Ti-Ni-C shape memory alloy with a pseudoelasticity and a wide elasticity range

Abstract: A shape memory alloy represented by a chemical formula or Ti a Ni b-c C c , a + b = 100, b = 50-52, c = 0.20-5.0, and having a pseudoelasticity at about 37 o C, and having an increased yield point of elasticity in comparison with Ti₅₀Ni₅₀.

Description: The present invention relates to a method of manufactoring a pseudoelastic spring alloy and, in particular, to improvement of an elasticity range of such an alloy. It is well known in the art that TiNi alloy and TiNiX alloy, where X is Cu, Cr, etc. indicates a remarkable shape memory effect. This effect is known to be caused by a reverse transition of thermoelastic martensitic transition. On the other hand, it is also well known that TiNi alloy has a pseudo elasticity which shows flexibility such as rubber. The pseudo elasticity is referred to as an unusual elastic behavior in a paper entitled "CRYSTAL STRUCTURE AND MICROSTRUCTURE OF A COLD WORKED TiNi ALLOY WITH UNUSUAL ELASTIC BEHAVIOR" by Tadaki et al, METALLURGICA, Vol. 14, 1980, Pergamon Press Ltd., pp. 911-914 (Reference 1). When a stress is loaded or applied to the TiNi alloy wire, the strain of the wire elastically increases by increase of the stress. When the stress exceeds a limit of the elasticity (yield point), the strain of the wire increases at the constant elasticity limit stress. Thereafter, when the stress is removed or unloaded, the strain also decreases with reduction of the internal stress due to discharge of the internal energy stored by the loaded force. Thereafter, the strain is rapidly decreased at a constant internal stress into its elastic condition or state, and then, elastically decreases to recover the original shape. Accordingly, the TiNi alloy is used in various applications, for example, eye glass frames, catheter guides, orthodontic implements and other spring parts. However, the TiNi alloy is usually low in the elasticity limit, and therefore, cannot be used for applications requiring a high elasticity limit and the pseudoelasticity. From JP-A-63-11637 a TiNi alloy is known in which intermediate-phase transformation appears by ageing treatment at ≤ 600°C, 0,5 ∼ 2,0 atomic percent C being substituted for a part of Ni. In a first heat treatment at 800°C a solution treatment is carried out, during a second heat treatment the alloy is heated for 40 hours at 400°C while mechanically restrained onto a copper pipe after the heat treatment. Such an alloy shows improvement in a property of shape memory effect. From JP-A-2-38547 an alloy constituted of 48,5 to 32,5 atomic percent Ni and the balance Ti is known wherein a part of Ti or Ni is substituted by one ore more kinds of C, O, and N. The alloy is subjected to work hardening and is restricted into a desired shape. In this condition it is subjected to a shape-memory heat treatment. From JP-A-1-56833 a Ti-Ni shape memory alloy is known having high density and excellent shape memory effects, by packing a powder mixture consisting of Ti, Ni, and additive carbon in a metallic cylindrical body. The cylindrical body is heat treated. From JP-A-59-150047 a shape memory alloy is known having small hysteresis in the transition of low temperature phase to high temperature phase transformation. Such a property is achieved by adding C, N, or O to shape memory Ti-Ni alloy. Therefore, it is the object of the present invention to provide a method of manufacturing a pseudo elastic spring alloy having a high elasticity limit and pseudo elastic properties. This object is achieved by the method as is defined in claim 1. Preferred developments of the method are given in claims 2 to 4. Preferred embodiments will be explained under reference to the accompanying figures. Fig. 1 shows stress to strain curves for the maximum 5% tension strain of Ti 50-x/2 Ni 50-x/2 C x alloys, where X = 0, 1, 2, 3; and Fig. 2 shows stress to strain curves for the maximum 5% tension strain of Ti 49.75-x/2 Ni 50.25-x/2 C x alloys, where X = 0, 1, 2, 3. The alloy according to the present invention comprises 0.20-5.0 at% C and the balance Ti and Ni, Ni/Ti = 1-1.08 (50/50-52/48). The addition of C (carbon) to the TiNi alloy lowers the austenitic and martensitic transition temperatures and also elevates the elasticity limit of the alloy. The minimum amount of C should be 0.2 at%. But, when C is added more than 5.0 at%, the cold working of the alloy is almost impossible. Therefore, the amount of C is limited 5.0 at% or less. In order to realize the austenitic and martensitic transition points below about 37°C and the pseudoelasticity at about 37°C, the total amount of Ni and C should be 50.0 at% or more. However, the maximum of the total amount is 55.0 at%, preferably 52.0 at%, because the increase of (Ni + C) amount makes the cold working of the alloy difficult. Examples Ingots of alloys containing various amounts of Ti, Ni and C were prepared by the melt method using a high frequency induction furnace. The ingots were reduced to wires of 1.3mm in diameter by means of hot working and cold working, thereafter solution treated at 950°C for 10 minutes, and reduced to wires of 1.0mm in diameter by means of cold working. The resultant wires were annealed at 500°C for 30 minutes and then subjected to the tensile test at 37°C by means of loading and unloading cycle. Fig. 1 shows various stress-strain curves of wires of Ti 50-x/2 Ni 50-x/2 C x alloys, where x = 0, 1, 2, 3. Fig. 2 also shows various stress-strain curves of wires of Ti 49.75-x/2 Ni 50.25-x/2 C x alloys, where x = 0, 1, 2, 3. Referring to Fig. 1, the samples of x = 1, x = 2 and x = 3 according to the present invention have high elasticity limit in comparison with the other sample of x = 0. The elasticity limit is elevated by increase of amount of C. Further, the samples of x = 1, x = 2 and x = 3 have the preferable pseudo elasticity but the sample of x = 0 does not have the pseudo elasticity. Referring to Fig. 2, it will be understood that samples of alloys Ti 49.75-x/2 Ni 50.25-x/2 C x alloys have a tendency similar to samples of Ti 50-x/2 Ni 50-x/2 C x . As mentioned above, the alloy according to the present invention has a pseudo elasticity with a high spring stiffness at a temperature of 37 o C so that the alloy can be widely applied as spring materials.

Claims: A method of manufacturing a pseudoelastic spring alloy represented by the chemical formula of Ti a-c/2 Ni b-c/2 C c , wherein a+b=100, b=50-52, and c=0.20-5.0 in atomic percent, said method comprising the steps of: reducing the sectional area of said alloy by coldworking, and followed by the final step of heattreating said alloy at a controlled temperature of 400 to 600°C for a controlled time period of 10-150 minutes, without mechanically restraining said alloy to a desired shape, whereby said alloy has, in comparison with Ti 50 Ni 50 , pseudoelasticity at about 37°C and an increased yield point of elasticity. The method according to claim 1, wherein the alloy is reduced to wires of 1.3mm in diameter by means of hot working and cold working and thereafter solution treated at 950°C for 10 minutes before the step of reducing the sectional area. The method according to claim 1 or 2, wherein the step of reducing the sectional area reduces the alloy to wires of 1.0mm, the step of heattreating said alloy takes place of 500°C for 30 minutes. The method according to one of claims 1 to 3, wherein the total amount of Ni and C is 50 - 52 atomic percent.

Inventors: SATO SHOICHI, TAKAARA HIDEO, YAMAUCHI KIYOSHI, SATO, SHOICHI, TAKAARA, HIDEO, YAMAUCHI, KIYOSHI, SATO, SHOICHI, TOKIN CORPORATION, TAKAARA, HIDEO, TOKIN CORPORATION, YAMAUCHI, KIYOSHI, TOKIN CORPORATION
Applicants: TOKIN CORP, TOKIN CORPORATION
Application year: 1991
Main classification: C22F1
IPC codes: C22C19, C22F1