EP0035070A1 - Memory alloy based on a highly cupriferous or nickelous mixed crystal - Google Patents

Abstract

Powder-metallurgically produced fine-grain copper-rich or nickel-rich memory alloy as a β-high-temperature phase with metal oxide particles embedded in the matrix and acting as grain growth-inhibiting dispersoids. Compared to cast and wrought alloys produced by melt metallurgy, mechanical properties such as elongation, toughness and working capacity are significantly improved.

Description

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

Copper-rich and nickel-rich memory alloys are known and have been described in various publications (e.g. U.S. Patent No. 3,783,037 and U.S. Patent No. 4,019,925). Such memory alloys, which are of the general type with the β high temperature phase, are generally made 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 improve these memory alloys metallurgically 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 / Al type by powder metallurgy, starting from finished starting alloys corresponding to the final composition (e.g. M. Follon, E. Aernoudt, Powder-metallurgically processed shape-memory. Alloys, 5th European Symposium on Powder Metallurgy, Stockholm 1978, pp. 275-281). 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 and Ni / Al, 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 Ni / Al, which form dense, compact bodies with good mechanical properties and at the same time to reproducible values of the transition temperature and others with the Have the memory effect process related quantities.

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 one A feature of the process is that it does not start from elementary powders or from a starting powder corresponding to the final alloy, but instead uses 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.

A1203, Y ₂ O ₃ and TiO ₂ or any mixtures of these oxides are particularly suitable as suitable dispersoids. Their content should preferably make up 0.5 to 2% by weight of the total mass of the alloy and their particles have an average diameter of 10 Å to 1 µ.

Al ₂ 0 ₃ can advantageously be introduced into the manufacturing process in the form of the oxide shell of the powder to be used made of aluminum or an aluminum-rich master alloy. In this case, the powder can be mixed in a tumble mixer. Y ₂ 0 ₃ and TiO ₂ are mixed into the metal powders separately in the form of very fine particles and ground and mechanically alloyed under an organic solvent (toluene, ethyl alcohol) that keeps atmospheric oxygen away.

The invention is described using the following exemplary embodiments:

Embodiment I:

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

• Pulver A: Cupro-Aluminium: 93 Gew.-% Cu; 7 Gew.-% Al, erschmolzen, atomisiert; Korngrösse 40 - 100 µ. Hersteller Baudier.
• Pulver B: Aluminium premixe 202 AC: 96 Gew.-% Al; 4 Gew.-% Cu, Korngrösse 23 - 28 µ. Hersteller Alcoa.
• Pulver C: Reinnickel: 100 Gew.-% Ni Korngrösse 44 µ. Mond-Nickel (z.B.. Int. Nickel Co.) Folgende Einwaage wurde während 10 min in einem Taumelmischer gemischt:
  

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 µ. Manufacturer Baudier.
• Powder B: aluminum premixe 202 AC: 96% by weight Al; 4% by weight of Cu, grain size 23-28 µ. Manufacturer Alcoa.
• Powder C: Pure nickel: 100% by weight Ni grain size 44 µ. Moon nickel (e.g. Int.Nickel Co.) The following sample was mixed in a tumble mixer for 10 min:
  

240 g of this powder mixture were filled into a rubber tube with an inner diameter of 20 mm and pressed isostatically at a pressure of 8000 bar to a cylinder with a diameter of 18 mm and a height of 220 mm. The green compact was reduced and presintered in a hydrogen stream at a temperature of 950 ° C. for 1 h and then sintered in a stream of argon at a temperature of 950 ° C. for 19 h. The raw sintered body was turned to a diameter of 17 mm, introduced into a soft-annealed copper tube with an outer diameter of 20 mm and completely encapsulated by covering the ends by means of plugs and soldering under an argon atmosphere. The workpiece formed in this way was then alternately subjected to thermomechanical processing and homogenization annealing in a stream of argon for 1 h at 950 ° C. In the present case, the thermomechanical processing consisted of round hammering at 950 ° C, with the diameter of the rod being 18 mm in the first round hammering stitch and every subsequent one Stitch was reduced by 2 mm each. The procedure was such that 2 thermomechanical operations were followed by homogenization annealing. The hammered down to 8 mm diameter rod was finally subjected to a final annealing in an argon stream for 15 minutes at a temperature of 950 ⁰ C and then immediately quenched in water. The test showed a density of 99.5 - 99.8% of the theoretical value for the workpiece. The aluminum oxide content present as a dispersoid was 1.8%.

Working example II:

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

The powders given in Example I were weighed out as follows and mixed in a tumble mixer for 12 minutes:

240 g of this powder mixture were placed in a soft annealed tombac tube of 20 mm inside diameter and 1.6 mm wall. starch filled 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 12,000 bar and the compact was reduced and presintered in a stream of hydrogen at a temperature of 850 ° C for 1 ψ2 h and then sintered in a stream of argon at a temperature of 820 ° C for 22 h. The workpiece was then reduced in 2 round hammer passes at a temperature of 900 ° C to 18 or 16 mm in diameter and homogenized in a stream of argon at 920 ° C for 1 h. This was followed by two round hammer stitches at 900 ^o C, so that the rod finally had a diameter of 13 mm. After repeated homogenization for 1 h at 920 ° C, the rod was rolled down in several successive hot: rolling operations, each with 20-25% reduction in cross section, to form a strip 1.5 mm thick and 20 mm wide. After a final annealing at 950 ° C. for 12 minutes, the strip was quenched in water. The dense _t e of the finished tape was 99.7% of the theoretical value. The aluminum oxide content (dispersoid) was 1.8%.

A comparison of this alloy with 13% by weight aluminum, 3% by weight nickel and 84% by weight copper serves to illustrate the differences in the mechanical properties compared to conventional alloys:

Test specimens made from this alloy were subjected to annealing at temperatures up to 950 ° C. for 50 h, 200 h and 500 h and checked. No drop in mechanical properties or grain growth could be found. Even after an arbitrary long annealing at 950 ° C the average crystallite diameter of 30 µ was retained.

Working example III:

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 µ. Hersteller Baudier.
• Pulver B: Aluminium premixe 202 AC: 96 Gew.-% Al; 4 Gew.-% Cu, Korngrösse 23 - 28 µ. Hersteller Alcoa.
• Pulver C: Reinnickel:100 Gew.-% Ni. Korngrösse 44 µ. Mond-Nickel (z.B. Int. Nickel Co.).
• Pulver D: Yttriumoxyd: 100 Gew.-% Y₂ ⁰ ₃, Korngrösse < 1 µ.

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

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 h and then in a stream of argon at a temperature of 800 ° C. for 25 h sintered. The workpiece was then alternately subjected to 2 round hammer operations and a homogenization anneal at 900 ° C each, similar to Example I. 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 temperature M _{S of} the martensitic transformation was 150 ° C. The average grain size was 28 µ.

Working example IV:

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 filled into a rubber tube with an inner diameter of 35 mm and pressed isostatically at a pressure of 14,000 bar to a cylinder with a diameter of 31 mm and a height of 80 mm. The green compact was reduced in a hydrogen stream at a temperature of 920 ° C for 1 h and presintered and subsequently finish-h in an argon stream at a temperature of 950 ⁰ C during the twentieth The raw sintered body was turned to a diameter of 30 mm, inserted into the recipient of an extrusion press and pressed at a temperature of 780 ° C. to form 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 Homogenize the rod at a temperature of 920 ° C for 30 min and then pull it down in 2 stitches 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 µ with a Vickers hardness (HV10) of 280 units.

Embodiment V:

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 µ.
• Pulver B₁: Reinnickel: 100 Gew.-% Ni. Korngrösse 44 µ. Mond-Nickel (z.B. Int. Nickel Co.).
• Pulver C₁: Yttriumoxyd: 100 Gew.-% Y₂0₃, Korngrösse < 1 u.

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 µ.
• Powder B ₁ : pure nickel: 100% by weight Ni. Grain size 44 µ. Moon nickel (e.g. Int.Nickel Co.).
• Powder C ₁ : yttrium oxide: 100% by weight Y ₂ 0 ₃ , grain size <1 u.

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, 1000 g of this powder mixture were filled 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 body was reduced in a hydrogen / nitrogen stream at a temperature of 1200 ° C. for 1 h and presintered and then sintered in a stream of argon at a temperature of 1250 ° C. for 25 h. 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 alternately upsetting and annealing at 1200 ° C, the height of the cylinder was successively reduced to approx. 32 mm, whereby the material condensed to approx. 95% of the theoretical density and now one had a diameter of 70 mm corresponding to the die. After an additional homogenization annealing at 1230 ° 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 1220 ° C and 1100 ° C the finished form forged. 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 1300 ° 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 MS of the martensitic transformation was 130 ° C.

Embodiment VI:

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 µ.
• Pulver B₂: Reinaluminium: 100 Gew.-% Al, Korngrösse 44 µ Hersteller Alcoa
• Pulver C₂: Nickel/Kobalt-Vormischung: 98,03 Gew.-% Ni; 1,97 Gew.-% Co; Korngrösse < 44 µ.
• Pulver D₂: Titandioxyd: 100 Gew.-% TiO₂, Korngrösse < 1 µ

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 µ.
• Powder B ₂ : pure aluminum: 100% by weight Al, grain size 44 µ manufacturer Alcoa
• Powder C ₂ : nickel / cobalt premix: 98.03% by weight Ni; 1.97 wt% Co; Grain size <44 µ.
• Powder D ₂ : titanium dioxide: 100% by weight TiO ₂ , grain size <1 µ

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, 2000 g of this powder mixture were filled into a plastic tube with an inner diameter of 66 mm and pressed 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 body was reduced in a hydrogen / nitrogen stream at a temperature of 1180 ° C. for 1 h and presintered and then sintered in a stream of argon at a temperature of 1220 ° C. for 25 h. The raw Sin The 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 placing covers 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 alternately upsetting and annealing at 1180 ° C, the height of the cylinder was successively reduced to approx. 64 mm, whereby the material condensed to approx. 95% of the theoretical density and now had a diameter of 70 mm corresponding to the recipient of the extrusion press . After an additional homogenization annealing at 1200 ° C. for 1 h, the preformed circular body was inserted into an extrusion press and pressed at a temperature of 1250 ° C. to 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 1300 ° 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 1250 to 1150 ° 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 1320 ° 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 μ. In general - depending on the choice of the starting powder - an average crystallite diameter of 30 µ 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:

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, whereas in the Be rich M _{S is} noticeably increased. By substituting nickel with manganese, M _{S is} significantly reduced in the entire range (for otherwise identical aluminum contents of approx. 11 to 14%). In all systems (both in the original Cu / Al / Ni system and in the substituted systems), M _S shows an extremely strong dependency on aluminum, such 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. Thanks to the possibility of inducing both the one-way and the two-way memory effect in these alloys, together with the wide range of variations in the temperature M _S and the excellent mechanical properties, there are wide areas of application ranging from temperature control to thermomechanical energy converters to overcurrent protection of electrical ones Attachments.

Claims (10)

1. Memory alloy on the basis of a copper-rich or nickel-rich mixed crystal, which is present as a β-phase, characterized in that it is made from master alloys and premixes by powder metallurgy, that it has a fine-grained structure with a crystallite diameter of at most 100 p and that in the through the beta phase formed matrix, at least one metal oxide is present as a finely divided dispersoid. 2. Memory alloy according to claim 1, characterized in that the dispersoid contains an aluminum oxide. 3. Memory alloy according to claim 1, characterized in that the dispersoid contains an yttrium oxide and / or a titanium oxide. 4. Memory alloy according to claim 1, characterized in that the content of metal oxide dispersoid makes up 0.5 to 2% by weight of the total mass of the alloy. 5. Memory alloy according to claim 1, characterized in that the average diameter of the dispersoid particles is 10 Å to 1 µ. 6. Memory alloy according to claim 1, characterized in that its matrix formed by the β phase consists of 10.5 to 15% by weight aluminum, 0 to 6% by weight nickel, the rest copper. 7. Memory alloy according to claim 6, characterized in that the nickel is partially or completely replaced by at least one of the elements manganese, iron, cobalt. 8. memory alloy according to claim 1, characterized in that its matrix formed by the β-phase consists of 17 to 26 wt .-% aluminum, the rest nickel plus cobalt. 9. Memory alloy according to claim 1, characterized in that its structure has an average crystallite diameter of 30 µ, which is retained even after an arbitrarily long annealing time at a temperature up to 950 ° C. 10. Memory alloy according to claim 1, characterized in that it has both a one-way and a two-way memory effect and that its point M _S of martensitic transformation is in the temperature range from - 200 to + 300 ° C.