Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1996-11-19
1999-01-26
Wyszomierski, George
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148563, 148402, 148403, 433 20, 606228, C22C 1903, C22K 100
Patent
active
058633600
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method of surface treatment of a shape memory alloy, and in particular the surface hardening of a nickel-titanium NiTi shape memory alloy by shot peening.
The term shape memory alloy is used in this Specification to refer to an alloy which recovers from a deformed shape to a pre-formed, substantially stress-free shape on being subjected to certain conditions. Such alloys are known within the art and have a variety of uses. For example, NiTi alloy is used in surgical remote tissue suturing wherein the suture may conveniently be introduced into the body in a deformed shape but subsequently may recover the pre-formed shape by virtue of the superelastic properties of the material, thereby automatically having the required suturing effect.
One particular disadvantage is that shape memory alloys are relatively soft materials, with the result that they may for this reason be unsuitable for certain uses to which they might otherwise be applied. Moreover, any surface hardening of the material attempted to overcome this problem must naturally not cause loss of desirable bulk characteristics of the material such as the shape memory behaviour or, in the cited example, the biocompatibility of the material.
The present invention seeks to mitigate or obviate these or other difficulties.
According to the invention there is provided a method of surface treating a shape memory alloy in which method a sample of the alloy to be treated is shot peened to modify the structure of a surface layer of the alloy, whereby bulk material characteristics of the sample are substantially unaffected.
According to the invention there is further provided a method of surface treating a NiTi shape memory alloy by shot peening a sample of the alloy to cause a crystal to amorphous transition of a surface layer of the alloy, whereby bulk material characteristics of the sample are substantially unaffected.
The method may further comprise preliminary forming and annealing of the sample to establish the shape memory. The annealed sample may be polished prior to peening.
Preferably there is used glass peening media. The pressure of the air jet carrying the media may be from 3 to 7 bar. Preferably the sample is maintained at a constant distance from the jet nozzle. The peening time may be between 30 and 60 seconds.
The method may further comprise the step of ultrasonically cleaning the sample after peening.
According to the invention there is further provided a shape memory alloy which has been surface treated by a method according to any of the preceding five paragraphs.
According to the invention there is further provided a tissue suture which has been treated by a method as hereinbefore described.
According to the invention there is further provided dental apparatus which has been treated by a method as hereinbefore described. The apparatus may be an orthodontic device, and may in particular be a dental archwire.
According to the invention there is further provided a method of remote tissue suturing comprising the steps of providing an elongate suture of a shape memory alloy, constraining the suture in a required suturing configuration and so treating the suture as to create a substantially stress-free structure in the suturing configuration, re-forming the suture into a further configuration, introducing the suture into the required suturing site whilst restrained in the further configuration and subsequently permitting the suture to return to the suturing configuration by virtue of its shape memory characteristics thereby effecting remote tissue suturing.
The alloy is preferably a nominally equiatomic NiTi alloy. The alloy may be selected or modified to have a shape memory retention temperature greater than the maximum temperature to which the suture is expected to be subjected in use, for example during sterilisation.
Preferably the suture is wound around a suitable mandrel to form a helix of a predetermined diameter. The suture may be subjected to a stress-relieving heat treatment to form a substantially stress-
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patent: 5232361 (1993-08-01), Sachdeva et al.
Oshida, Y. et al, "Effects of Shot-Peening on Surface Contact Angles of Biomaterials," Journal of Materials Science: Materials in Medicine, vol. 4, No. 5, 1993, pp. 443-447.
Koike, H. et al, "Crystal-to Amorphous Transformation of NiTi Induced by Cold Rolling," Journal of Materials Research, vol. 5, No. 7, Jul. 1990, pp. 1414-1418.
Grant David Malcolm
Green Sarah Margaret
Wood John Vivian
The University Nottingham
The University of Dundee
Wyszomierski George
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