Metal treatment – Stock – Titanium – zirconium – or hafnium base
Reexamination Certificate
1999-11-22
2001-11-20
Sheehan, John (Department: 1742)
Metal treatment
Stock
Titanium, zirconium, or hafnium base
C148S669000, C420S420000
Reexamination Certificate
active
06319340
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a superelastic alloy not containing nickel, more specifically to a Ti—V—Al based superelastic alloy which has high corrosion resistance and is lightweight and to processes for its preparation.
BACKGROUND ART
Known as functional alloys exhibiting superelasticity are Au—Cd alloys, Cu—Zn—Al alloys, Cu—Al—Ni alloys, Ni—Ti alloys and the like.
Of those functional alloys, Ni—Ti alloys have attracted attention, for example, as orthopedic implant materials due to their superiority in corrosion resistance. However, Ni—Ti alloys, in contact with body tissue, are known to cause inflammation to the contact part. Moreover, the effect of dissolved Ni ions on body tissue (e.g., carcinogenesis) has not been fully studied and safety has not yet been confirmed. Therefore, Ni—Ti alloys cannot be implanted into the body as they are (i.e., without coating). Furthermore, since nickel in Ni-containing articles such as pierced earrings, wristwatch bands or the like, which are used in contact with the skin, dissolves in sweat and induces allergies, it might be highly dangerous to use Ni—Ti alloys as implant materials.
Known as Ni-free shape memory alloys are Ti alloys (U.S. Pat. No. 4,412,872), etc. However, there are no known Ni-free superelastic alloys that are easy to use as structural materials for industrial use and have high corrosion resistance and improved workability.
OBJECT OF THE INVENTION
A primary object of the present invention is to provide a Ni-free superelastic alloy that is easy to use as a structural material for industrial use and has high corrosion resistance and improved workability.
DISCLOSURE OF THE INVENTION
In view of the state of the prior art, the present inventors carried out extensive research and found that Ti—V—Al based alloys having a specific composition range exhibit superelasticity over a broad range of temperatures and have various excellent properties such as high corrosion resistance and improved workability.
The present invention provides the following Ti—V—Al based alloy and processes for its preparation.
1. A Ti—V—Al based superelastic alloy wherein the proportions of Ti, Al and V, based on the total weight of the three components, fall within the range defined by the lines joining the following points of A, B, C and D shown in the ternary composition diagram of accompanying FIG.
1
:
A: 79.8% Ti, 17.5% V, 2.7% Al,
B: 76.8% Ti, 20.5% V, 2.7% Al,
C: 73.8% Ti, 20.5% V, 5.7% Al,
D: 76.8% Ti, 17.5% V, 5.7% Al.
2. A process for preparing a Ti—V—Al based superelastic alloy, which comprises melt-forming an alloy wherein the proportions of Ti, Al and V, based on the total weight of the three components, fall within the range defined by the lines joining the following points of A, B. C and D shown in the ternary composition diagram of accompanying
FIG. 1
, followed by heat treatment at 800° C. to 1200° C. and quenching:
A: 79.8% Ti, 17.5% V, 2.7% Al,
B: 76.8% Ti, 20.5% V, 2.7% Al,
C: 73.8% Ti, 20.5% V, 5.7% Al,
D: 76.8% Ti, 17.5% V, 5.7% Al.
3. A process for preparing a Ti—V—Al based superelastic alloy, which comprises melt-forming an alloy wherein the proportions of Ti, Al and V, based on the total weight of the three components, fall within the range defined by the lines joining the following points of A, B, C and D shown in the ternary composition diagram of accompanying
FIG. 1
, followed by heat treatment at 800° C. to 1200° C. and quenching and then aging the alloy at temperatures no higher than 200° C.:
A: 79.8% Ti, 17.5% V, 2.7% Al,
B: 76.8% Ti, 20.5% V, 2.7% Al,
C: 73.8% Ti, 20.5% V, 5.7% Al,
D: 76.8% Ti, 17.5% V, 5.7% Al.
The superelastic alloy of the present invention is prepared by melt-forming a basic alloy according to a conventional method, the proportions of Ti, Al and V in the alloy being selected from the range defined by the lines joining the points of A (79.8% Ti, 17.5% V, 2.7% Al), B (76.8% Ti, 20.5% V, 2.7% Al), C (73.8% Ti, 20.5% V, 5.7% Al) and D (76.8% Ti, 17.5% V, 5.7% Al) shown in the ternary composition diagram of accompanying
FIG. 1
, followed by subjecting the basic alloy to heat treatment at 800° C. to 1200° C. and quenching.
The superelastic alloy according to the present invention may contain unavoidable impurities as long as the properties of the alloy are not impaired.
According to the present invention, &bgr; phase formed by heat treatment is prevented from transforming to &agr; phase, &agr;+&bgr; phase, &ohgr; phase or the like by quenching the Ti—V—Al alloy from the heat treatment temperature. Therefore, the alloy exhibits satisfactory superelasticity over a broad range of temperatures from liquid nitrogen temperature to about 80° C.
There is no specific limitation on the alloy-quenching method and quenching rate. A typical alloy-quenching method comprises immersing an ally ingot in a sufficient amount of water to quench the ingot from the heat treatment temperature. For example, when 30 g of a button-shaped alloy ingot obtained according to an embodiment of the present invention is immersed in about 20° C. water, the quenching rate is at least about 300° C./sec.
Especially, when the thus obtained Ti—V—Al based superelastic alloy of the present invention is further aged within the temperature range where &ohgr; phase does not form (about 200° C. to about −30° C.), the alloy exhibits superelasticity throughout a broad range of temperatures from about 80° C. to about liquid nitrogen temperature (−196° C.).
To improve cold workability, it is preferable for the superelastic alloy of the present invention to have a Ti—Al—V composition selected from the range defined by the lines joining the points of A (79.8% Ti, 17.5% V, 2.7% Al), B (76.8% Ti, 20.5% V, 2.7% Al), E (75.0% Ti, 20.5% V, 4.5% Al) and F (78.0% Ti, 17.5% V, 4.5% Al) shown in the ternary Ti—Al—V composition diagram of FIG.
1
.
To reduce costs without substantially impairing the properties of the alloy, it is preferable for the superelastic alloy of the present invention to have a Ti—Al—V composition selected from the range defined by the lines joining the points of A (79.8% Ti, 17.5% V, 2.7% Al), G (77.8% Ti, 19.5% V, 2.7% Al), H (74.8% Ti, 19.5% V, 5.7% Al) and D (76.8% Ti, 17.5% V, 5.7% Al).
To improve cold workability and reduce costs, it is preferable for the superelastic alloy of the present invention to have a Ti—Al—V composition selected from the range defined by the lines joining the points of A (79.8% Ti, 17.5% V, 2.7% Al), G (77.8% Ti, 19.5% V, 2.7% Al), I (76.0% Ti, 19.5% V, 4.5% Al) and F (78.0% Ti, 17.5% V, 4.5% Al).
The Ti—V—Al based superelastic alloy of the present invention has high corrosion resistance similarly to Ti—V—Al alloys that do not have superelastisity but are useful as aircraft materials.
As compared with Ti—Ni based superelastic alloys, the superelastic alloy of the invention is lightweight.
Further, since the superelastic alloy of the present invention is comparatively low in hardness and has improved press workability, end cracking and warping associated with cold working are less likely to occur.
Still further, like known Ti—V—Al based alloys utilized as implant materials, the superelastic alloy of the present invention is highly safe for use within the body and does not cause inflammation to body tissues such as the skin, even when in contact with the body tissue for a long period of time.
In the substantially ternary superelastic alloy according to the present invention, aging can be facilitated by either adding at least one of elements such as Cr, Nb, Mo and the like to the alloy, or replacing a part of Al present in the alloy with Si and/or Ge (a quaternary or higher alloy). In more detail, if the quaternary alloy is aged under the conditions where the martensitic transformation occurs while austenite as the parent phase is dominant, the alloy acquires superelasticity and has the same functional properties as the ternary alloy of the present invention.
Of conventional Ti—V—Al alloys and other Ti alloys, there are no known functional alloys that acquire s
Inoue Kanryu
Tada Hiroyuki
Takeuchi Mikio
Larson & Taylor PLC
Sheehan John
LandOfFree
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