Patent Application
20240376574
The present invention relates to a cobalt-chromium alloy member suitable for use in medical devices cu has stents, medical tubes, and medical guide wires, devices for gas turbines used in a high-temperature environment or a corrosive environment, or devices for other industrial equipment, and a method of producing the same. In particular, the present invention relates to improvement of a cobalt-chromium alloy material that has excellent corrosion resistance properties, excellent biocompatibility, high strength, and excellent ductility, and is suitable for indwelling medical devices.
As a metal member used in medical devices, particularly, a metal member to be implanted in the body, a metal having excellent corrosion resistance properties, excellent biocompatibility, and high mechanical properties is desired, and stainless steel, a nickel-titanium alloy, a cobalt-chromium alloy, or the like has been used. As such a biocompatible alloy, for example, a dental casting cobalt-chromium alloy (JIS T6115) has been known. As a nickel-containing alloy, a dental stainless steel wire (JIS T6103) has been known.
Among cobalt-chromium alloy members, a stent is a hollow tubular object intended to dilate stenosed internal vessels and maintains the dilated vessels and is roughly classified into a self-expandable stent and a balloon-expandable stent.
The self-expandable stent is fixed to the tip of a catheter and given self-expandability by using a superelastic alloy/shape memory alloy from the catheter at a predetermined position. For example, a stent using a nickel-titanium alloy has been put to practical use.
The balloon-expandable stent is a stent that is fixed to a balloon catheter by reducing the tube diameter and increases the tube diameter by expanding the balloon at a predetermined position. Mainly, stainless steel SUS316L and a cobalt-chromium alloy have been put to practical use. For example, in the case where stenosis occurs in a blood vessel, the stent is placed after dilating the stenosed portion by the balloon catheter and is used to support the inner wall of the blood vessel from the inside and prevent restenosis. Regarding the insertion of the stent, the stent is attached to the tip of the catheter in a reduced-diameter state outside the deflated balloon and is inserted into the blood vessel together with the balloon portion. After the balloon portion is positioned at the stenosed site, the balloon portion is inflated to expand the stent, the stent is left while the stenosed portion is dilated, and the balloon catheter is withdrawn.
As an alloy for balloon-expandable stents, ASTMF90-14 (Co-20Cr-15W-10Ni alloy (L605 alloy)), ASTMF562-13 (Co-20Cr-10Mo-35Ni alloy (MP35N alloy)), and SUS316L have been known as surgical implant materials (see Non-Patent Literatures 1, 3, and 4).
Meanwhile, rupture of the implanted metal in the field of orthopedic surgery and premature rupture of a stent in the field of cardiovascular medicine have been reported, and there is a demand for a metal member having more excellent fatigue properties. We have proposed an alloy having improved low cycle fatigue properties with respect to an L605 (Co-20Cr-15W-10Ni) alloy and an MP35N (Co-20Cr-10Mo-35Ni) alloy that are most commonly used as coronary stent materials (see Patent Literature 1). This alloy has a composition of, in terms of mass %, 10 to 27% of Cr, 3 to 12% of Mo, and 22 to 34% of Ni, a remainder thereof substantially contains Co and an unavoidable impurity, and Co is desirably 37 to 48%.
A guide wire assists in inserting a diagnostic or therapeutic catheter used in a blood vessel to a predetermined position in the blood vessel and has a structure in which a thin wire is wound around a core wire. The guide wire is required to have torque transmissibility that the rotation of the tip follows the rotation at hand and sufficient strength and ductility such that it does not rupture during treatment. Note that Non-Patent Literature 2 describes a general relationship between the strength and hardness.
L605, which is a Co—Cr alloy currently used, and a Ti—Ni alloy are materials that are difficult to cold work and the processing cost is very high as compared with SUS316.
Further, recently, a cobalt-chromium alloy member that is suitable for medical devices, devices for gas turbines, and devices for other industrial equipment and has high mechanical strength and ductility is desired.
In particular, there is a demand for the use of indwelling medical devices such as stents in blood vessels having fine and complicated shapes such as nerve vessels and cerebral vessels. In order to achieve this, it is necessary to thin, using a thin tube, the strut that is a metal portion of the stent, and a material having as high strength as possible is required in order to achieve a sufficient blood-vessel-holding force. This also leads to a reduction in the amount of metal placed in the body.
Using as thin a wire as possible as a guide wire makes it easier to insert the guide wire into fine blood vessels, but it needs to have as high strength as possible in order to achieve further favorable torque transmissibility. Further, a material having ductility is desirable to prevent rupture during use.
It is an object of the present invention to provide a cobalt-chromium alloy member suitable for use in medical devices, devices for gas turbines, and devices for other industrial equipment.
In particular, it is another object of the present invention to provide a cobalt-chromium alloy member suitable for a guide wire that makes it easier to insert an indwelling medical device such as a stent into fine blood vessels.
In order to achieve the above-mentioned object, a cobalt-chromium alloy member according to the present invention adopted the following configurations.
[1] A cobalt-chromium alloy member, which has
[2] The cobalt-chromium alloy member according to [1] is favorably obtained by performing heat treatment at a heat treatment temperature exceeding a recrystallization temperature of a cobalt-chromium alloy material having the composition on a cobalt-chromium alloy as-processed material obtained by causing the cobalt-chromium alloy material to be subjected to cold plastic working into a predetermined shape.
[3] The cobalt-chromium alloy member according to [1] or [2] favorably has a composition of, in terms of mass %, 25 to 29% of Ni, 37 to 48% of Co, and 9 to 11% of Mo, a remainder thereof containing Cr and an unavoidable impurity, the composition satisfying a relationship of
[4] The cobalt-chromium alloy member according to [3] is favorably obtained by performing heat treatment for 1 minute or more and 60 minutes or less at 800° C. or more and 1100° C. or less as heat treatment performed at a heat treatment temperature exceeding a recrystallization temperature of a cobalt-chromium alloy material having the composition on a cobalt-chromium alloy as-processed material obtained by causing the cobalt-chromium alloy material to be subjected to cold plastic working into a predetermined shape.
[5] In the cobalt-chromium alloy member according to any one of [1] to [4], the unavoidable impurity favorably contains, in terms of mass %, 1.0% or less of Ti, 1.0% or less of Mn, 1.0% or less of Fe, 1.0% or less of Nb, 1.0% or less of W, 0.5% or less of Al, 0.1% or less of Zr, 0.01% or less of B, and 0.1% or less of C as contents of Ti, Mn, Fe, Nb, W, Al, Zr, B, and C.
[6] In the cobalt-chromium alloy member according to any one of [1] to [5],
[7] In the cobalt-chromium alloy member according to any one of [1] to [5],
[8] A device favorably uses the cobalt-chromium alloy member according to any one of [1] to [7]. Favorably, this device is a medical device, a device for gas turbines, or a device for other industrial equipment.
[9] The cobalt-chromium alloy member according to [8] is favorably a medical device selected from a stent, a tube, a wire, and an implant.
[10] The cobalt-chromium alloy member according to [9] is favorably a device for gas turbines selected from a combustor and an exhaust component for aviation and industrial gas turbine engines, such as a tail tube, a combustion tube, a spray bar, a frame holder, an afterburner, and a tail pipe.
[11] The cobalt-chromium alloy member according to [9] is favorably a device for industrial equipment used in waste incinerators, boilers, high-temperature reaction vessels, rotary calciners, and a production plant and a synthesis gas plant of petrochemical products.
[12] A method of producing a cobalt-chromium alloy member, including:
The cobalt-chromium alloy member according to the present invention has excellent mechanical properties such as improved strength and ductility and is more reliable than existing products because it has a crystal structure including a face-centered cubic lattice (fcc) or a crystal structure including a face-centered cubic lattice (fcc) and a hexagonal lattice (hcp), an average value of a crystal grain size of 2 to 15 μm, and a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less by heat treatment exceeding the recrystallization temperature after being subjected to cold plastic working. For this reason, for example, when an indwelling medical device such as a stent is prepared using the cobalt-chromium alloy member according to the present invention, the reliability of the stent during mounting is enhanced and the mounting of the stent on an affected area is made easier.
In the cobalt-chromium alloy member according to the present invention, the face-centered cubic lattice (fcc) phase is stabilized by causing an alloy containing Co, Ni, Cr, and Mo as main constituents to be subjected to cold plastic working and then performing heat treatment exceeding the recrystallization temperature. As a result, in the formed fcc phase, when the cobalt-chromium alloy member deforms, an fcc twin deformation and a deformation-induced transformation from an fcc to a hexagonal lattice (hcp) occurs and high work hardenability and excellent mechanical strength/ductility are exhibited.
Note that in the case where the cobalt-chromium alloy member according to the present invention further contains solute atoms such as Mo and Nb, segregation at dislocation cores or stacking faults of extended dislocations is capable of making it difficult for cross slip to occur and work hardening further increases the mechanical strength.
The cobalt-chromium alloy member according to the present invention is capable of obtaining a member that has a crystal structure including a face-centered cubic lattice (fcc) or a crystal structure including a face-centered cubic lattice (fcc) and a hexagonal lattice (hcp), an average value of a crystal grain size of 2 to 15 μm, and a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less and exhibits a tensile strength of 800 to 1200 MPa and a breaking elongation of 30 to 80% by performing specific heat treatment at a temperature exceeding a recrystallization temperature on a cobalt-chromium alloy as-processed material obtained by causing a cobalt-chromium alloy material having a specific composition to be subjected to cold plastic working (hereinafter, referred to simply also as “cold working”) into a predetermined shape such as a tubular shape and a wire shape. In particular, the cobalt-chromium alloy member according to the present invention is characterized in that a member having a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less can be obtained. As a result, a cobalt-chromium alloy member exhibiting high work hardenability and excellent mechanical strength/ductility is obtained.
Details of the present invention will be described below.
A cobalt-chromium alloy material according to the present invention has a composition of 23 to 32% of Ni, 37 to 48% of Co, and 8 to 12% of Mo, a remainder thereof containing Cr and an unavoidable impurity, the composition satisfying a relationship of 20≤[Cr %]+[Mo %]+ [unavoidable impurity %]≤40.
The unavoidable impurity is not an intentionally added constituent but is a constituent that is unavoidably mixed due to the material or process. The constituents of the unavoidable impurity are not particularly limited and may be, for example, Ti, Mn, Fe, Nb, W, Al, Zr, B, C, and the like, which do not necessarily need to be contained.
Further, the cobalt-chromium alloy material according to the present invention is not particularly limited as long as it has a specific composition range, and may be homogenized, subjected to hot working such as hot rolling and hot forging, or processed into a specific shape by cutting or the like, as will be described below.
The reason for limiting the composition range of the cobalt-chromium alloy material according to the present invention will be described below.
Note that the content of each constituent of the cobalt-chromium alloy material is the content (mass %, referred to simply as “%” below) when the entire cobalt-chromium alloy material is 100 mass %.
Further, the numerical range in the present invention includes the upper limit value and the lower limit value. The same applies not only to the composition range shown below but also to the range of temperature treatment, the range of a tensile strength, and the range of a breaking elongation and a uniform elongation. However, this does not apply when it is clearly stated that the numerical range does not include the upper limit value or the lower limit value, such as “exceeding” and “less than”.
Ni (nickel) has effects of stabilizing the face-centered cubic lattice phase, maintaining the workability, increasing the corrosion resistance, improving the low cycle fatigue life, and improving the strength and ductility by heat treatment exceeding the recrystallization temperature after cold working. However, since it is difficult to achieve the effect of improving the strength and ductility by the heat treatment when the content of Ni is less than 23% and it is difficult to achieve the effect of improving the strength and ductility by the heat treatment when the content of Ni exceeds 32% in the composition range of Co, Cr, and Mo in the cobalt-chromium alloy material according to the present invention, the content of Ni in the present invention is 23 to 32%, favorably 25 to 29%. As a result, the effect of further improving the strength and ductility can be achieved.
Co (cobalt) itself has large work hardenability and has the effects of reducing the notch brittleness, increasing the fatigue strength, increasing the high-temperature strength, improving the low cycle fatigue life, and improving the strength and ductility by heat treatment exceeding the recrystallization temperature after cold working.
When the content of Co is less than 37%, the effect of Co is weak. In this composition, when the content of Co exceeds 48%, the matrix becomes too hard and difficult to process, and the effect of improving the strength and ductility by heat treatment exceeding the recrystallization temperature after cold working is lost. For this reason, the content of Co in the present invention is 37 to 48%, favorably 40 to 45%. As a result, the effect of further improving the strength and ductility is achieved.
Mo (molybdenum) has the effects of being dissolved in the matrix to strengthen this, increasing the work hardenability, and increasing the corrosion resistance in coexistence with Cr. However, a desired effect cannot be achieved when the content of Mo is less than 8%, and the workability is rapidly reduced and a brittle σ phase is likely to be generated when the content of Mo exceeds 12%. For this reason, the content of Mo in the present invention is 8 to 12%, favorably 9 to 11%. As a result, the effect of further improving the strength and ductility is achieved.
When the total content of Cr, Mo, and the unavoidable impurity is less than 20% with the entire cobalt-chromium alloy material as 100%, the hexagonal lattice (hcp) phase becomes stable. When the total content exceeds 40%, the face-centered cubic lattice (fcc) phase becomes unstable and a body-centered cubic lattice (bcc) phase tends to appear. That is, in the case where the total content of Cr, Mo, and the unavoidable impurity is not 20 to 40%, the fcc phase is difficult to stabilize, the fcc twin deformation and the deformation-induced transformation from fcc to hcp are difficult to occur when the cobalt-chromium alloy member thus obtained is deformed, and excellent ductility and a low cycle fatigue life cannot be achieved. For this reason, the total content of Cr, Mo, and the unavoidable impurity in the present invention is 20 to 40%, favorably 23 to 38%. As a result, excellent ductility and a low cycle fatigue life can be achieved.
Note that the content of the unavoidable impurity may be 0%. In the case where the content of the unavoidable impurity exceeds 0%, the composition ratio of the unavoidable impurity is adjusted such that the total is 100% with reference to the composition ratio of Co, Ni, Cr, and Mo.
Cr (chromium) is an essential constituent for achieving the corrosion resistance and has an effect of strengthening the matrix. In the case where the unavoidable impurity is 0%, the content of Cr in the present invention is favorably 12 to 28%, more favorably 14 to 27%, and still more favorably 18 to 22%. Excellent corrosion resistance is likely to be achieved when the content of Cr is 12% or more, and it is difficult for the workability and toughness to rapidly decrease when the content of Cr is 28% or less. As a result, more excellent corrosion resistance can be achieved while achieving the workability and toughness.
Ti (titanium) has effects of strong deoxidizing, denitrifying, and desulfurizing, but inclusions increase in an alloy and a n phase (Ni3Ti) is precipitated to reduce the toughness when too much Ti is contained. For this reason, the content of Ti in the present invention is desirably 1.0% or less as the unavoidable impurity.
Mn (manganese) has effects of deoxidizing, desulfurizing, and stabilizing the face-centered cubic lattice phase, but the corrosion resistance and the oxidation resistance are deteriorated when too much Mn is contained. For this reason, the content of Mn in the present invention is desirably 1.5% or less. The upper limit thereof as the unavoidable impurity is more desirably 1.0% or less.
Fe (iron) has the functions of stabilizing the face-centered cubic lattice phase and improving the workability, but the oxidation resistance is reduced when too much Fe is contained. For this reason, the content of Fe in the present invention is desirably 1.0% or less as the unavoidable impurity.
C (carbon) has the effects of forming a carbide with Cr, Mo, or the like to prevent coarsening of crystal grains as well as being dissolved in the matrix, but the toughness is reduced and the corrosion resistance is deteriorated when too much C is contained. For this reason, the content of C in the present invention is desirably 0.1% or less.
Nb (niobium) has the effects of being dissolved in the matrix to strengthen this and increasing the work hardenability, but a σ phase and a 8 phase (Ni3Nb) are precipitated to reduce the toughness when the content of Nb exceeds 3.0%. For this reason, the content of Nb in the present invention is desirably 3.0% or less. The upper limit thereof as the unavoidable impurity is more desirably 1.0% or less.
W (tungsten) has the effects of being dissolved in the matrix to strengthen this and significantly increasing the work hardenability, but a σ phase is precipitated to reduce the toughness when the content of W exceeds 5.0%. For this reason, the content of W in the present invention is desirably 5.0% or less. The upper limit thereof as the unavoidable impurity is more desirably 1.0% or less.
Al (aluminum) has the effects of deoxidizing and improving the oxidation resistance, but the corrosion resistance is deteriorated when too much Al is contained. For this reason, the content of Al in the present invention is desirably 0.5% or less.
Zr (zirconium) has the effect of increasing the grain boundary strength at high temperatures to improve the hot workability, but the workability is deteriorated conversely when too much Zr is contained. For this reason, the content of Zr in the present invention is desirably 0.1% or less.
B (boron) has the effect of improving the hot workability, but the workability is reduced conversely and cracks easily occur when too much B is contained. For this reason, the content of B in the present invention is desirably 0.01% or less.
(Cobalt-Chromium Alloy as-Processed Material)
A cobalt-chromium alloy as-processed material according to the present invention is obtained by causing the cobalt-chromium alloy material described above to be subjected to cold working into a predetermined shape.
In the present invention, an fcc deformation twin and an hcp phase (& phase) are introduced by a twin deformation and an induced transformation during cold working, and a belt-like deformed band tissue having high density is formed. As a result, very high strength is achieved. In addition, in the present invention, cold working refines the crystal grains and makes it easier to achieve higher strength.
The predetermined shape is not particularly limited, but is favorably a tubular shape or a wire shape, for example. This allows for use in a medical or aerospace device having a tubular shape or a wire shape.
The cobalt-chromium alloy member according to the present invention is obtained by performing specific heat treatment at a crystallization temperature or more on the cobalt-chromium alloy as-processed material described above.
The cobalt-chromium alloy member according to the present invention has a composition similar to that of the cobalt-chromium alloy material described above, has a composition of, in terms of mass %, 23 to 32% of Ni, 37 to 48% of Co, and 8 to 12% of Mo, a remainder thereof containing Cr and an unavoidable impurity, the composition satisfying a relationship of 20≤[Cr %]+[Mo %]+[unavoidable impurity %]≤40, and favorably has a composition of, in terms of mass %, 25 to 29% of Ni, 37 to 48% of Co, and 9 to 11% of Mo, a remainder thereof containing Cr and an unavoidable impurity, the composition satisfying a relationship of 23≤[Cr %]+[Mo %]+ [unavoidable impurity %]≤38.
The unavoidable impurity favorably contains, in terms of mass %, 1.0% or less of Ti, 1.0% or less of Mn, 1.0% or less of Fe, 1.0% or less of Nb, 1.0% or less of W, 0.5% or less of Al, 0.1% or less of Zr, 0.01% or less of B, and 0.1% or less of C as contents of Ti, Mn, Fe, Nb, W, Al, Zr, B, and C.
This makes it easy to achieve high work hardenability and excellent mechanical strength/ductility.
The cobalt-chromium alloy member according to the present invention has a crystal structure including a face-centered cubic lattice (fcc) or a crystal structure including a face-centered cubic lattice (fcc) and a hexagonal lattice (hcp).
That is, in the present invention, the fcc deformation twin or the hcp phase in the cobalt-chromium alloy as-processed material changes to the fcc phase by performing heat treatment. Due to the formation of the fcc phase, an fcc twin deformation or a deformation-induced transformation from fcc to hcp occurs again when the cobalt-chromium alloy member is deformed. The cobalt-chromium alloy member according to the present invention in which such a deformation or transformation occurs has excellent mechanical strength and excellent ductility.
The cobalt-chromium alloy member according to the present invention has a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less.
The KAM value is, for example, a local change in crystal orientation obtained by electron backscatter diffraction (EBSD) measurement, and can be represented as a local misorientation (Kernel Average Misorientation: KAM) defined by the following formula (1).
Here, αij indicates the crystal orientation difference between measurement points i and j. The specific calculation procedure is described in, for example, Kouei Sasaki, et. al., “Correlation between Microstructural Scale Plastic Strain and Misorientation”, Journal of the Japan Institute of Metals, 74 (2010), pp. 467-474. The KAM value takes a high value in a region where the density of lattice defects such as dislocations is high or in a region where the curvature of the crystal lattice plane is significant.
The strain distribution within crystal grains can be evaluated by the KAM value. In the present invention, since the KAM value is as low as 0.0 or more and 1.0 or less and the density of lattice defects such as dislocations is low, the cobalt-chromium alloy member according to the present invention has excellent mechanical strength. Further, since the crystal grains are easily made uniform, homogenized mechanical properties with excellent crystallinity are easily achieved.
The average value of the crystal grain size of the cobalt-chromium alloy member according to the present invention is 2 μm or more and 15 μm or less, favorably 4 μm or more and 15 μm or less, and more favorably 4 μm or more and 10 μm or less. This makes it easy to achieve high mechanical strength.
The average value of a crystal grain size is calculated by the area-fraction method using EBSD. In detail, the average value of a crystal grain size can be calculated in accordance with JIS G0551 “Steels-Micrographic determination of the apparent grain size” or ASTM E112-13 “Standard Test Methods for Determining Average Grain Size”.
The cobalt-chromium alloy member according to the present invention has a tensile strength of 800 to 1200 MPa.
The cobalt-chromium alloy member has a breaking elongation of 30 to 80%, favorably 30 to 60%, and still more favorably 50 to 60%.
The tensile strength and the breaking elongation are measured by, for example, a tensile test using an autograph manufactured by Shimadzu Corporation.
The cobalt-chromium alloy member having the physical properties described above has excellent mechanical strength and excellent ductility.
The cobalt-chromium alloy member favorably has a uniform elongation of 25 to 60%, more favorably 30 to 60%, and still more favorably 50 to 60%.
The uniform elongation is measured by, for example, a tensile test using an autograph manufactured by Shimadzu Corporation.
The cobalt-chromium alloy member having the physical properties described above has more excellent mechanical strength and ductility.
In particular, in the case where the cobalt-chromium alloy member according to the present invention has a tubular shape that is hollow inside with the circumferential surface surrounded with a cobalt-chromium alloy, it is favorable that the cobalt-chromium alloy member has an average value of a crystal grain size of 2 to 15 μm and a change amount in local crystal orientation (KAM value) of 0.1 or more and 0.8 or less and exhibits a tensile strength of 800 to 1000 MPa and a breaking elongation of 30 to 80%.
In the case where the cobalt-chromium alloy member according to the present invention has a wire shape whose cross-sectional shape is a circular cross section, an oval cross section, a flat cross section, a recessed or projecting irregular cross section, or the like, it is favorable that the cobalt-chromium alloy member has an average value of a crystal grain size of 4 to 15 μm and a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less and exhibits a tensile strength of 1000 to 1200 MPa and a breaking elongation of 30 to 60%.
As a result, higher strength and excellent ductility are achieved.
The cobalt-chromium alloy member according to the present invention is favorably obtained by heat treatment under the following conditions.
The temperature of the heat treatment according to the present invention favorably exceeds the recrystallization temperature of the cobalt-chromium alloy material and is favorably 1100° C. or less, and is more favorably 800° C. or more and 1100° C. or less, and still more favorably 900° C. or more and 1100° C. or less. The recrystallization temperature of the cobalt-chromium alloy material is, for example, within the range of 780° C. to 820° C. in the case of a Co-20Cr-10Mo-26Ni alloy having the composition according to this Example, but is within the range of 750° C. to 1000° C. depending on the alloy composition of the cobalt-chromium alloy material.
By setting the temperature to the recrystallization temperature or more, recrystallization is performed and the fcc phase is stabilized. By setting the temperature to 1100° C. or less, the coarsening of the crystal grain size is suppressed.
As a result, a cobalt-chromium alloy member having a tensile strength, a uniform elongation, and a breaking elongation in the range described above, high mechanical strength, and high ductility is obtained.
The time of the heat treatment according to the present invention is favorably 1 minute or more and 60 minutes or less. By setting the time to 1 minute or more, recrystallization is sufficiently performed and the fcc phase is stabilized. By setting the time to 60 minutes or less, the coarsening of the crystal grain size is suppressed.
As a result, a cobalt-chromium alloy member having a tensile strength, a uniform elongation, and a breaking elongation in the range described above, high mechanical strength, and high ductility is easily obtained.
In particular, the cobalt-chromium alloy member is favorably obtained by performing heat treatment for 1 minute or more and 60 minutes or less at 800° C. or less and 1100° C. or less as heat treatment performed at a heat treatment temperature exceeding the recrystallization temperature of the cobalt-chromium alloy material on the cobalt-chromium alloy as-processed material.
The cobalt-chromium alloy member according to the present invention may include a belt-like deformed band tissue. The belt-like deformed band tissue according to the present invention is an aggregate tissue of dislocation cells in which a large number of dislocations generated by cold working are densely packed and is a tissue in the vicinity of the fcc deformation twin and the hcp phase (ε phase) introduced during cold working.
The cobalt-chromium alloy member according to the present invention has a low stacking fault energy and achieves high work hardenability by the movement of partial dislocations and the formation of fine plate-like fcc twins and hcp phases during deformation. Further, high work hardenability is exhibited because solute atoms such as Mo and Nb having large or approximate atomic radii are strongly attracted to dislocation cores or stacking faults of extended dislocations to segregate, making it difficult for cross slip to occur, as compared with Co, Ni, and Cr whose atomic radius is 1.25 Å.
Further, since the high work hardenability of the cobalt-chromium alloy member according to the present invention is exhibited not only at temperatures around the body temperature but also at high temperatures, it has characteristics of high-temperature strength properties. Therefore, the use of the cobalt-chromium alloy member is not limited to medical use, and the cobalt-chromium alloy member according to the present invention withstands used under more severe conditions as industrial equipment use such as aerospace use and steam turbine use.
A method of producing the cobalt-chromium alloy member includes a step of preparing a cobalt-chromium alloy material; a step of homogenizing the prepared cobalt-chromium alloy material at 1100° C. to 1300° C.; a step of causing the homogenized cobalt-chromium alloy material to be subjected to cold plastic working into a tubular shape or a wire shape to obtain a cobalt-chromium alloy as-processed material; and a step of performing heat treatment for 1 minute or more and 60 minutes or less at a temperature exceeding a recrystallization temperature of the cobalt-chromium alloy material and not more than 1100° C. on the cobalt-chromium alloy as-processed material that has been subjected to the cold plastic working. The recrystallization temperature of the cobalt-chromium alloy material is, for example, 800° C.
As a result, a cobalt-chromium alloy member having high mechanical strength and high ductility is obtained.
In the step of preparing a cobalt-chromium alloy material, the cobalt-chromium alloy material described above is used.
In the step of causing the homogenized cobalt-chromium alloy material to be subjected to cold plastic working, the cobalt-chromium alloy as-processed material that has been subjected to cold working into a tubular shape or a wire shape is obtained.
In the step of performing heat treatment on the cobalt-chromium alloy as-processed material, the cobalt-chromium alloy member described above is obtained.
In the homogenizing, by performing heat treatment on the cobalt-chromium alloy material at 1100° C. to 1300° C., each composition is uniformly dispersed. As a result, the uniformity of the mechanical properties is achieved in the cold working in the subsequent step.
By setting the homogenizing temperature to 1100° C. or more, it is possible to efficiently homogenize the material. By setting the homogenizing temperature to 1300° C. or less, it is possible to prevent the excessive coarsening of crystal grains and significant oxidation of the material surface. Other homogenizing conditions can be appropriately set within the range that does not impair the physical properties of the cobalt-chromium alloy member to be obtained.
The cobalt-chromium alloy material to be homogenized only needs to be a cobalt-chromium alloy material having the specific composition described above, and may be, for example, an alloy ingot prepared by high-frequency melting.
Further, the homogenized cobalt-chromium alloy material may be subjected to hot working into a shape that is easy to apply cold working, such as a round bar.
Further, in the method of producing a cobalt-chromium alloy member according to the present invention, after performing heat treatment at a temperature of the recrystallization temperature or more and 1100° C. or less on a cobalt-chromium alloy as-processed material obtained by causing a cobalt-chromium alloy material to be subjected to cold working into a plate material for a stent, aging treatment at a temperature of 200° C. or more and the recrystallization temperature or less may be performed. As a result, higher strength properties are achieved by so-called static strain aging, in which solute atoms such as Mo are attracted to dislocation cores or stacking faults of extended dislocations to fix the dislocations.
The cobalt-chromium alloy material according to the present invention is obtained by preparing an alloy ingot having a composition similar to that of the cobalt-chromium alloy material described above by high-frequency melting, performing hot forging and homogenizing thereon at 1100° C. to 1300° C., and producing a round bar having a diameter of 8 mm and a length of 270 mm by hot rolling and cutting.
By the production method according to the present invention described above, a cobalt-chromium alloy member characterized by having a crystal structure including a face-centered cubic lattice (fcc) or a crystal structure including a face-centered cubic lattice (fcc) and a hexagonal lattice (hcp), an average value of a crystal grain size of 2 to 15 μm, and a change amount in local crystal orientation (KAM value) of 0.0 or more and 1.0 or less is obtained.
A First Example of the present invention is a member having a tubular shape using the cobalt-chromium alloy material according to the present invention.
That is, the cobalt-chromium alloy material described above was subjected to cold working to obtain a tube material having a diameter of 1.6 mm, a thickness of 0.1 mm, and a length of 1 m. This tube material corresponds to a cobalt-chromium alloy as-processed material. Further, predetermined heat treatment was performed on this tube material to impart ductility thereon, thereby obtaining a cobalt-chromium alloy member as a tube material.
The composition of the cobalt-chromium alloy material used in this Example is shown in Table 1. The unit is mass %.
In the Examples 1 to 4, the contents of Cr and Mo were respectively made constant, i.e., 20 mass % and 10 mass %, and the content of Co was changed with respect to the content of Ni. The content of Ni was changed in the range of 23 to 32 mass %.
In the Comparative Examples 1 to 4, a commercially available Co-20Cr-10Mo-35Ni alloy (hereinafter, referred to simply as “MP35N alloy”), a Co-20Cr-10Mo-20Ni alloy, a Co-20Cr-15W-10Ni alloy (hereinafter, referred to simply as “L605 alloy”), and SUS316L (manufactured by Hayes) were used as comparative materials, respectively. In the Examples 1 to 4, it was confirmed that they had a crystal structure including a face-centered cubic lattice (fcc) or a crystal structure including a face-centered cubic lattice (fcc) and a hexagonal lattice (hcp).
A low cycle fatigue at a strain amplitude of 0.01 was performed on the cobalt-chromium alloy materials having the compositions according to the Examples 1 to 4 and the alloys having the compositions according to the Comparative Examples 1 to 4, which have been subjected to hot working into a rod shape and then heat treatment for 1 minute at 1200° C.
The test results are shown in
Further, a tensile test at a strain rate of 2.5×10−4 s−1 was performed using a tensile tester manufactured by A&D Company, Limited on the cobalt-chromium alloy materials having the compositions according to the Examples 1 to 4 and the alloys having the compositions according to the Comparative Examples 1 to 4, which have been subjected to hot working into a rod shape and then heat treatment for 1 minute at 1200° C., and the results thereof were shown in Table 2. In the cobalt-chromium alloy materials according to the Examples 1 to 4, tensile strengths of 848 to 886 MPa were shown and high tensile strength peculiar to the cobalt-chromium alloys equivalent to that of the MP35N alloy (Comparative Example 1) were shown.
Further, Table 3 shows the 0.2% proof stress [MPa], the tensile strength [MPa], and the breaking elongation [%] obtained from
What appears to be linear in
The as-processed material and the 650° C./750° C. heat-treated material have a high dislocation density, the processed structure remains, and the average KAM value is as high as 1 or more. However, the heat-treated material obtained by performing heat treatment at a temperature of 850° C. or more has a KAM value as low as 1 or less, and has a recrystallized structure with a low defect density. Further, as the heat treatment temperature increases, the crystal grain size increases.
That is, while the KAM value of the as-processed material is 1.32±0.74, the KAM value of the 650° C. heat-treated material treated at a temperature lower than the recrystallization temperature is 1.26±0.71, and the KAM value of the 750° C. heat-treated material is 1.25±0.69. Meanwhile, the KAM value of the 850° C. heat-treated material treated at a temperature higher than the recrystallization temperature is 0.48±0.30, the KAM value of the 950° C. heat-treated material is 0.47±0.30, and the KAM value of the 1050° C. heat-treated material is 0.32=0.15.
While the average crystal grain size of the as-processed material is 5.1 μm, the average crystal grain size of the 650° C. heat-treated material treated at a temperature lower than the recrystallization temperature is 5.3 μm, and the average crystal grain size of the 750° C. heat-treated material is 4.3 μm. Meanwhile, the average crystal grain size of the 850° C. heat-treated material treated at a temperature higher than the recrystallization temperature is 2.3 μm, the average crystal grain size of the 950° C. heat-treated material is 3.2 μm, and the average crystal grain size of the 1050° C. heat-treated material is 7.6 μm. In the heat treatment at 850° C., fine crystal grains of 2.3 μm have been obtained by recrystallization.
A Second Example of the present invention is a member having a wire shape using the cobalt-chromium alloy material according to the Example 3 of the present invention. That is, the cobalt-chromium alloy material was subjected to cold working to obtain a wire material having a diameter of 0.5 mm and a length of 1 m. This wire material corresponds to a cobalt-chromium alloy as-processed material. Further, predetermined heat treatment was performed on this wire material to impart ductility thereon, thereby obtaining a cobalt-chromium alloy member as a wire material.
Table 4 shows the tensile strength [MPa] and the breaking elongation [%] of the cobalt-chromium alloy as-processed material that has been subjected to cold working into a wire shape according to the present invention, the wires as the cobalt-chromium alloy members obtained by performing heat treatment for 5 minutes at 450° C., 650° C., 850° C., and 1050° C. on the as-processed material having this wire shape, and the wire of the comparative material.
Table 5 compares the tensile strength and the breaking elongation of the wire as the cobalt-chromium alloy member according to an Example of the present invention with those of the SUS316L, the L605 alloy, and the MP35N alloy. The cobalt-chromium alloy member prepared in Table 5 was prepared under the same conditions as those of the cobalt-chromium alloy member prepared in Table 4, and similar results were obtained.
Comparative Example SUS316L: a tensile strength of 480 MPa and a breaking elongation of 40%
The numerical values (%) of the “Comparative Example L605” and “Comparative Example MP35N” in Table 5 indicate the rate of cold working.
The wire as a cobalt-chromium alloy member according to an Example of the present invention exhibited a strength exceeding that of SUS316L that is most widely used as a guide wire and exhibited a tensile strength and a breaking elongation comparable to those of the wires of the L605 alloy and MP35N (
That is, while the KAM value of the as-processed material is 1.76±0.93, the KAM value of the 450° C. heat-treated material treated at a temperature lower than the recrystallization temperature is 2.34±1.07 and the KAM value of the 650° C. heat-treated material is 2.04±1.05. Meanwhile, the KAM value of 850° C. heat-treated material treated at a temperature higher than the recrystallization temperature is 0.33±0.43, and the KAM value of the 1050° C. heat-treated material is 0.96±0.61.
The as-processed material and the 450° C./650° C. heat-treated material have a KAM value as high as 1.76 to 2.01 and a high dislocation density, and the processed structure remains. The samples obtained by performing heat treatment at a temperature of 850° C. or more, which is a temperature higher than the recrystallization temperature, have a reduced KAM value of 1 or less and a reduced density of defects such as dislocations.
That is, while the average crystal grain size of the as-processed material is 9.04 μm, the average crystal grain size of the 450° C. heat-treated material treated at a temperature lower than the recrystallization temperature is 10.3 μm and the average crystal grain size of the 650° C. heat-treated material is 7.78 μm. Meanwhile, the average crystal grain size of the 850° C. heat-treated material treated at a temperature higher than the recrystallization temperature is 4.43 μm, and the average crystal grain size of the 1050° C. heat-treated material is 12.1 μm. In the heat treatment at 850° C. that is a temperature higher than the recrystallization temperature, fine crystal grains of 4.4 μm have been obtained by recrystallization.
As described above in detail, a cobalt-chromium alloy member having high strength and high ductility is obtained by preparing a cobalt-chromium alloy material having an alloy composition according to the present invention into a predetermined shape such as a tube and a wire by cold working and then performing heat treatment exceeding the recrystallization temperature of the cobalt-chromium alloy material thereon. Such a cobalt-chromium alloy member is suitable for use in a medical device, a device for gas turbines, or a device for other industrial equipment because it uses a cobalt-chromium alloy member having a long fatigue life.
Examples of the medical device include an indwelling medical device such as a stent, a catheter, a fastening cable, a guide rod, an orthopedic cable, a heart valve, and an implant. As another medical device, it can be used as a bone drill bit and a gallstone removal wire.
Examples of the device for gas turbines include a combustor and an exhaust component for aviation and industrial gas turbine engines, such as a tail tube, a combustion tube, a spray bar, a frame holder, an afterburner, and a tail pipe. Examples of the device for industrial equipment include those used in waste incinerators, boilers, high-temperature reaction vessels, rotary calciners, and a production plant and a synthesis gas plant of petrochemical products.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-137738 | Aug 2021 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/031532 | 8/22/2022 | WO |
