Metal treatment – Stock – Mechanical memory
Reexamination Certificate
1999-03-05
2001-07-10
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Mechanical memory
C148S421000, C148S671000, C420S418000, C420S420000
Reexamination Certificate
active
06258182
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of metallurgy and the uses thereof, and, more particularly to shape memory alloys which are particularly suitable for medical uses and which do not use nickel.
BACKGROUND OF INVENTION
Shape Memory Effect and Pseudo-elasticity
Materials which undergo martensite transformation may exhibit “Shape Memory Effect” and “Pseudo-elasticity.” During the transformation on cooling, the high temperature phase known as“austenite” changes its crystalline structure through a diffusionless shear process adopting a less symmetrical structure called “martensite”, and, on heating, the reverse transformation occurs. The starting temperature of the cooling transformation is referred to as the M
s
temperature and the finishing temperature, M
f
. The starting and finishing temperatures of the reverse transformation on heating are referred to as A
s
and A
f
respectively.
Materials exhibiting Shape Memory Effect can be deformed in their martensitic phase and upon heating recover their original shapes. These materials can also be deformed in their austenitic phase above the A
f
temperature through stress-induced martensitic transformation and recover their original shapes upon unloading. This strain recovery, referred to as“pseudo-elasticity” [sometimes referred to herein as “PE”] is associated with the reversion of stress-induced martensite back to austenite. A well known shape memory alloy is nitinol, a near-stoichiometric alloy of nickel and titanium.
The Alloy Material
Pure titanium has an isomorphous transformation at 882° C. The body centered cubic (bcc) structure, so called &bgr;-Ti, is stable above the isomorphous point and the hexagonal close packed (hcp) structure, so called &agr;-Ti, is stable below. When alloyed with elements such as vanadium, molybdenum, or niobium, the resulting alloys have an extended &bgr; phase stability below 882° C. On the contrary, when alloyed with elements such as Al or oxygen, the temperature range of stable &agr; phase extends above the isomorphous point. Elements which have the effect of extending the &bgr; phase temperature range are called the &bgr; stabilizers while those capable of extending the &agr; phase temperature range are called the a stabilizers.
For alloys with a high enough concentration of &bgr; stabilizer elements, the material would be sufficiently stabilized to obtain a meta-stable &bgr; phase structure at room temperature. The alloys showing such a property are called &bgr; titanium alloys. Martensite transformations are commonly found among &bgr; titanium alloys. The M
S
temperatures in &bgr;-Ti alloys decrease with increasing amount of &bgr; stabilizer in the alloys, while increasing amount of &agr; stabilizer raises the M
S
. The dependence of M
S
on the concentration of some transition metals in binary titanium alloys is shown in
FIG. 14
[‘The Martensite Transformation Temperature in Titanium Binary Alloys’, Paul Duwez, Trans. ASM, vol. 45, pp.934-940, 1953]. Therefore, depending on the extent of stabilization, &bgr;-Ti alloys may exhibit martensitic transformation when cooled very quickly from temperatures above the &bgr; transus, the temperatures above which &bgr; is the single phase at equilibrium.
To exhibit PE at room temperature, the alloys must be sufficiently &bgr; stabilized to have the A
f
point suppressed to below the ambient, but still allow the formation of stress-induced martensite before plastic deformation occurs. That is, the stress level for the martensite to form must be lower than that of plastic deformation. Shape memory effect, on the other hand, is observed when an alloy has an A
s
point higher than and M
S
temperature slightly below room temperature. Stress-induced martensite transformations have also been observed in &bgr; titanium alloys [‘Formation and Reversion of Stress Induced Martensite in Ti-10V-2Fe-3A1’, T. W. Duerig, J Albrecht, D. Richter and P. Fischer, Acta Metall., vol. 30, pp.2161-2172, 1982].
Both shape memory effect and pseudo-elasticity have been observed in certain Ti—Mo—Al &bgr; titanium alloys [‘Shape Memory Effect in Ti—Mo—Al Alloys’, Hisaoki Sasano and Toshiyuki Suzuki, Proc. 5th Int. Conf. on Titanium, Munich, Germany, pp.1667-1674, 1984]. In order to obtain SME or PE at room temperature the material has to be properly heat treated to produce the uniform &bgr; phase structure. The heat treatment to achieve that goal is called a solution treatment in which the test sample is heated to temperatures slightly above the &bgr; transus for a period of time long enough to allow for full austenization and then immediately cooled to room temperature.
Some &bgr;-Ti alloys, for example, TMA (Registered trade mark of Ormco, Glendora, Calif.), has been successfully commercialized for orthodontic arch wire application. The detailed description of the applications and properties of &bgr; titanium wires can be found in U.S. Pat. No. 4,197,643. The TMA wires show a unique balance of low stiffness, high spring-back, good formability [‘Beta titanium: A new orthodontic alloy’, C. Burstone and A. Jon Goldberg, American Journal of orthodontics, pp.121-132, Feb. 1980], and weldability. [‘Optimal welding of beta titanium orthodontic wires’, Kenneth R. Nelson et al, American Journal of Orthodontics and Dentofacial Orthopedics, pp.213-219, Sept., 1987] The nickel-free chemistry of the alloy makes it more tolerable to some patients. However, TMA wires utilize the inherent mechanical properties of the material through thermo-mechanical processing. The material does not exhibit PE due to the occurrence and reversion of stress-induced martensite in the material.
SUMMARY OF THE PRESENT INVENTION
An object of the present invention is to provide a titanium nickel-free SME alloy which is particularly useful for medical applications.
Another object of the present invention is to provide an alloy having pseudo-elastic properties and which is useful for medical applications.
A further object of the present invention is to provide super-elastic springs made from formable, weldable nickel-free shape memory alloy.
Still a further object of the present invention is to provide shape memory orthodontic springs having constant and readily controlled force.
Yet a further object of the present invention is to provide nickel-free shape memory or pseudo-elastic compositions for use in orthodontic, maxillo-facial and other surgical applications.
These and other objects of the present invention are accomplished by providing a nickel-free &bgr; titanium alloy characterized by exhibiting pseudo-elasticity at −25° C. to 50° C. or greater due to the formation and reversion of stress-induced martensite. Such an alloy exhibits SME at room temperature when the A
S
temperature is higher than room temperature. Furthermore, the alloy exhibits pseudo-elasticity with lower stiffness and force output magnitude than the conventional &bgr; titanium alloy TMA, better formability than Nitinol, the ability to be welded to other appliances, and good corrosion resistance.
It is capable of being cold worked to 20% without significantly reducing the pseudo-elastic performance, whereby it can be cold formed into various shapes at ambient temperature while retaining the high spring-back characteristics of the pseudo-elastic phenomenon, and it can be made so that it exhibits pseudo-elasticity at ambient and/or body temperature. The alloy can have a strain recovery of up to approximately 3.5% when tensile loaded to 4% strain at room temperature in the as-solution treated condition. The nickel-free &bgr; titanium alloy can be used for a medical device within a living body, such as an orthodontic arch wire, a stent, a catheter introducer, oral pins and/or plates used in maxillofacial reconstructive procedures, oviduct clamp, and bone staples, for example. It can also be used for eyeglasses. A nickel-free &bgr; titanium having super-elastic properties by being cold worked in its martensitic state, said alloy exhibiting
Burstone Charles J.
Loi Chi-Yuan
Schetky L. McDonald
Wu Ming H.
Cohen Jerry
Kaye Harvey
Memry Corporation
Perkins Smith & Cohen
Wyszomierski George
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