EP0035070B1 - Memory alloy based on a highly cupriferous or nickelous mixed crystal - Google Patents

Description

The invention is based on a memory alloy according to the preamble of claim 1.

Copper-rich memory alloys are known and have been described in various publications (e.g., U.S. Patent No. 3,783,037). Such memory alloys, which belong to the general type with the β high-temperature phase, are generally produced by melt metallurgy.

When these alloys are cast, a coarse-grained structure is generally obtained, which is further coarsened by grain growth due to the subsequent annealing in the region of the β mixed crystals and cannot be reversed by thermomechanical treatment. Accordingly, the mechanical properties, especially the elongation and notch toughness of memory alloys produced in this way are relatively poor and their field of application is limited.

There is therefore a need to metallurgically improve these memory alloys in such a way that further practical fields of application can be opened up for them. It has already been proposed to produce memory alloys of the Cu / Zn / AI type by powder metallurgy, starting from finished starting alloys corresponding to the final composition (e.g. BM Follon, E. Aernoudt, Powder-metallurgically processed shape-memory alloys, 5th European Symposium on Powder Metallurgy , Stockholm 1978, pp. 275-281; DE-A-2856082). The finished powder is encapsulated, cold compressed, hot compressed and extruded.

However, this method is unsuitable for the production of compact and dense finished parts made of Cu / Al / Ni, Cu / Al and NiAI, since the powder cannot be compacted and disintegrates again.

The invention has for its object to provide memory alloys based on Cu / Al, Cu / Al / Ni and NiAI, which form dense, compact bodies with good mechanical properties and at the same time to reproducible values of the transition temperature and others related to the memory effect Have sizes processed.

According to the invention, this object is achieved by the features of claim 1.

The essence of the invention consists in the presence of metal oxides which are embedded in the form of finely divided dispersoids in the metallic matrix of the alloys. Metal oxides of this type can be introduced into the end product both as separate powders and as natural contents of the starting substances, the production of which is advantageously carried out by powder metallurgy. An essential feature of the process consists in neither starting from elementary powders nor from a starting powder corresponding to the final alloy, but instead using a mixture of pre-alloyed powders and specially composed powder mixtures. This allows the required ductility to be optimally adapted to the processing process with extensive freedom in terms of composition. The grain size of the crystallite of the finished body can largely be predetermined. Grain growth is not to be feared thanks to the presence of the dispersoids. Coherent oxide skins that prevent homogenization and impair mechanical properties are avoided.

The dispersoids are Y ₂ 0 ₃ and Ti0 ₂ or any mixtures of these oxides. Their content is 0.5 to 2% by weight of the total mass of the alloy and their particles have an average diameter of 10 Å (1 nm) to 1 µm.

Y ₂ 0 ₃ and Ti0 ₂ are separately mixed into the metal powders in the form of very fine particles and ground and mechanically alloyed under an organic solvent (toluene, ethyl alcohol) which keeps out atmospheric oxygen in a ball mill or attritor.

The invention is described using the following exemplary embodiments:

Embodiment I

A round rod was made from a memory alloy with the following final composition:

• Pulver A : Cupro-Aluminium : 93 Gew.-% Cu ; 7 Gew.-% Al, erschmolzen, atomisiert ; Korngrösse 40-100 wm. Hersteller Baudier.
• Pulver B : Aluminium premixe 202 AC : 96 Gew.-% AI ; 4 Gew.-% Cu, Korngrösse 23-28 µm. Hersteller Alcoa.
• Pulver C : Reinnickel : 100 Gew.-% Ni, Korngrösse 44 µm. Mond-Nickel (z. B. Int. Nickel Co.).
• Pulver D : Yttriumoxyd : 100 Gew.-% Y₂0₃, Korngrösse < 1 µm.

The following powders were used as starting materials:

• Powder A: Cupro aluminum: 93% by weight Cu; 7% by weight Al, melted, atomized; Grain size 40-100 wm. Manufacturer Baudier.
• Powder B: aluminum premixe 202 AC: 96% by weight AI; 4% by weight of Cu, grain size 23-28 µm. Manufacturer Alcoa.
• Powder C: pure nickel: 100% by weight Ni, grain size 44 µm. Moon nickel (e.g. Int.Nickel Co.).
• Powder D: yttrium oxide: 100% by weight Y ₂ 0 ₃ , grain size <1 µm.

The following sample was mixed, milled and mechanically alloyed for 8 hours under toluene in the attritor:

In order to volatilize the toluene, the powder mixture was dried and then 240 g were poured into a soft annealed copper tube with an inside diameter of 18 mm and a wall thickness of 2 mm and completely encapsulated by covering the ends and soldering under an argon atmosphere. The tube and powder were then isostatically pressed at a pressure of 10,000 bar and the compact was reduced and presintered in a hydrogen / nitrogen stream at a temperature of 750 ° C. for 2 hours and then in a stream of argon at a temperature of 800 ° C. for 25 hours finish sintered. The workpiece was then alternately subjected to 2 round hammer operations and a homogenization anneal at 900 ° C each. The rod hammered down to 6 mm was subjected to a final annealing at 1000 ° C. for 10 minutes in a stream of argon and quenched in water. The density of the matrix was 99.5% of the theoretical value. The martensitic transformation temperature Mg was 150 ° C. The average crystallite diameter was 28 µm.

Embodiment II

A square bar was made from a memory alloy of the following final composition:

The powders A, B, C and D ^* (100% titanium dioxide) were weighed out as follows and mixed, ground and mechanically alloyed in a ball mill for 12 hours under ethyl alcohol:

After the volatilization of the ethyl alcohol, 250 g of this powder mixture were poured into a rubber tube with an inner diameter of 35 mm and pressed isostatically to a cylinder with a diameter of 31 mm and a height of 80 mm at a pressure of 14,000 bar. The green body was reduced in a hydrogen stream at a temperature of 920 ° C. for 1 h and presintered and then sintered in a stream of argon at a temperature of 950 ° C. for 20 h. The raw sintered body was turned to a diameter of 30 mm, inserted in the recipient of an extrusion press and pressed at a temperature of 780 ° C. to a square bar with a square cross section and an edge length of 8 mm. The reduction ratio (decrease in cross-section) was 11: 1. The rod was then homogenized at a temperature of 920 ° C. for 30 minutes and then pulled down in 2 passes on a warm drawing bench at a temperature of 750 ° C. to an edge length of 6 mm. After the final annealing at 900 ° C. for 15 minutes in a stream of argon, the rod was quenched in water. The density of the matrix of the finished rod was 99.8% of the theoretical value. The transition temperature M _s was 170 ° C. The average crystallite diameter was 26 µm with a Vickers hardness (HV10) of 280 units.

Embodiment III

A round plate was made from a memory alloy of the following final composition:

• Pulver A₁ : Nickel/Aluminium-Vorlegierung : 50 Gew.-% Ni ; 50 Gew.-% Al, erschmolzen, atomisiert, Korngrösse 44-100 µm.
• Pulver B_, : Reinnickel : 100 Gew.-% Ni, Korngrösse 44 µm. Mond-Nickel (z. B. Int. Nickel Co.).
• Pulver C₁ : Yttriumoxyd : 100 Gew.-% Y₂0₃, Korngrösse < 1 µm.

The following powders were used as starting materials:

• Powder A ₁ : nickel / aluminum master alloy: 50% by weight Ni; 50% by weight Al, melted, atomized, grain size 44-100 µm.
• Powder _B: Nickel: 100 wt .-% Ni, particle size 44 microns. Moon nickel (e.g. Int.Nickel Co.).
• Powder C ₁ : yttrium oxide: 100% by weight Y ₂ 0 ₃ , grain size <1 µm.

The following sample was mixed, milled and mechanically alloyed under toluene in the attritor for 20 h:

After the toluene had been removed by drying, 1,000 g of this powder mixture were poured into a plastic tube with an internal diameter of 66 mm and compressed isostatically at a pressure of 12,000 bar to a cylinder with a diameter of 60 mm and a height of 80 mm. The green compact was reduced and presintered in a hydrogen / nitrogen stream at a temperature of 1200 ° C. for 1 h and then sintered in a stream of argon at a temperature of 1200 ° C. for 25 hours. The raw sintered body was turned to a diameter of 58 mm, introduced into a soft-annealed can made of soft iron with an outside diameter of 64 mm and completely encapsulated by placing the lid on and soldering in an argon atmosphere. The workpiece produced in this way was subjected to thermomechanical processing under a hot press, interrupted by homogenization annealing. By alternating upsetting and annealing at 1200 ° C, the height of the cylinder was successively reduced to approximately 32 mm, the material condensing to approximately 95% of the theoretical density and now having a diameter of 70 mm corresponding to the die. After an additional homogenization annealing at 1 230 ° C for 1 h, the preformed circular plate with parallel flat end faces was inserted in a forging die with a reduced diameter and in several operations, which were interrupted by intermediate annealing, at temperatures between 1 220 ° C and 1 100 ° C forged to the finished shape. With a maximum outer diameter of 90 mm, the 20 mm thick plate had a radial bead of 5 x 5 mm on the upper side and a central recess of 20 mm diameter and 5 mm axial depth on the lower side. After a final annealing at 1,300 ° C. for 15 minutes, the plate was quenched in water. The density of the matrix was 99.2-99.5% of the theoretical value. The temperature Mg of the martensitic transformation was 130 ° C.

Embodiment IV

A sheet was made from a memory alloy of the following final composition:

• Pulver A₂: Nickel/Aluminium-Vorlegierung : 50 Gew.-% Ni ; 50 Gew.-% Al, erschmolzen, atomisiert, Korngrösse 44-100 µm.
• Pulver B₂ : Reinaluminium : 100 Gew.-% Al, Korngrösse 44 µm. Hersteller Alcoa.
• Pulver C₂: Nickel/Kobalt-Vormischung ; 98,03 Gew.-% Ni ; 1,97 Gew.-% Co ; Korngrösse < 44 µm.
• Pulver D₂: Titandioxyd : 100 Gew.-% Ti0₂, Korngrösse < 1 µm.

The following powders were used as starting materials:

• Powder A ₂ : nickel / aluminum master alloy: 50% by weight Ni; 50% by weight Al, melted, atomized, grain size 44-100 µm.
• Powder B ₂ : pure aluminum: 100% by weight Al, grain size 44 µm. Manufacturer Alcoa.
• Powder C ₂ : nickel / cobalt premix; 98.03 wt% Ni; 1.97 wt% Co; Grain size <44 µm.
• Powder D ₂ : titanium dioxide: 100% by weight Ti0 ₂ , grain size <1 µm.

The following initial weight was mixed, ground and mechanically alloyed in a ball mill under ethyl alcohol for 25 h:

After the volatilization of the ethyl alcohol, 2,000 g of this powder mixture were filled into a plastic tube with an inner diameter of 66 mm and compressed isostatically at a pressure of 12,000 bar to a cylinder with a diameter of 60 mm and a height of 160 mm. The green compact was reduced in a hydrogen / nitrogen stream at a temperature of 1,180 ° C. for 1 hour and presintered and then sintered in a stream of argon at a temperature of 1,220 ° C. for 25 hours. The raw sintered body was turned to a diameter of 58 mm, inserted into a soft-annealed tube made of corrosion-resistant steel with an outside diameter of 64 mm and completely encapsulated by putting on lids and soldering in an argon atmosphere. The workpiece produced in this way was subjected to thermomechanical processing under a hot press, interrupted by homogenization annealing. By alternating upsetting and annealing at 1,180 ° C, the height of the cylinder was successively reduced to approx. 64 mm, the material condensing to approx. 95% of the theoretical density and now having a diameter of 70 mm corresponding to the recipient of the extrusion press . After an additional homogenization annealing at 1200 ° C. for 1 hour, the preformed circular body was inserted into an extrusion press and pressed at a temperature of 1200 ° C. to form a flat bar of rectangular cross section of 10 × 50 mm. The reduction ratio (decrease in cross section) was 7.8: 1. The rod was then homogenized at a temperature of 1,300 ° C. for 30 minutes and then a piece of 250 mm in length was cut out. This piece was rolled down in several successive hot rolling operations with corresponding intermediate annealing at 1,250 to 1,150 ° C. to a sheet of 2 mm thickness. Each 2 stitches of cross rolls, each with a 20% reduction in cross-section, were interrupted by 1 stitch each of longitudinal rolls with a 5% reduction in cross-section (stitch for straightening the sheet). There was an intermediate annealing of 15 minutes for every two cross and one longitudinal stitch. After a final annealing at 1,320 ° C. for 10 minutes, the sheet was quenched in water. The density of the matrix of the finished sheet was 99.8%. The temperature M _{s of} the martensitic transformation was 200 ° C.

The dispersion memory alloys according to the invention produced by the aforementioned processes have a fine-grained structure with a crystallite diameter of at most 100 μm. In general - depending on the choice of the starting powder - an average crystallite diameter of 30 _I Lm and less can be achieved. The invention is not restricted to the parameters specified in the examples. In general, the powder compositions and mixtures can be varied and substituted in such a way that the following material limits for the metallic matrix are reached:

Nickel can also be partially or completely replaced by at least one of the following elements:

Nickel can also be partially or completely replaced by cobalt.

In the Cu / Al / Ni system, the substitution of nickel by iron in the range of 2-3% has hardly any effect on the level of the transition temperature, while Mg is noticeably increased in areas above. The substitution of nickel with manganese significantly reduces Ms over the entire range (for otherwise the same aluminum content of approx. 11 to 14%). In all systems (both in the original Cu / AI / Ni system and in the substituted systems), M _s shows an extremely strong dependence on aluminum, in such a way that M _s decreases with increasing aluminum content. So there is a very wide margin for the point of martensitic transformation, which can range from - 200 ° C to + 300 ° C. With the ability in these alloys both the one-way to induce such as the two-way memory effect, the temperature _Ms and the excellent mechanical properties result together with the ^g Rossen variation wide application areas of the Te _m p _e itself - raturkontrolle thermomechanical Extend energy converters to overcurrent protection of electrical systems.

Claims (2)

1. Memory alloy containing a dispersoid and based on a mixed crystal present as a β-phase, characterised in that it has been prepared by powder metallurgy from prealloys and premixes, that its matrix, formed by the ß-phase, consists of 10.5 to 15 % by weight of aluminium and 0 to 6 % by weight of nickel, the remainder being copper, it being possible for the nickel to be replaced partially or wholly by at least one of the elements manganese, iron or cobalt, or of 17 to 26 % by weight of aluminium, the remainder being nickel plus cobalt, that it has a fine-grained structure with a crystallite diameter of at most 100 µm, and that 0.5 to 2 % by weight of Y 0₃ and/or Ti0₂ are present as a finely distributed dispersoid having a mean particle diameter from 10 Å (1 nm) to 1 µm in the matrix formed by the β-phase.

2. Memory alloy according to Claim 1, characterised in that its structure has a mean crystallite diameter of 30 µm, which is preserved even after any length of annealing time at a temperature of up to 950 °C.