JP5112132B2 - Ferromagnetic shape memory alloy and method for producing sintered ferromagnetic shape memory alloy - Google Patents

Description

  The present invention relates to a ferromagnetic shape memory alloy that undergoes magnetic field induced reverse transformation and recovers its shape with a magnetic change.

  Shape memory alloys have a significant shape memory effect associated with the reverse transformation of martensite transformation, and are useful as actuator materials and the like. An actuator made of a shape memory alloy is usually thermally driven by martensitic transformation by cooling and reverse transformation by heating. For shape memory alloys, the reverse transformation temperature during heating is generally higher than the transformation temperature during cooling. The difference between the transformation temperature and the reverse transformation temperature is called temperature hysteresis. In the thermoelastic martensitic transformation with small temperature hysteresis, a large shape recovery strain of about 5% is usually obtained. However, the heat-driven actuator has a problem that the response speed is slow because the cooling process is rate-controlled by heat dissipation.

  Therefore, ferromagnetic shape memory such as Ni—Co—Al alloy and Ni—Mn—Ga alloy having shape memory effects such as inducing martensitic transformation by magnetic field and twin deformation of martensite phase. Alloys are attracting attention. Ferromagnetic shape memory alloys are capable of magnetic field induced reverse transformation, have high response speed, and are promising as actuator materials.

  For example, Patent Document 1 discloses a ferromagnetic material having a two-phase structure which is a Ni—Co—Al-based alloy and includes a β phase having a B2 structure and a γ phase having an fcc structure existing at a grain boundary of the β phase. Shape memory alloys are disclosed. The production of a ferromagnetic shape memory alloy having a two-phase structure is obtained by subjecting an ingot obtained by melting and solidifying an alloy having a predetermined composition to two-phase separation by subjecting the ingot to one step or two or more steps. . When a large amount of γ phase is generated at the grain boundary of the β phase, the γ phase, which is rich in ductility, supplements the grain boundary of the β phase and the destruction that occurs when the β phase alone is prevented, but compared with the case of the β phase alone. The ability as a shape memory alloy is reduced. Therefore, it is necessary to adjust the amount of γ phase generated by the heat treatment, but it is difficult to precisely control the structure by the heat treatment.

Patent Document 2 discloses at least one selected from the group consisting of nickel (Ni), manganese (Mn), indium (In), tin (Sn) and antimony (Sb), cobalt (Co) and A ferromagnetic shape memory alloy comprising / and iron (Fe) is disclosed. The alloy disclosed in Patent Document 2 exhibits excellent shape memory characteristics in a practical temperature range (−40 to + 200 ° C.), and recovers the shape with a magnetic change by magnetic field induced reverse transformation in the practical temperature range. Such alloys are used as magnetic field driving elements and thermomagnetic driving elements.
JP 2004-277865 A International Publication No. 2007/001009 Pamphlet

  In patent document 2, melt casting is mentioned as a manufacturing method of a ferromagnetic shape memory alloy. However, this ferromagnetic shape memory alloy is a typical intermetallic compound. For this reason, the polycrystalline body of ferromagnetic shape memory alloy produced by ordinary melt casting has a single-phase structure, and due to the brittleness at the grain boundaries peculiar to intermetallic compounds, the force acting from the outside, sometimes the temperature Or there exists a problem that it is easy to collapse by the transformation by a magnetic field change. In addition, since this ferromagnetic shape memory alloy is essentially an equilibrium phase, a structure composed of a single phase cannot be separated into two phases by heat treatment after casting as described in Patent Document 1.

  That is, the ferromagnetic shape memory alloy described in Patent Document 2 is desirably used as a single crystal. However, since the manufacturing process of a single crystal is complicated, the manufacturing cost is higher than that of a polycrystal, and its application is limited. Therefore, development of a ferromagnetic shape memory alloy having a high mechanical strength even for a polycrystalline body and a method for producing the same is demanded.

  In the ferromagnetic shape memory alloy comprising at least one selected from the group consisting of Ni, Mn, In, Sn and Sb, Co and / or Fe, and inevitable impurities, the inventors It was newly found that a phase having an fcc structure as a phase has high toughness. And, the structure of this ferromagnetic shape memory alloy is a two-phase structure consisting of a phase exhibiting martensitic transformation and a phase rich in toughness, thereby exhibiting high mechanical strength even in a polycrystalline body. I came up with it.

  That is, an object of the present invention is to provide a ferromagnetic shape memory alloy having a high mechanical hardness and a production method capable of easily producing a ferromagnetic shape memory alloy having a high mechanical hardness.

The ferromagnetic shape memory alloy of the present invention comprises at least one selected from the group consisting of nickel (Ni), manganese (Mn), indium (In), tin (Sn), and antimony (Sb), cobalt (Co ) And / or iron (Fe), and a ferromagnetic shape memory alloy,
It has a structure consisting of a first phase having a bcc structure exhibiting martensitic transformation and a second phase surrounding the first phase and having an fcc structure, and the first phase has a total of 100 atoms. %, Mn is 25 to 50 atomic%, at least one selected from the group consisting of In, Sn and Sb is 5 to 18 atomic% in total, and Co and / or Fe is 0.1 to 15 It is characterized in that it has a composition comprising atomic% and the balance including Ni and inevitable impurities.

  The ferromagnetic shape memory alloy comprising Ni, Mn, at least one selected from the group consisting of In, Sn, and Sb, and Co and / or Fe is a polycrystalline single-phase structure as described above. In the case of the body, it is highly brittle and easily collapses from the grain boundary. Therefore, in the present invention, a two-phase structure including a phase (first phase) having a bcc structure exhibiting martensitic transformation and a phase (second phase) surrounding the phase and having an fcc structure is adopted. Although the first phase is fragile alone, it is surrounded by a high-strength second phase rich in toughness, so that fracture is suppressed and the mechanical strength of the ferromagnetic shape memory alloy is improved.

The method for producing a ferromagnetic shape memory alloy sintered body of the present invention is one of the methods for producing the ferromagnetic shape memory alloy of the present invention. The manufacturing method of the ferromagnetic shape memory alloy sintered body according to the present invention has a manganese (Mn) content of 25 to 50 atomic%, indium (In), tin (Sn), and antimony (Sb) when the whole is 100 atomic%. At least one selected from the group consisting of 5 to 18 atomic%, cobalt (Co) and / or iron (Fe) 0.1 to 15 atomic%, the balance being nickel (Ni) and inevitable A powder manufacturing process for manufacturing an alloy powder of a ferromagnetic shape memory alloy made of impurities and having a bcc structure;
A molding step of molding the alloy powder into a molded body;
A sintering step in which a surface layer portion of the alloy powder is heated so that the surface layer portion has an fcc structure and the compact is a sintered body;
It is characterized by including.

  The ferromagnetic shape memory alloy of the present invention can be easily manufactured by using a sintering method.

  The best mode for carrying out the manufacturing method of the ferromagnetic shape memory alloy and the ferromagnetic shape memory alloy sintered body of the present invention will be described below.

[Ferromagnetic shape memory alloy]
The ferromagnetic shape memory alloy of the present invention mainly comprises at least one selected from the group consisting of nickel (Ni), manganese (Mn), indium (In), tin (Sn) and antimony (Sb), cobalt A ferromagnetic shape memory alloy comprising (Co) and / or iron (Fe).

  The ferromagnetic shape memory alloy of the present invention has a structure composed of a first phase having a bcc structure exhibiting martensitic transformation and a second phase surrounding the first phase and having an fcc structure. FIG. 1 is an explanatory view schematically showing a cross-sectional structure of a ferromagnetic shape memory alloy of the present invention. In the ferromagnetic shape memory alloy A, the first phase 1 is a phase having a bcc structure showing martensitic transformation. That is, the first phase is a parent phase having a bcc structure when the martensite transformation end temperature (Mf) is lower than room temperature, and is a martensite phase (hereinafter abbreviated as “M phase”) when Mf is higher than room temperature. is there. The second phase 2 surrounds the first phase 10. The second phase is a non-equilibrium phase that is not normally generated in a ferromagnetic shape memory alloy having the above composition, and has an fcc structure. Hereinafter, the first phase and the second phase will be described in detail.

[Phase 1]
The first phase has a total Mn content of 25 to 50 atom% and at least one selected from the group consisting of In, Sn and Sb in a total amount of 5 to 18 atom% when the entire first phase is 100 atom%, and Co and / or Fe is contained in an amount of 0.1 to 15 atomic%, and the balance is composed of Ni and inevitable impurities.

  Mn is an element that promotes the formation of a ferromagnetic matrix having a bcc structure. By adjusting the content of Mn, the start temperature (Ms) and end temperature (Mf) of the martensite transformation, the start temperature (As) and end temperature (Af) of the martensite reverse transformation, and the Curie temperature (Tc) Can be changed. When the amount of Mn added is less than 25 atomic%, martensitic transformation does not occur. On the other hand, if it exceeds 50 atomic%, the ferromagnetic shape memory alloy does not become a single phase of the parent phase. A preferable Mn content is 28 to 45 atomic%, and further 36 to 42 atomic%.

  In, Sn, and Sb are elements that improve magnetic properties. By adjusting the content of these elements, Ms and Tc can be changed, and the base organization is strengthened. When the total content of these elements is less than 5 atomic%, Ms becomes Tc or more. On the other hand, if it exceeds 18 atomic%, martensitic transformation does not occur. The total content of these elements is preferably 7 to 16 atomic%, more preferably 9 to 15 atomic%, and particularly preferably 10 to 13 atomic% when Sn is used alone.

  Co and Fe have an effect of increasing Tc. If the total content of these elements exceeds 15 atomic%, the brittleness of the first phase is increased and the mechanical strength of the ferromagnetic shape memory alloy may be lowered. The total content of these elements is preferably 0.5 to 10 atomic%, more preferably 4 to 8 atomic%, and particularly preferably 6 to 8 atomic% when Co is used alone.

  Ni is an element that improves shape memory characteristics and magnetic characteristics. When the Ni content is insufficient, the ferromagnetism disappears, and when it is excessive, the shape memory effect is not exhibited. In order to obtain excellent shape memory characteristics and ferromagnetism, the Ni content is preferably more than 35 atomic%, more preferably 40 atomic% or more, and particularly preferably 42 atomic% or more.

  In the ferromagnetic shape memory alloy of the present invention, the first phase further includes titanium (Ti), palladium (Pd), platinum (Pt), aluminum (Al), gallium (Ga), silicon (Si), germanium (Ge). ), At least one metal selected from the group consisting of lead (Pb) and bismuth (Bi) may be contained in a total amount of 0.1 to 15 atomic%. At this time, the Ni content is preferably 40 atomic% or more from the viewpoint of shape memory characteristics and magnetic characteristics. At least one metal selected from the group consisting of Ti, Pd, Pt, Al, Ga, Si, Ge, Pb, and Bi improves shape memory characteristics. Moreover, Ms and Tc are changed by adjusting the content. Among these, Ti, Al, Ga, Si, and Ge have an effect of stabilizing the M-phase long-period stacked structure. Pd, Pt, Pb and Bi have an action of stabilizing the paramagnetic phase, antiferromagnetic phase or ferrimagnetic phase constituting the M phase, particularly the paramagnetic phase or antiferromagnetic phase. If the total content of these elements exceeds 15 atomic%, the brittleness of the first phase is increased and the mechanical strength of the ferromagnetic shape memory alloy may be lowered. The total content of these elements is preferably 0.5 to 8 atomic%.

  The first phase having the composition detailed above has an Mf higher than the practical temperature range (−40 to + 200 ° C.) and is in the martensitic phase state in the practical temperature range, and thus stably exhibits good shape memory characteristics. . The shape recovery rate [= 100 × (strain−residual strain) / strain] in the case of the first phase single phase is 95% or more, and is substantially 100%. The first phase has an Af lower than the practical temperature range, and exhibits stable and good superelasticity in the practical temperature range. Usually, even when the strain is 6 to 8%, the shape recovery rate after the deformation is released is 95% or more.

  The first phase performs thermoelastic martensitic transformation and reverse transformation between the parent phase and the M phase and between the parent phases, respectively. M phase is a laminated structure of 2M, 6M, 10M, 14M, 4O, etc. (the number indicating the laminated structure represents the <001> plane lamination period, which is a fine surface, and the symbol M indicating the laminated structure represents a monoclinic crystal. The symbol O represents orthorhombic crystal), but a long-period laminated structure such as 6M, 10M, 14M, 4O or the like is preferable in order to reduce temperature hysteresis.

[Second phase]
The second phase surrounds the first phase and improves the mechanical strength of the ferromagnetic shape memory alloy of the present invention.

  The second phase is a non-equilibrium phase that is not normally generated in a ferromagnetic shape memory alloy comprising Ni, Mn, at least one selected from the group consisting of In, Sn and Sb, and Co and / or Fe. And has an fcc structure. If the composition of the second phase is defined, the second phase is at least selected from the group consisting of 34 to 42 atomic%, In, Sn, and Sb when Mn is 100 atomic%. It is preferable to have a composition of 7 to 15 atom% in total, 4 to 12 atom% of Co and / or Fe, and the balance of Ni and inevitable impurities. If the composition is within this range, the second phase has an fcc structure and is rich in toughness.

  The second phase needs to have a composition similar to the first phase. This is because in order to increase the strength of the ferromagnetic shape memory alloy of the present invention, the first phase and the second phase must have similar crystal structures and thermal expansion coefficients. Therefore, the preferable Mn content in the second phase is 36 to 40 atomic%, further 37 to 39 atomic%. The total content of In, Sn and Sb is preferably 8 to 12 atomic%, more preferably 8 to 10 atomic%, and particularly preferably 7 to 10 atomic% when Sn is used alone. The total content of Co and Fe is preferably 6 to 11 atomic%, more preferably 8 to 10 atomic%, and particularly preferably 8 to 10 atomic% when Co is used alone. The Ni content is preferably 38 atomic% or more, 40 atomic% or more, and more preferably 42 atomic% or more.

  The second phase may further contain carbon (C). The second phase may contain carbon by being sintered in a graphite mold. The carbon content is preferably 5 atomic percent or less, and more preferably 0.001 to 3 atomic percent, assuming that the entire second phase is 100 atomic percent. The second phase may further contain at least one metal selected from the group consisting of Ti, Pd, Pt, Al, Ga, Si, Ge, Pb, and Bi. The content of these elements is preferably 0.01 to 5 atomic% in total when the entire second phase is 100 atomic%.

  Note that the structures of the first phase and the second phase described above are determined by, for example, X-ray diffraction (XRD) or electron diffraction. The composition may be analyzed by energy dispersive X-ray fluorescence analysis (EDX), X-ray photoelectron spectroscopy (XPS), electron beam microanalyzer (EPMA), or the like.

Unlike the first phase, the second phase does not exhibit martensitic transformation. As for the magnetic characteristics, the first phase is ferromagnetic in the parent phase, paramagnetic, antiferromagnetic or ferrimagnetic in the M phase, but the second phase is always ferromagnetic. Therefore, in order to maximize the transformation characteristics of the ferromagnetic shape memory alloy of the present invention, it is preferable that the volume fraction of the second phase in the entire alloy is small. Therefore, when the volume ratio of the second phase is 100% by volume of the total of the first phase and the second phase,
It is good that it is 2-30 volume% further 2-20 volume%. If the volume fraction of the second phase is less than 2% by volume, it may be undesirably self-collapsed due to changes in temperature or magnetic field.

  The ferromagnetic shape memory alloy of the present invention has a total of at least one selected from the group consisting of 25 to 50 atomic% Mn, In, Sn and Sb when the total alloy is 100 atomic%. It is preferable that it is ˜18 atomic%, and Co and / or Fe is 0.1 to 15 atomic%, and the balance is made of Ni and inevitable impurities. In addition, although the ferromagnetic shape memory alloy of the present invention has a two-phase structure, the range of the above composition indicates the entire composition of the alloy. When the overall composition is in the above range, both the composition preferable for the first phase to exhibit martensitic transformation and the composition preferable for forming the second phase having the fcc structure are maintained. The more preferable content of Mn is 40 to 46 atomic%. The more preferable content of at least one selected from the group consisting of In, Sn and Sb is 8 to 14 atomic%, and when Sn is used alone, it is particularly preferably 9 to 13 atomic%. A more preferable content of Co and / or Fe is 4 to 10 atomic%, and when Co is used alone, it is particularly preferably 5 to 9 atomic%.

  In the ferromagnetic shape memory alloy of the present invention, even when the first phase is surrounded by the second phase, the transformation characteristics of the first phase are exhibited. Hereinafter, the transformation characteristics (magnetic field induced reverse transformation characteristics, thermoelastic transformation characteristics, stress induced transformation characteristics) and electrical resistance characteristics of the ferromagnetic shape memory alloy of the present invention will be described.

(I) Magnetic-field-induced reverse transformation characteristics When a magnetic field is applied to a ferromagnetic shape memory alloy in the M-phase state in which the first phase is paramagnetic, antiferromagnetic, or ferrimagnetic, the M phase is martensite-reversed to the ferromagnetic matrix. When transformed and the magnetic field is removed, it transforms to martensite and returns to the M phase. Therefore, the ferromagnetic shape memory alloy of the present invention has a two-way shape memory effect.

  The first phase stores magnetic energy (Zeeman energy) of the magnetic field in the parent phase state, but does not store in the M phase state, so there is a large magnetization difference between the parent phase and the M phase. When a magnetic field is applied to the ferromagnetic shape memory alloy, Ms, Mf, As, and Af are greatly reduced by Zeeman energy, and the M phase is transformed back into a stable matrix. Although not limited, in order to cause martensitic reverse transformation in the ferromagnetic shape memory alloy of the present invention in a practical temperature range (−40 to + 200 ° C.), the strength of the magnetic field is 0.5 to 100 kOe (398 to 7958 kA). / M).

(II) Thermoelastic transformation characteristics The first phase causes thermoelastic martensitic transformation / reverse transformation. Ms and As in the magnetic field of the ferromagnetic shape memory alloy of the present invention are usually in the range of −200 ° C. to + 100 ° C. Further, the difference between Tc and Ms is 40 ° C. or more, and a ferromagnetic parent phase exists in a wide temperature range. Ms can be adjusted by the mixing ratio of each of the above elements. In the ferromagnetic shape memory alloy of the present invention, the first phase in the M phase state has paramagnetism, antiferromagnetism, or ferrimagnetism, but in the case of antiferromagnetism or ferrimagnetism, the transformation efficiency of transformation energy is higher than that in the case of paramagnetism. Is expensive.

(III) Stress-induced transformation characteristics When stress is applied to the first phase of the parent phase, martensitic transformation occurs, and when stress is removed, martensitic reverse transformation occurs.

(IV) Electrical resistance characteristics The electrical resistance of the first phase is much higher in the M phase than in the mother phase. In the absence of a magnetic field, the ratio [rho _{M /} [rho _P electrical resistance [rho _M of M phase to the electric resistance [rho _P of the matrix phase is 2 or more. Therefore, the ferromagnetic shape memory alloy of the present invention provides an element whose electrical resistance changes due to martensite transformation / reverse transformation induced by temperature, magnetic field or stress. In particular, when a magnetic field is applied and removed at a temperature of (Mf-100 ° C.) or more and less than Mf, a giant magnetoresistance effect is obtained in which the electrical resistance reversibly changes.

[Usage]
The ferromagnetic shape memory alloy of the present invention has excellent shape memory characteristics and magnetic change characteristics in a practical temperature range (−40 to + 200 ° C.). Therefore, applications include magnetic field drive elements, thermomagnetic drive elements, heat-generating heat-absorbing elements (especially magnetic refrigerating materials), stress-magnetic characteristics, stress-resistance characteristics, and magnetic-resistance having high response speed and energy efficiency in the practical temperature range. Element, etc.

  When the ferromagnetic shape memory alloy of the present invention including the first phase structure that undergoes the reverse transformation of the magnetic field induced martensite is used, a magnetic field driving element such as a magnetic field driving microactuator and a magnetic field driving switch having a high response speed and a large output can be obtained. The magnetic field driving element includes a driving body (rotating body, deformable body, moving body, etc.) made of the ferromagnetic shape memory alloy of the present invention, and utilizes a shape change and / or a magnetic change generated in the driving body by applying a magnetic field. However, it is not necessarily limited to this. When a pulse magnetic field is applied, the response speed of the magnetic field driving element increases. In order to continuously operate the magnetic field driving element at a high response speed, it is preferable to use the magnetic field driving element at a temperature lower than Mf.

  When the ferromagnetic shape memory alloy of the present invention is used as a temperature-sensitive magnetic material, a thermomagnetic drive element with high energy efficiency can be obtained. The thermomagnetic drive element includes, for example, a driving body (rotating body, deformable body, moving body, etc.) made of the ferromagnetic shape memory alloy of the present invention, heating means (laser light irradiation apparatus, infrared irradiation apparatus, etc.), and magnetic field application means ( A permanent magnet or the like), and power is generated using a magnetic change generated in the driving body by heating, but is not necessarily limited thereto. As an example of a thermomagnetic drive element using the ferromagnetic shape memory alloy of the present invention, a current switch and a fluid control using the principle of adsorbing to a permanent magnet when the temperature-sensitive magnetic body is heated and releasing from the magnet when it is cooled Examples thereof include a thermomagnetic motor that drives a temperature-sensitive magnetic body by heating a part of a valve and a temperature-sensitive magnetic body to make it ferromagnetic and a permanent magnet is applied thereto. Details of these thermomagnetic drive elements are described in JP-A No. 2002-129273.

  When a magnetic field is applied to a ferromagnetic shape memory alloy having a first phase in the M phase, a martensitic reverse transformation accompanied by endotherm occurs, and a large magnetic entropy change occurs in a practical temperature range (−40 to + 200 ° C.). For example, the magnetic entropy change for a magnetic field change of 0 to 90 kOe (0 to 7162 kA / m) at 21 ° C. is about 20 J / kgK. Due to such a large magnetic endothermic effect, a magnetic refrigeration material having a high refrigeration capacity is obtained. Using this magnetic refrigerating material, for example, a working chamber filled with the magnetic refrigerating material, a magnetic field applying permanent magnet disposed in the vicinity of the magnetic freezing chamber, a refrigerant that exchanges heat with the magnetic refrigerating material, and a pipe for circulating the refrigerant An equipped magnetic refrigeration system can be obtained.

  By using the ferromagnetic shape memory alloy of the present invention, a heat generating element using heat generated by martensitic transformation or a heat absorbing element using heat absorbed by martensitic reverse transformation can be obtained. The exothermic endothermic element can be used as an element for automatic temperature control, for example. The configuration of the exothermic heat-absorbing element is not particularly limited, and the exothermic body and / or the endothermic body made of the ferromagnetic shape memory alloy of the present invention may be provided.

  The ferromagnetic shape memory alloy of the present invention capable of undergoing stress-induced martensitic transformation / reverse transformation above Af temperature can be used for a stress-magnetic element by utilizing a magnetic change accompanying transformation / reverse transformation. Examples of the stress-magnetic element include a strain sensor (stress sensor) that detects a magnetic change caused by applying or removing stress. The configuration of the stress-magnetic element is not particularly limited, and includes, for example, a detection body made of the ferromagnetic shape memory alloy of the present invention and means for detecting a magnetic change generated in the detection body (for example, a magnetic sensor such as a pickup coil). do it.

  By using the ferromagnetic shape memory alloy of the present invention, a stress-resistance element such as a strain sensor (stress sensor) using an electrical resistance change accompanying stress-induced martensitic transformation / reverse transformation is obtained. The configuration itself of the stress-resistance element is not particularly limited, and may include, for example, a detection body made of a ferromagnetic shape memory alloy and a means (for example, an ammeter) for detecting a change in electric resistance generated in the detection body.

  The ferromagnetic shape memory alloy of the present invention having a magnetoresistive effect can be used for a magnetoresistive element for detecting a magnetic field. The configuration itself of the magnetoresistive element is not particularly limited, and for example, electrodes may be attached to two points of the element made of the ferromagnetic shape memory alloy of the present invention. The magnetoresistive element using the ferromagnetic shape memory alloy of the present invention can be used for a magnetic head, for example.

  When a magnetic sensor such as a pickup coil is attached to a plurality of members made of the ferromagnetic shape memory alloy of the present invention and having different Ms, a ferromagnetic shape memory alloy member (Ms is known) that changes in magnetism in response to a temperature change is specified. As a result, a temperature sensor is obtained.

[Method for producing sintered ferromagnetic shape memory alloy]
The ferromagnetic shape memory alloy of the present invention described above is preferably a sintered body produced by a sintering method. That is, the ferromagnetic shape memory alloy of the present invention comprises particles having an inner portion made of the first phase and a surface layer portion made of the second phase, and a grain boundary bonding portion made of the second phase and bonding between the particles. A sintered body is preferred. FIG. 2 is an explanatory view schematically showing a cross-sectional structure when the ferromagnetic shape memory alloy of the present invention is a sintered body. FIG. 2 shows a case where four true spherical particles are sintered for simplification for explanation. In the ferromagnetic shape memory alloy sintered body A ′, the particle 10 is composed of an inner layer 11 composed of a first phase (corresponding to “1” in FIG. 1) and a surface layer composed of a second phase (corresponding to “2” in FIG. 1). Part 12. That is, the first phase is surrounded by the second phase. Further, the adjacent particles 10 are bonded by the grain boundary bonding portion 20 made of the second phase, thereby forming a sintered body A ′. The gap between the particle 10 and the coupling portion 20 becomes a pore 30. Below, the manufacturing method of a ferromagnetic shape memory alloy sintered compact is demonstrated.

  The method for producing a ferromagnetic shape memory alloy sintered body of the present invention mainly includes a powder production process, a forming process, and a sintering process.

  The powder manufacturing process comprises at least one selected from the group consisting of 25-50 atomic% manganese (Mn), indium (In), tin (Sn), and antimony (Sb) when the whole is 100 atomic%. Ferromagnetic shape memory having a bcc structure consisting of 5 to 18 atomic% in total, 0.1 to 15 atomic% of cobalt (Co) and / or iron (Fe), the balance being nickel (Ni) and inevitable impurities This is a process for producing an alloy powder of an alloy. The composition of the alloy powder may be the entire composition of the ferromagnetic shape memory alloy of the present invention already described, but the alloy powder having the desired mechanical strength can be obtained even if the alloy powder having the first phase composition is prepared. A magnetic shape memory alloy sintered body is obtained.

  A powder manufacturing process will not be specifically limited if it is performed by the normal method conventionally performed. The alloy powder is preferably manufactured by a gas atomization method in which a gas such as air, nitrogen gas, or argon gas is collided against a molten metal flow of a ferromagnetic shape memory alloy having the above composition to powder. In addition, by a pulverization method that mechanically pulverizes a solid ferromagnetic shape memory alloy having the above composition, various atomization methods that use a liquid such as water or oil as an atomizing medium, or a centrifugal force, etc. Manufacturing is possible. As the particle diameter of the alloy powder, it is desirable that the average particle diameter is 5 to 300 μm, and further 10 to 100 μm. The smaller the particle diameter, the more dense and high-strength sintered body can be obtained. However, when the average particle diameter is less than 5 μm, most of the particles have an fcc structure depending on the sintering conditions. The volume ratio of the second phase is increased, and the shape memory characteristics of the first phase may not be sufficiently exhibited. When the average particle size exceeds 300 μm, the strength as a sintered body is lowered, which is not desirable.

  Moreover, it is desirable to include the solution treatment process which solution-treats the alloy powder obtained after the powder manufacturing process. In the solution treatment, the alloy powder obtained in the powder production process is heated to a solid solution temperature to make the structure a single phase of the matrix, and then rapidly cooled. The solid solution temperature is desirably 700 ° C. or higher, more desirably 750 to 1100 ° C., and further desirably 800 to 1000 ° C. The holding time at the solution temperature may be 1 minute or longer. The quenching rate is not particularly limited, but the quenching rate is preferably 50 ° C./second or more. In addition, although the alloy powder which has a parent phase structure | tissue is obtained by rapidly cooling after a heating, when Mf of an alloy is less than room temperature, the structure | tissue of an alloy powder becomes a substantially M phase.

  Moreover, you may perform an aging treatment after the solution treatment process. The aging treatment strengthens the base of the alloy powder and improves the shape memory characteristics. The aging treatment is performed at a temperature of 100 ° C. or higher. If it is less than 100 ° C., sufficient aging effect cannot be obtained. The upper limit of the aging treatment temperature is not particularly limited, but is preferably less than 700 ° C. The aging treatment time varies depending on the aging treatment temperature and the composition of the alloy powder, but is preferably 1 minute or more, and more preferably 30 minutes or more. The upper limit of the aging treatment time is not particularly limited as long as the parent phase does not precipitate.

  The forming step is a step of forming the alloy powder into a formed body. In the molding step, the alloy powder may be filled in a mold having a predetermined shape. The filled alloy powder may be subjected to a sintering step after the alloy powder is pressure-molded, or may be sintered at the same time as being pressed in a mold.

  The sintering step is a step in which the surface layer portion of the alloy powder is heated so that the surface layer portion has an fcc structure and the formed body is a sintered body. As the sintering method, it is desirable to use a method in which the temperature of the surface layer of the alloy powder is selectively increased. As the sintering method, an electric current sintering method in which sintering is performed by utilizing a discharge phenomenon generated in the gap between the alloy powders by energizing the compact is particularly desirable. When a direct current pulse current is passed through the compact, discharge occurs in the gap between the alloy powders. Due to this discharge phenomenon, a second phase which is a non-equilibrium phase is easily formed in the surface layer portion of the alloy powder.

  When the sintering step is a step of sintering the formed body by a spark plasma sintering (SPS) method, a surface layer portion having an fcc structure is easily formed in the alloy powder. In the SPS method, a DC ON-OFF pulse current is applied to the molded body in a compressed state. Since a high-temperature discharge plasma is generated in the gap between the alloy powders when ON, the surface of the alloy powder is activated, and a second phase that is a non-equilibrium phase is easily formed on the surface layer portion of the alloy powder. Since the alloy powders adjacent to each other are bonded at the surface layer portion, they are hardened in a molded shape.

  By adjusting the sintering conditions in the sintering step, the ratio of the second phase can be easily adjusted. As a sintering temperature, 700-1000 degreeC is desirable, More preferably, it is 800-950 degreeC. If the sintering temperature is less than 700 ° C., the second phase is difficult to form, and the mechanical strength of the obtained sintered body may not be sufficiently exhibited. As the sintering temperature is higher, a dense sintered body is obtained. However, if the sintering temperature exceeds 1000 ° C., the volume ratio of the first phase exhibiting martensitic transformation decreases, which is not desirable. Moreover, it is good to hold | maintain at the sintering temperature of the said range for 1 to 60 minutes further 10 to 20 minutes.

  As mentioned above, although embodiment of the manufacturing method of the ferromagnetic shape memory alloy and ferromagnetic shape memory alloy sintered compact of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be specifically described with reference to examples of the method for producing a ferromagnetic shape memory alloy and a ferromagnetic shape memory alloy sintered body according to the present invention.

[Production of alloy powder]
The average particle size of Ni ₄₃ Co ₇ Mn ₃₉ Sn ₁₁ alloy (43.2% Ni-38.8% Mn-6.9% Co-11.0% Sn by weight composition) by gas atomization method using argon gas Was in a powder form of about 50 μm. An optical micrograph of the obtained alloy powder is shown in FIG. Next, the obtained powder was subjected to a solution treatment. The solution treatment was performed in vacuum at 800 ° C. or 900 ° C. for 24 hours to obtain two types of alloy powders having different solution treatment temperatures. An optical micrograph of the alloy powder after solution treatment at 800 ° C. is shown in FIG. By performing the solution treatment, a single-phase structure having a bcc structure was obtained. These alloy powders were sieved to a particle size of 25 to 63 μm.

[Magnetic change characteristics of alloy powder]
For alloy powder P ₉ was treated solution in the alloy powder P ₈ and 900 ° C. treated solution at the solution treatment before the alloy powder P _0, 800 _℃, was measured thermal magnetization curve. The thermal magnetization curve was measured by cooling / heating (heating / cooling rate; 2 ° C./min) between −268 and + 120 ° C. in a magnetic field of 500 Oe (40 kA / m), and SQUID (superconducting quantum interference device). ). The measurement result of the thermal magnetization curve of each powder is shown in FIG. In the graph of FIG. 5, the horizontal axis represents temperature (unit: K), and the vertical axis represents magnetization intensity. In the alloy powder P ₀ before the solution treatment, although there was a change in magnetization due to martensitic transformation, the amount of change was small, and strong magnetization remained even in a low temperature range. On the other hand, the alloy powder P ₈ and P ₉ was treated solution was greater variation in magnetization. That is, the alloy powder P ₈ and P ₉ became single-phase structure showing the martensitic transformation by the solution. Additionally, the solution treatment temperature is high alloy powder P _9, compared to the alloy powder P _8, change in magnetization by the martensitic transformation was steep. That is, it was found that the solution treatment was sufficiently performed when the temperature of the solution treatment was higher.

[Preparation of sintered ferromagnetic shape memory alloy]
Using an alloy powder P _9, to produce a ferromagnetic shape memory alloy sintered body. For the sintering, a discharge plasma sintering apparatus (model number: DR SINTER) manufactured by SPS Shintex Co., Ltd. was used. The basic configuration of this SPS device is shown in FIG. The SPS device 90 has an upper punch 91 and a lower punch 92 made of graphite that are opposed to each other in a columnar shape, and a cylindrical (inner diameter 15 mmφ) sintered graphite die 93 that is disposed around the opposed portions, Is provided. Upper punch 91, the alloy powder _{P 9} is filled into the lower punch 92, and the sintering die 93 by the space defined form (cavity 94). One end of each of the upper punch electrode 95 and the lower punch electrode 96 is provided at each end of the upper punch 91 and the lower punch 92 opposite to the opposing ends, and is accommodated in the vacuum chamber 97. The other ends of the upper punch electrode 95 and the lower punch electrode 96 are drawn out from the upper and lower ends of the vacuum chamber 97 and project outside. A pressurizing mechanism and a sintering power source (not shown) are connected to the other ends of the upper punch electrode 95 and the lower punch electrode 96, respectively.

The alloy powder P ₉ as a starting material, to prepare three different kinds of sintered bodies of sintering conditions. First, a predetermined amount of alloy powder P ₉ was filled into the cavity 94 with the end face of the lower punch 92 positioned in the cavity 94. Then, by operating the pressing mechanism, to obtain a pressed green body at 50MPa by the punches 91 and 92 the alloy powder P ₉ filled. The sintered compact was operated by operating the sintering power source in the pressurized state. Sintering was performed by raising the temperature to about 800 ° C. in 10 minutes, then raising the temperature to a predetermined sintering temperature in 2 to 3 minutes, and holding at the sintering temperature for 5 minutes or 15 minutes. In addition, sintering was performed in a vacuum atmosphere by exhausting the inside of the vacuum chamber 97 to about 1 Pa before raising the temperature.

In the above procedure, sintered body S ₈₀ (sintered at 800 ° C. for 15 minutes), sintered body S ₈₅ (sintered at 850 ° C. for 15 minutes), sintered body S ₉₀ (sintered temperature) Four types of sintered bodies were produced: sintered at 900 ° C. for 15 minutes) and sintered body S ₉₀ ′ (sintered at 900 ° C. for 5 minutes). The sintering temperature is a set temperature of the discharge plasma sintering apparatus used for producing the sintered body. The temperature is measured by a thermocouple 93 t inserted into the graphite sintered die 93 from the outside toward the wall surface of the cavity 94.

The sintered body S ₈₀ , the sintered body S ₈₅ and the sintered body S ₉₀ were observed with an optical microscope and measured for a thermal magnetization curve. Optical microscope observation observed the cross section of each sintered compact. The thermal magnetization curve was measured by cooling / heating each sintered body between −268 to + 120 ° C. in a magnetic field of 20 kOe (1600 kA / m) and 500 Oe (40 kA / m) (temperature increase / temperature decrease rate; 2 ° C. / Min) and measured using SQUID. The results are shown in FIGS. 7 and 8 (sintered body S ₈₀ ), FIGS. 9 and 10 (sintered body S ₈₅ ), and FIGS. 11 and 12 (sintered body S ₉₀ ), respectively. In each graph, the horizontal axis represents temperature (unit: K), and the vertical axis represents the strength of magnetization.

[Structure and magnetic change characteristics of ferromagnetic shape memory alloy sintered body]
From the respective optical micrographs, it was found that the higher the sintering temperature, the smaller the volume fraction of pores (black parts) and the more dense. The sintered body S ₈₀ sintered at 800 ° C. had a structure in which the alloy powder was connected only at the contact portion (FIG. 7). In addition, in the sintered body S ₈₅ sintered at 850 ° C. and the sintered body S ₉₀ sintered at 900 ° C., a bright phase (first phase) and a dark phase (first phase) surrounding the same (first phase). A two-phase structure consisting of two phases) was clearly observed (FIGS. 9 and 11). The first phase is located inside the particles derived from the alloy powder, and the second phase binds adjacent particles at the surface layer of the particles. This second phase was found to have an fcc structure from the results of measurement by EPMA. The first phase and the second phase were measured by EDX. The composition of the first layer was Ni ₄₄ Co ₇ Mn ₃₈ Sn ₁₁ , and the composition of the second phase was Ni ₄₄ Co ₉ Mn ₃₈ Sn ₉ .

Further, from the thermal magnetization curve of each sintered body, a large change in magnetization due to martensitic transformation was confirmed in any sintered body. Here, the thermal magnetization curve of before sintering alloy powder P ₉ (FIG. 5) is compared with the thermal magnetization curve of each sintered body was higher intensity of magnetization of the sintered body at low temperatures. This is because a second phase having an fcc structure that does not exist in the alloy powder because it is a non-equilibrium phase is produced by sintering. The second phase having the fcc structure exhibits ferromagnetism even at a low temperature. Since ferromagnetism at low temperatures appeared in all the sintered bodies, it was found that the second phase was generated in all the sintered bodies.

  Note that a polycrystalline body having the same alloy composition produced by melt casting was self-destructed only by changing the temperature to martensite transformation. On the other hand, in the above sintered body, even if the temperature was changed in the measurement of the thermal magnetization curve, the sintered body did not collapse during the measurement.

[Mechanical properties]
With respect to the sintered body S ₈₀ , the sintered body S ₉₀ ′, and the sintered body S ₉₀ , fracture strain was measured. Each sintered body was cut out to obtain a plate-shaped test piece of about 6 mm × 3 mm × 3 mm, and the test piece was compressed in the longitudinal direction using a universal testing machine. The compression was performed at room temperature under the condition of a crosshead speed of 0.1 mm / min, and compressed until the specimen was broken. Table 1 shows the strain when the test piece was broken.

  Since it can be used as a shape memory alloy if the fracture strain is about 1 to 2%, it has been found that these sintered bodies can be sufficiently used as a shape memory alloy. Moreover, since it can endure a deformation | transformation of 5% or more, it turned out that workability is also good.

[Shape memory characteristics]
Using the above test pieces, the shape memory characteristics of the sintered body S ₈₀ , the sintered body S ₉₀ ′, and the sintered body S ₉₀ were evaluated. First, the length of each test piece in the longitudinal direction was measured (“Initial” in Table 2). Next, a compressive stress was applied to the test piece to a strain of about 2% with a compression tester, and the length in the longitudinal direction of the test piece after unloading was measured ("after compression" in Table 2). The compressed test piece was heated at 140 ° C. for 5 minutes, and then the length of the test piece in the longitudinal direction was measured (“after heating” in Table 2). Table 2 shows the measured values. Table 3 shows the strain after compression, the strain after heating, and the shape recovery rate calculated from these values.

  After deforming the test piece, the shape was recovered by heating by about 50%. That is, it was found that these sintered bodies have shape memory characteristics.

[Electrical resistance characteristics]
Using an electrical resistance measuring device, the electrical resistance change of the sintered body S ₉₀ accompanying the temperature change was measured by the four probe method. Resistance measurement was performed by cooling / heating each sintered body between −268 and + 80 ° C. in a magnetic field of 40 kOe (3200 kA / m) or 80 kOe (6400 kA / m) (heating / cooling rate; 2). (C / min). The results are shown in FIG. In the graph of FIG. 13, the horizontal axis represents temperature (unit: K), and the vertical axis represents electrical resistance. With the transformation from the mother phase to the M phase, the electrical resistance increased significantly.

[Magnetic strain measurement]
After applying a compressive strain of 2% to the sintered body S _90, a magnetic field is applied at a temperature 310K, it was measured magnetostriction by three-terminal capacitance method. The results are shown in FIG. In FIG. 14, the horizontal axis represents the external magnetic field, and the vertical axis represents the shape recovery strain amount. As the applied magnetic field increased, a shape change accompanied with the martensite reverse transformation occurred, and a shape recovery rate of about 0.6% was obtained when 80 kOe (6400 kA / m) was applied. That is, the shape memory effect by a magnetic field was observed.

It is explanatory drawing which shows typically the cross-sectional structure | tissue of the ferromagnetic shape memory alloy of this invention. It is explanatory drawing which shows typically a cross-sectional structure | tissue in case the ferromagnetic shape memory alloy of this invention is a sintered compact. Is an optical micrograph of _Ni 43 _Co 7 _Mn 39 _{Sn 11} alloy powder obtained by a gas atomizing method. Is an optical micrograph after solution treatment _Ni 43 _Co 7 _Mn 39 _{Sn 11} alloy powder obtained by a gas atomizing method. Is a graph showing the thermal magnetization curve when cooling / heating the alloy powder P _0, P ₈ and P ₉ in a magnetic field. It is the schematic which shows the axial cross section of a discharge plasma sintering apparatus. The sintered body S ₈₀ is a cross-sectional structure photograph obtained by observing with an optical microscope. Is a graph showing the thermal magnetization curve when cooling / heating the sintered body S ₈₀ in a magnetic field of different strengths. The sintered body S ₈₅ is a cross-sectional structure photograph obtained by observing with an optical microscope. Is a graph showing the thermal magnetization curve when cooling / heating the sintered body S ₈₅ in a magnetic field of different strengths. The sintered body S ₉₀ is a cross-sectional structure photograph obtained by observing with an optical microscope. Is a graph showing the thermal magnetization curve when cooling / heating the sintered body S ₉₀ in a magnetic field of different strengths. Magnetic field upon cooling / heating the sintered body S ₉₀ in a magnetic field of different strengths - is a graph showing the electrical resistance curve. Shape recovery strain of the sintered body S ₉₀ - is a graph showing the magnetic field curve.

Explanation of symbols

A: ferromagnetic shape memory alloy 1: first phase 2: second phase A ′: sintered ferromagnetic shape memory alloy 10: particles 11: inside 12: surface layer portion 20: coupling portion 30: pores

Claims (12)

At least one selected from the group consisting of nickel (Ni), manganese (Mn), indium (In), tin (Sn) and antimony (Sb), cobalt (Co) and / or iron (Fe), A ferromagnetic shape memory alloy comprising:
It has a structure consisting of a first phase having a bcc structure exhibiting martensitic transformation and a second phase surrounding the first phase and having an fcc structure, and the first phase has a total of 100 atoms. %, Mn is 25 to 50 atomic%, at least one selected from the group consisting of In, Sn and Sb is 5 to 18 atomic% in total, and Co and / or Fe is 0.1 to 15 A ferromagnetic shape memory alloy having a composition comprising atomic%, the balance including Ni and inevitable impurities.   The second phase has a total Mn content of 34 to 42 atomic% and at least one selected from the group consisting of In, Sn and Sb when the entire second phase is 100 atomic%, 2. The ferromagnetic shape memory alloy according to claim 1, wherein the ferromagnetic shape memory alloy has a composition comprising Co and / or Fe in an amount of 4 to 12 atomic%, the balance being Ni and inevitable impurities.   3. The ferromagnetic shape memory alloy according to claim 2, wherein the second phase further contains 5 atomic% or less of carbon (C) when the entire second phase is 100 atomic%.   When the total ferromagnetic shape memory alloy is 100 atomic%, Mn is 25 to 50 atomic%, at least one selected from the group consisting of In, Sn and Sb is 5 to 18 atomic% in total, and Co and 2. The ferromagnetic shape memory alloy according to claim 1, wherein Fe is contained in an amount of 0.1 to 15 atomic% and the balance is made of Ni and inevitable impurities.   2. A sintered body comprising particles having an inner portion made of the first phase and a surface layer portion made of the second phase, and a grain boundary bonding portion made of the second phase and bonding the particles. The ferromagnetic shape memory alloy as described.   The ferromagnetic shape memory alloy according to claim 5, wherein the sintered body is sintered by spark plasma sintering.   The ferromagnetic shape memory alloy according to claim 1, wherein the first phase is ferromagnetic in the parent phase, paramagnetic, antiferromagnetic or ferrimagnetic in the martensite phase, and the second phase is ferromagnetic. When the total is 100 atomic%, manganese (Mn) is 25 to 50 atomic%, and at least one selected from the group consisting of indium (In), tin (Sn) and antimony (Sb) is 5 to 18 in total. And an alloy powder of a ferromagnetic shape memory alloy having a bcc structure comprising 0.1% to 15% by atom of cobalt (Co) and / or iron (Fe), the balance being nickel (Ni) and inevitable impurities. A powder manufacturing process to manufacture;
A molding step of molding the alloy powder into a molded body;
A sintering step in which a surface layer portion of the alloy powder is heated so that the surface layer portion has an fcc structure and the compact is a sintered body;
The manufacturing method of the ferromagnetic shape memory alloy sintered compact characterized by including this.   The ferromagnetic shape memory alloy firing according to claim 8, wherein the sintering step is a step of sintering by an electric current sintering method in which sintering is performed by a discharge phenomenon generated between the alloy powders by energizing the compact. A method for producing a knot.   The method of manufacturing a ferromagnetic shape memory alloy sintered body according to claim 9, wherein the electric current sintering method is a discharge plasma sintering method.   The manufacturing method of the ferromagnetic shape memory alloy sintered compact of Claim 8 including the solution treatment process of solution-treating the alloy powder obtained after the said powder manufacturing process.   A ferromagnetic shape memory alloy sintered body produced by the method for producing a ferromagnetic shape memory alloy sintered body according to claim 8.