/or an elastic limit in the TL direction of at least 200 MPa and also a homogeneous granular structure in the thickness. The silicon content is between 0.4 and 0.7% by weight and preferably between 0.5% and 0.6% by weight. The magnesium content is between 0.4 and 1.0% by weight. Preferably the minimum magnesium content is 0.5% by weight. Preferably, the maximum magnesium content is 0.7% by weight and more preferably 0.6% by weight. In an advantageous embodiment the magnesium content is 0.4 to 0.7% by weight and preferably 0.5 to 0.6% by weight. The preferred contents of silicon and/or magnesium make it possible in particular to achieve, both on the surface and at mid-thickness, durations of appearance of hydrogen bubbles in the bubble test that are particularly remarkable for the products according to the invention. In addition, the ratio in % by weight Mg/Si must remain less than 1.8 and preferably less than 1.5. The present inventors have in fact observed that if this ratio is too high, the resistance to creep deformation at high temperature decreases. The present inventors believe that a Mg content too high in solid solution could affect the resistance to creep deformation at high temperatures.

The present inventors have found that surprisingly too little iron affects the resistance to creep deformation at high temperatures. Thus the minimum iron content is 0.08% by weight and preferably 0.10% by weight. Too much iron can have a detrimental effect on the properties of the anodic oxide layer. Thus the iron content is at most 0.25% by weight and preferably at most 0.20% by weight. In an advantageous embodiment of the invention, the iron content is 0.10 to 0.20% by weight.

Adding too much copper can have an adverse effect on the resistance to creep deformation at high temperatures. The copper content is therefore less than 0.35% by weight. In addition, a high copper content can degrade the properties of the protective oxide layer and/or contaminate the products manufactured in the vacuum chambers. Preferably the copper content is less than 0.05% by weight, preferably less than 0.02% by weight and preferably less than 0.01% by weight.

Too much titanium can also have a detrimental effect on the properties of the anodic oxide layer. Thus the titanium content is less than 0.15% by weight. However, the addition of a small quantity of titanium has a favorable effect on the granular structure and its homogeneity, thus the titanium content is at least 0.01% by weight. In an advantageous embodiment of the invention, the titanium content is 0.01 to 0.1% by weight and preferably 0.01 to 0.05% by weight. Advantageously, the titanium content is at least 0.02% by weight and preferably at least 0.03% by weight.

Too much chromium can also have a detrimental effect on resistance to creep deformation at high temperatures. Thus the chromium content is less than 0.25% by weight. However, the addition of a small amount of chromium can have a favorable effect on the granular structure, so the chromium content is preferably at least 0.01% by weight. In an advantageous embodiment of the invention, the chromium content is 0.01 to 0.04% by weight and preferably 0.01 to 0.03% by weight. The simultaneous addition of chromium and titanium is advantageous because it makes it possible in particular to improve the granular structure and in particular to reduce the anisotropy index of the grains.

Controlling the maximum levels of certain other elements is important because these elements can, if they are present at levels higher than those recommended, degrade the properties of the anodic oxide layer and/or contaminate products manufactured in vacuum chambers. Thus, the manganese content is less than 0.4% by weight, preferably less than 0.04% by weight and preferably less than 0.02% by weight. The zinc content is less than 0.04% by weight, preferably less than 0.02% by weight and preferably less than 0.001% by weight.

The aluminum alloy sheets according to the invention have a thickness of at least 10 mm. Advantageously, the aluminum alloy sheets according to the invention have a thickness of between 20 and 110 mm and preferably between 30 and 90 mm. In one embodiment of the invention, the aluminum alloy sheets according to the invention have a thickness of at least 50 mm and preferably at least 60 mm.

The sheets according to the invention have a grain size such as the average linear intercept length ℓ , measured in the L/TC plane according to the ASTM E112 standard, is at least equal to 350 µm between surface and ½ thickness, and preferably at least equal to 400 µm between surface and ½ thickness, which contributes to obtaining resistance to high temperature creep deformation. Advantageously, the grain size is particularly homogeneous in the thickness, and the sheet is such that the variation in the thickness of the average linear intercept length in the plane L/TC in the transverse direction, called ℓ _{ℓ (90°)} according to ASTM E112, is less than 30% and preferably less than 20%. The grain size variation is calculated by taking the difference between the maximum value and the minimum value at ½ thickness, ¼ thickness and area and dividing by the average of the values at ½ thickness, ¼ thickness and area. Preferably, the average linear intercept length measured in the L/TC plane according to ASTM E112 in the transverse direction ℓ _{ℓ (90°)} is at least equal to 200 µm and preferably at least equal to 230 µm between surface and ½ thickness.

The sheets according to the invention have resistance to creep deformation at high temperatures. Thus, advantageously, the creep deformation under a stress of 5 MPa at 420°C is after 10 hours at most 0.40% and preferably at most 0.27%.

The sheets according to the invention are suitable for machining. Thus the stored elastic energy density Wt,t, the measurement of which is described in Example 1, for the sheets according to the invention whose thickness is between 20 and 80 mm is advantageously less than 0.2 kJ/ m ³ .

1. a. on coule une plaque de laminage en alliage d'aluminium de composition selon l'invention,
2. b. optionnellement, on homogénéise ladite plaque de laminage,
3. c. on lamine ladite plaque de laminage à une température supérieure à 400 °C pour obtenir une tôle d'épaisseur au moins égale à 10 mm,
4. d. on réalise un traitement de mise en solution de ladite tôle, optionnellement précédé par une opération d'écrouissage à froid, et on la trempe,
5. e. on détensionne ladite tôle ainsi mise en solution et trempée par traction contrôlée avec un allongement permanent de 1 à 5%,
6. f. on réalise un revenu de la tôle ainsi tractionnée,
7. g. optionnellement on réalise une déformation à froid supplémentaire d'au moins 3% et un traitement de recuit à une température d'au moins 500 °C, le traitement de recuit pouvant être réalisé avant ou après les étapes h ou i d'usinage et de traitement de surface,
8. h. on usine la tôle ainsi revenue en élément de chambre à vide,
9. i. on réalise un traitement de surface de l'élément de chambre à vide ainsi obtenu comprenant de préférence une anodisation réalisée à une température comprise entre 10 et 30 °C avec une solution comprenant 100 à 300 g/l d'acide sulfurique et 10 à 30 g/l d'acide oxalique et 5 à 30 g/l d'au moins un polyol,

le procédé comprenant des étapes de laminage et/ou de mise en solution et/ou de déformation à froid supplémentaire et recuit adaptées pour obtenir une taille de grain telle que longueur moyenne d'interception linéaire ℓ, mesurée dans le plan L/TC selon la norme ASTM E112, soit au moins égale à 350 µm entre surface et ½ épaisseur.The vacuum chamber elements according to the invention are obtained by a process in which

1. has. a rolling plate of aluminum alloy of composition according to the invention is cast,
2. b. optionally, said rolling plate is homogenized,
3. vs. said rolling plate is rolled at a temperature above 400°C to obtain a sheet with a thickness of at least 10 mm,
4. d. a solution treatment is carried out on said sheet, optionally preceded by a cold work hardening operation, and it is quenched,
5. e. said sheet thus put into solution and quenched is relieved by controlled traction with a permanent elongation of 1 to 5%,
6. f. an income is produced from the sheet thus pulled,
7. g. optionally, an additional cold deformation of at least 3% is carried out and an annealing treatment at a temperature of at least 500 °C, the annealing treatment being able to be carried out before or after steps h or i of machining and surface finishing,
8. h. the sheet thus returned is machined into a vacuum chamber element,
9. i. a surface treatment is carried out on the vacuum chamber element thus obtained preferably comprising an anodization carried out at a temperature between 10 and 30 ° C with a solution comprising 100 to 300 g/l of sulfuric acid and 10 to 30 g/l of oxalic acid and 5 to 30 g/l of at least one polyol,

the process comprising steps of rolling and/or solution and/or additional cold deformation and annealing adapted to obtain a grain size such as average linear intercept length ℓ , measured in the L/TC plane according to the ASTM E112 standard, i.e. at least equal to 350 µm between surface and ½ thickness.

Homogenization is advantageous; it is preferably carried out at a temperature between 540°C and 600°C. Preferably the homogenization time is at least 4 hours.

When homogenization is carried out, the plate can be cooled after homogenization then reheated before hot rolling or directly rolled after homogenization without intermediate cooling.

Hot rolling conditions are important to obtain the desired microstructure, in particular to improve the corrosion resistance of the products. In particular, the rolling plate is maintained at a temperature above 400°C throughout hot rolling. Preferably, the temperature of the metal is at least 450°C during hot rolling. The sheets according to the invention are rolled to a thickness of at least 10 mm.

A solution treatment of the sheet is then carried out, optionally preceded by a cold work hardening operation, and it is quenched. Quenching can be carried out in particular by spraying or by immersion. The solution is preferably carried out at a temperature between 540°C and 600°C. Preferably the solution duration is at least 15 min, the duration being adapted according to the thickness of the products.

The sheet thus put into solution is then relieved by controlled traction with a permanent elongation of 1 to 5%.

The sheet thus pulled is then tempered. The tempering temperature is advantageously between 150°C and 190°C. The duration of income is typically between 5 hours and 30 hours. Preferably, a peak income is produced to achieve a maximum elastic limit and/or a T651 state.

Optionally, an additional cold deformation of at least 3% is carried out and an annealing treatment at a temperature of at least 500 °C, the annealing treatment being able to be carried out before or after the machining or surface treatment steps. .

To obtain a grain size according to the invention, the steps of rolling and/or solution and/or additional cold deformation and annealing are adapted.

où ε̇ est la vitesse de déformation moyenne dans l'épaisseur exprimée en s^-1, Q est l'énergie d'activation de 156 kJ/mol, R est la constante des gaz parfaits 8,31 J K^-1 mol^-1, T est la température de laminage exprimée en Kelvin.In a first embodiment, the rolling temperature is maintained at a temperature above 500°C and preferably above 525°C during all the rolling stages. Advantageously in this first embodiment, the natural logarithm of the Zener-Hollomon parameter Z defined by equation (1), ln Z, is between 21 and 25 and preferably between 21.5 and 24.5 for the majority passes and preferably for all of the passes carried out during hot rolling. $$\mathrm{Z}=\dot{\epsilon }{e}^{Q/\left(\mathit{RT}\right)}$$

where ε̇ is the average strain rate in the thickness expressed in s ^-1 , Q is the activation energy of 156 kJ/mol, R is the ideal gas constant 8.31 JK ^-1 mol ^-1 , T is the rolling temperature expressed in Kelvin.

In this first embodiment the last rolling pass is advantageously such that L/H is at least 0.6 where H is the thickness at the rolling mill inlet and L is the contact length in the rolling mill.

In a second embodiment, the solution duration and/or temperature are modified relative to the solution duration and/or temperature necessary to dissolve the alloy elements, so as to obtain a grain enlargement. Typically, the duration used is at least double and/or the temperature is at least 10°C higher than the duration and/or the solution temperature necessary to dissolve the alloy elements.

In a third embodiment, the solution is preceded by a cold work hardening operation by rolling or traction with a deformation of at least 4% and preferably at least 7%.

In a fourth embodiment, an additional cold deformation of at least 3% is carried out after the tempering step and an annealing treatment at a temperature of at least 500°C, and preferably at least 525°C. C, the annealing treatment can be carried out before or after the machining or surface treatment steps.

The four embodiments can be combined to obtain the grain size according to the invention.

A vacuum chamber element is obtained by machining and surface treatment of a sheet with a thickness of at least 10 mm according to the invention.

The surface treatment preferably comprises an anodizing treatment to obtain an anodic layer whose thickness is typically between 20 and 80 µm.

The surface treatment preferably comprises degreasing and/or pickling before anodizing with known products, typically alkaline products. Degreasing and/or stripping may include a neutralization operation, particularly in the case of alkaline stripping, typically with an acidic product such as nitric acid, and/or at least one rinsing step.

Anodizing is carried out using an acid solution. It is advantageous for the surface treatment to include, after anodization, hydration (also called “sealing”) of the anodic layer thus obtained.

In an advantageous embodiment, anodizing is carried out at a temperature between 10 and 30°C with a solution comprising 100 to 300 g/l of sulfuric acid and 10 to 30 g/l of acid. oxalic acid and 5 to 30 g/l of at least one polyol and advantageously the product thus anodized is hydrated in deionized water at a temperature of at least 98°C preferably for a period of at least approximately 1 hour. These advantageous anodizing conditions make it possible to achieve particularly remarkable durations of appearance of hydrogen bubbles in the bubble test, both on the surface and at mid-thickness, in particular for the preferred products according to the invention whose content of Mg is between 0.4 and 0.7% by weight, the Si content is between 0.4 and 0.7% by weight and the Cu content is less than 0.05% by weight for which the durations to the bubble test are preferably at least 750 min. Preferably, the aqueous solution used for the anodization of this advantageous surface treatment does not contain titanium salt. The presence of at least one polyol in the anodizing solution also contributes to improving the corrosion resistance of the anodic layers. Ethylene glycol, propylene glycol or preferably glycerol are advantageous polyols. Anodizing is preferably carried out with a current density of between 1 and 5 A/dm ² . The anodizing duration is determined so as to achieve the desired anodic layer thickness.

After anodization, it is advantageous to carry out a hydration step (also called sealing) of the anodic layer. Preferably the hydration is carried out in deionized water at a temperature of at least 98°C, preferably for a period of at least approximately 1 hour. The present inventors have observed that it is particularly advantageous to carry out the hydration subsequent to anodization in two stages in deionized water, a first stage lasting at least 10 min at a temperature of 20 to 70°C and a second stage lasting at least approximately 1 hour at a temperature of at least 98°C. Advantageously an anti-powder additive derived from triazine such as Anodal-SH1 ^® is added to the deionized water used for the second stage of hydration.

The vacuum chamber elements treated with the advantageous surface treatment process and obtained from sheets whose thickness is between 20 and 80 mm easily reach at mid-thickness a duration of appearance of hydrogen bubbles in a 5% hydrochloric acid solution (“bubble test”) for at least around 400 min and preferably at least 750 min and even at least around 900 min, at least for the part corresponding to the surface of prison. The vacuum chamber elements obtained from an alloy sheet according to the invention whose thickness is between 60 and 80 mm and with the advantageous surface treatment process can achieve a duration on the surface of the sheet appearance of hydrogen bubbles in a 5% hydrochloric acid solution for at least 500 min and preferably at least 900 min at mid-thickness.

The preferred products according to the invention whose Mg content is between 0.4 and 0.7% by weight, the Si content is between 0.4 and 0.7% by weight and the Cu content is lower at 0.05% by weight reach at mid-thickness a duration of appearance of hydrogen bubbles in a 5% hydrochloric acid solution (“bubble test”) of at least at least 750 min and a creep deformation under a stress of 5 MPa at 420 °C is after 10 hours at most 0.27%.

The use of the vacuum chamber elements according to the invention in vacuum chambers is particularly advantageous because their properties are very homogeneous and moreover, particularly for the anodized elements with the advantageous surface treatment process, the corrosion resistance is high, which makes it possible to avoid pollution of products manufactured in the rooms such as, for example, microprocessors or panels for flat screens.

Examples Example 1

In this example, 6xxx alloy sheets with a thickness of 16 mm were prepared.

Plates whose composition is given in Table 1 were cast. Table 1 - composition of alloys (% by weight) Alloy If Fe Cu Mn Mg Cr Ti Mg/Si A (Invention) 0.6 0.23 0.30 0.12 1.0 0.20 0.06 1.7 B (Reference) 0.6 0.23 0.29 0.12 1.2 0.20 0.07 2.0 C (Reference) 0.4 0.24 0.29 0.12 1.0 0.19 0.06 2.5 D (Reference) 0.6 0.07 0.29 0.12 1.0 0.20 0.06 1.7 E (Reference) 0.6 0.06 0.29 <0.01 1.0 0.30 0.06 1.7

The plates were homogenized at a temperature of 560°C for 2 hours, hot rolled to a thickness of 16 mm at a temperature of at least 400°C. The sheets thus obtained were put in solution for 2 hours at a temperature of 575 ° C (A, D, E), 545°C (C) or 570°C (B) adapted to their composition, quenched and tensile. The sheets obtained underwent an appropriate tempering to reach a T651 state. The duration and temperature of the solution were intended to obtain a grain size such as the average linear intercept length in the L/TC plane measured according to the ASTM standard.

E112, named ℓ , or at least equal to 350 µm between surface and ½ thickness. The micrograph obtained for sheet A, representative of all the sheets, is presented on the Figure 1 .

Resistance to creep deformation at high temperature was evaluated on specimens as described in Figure 2 , at a temperature of 420 °C under a stress of 5 MPa. The deformation after 10 hours is provided in Table 2 Table 2 - Deformation after 10 hours of creep test at 420°C under a stress of 5 MPa. Alloy Deformation (%) A (Invention) 0.15 B (Reference) 0.29 C (Reference) 0.45 D (Reference) 0.46 E (Reference) 0.61

Sheet A has undergone machining and surface treatment. In the surface treatment the product is degreased, pickled using an alkaline solution, then neutralized with a nitric acid solution before undergoing anodizing at a temperature of approximately 20°C in a sulpho-oxalic bath (sulfuric acid 160 g/l + oxalic acid 20g/l + 15 g/l glycerol). After anodization, a hydration treatment of the anodic layer is carried out in two stages: 20 min at 50 °C in deionized water then approximately 80 min in boiling deionized water in the presence of a derived anti-powder additive. of triazine Anodal-SH1 ^® . The anodic layer obtained had a thickness of approximately 50 μm.

The anodic layer obtained was characterized by the following tests.

The breakdown voltage characterizes the voltage at which a first electric current passes through the anodic layer. The measurement method is described in standard EN ISO 2376:2010. The value obtained was 2.6 kV.

The “bubble test” is a corrosion test which makes it possible to characterize the quality of the anodic layer by measuring the duration of appearance of the first bubbles in a hydrochloric acid solution. A flat surface of 20 mm in diameter of the sample is brought into contact at room temperature with a 5% solution by mass of HCl. The characteristic duration is the duration from which a continuous flow of gas bubbles originating from at least one discrete point on the surface of the anodized aluminum is visible. The result obtained was 450 minutes.

Example 2

In this example, alloy sheets of composition as indicated in Table 3 and of thickness 280 mm were prepared by homogenization and hot rolling at a temperature above 400 ° C. Table 3 - alloy composition (% by weight) Alloy If Fe Cu Mn Mg Cr Ti Mg/Si F 0.56 0.13 0.011 0.016 0.54 0.021 0.018 1

One F-1 sheet then underwent 8% tension while the other F-2 did not receive this treatment. The sheets thus obtained were put in solution for 6 hours at a temperature of 500 °C, quenched and tensile. The sheets obtained underwent an appropriate tempering to reach a T651 state.

The granular structure of the different products obtained was observed at mid-thickness on L/TC sections by optical microscopy after Barker attack. The micrographs are presented on the Figure 3A (plate F1) and 3B (plate F-2).

The grain sizes measured in the L - TC plane are presented in Table 4 Table 4 - grain size in the L plane - TC (µm) Alloy Position ℓ _{ℓ (90°)} µm ℓ _{ℓ (0°)} . µm ℓ µm AI _ℓ (L/TC) F1 ½ thickness 435 567 four hundred ninety seven 1.3 F2 ½ thickness 223 359 283 1.6

Resistance to creep deformation at high temperature was evaluated on specimens as described in Figure 2 , at a temperature of 420 °C under a stress of 5 MPa. The deformation after 10 hours is provided in table 5. Table 5 - Deformation after 10 hours of creep test at 420°C under a stress of 5 MPa. Alloy Deformation (%) F-1 (Invention) 0.08% F-2 (Reference) 0.7%

Example 3

In this example, 6xxx alloy sheets with a thickness of 64 mm were prepared. Plates whose composition is given in Table 6 were cast. Table 6 - composition of alloys (% by weight) Alloy If Fe Cu Mn Mg Cr Ti Mg/Si G 0.6 0.14 <0.01 <0.01 0.6 0.02 0.04 1.0 H 0.5 0.13 <0.01 <0.01 0.5 0.04 0.03 1.0

The plates were homogenized at a temperature of 595°C for 12 hours.

Plate G was hot rolled to a thickness of 64 mm at a temperature of at least 530 °C and maintaining the Zener - Hollomon parameter for each rolling pass such that ln Z is between 22 and 24 ,5.

The H plate was hot rolled to a thickness of 64 mm at a temperature of between 480 and 500 °C, the Zener - Hollomon parameter being such that ln Z was greater than 26 for the majority of passes rolling.

The sheets thus obtained were put in solution for 4 hours at a temperature of 535 °C and tensile by 3%. The sheets obtained underwent an appropriate tempering to reach a T651 state.

The mechanical properties in the TL direction were measured at quarter thickness and are reported in Table 7 Table 7 - quarter-thickness mechanical properties in TL direction Alloy Rp0.2 (MPa) Rm (MPa) HAS(%) G 268 289 7.2 H >220 >260 >5

Resistance to creep deformation at high temperature was evaluated on specimens as described in Figure 2 , at a temperature of 420 °C under a stress of 5 MPa. The deformation after 10 hours is provided in table 8. Table 8 - Deformation after 10 hours of creep test at 420°C under a stress of 5 MPa. Alloy Deformation (%) G 0.26% H 2.5%

The granular structure of the different products obtained was observed on L/TC sections by optical microscopy after Barker etching, on the surface at quarter and mid-thickness. The micrographs are presented on the Figure 4 .

The average grain sizes measured in the L/TC plane according to the intercept method of the standard (ASTM E112-96 §16.3) are presented in Table 9. Table 9 - grain size in the L plane - TC (µm) Alloy Position ℓ _{ℓ (90°)} µm ℓ _{ℓ (0°)} . µm ℓ µm AI _ℓ (L/TC) Δ ℓ _{ℓ (90°)} G Surface 246 770 435 3.1 14% ¼ thickness 264 682 424 2.6 ½ thickness 284 732 456 2.6 H Surface 185 364 259 2.0 31% ¼ thickness 226 688 394 3.0 ½ thickness 254 738 433 2.9

It can be seen that product G according to the invention has a larger grain size than product H and is also more homogeneous in thickness.

The residual stresses in the thickness were evaluated using the step-by-step machining method of rectangular bars taken at full thickness in the L and TL directions, described for example in the publication " Development of New Alloy for Distortion Free Machined Aluminum Aircraft Components", F.Heymes, B.Commet, B.Dubost, P.Lassince, P.Lequeu, GM.Raynaud, in 1st International Non-Ferrous Processing & Technology Conference, 10- March 12, 1997 - Adams's Mark Hotel, St Louis, Missouri . This method mainly applies to plates whose length and width are significantly greater than the thickness and for which the residual stress state can reasonably be considered to be biaxial with its two main components in the directions L and T ( ie no residual stress in the S direction) and such that the level of residual stresses varies only in the S direction. This method is based on measuring the deformation of two full-thickness rectangular bars which are cut from the plate along directions L and TL. These bars are machined downward in the S direction step by step, and at each step the deflection is measured, as well as the thickness of the machined bar.

The width of the bar was 30 mm. The bar must be long enough to avoid any edge effect on the measurements. A length of 400 mm was used.

Measurements are taken after each machining pass.

After each machining pass, the bar is removed from the vice, and a stabilization time is observed before the deformation measurement is carried out, so as to obtain a uniform temperature in the bar after machining.

At each step i, the thickness h(i) of each bar and the deflection f(i) of each bar are collected.

$$S{\left(i\right)}_L=\frac{4E}{{\mathit{lƒ}}^2}{\displaystyle \sum _{k=1}^{i-1}\left[ƒ{\left(k+1\right)}_L-ƒ{\left(k\right)}_L\right]\left[-(h\left(i\right)+\left(h\left(i+1\right)\right)+\frac{h\left(k+1\right)\left(3h\left(k\right)-h\left(k+1\right)\right)}{3h\left(k\right)}\right]}$$

$$\sigma {\left(i\right)}_L=\frac{u{\left(i\right)}_L+\mathit{\nu u}{\left(i\right)}_{\mathit{LT}}}{1-{\nu }^2}$$

$$\sigma {\left(i\right)}_{\mathit{LT}}=\frac{u{\left(i\right)}_{\mathit{LT}}+\mathit{\nu u}{\left(i\right)}_L}{1-{\nu }^2}$$

These data make it possible to calculate the residual stress profile in the bar, corresponding to the stress σ(i) _L and the stress σ(i) _LT in the form of an average in the layer removed during the i step, given by the following formulas, in which E is the Young's modulus, lf is the length between the supports used for the deflection measurement and v is the Poisson's ratio:
from i = 1 to N-1 $$u{\left(i\right)}_L=-\mathrm{<figure-callout\; id="4"\; label="E"\; filenames="imgf0001.png,imgf0004.png"\; state="\{\{state\}\}">E</figure-callout>}\frac{4}{3}\frac{E}{{\mathit{lƒ}}^2}\left[ƒ{\left(i+1\right)}_L-ƒ{\left(i\right)}_L\right]\frac{h^3\left(i+1\right)}{h\left(i\right)(h\left(i\right)-\left(h\left(i+1\right)\right)}-S{\left(i\right)}_L$$

$$S{\left(i\right)}_L=\frac{4E}{{\mathit{lƒ}}^2}{\displaystyle \sum _{k=1}^{i-1}\left[ƒ{\left(k+1\right)}_L-ƒ{\left(k\right)}_L\right]\left[-(h\left(i\right)+\left(h\left(i+1\right)\right)+\frac{h\left(k+1\right)\left(3h\left(k\right)-h\left(k+1\right)\right)}{3h\left(k\right)}\right]}$$

$$\sigma {\left(i\right)}_L=\frac{u{\left(i\right)}_L+\mathit{\nu u}{\left(i\right)}_{\mathit{L.T.}}}{1-{\nu }^2}$$

$$\sigma {\left(i\right)}_{\mathit{L.T.}}=\frac{u{\left(i\right)}_{\mathit{L.T.}}+\mathit{\nu u}{\left(i\right)}_L}{1-{\nu }^2}$$

avec $$W_L\left(\mathit{kJ}/m^3\right)=\frac{500}{\mathit{Eth}}{\displaystyle \sum _{i=1}^{N-1}{\sigma }_L\left(i\right)\left[{\sigma }_L\left(i\right)-{\mathit{\nu \sigma }}_{\mathit{LT}}\left(i\right)\right]\mathit{dh}\left(i\right)}$$

$$W_{\mathit{LT}}\left(\mathit{kJ}/m^3\right)=\frac{500}{\mathit{Eth}}{\displaystyle \sum _{i=1}^{N-1}{\sigma }_{\mathit{LT}}\left(i\right)\left[{\sigma }_{\mathit{LT}}\left(i\right)-{\mathit{\nu \sigma }}_L\left(i\right)\right]\mathit{dh}\left(i\right)}$$

Finally, the elastic energy density stored in the bar W _tot can be calculated from the residual stress values using the following formulas: $$W_{\mathit{early}}=W_L+W_{\mathit{L.T.}}$$

with $$W_L\left(\mathit{K\; J}/m^3\right)=\frac{500}{\mathit{Eth}}{\displaystyle \sum _{i=1}^{\mathrm{NOT}-1}{\sigma }_L\left(i\right)\left[{\sigma }_L\left(i\right)-{\mathit{\nu \sigma }}_{\mathit{L.T.}}\left(i\right)\right]\mathit{dh}\left(i\right)}$$

$$W_{\mathit{L.T.}}\left(\mathit{K\; J}/m^3\right)=\frac{500}{\mathit{Eth}}{\displaystyle \sum _{i=1}^{\mathrm{NOT}-1}{\sigma }_{\mathit{L.T.}}\left(i\right)\left[{\sigma }_{\mathit{L.T.}}\left(i\right)-{\mathit{\nu \sigma }}_L\left(i\right)\right]\mathit{dh}\left(i\right)}$$

The stress profile in the thickness for the direction L is given in the figure 5 . The total measured energy W _tot was 0.18 kJ/m ³ for sample G and 0.17 kJ/m ³ for sample H.

The products have undergone machining and surface treatment. In the surface treatment the product is degreased, pickled using an alkaline solution, then neutralized with a nitric acid solution before undergoing anodizing at a temperature of approximately 20°C in a sulpho-oxalic bath (sulfuric acid 160 g/l + oxalic acid 20g/l + 15 g/l glycerol). After anodization, a hydration treatment of the anodic layer is carried out in two stages: 20 min at 50 °C in deionized water then approximately 80 min in boiling deionized water in the presence of a derived anti-powder additive. of triazine Anodal-SH1 ^® . The anodic layer obtained had a thickness of approximately 50 μm.

The anodic layers were characterized by the following tests.

The breakdown voltage characterizes the voltage at which a first electric current passes through the anodic layer. The measurement method is described in standard EN ISO 2376:2010. The values are indicated in absolute value after direct current (DC) measurement.

The “bubble test” is a corrosion test which makes it possible to characterize the quality of the anodic layer by measuring the duration of appearance of the first bubbles in a hydrochloric acid solution. A flat surface of 20 mm in diameter of the sample is brought into contact at room temperature with a 5% solution by mass of HCl. The characteristic duration is the duration from which a continuous flow of gas bubbles originating from at least one discrete point on the surface of the anodized aluminum is visible.

The results measured on the surface and at mid-thickness are presented in table 10. Table 10 - characterization of products after anodization Position Product Bubble test (min) Breakdown voltage (KV) Surface G 1020 2.0 H 1380 2.6 ¼ thickness G > 1440 2.0 H > 1500 3.3 ½ thickness G 900 2.0 H 1320 2.8

The product according to the invention has excellent properties after surface treatment.

Claims (13)

1. Vacuum chamber element obtained by machining and surface treatment of a plate of thickness at least equal to 10 mm made of aluminium alloy composed as follows (as percentages by weight), Si: 0.4 - 0.7, Mg: 0.4 - 1.0; the Mg/Si ratio as a percentage by weight being less than 1.8; Ti: 0.01 - 0.15, Fe 0.08 - 0.25; Cu <0.35; Mn <0.4; Cr: <0.25; Zn <0.04; other elements <0.05 each and <0.15 in total, the rest aluminium, characterized in that the grain size of said plate is such that the mean linear intercept length ℓ, measured in plane L/TC according to standard ASTM E112, is at least equal to 350 µm between surface and ½ thickness.
2. Element according to claim 1 characterized in that the grain size of said plate is such that the variation in the thickness of the average linear intercept length in plane L/TC in the transverse direction, called ℓ _ℓ(90°) according to standard ASTM E112, is less than 30% and preferably less than 20%.
3. Element according to claim 1 or claim 2 wherein the creep deformation at a temperature of 420°C under a stress of 5 MPa is at most 0.40% after 10 hours and preferably at most 0.27%.
4. Element according to any one of claims 1 to 3 wherein the magnesium content is 0.4 to 0.7 as percentage by weight and preferably 0.5 to 0.6% by weight.
5. Element according to any one of claims 1 to 4 wherein the copper content is less than 0.05% by weight, preferentially less than 0.02% by weight and preferably less than 0.01% by weight.
6. Element according to any one of claims 1 to 5 wherein said plate is such that its thickness is between 20 and 80 mm and stored elastic energy density W_tot is less than 0.2 kJ/m³.
7. Element according to any one of claims 1 to 6 wherein said surface treatment comprises anodization carried out at a temperature between 10 and 30°C with a solution comprising 100 to 300 g/l of sulphuric acid and 10 to 30 g/l of oxalic acid and 5 to 30 g/l of at least one polyol and wherein said plate is such that its thickness is between 20 and 80 mm, that it has at mid-thickness a hydrogen bubble appearance duration in a 5% hydrochloric acid solution greater than 400 min and preferably wherein said plate is such that its thickness is greater than 60 mm and has at its surface a hydrogen bubble appearance duration in a solution of 5% hydrochloric acid of at least 500 min.
8. Element according to claims 7 wherein the Mg content is between 0.4 and 0.7% by weight, the Si content is between 0.4 and 0.7% by weight and the Cu content is lower than 0.05% by weight for which at mid-thickness the hydrogen bubble appearance duration in a 5% hydrochloric acid solution ("bubble test") is at least 750 min and for which the creep deformation under a stress of 5 MPa at 420°C is after 10 hours at most 0.27%.
9. Method of manufacturing a vacuum chamber element wherein successively

   a. an aluminium alloy rolling slab is cast, of composition (as percentages by weight) Si: 0.4 - 0.7, Mg: 0.4 - 1.0; the Mg/Si ratio as a percentage by weight being less than 1.8; Ti: 0.01 - 0.15, Fe 0.08 - 0.25; Cu <0.35; Mn <0.4; Cr: 0.25 - 0.04; other elements < 0.05 each and < 0.15 in total, the rest aluminium,

   b. optionally, said rolling slab is homogenized,

   c. said rolling slab is rolled at a temperature above 400 °C to obtain a plate having a thickness at least equal to 10 mm,

   d. said plate undergoes solution heat treatment, optionally preceded by a cold working operation, and is quenched,

   e. after solution heat treatment and quenching, said plate is stress-relieved by controlled stretching with permanent elongation of 1 to 5%,

   f. the stretched plate then undergoes ageing,

   g. optionally, additional cold working of at least 3% and an annealing treatment at a temperature of at least 500°C are carried out; the annealing treatment can be carried out before or after steps h or i of machining and surface treatment,

   h. the aged plate is machined into a vacuum chamber element,

   i. surface treatment of the vacuum chamber element obtained in this way, preferably comprising anodization carried out at a temperature of between 10 and 30°C, is performed with a solution comprising 100 to 300 g/l of sulphuric acid and 10 to 30 g/l of oxalic acid and 5 to 30 g/l of at least one polyol,

   said method comprising appropriate additional annealing and/or solution heat treatment and/or cold working and/or annealing steps to obtain a grain size such that the average linear intercept length . , measured in plane L/TC according to standard ASTM E112, is at least 350 µm between surface and mid-thickness.
10. Method according to claim 9 wherein the rolling temperature is maintained at a temperature above 500°C and preferably at a temperature above 525°C.
11. Method according to claim 10 wherein the natural logarithm of the Zener-Hollomon parameter Z defined by equation (1), In Z is between 21 and 25 and preferably between 21.5 and 24.5 for the majority of passes and preferably for all passes made during hot rolling. $$\mathrm{Z}=\dot{\epsilon }{e}^{Q/\left(\mathit{RT}\right)}$$

    
12. Method according to any one of claims 9 to 11 wherein solution heat treatment is preceded by cold working by rolling or stretching with a deformation of at least 4% and preferably at least 7%.
13. Method according to any one of claims 9 to 12 wherein additional cold working of at least 3% is carried out after the ageing step and annealing treatment at a temperature of at least 500°C, and preferably at least 525°C; the annealing treatment can be performed before or after the machining and surface treatment steps.

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