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Heat Treating and Shape Setting

Nitinol and other shape memory alloy mill products - bar, wire, ribbon and sheet are normally finished by cold working to achieve dimensional control and enhance surface quality. Cold working suppresses the shape memory response of these alloys. It also raises the strength and decreases the ductility. However, cold work does not raise the stiffness of the material. Heat treating after cold working diminishes the effects of cold working and restores the shape memory response of these alloys. Therefore, in order to optimize the physical and mechanical properties of a Nitinol product and achieve shape memory and /or superelasticity, the material is cold worked and heat treated.

The mill product supplier normally provides the material in the cold worked condition. The maximum practical level of cold work will be limited by the alloy and by the product section size. Binary superelastic NiTi alloy fine wires with As in the range of –25 to +95°C are typically supplied with cold reduction after the final anneal in the range of 30 to 50%. Higher reductions are sometimes used for very fine wires. These same alloys will be limited to about 30% maximum cold reduction in larger diameter bar sizes. Binary NiTi alloys with very low As in the range of –50 to –60 °C will not sustain the higher levels of cold work without cracking.

Both superelastic and shape memory properties are optimized by cold work and heat treatment. This thermo-mechanical process is applied to all Nitinol alloys although different amounts of cold work and different heat treatments may be used for different alloys and property requirements.

Shape setting is accomplished by deforming the Nitinol to the shape of a desired component, constraining the Nitinol by clamping and then heat treating. This is normally done with material in the cold worked condition, for example cold drawn wire. However, annealed wire may be shape set with a subsequent lower temperature heat treatment.

In shape setting cold worked material, care must be taken to limit deformation strain to prevent cracking of the material. Another approach is to partially anneal the wire prior to shape setting. Yet another option is to shape set in incremental steps. Smith et al. reviewed the types of furnaces and fixturing hardware or mandrels that have been used in heat treatment. Many types of furnaces have been used including box furnaces, continuous belt hearth furnaces, tube furnaces, heated platen presses, vacuum furnaces, induction heaters, salt baths and fluidized bed furnaces. The electrical resistance of Nitinol makes it a good candidate for self heating by electric current. Nitinol will be oxidized when heat treated in air. Therefore, surface requirements and atmosphere control are important considerations.

Shape setting can be done over a wide temperature range from 300 °C to 900°C. However, heat treating temperatures for binary NiTi alloys are usually chosen in the narrower range of 325 to 525°C in order to optimize a combination of physical and mechanical properties. Heat treating times are typically 5 minutes to 30 minutes. Consideration must be given to the mass of the heat treating fixture as well as the mass of the product. Sufficient time must be allowed in the furnace to get the entire mass to the desired temperature.

The shape setting heat treatment changes the physical and mechanical properties of Nitinol. Morgan and Broadly mapped the effect of temperature and time at temperature on shape set wire properties. Their response curves illustrate that physical and mechanical property do not always change in the same direction. Also, some properties are not monotonic functions of time at temperature. For example, upper plateau stress goes through a minimum as a function of time at temperature when heat treating a superelastic alloy in the range of 450°C to 550°C. This can be understood in terms of the complex precipitation response of the nickel rich Ni – Ti alloys.

Precipitation processes in nickel rich NiTi were studied in detail by Nishida et al. Their TTT diagram shows that in a Ti – 52 atomic % Ni alloy heat treated below 820oC, precipitation starts as fine Ti11Ni14 transitions over time to Ti2Ni3 and terminates after long time as TiNi3 in equilibrium with the NiTi matrix.  All the while, the NiTi ratio in the matrix is being shifted towards higher Ti content and higher transformation temperature. Pelton et al. reported on the combined effects of non-isothermal and isothermal heat treatment on the physical and mechanical properties of Nitinol wire.  This work suggests that for a 50.8 atomic % Ni alloy Ti11Ni14 dissolves at about 500oC and Ti2Ni3 will start to precipitate at 550oC.  This results a minimum or maximum in properties as NiTi ratio in the matrix goes through a peak.  Furthermore, this analysis suggests that the temperature for transition from Ti11Ni14 precipitation to Ti2Ni3 precipitation occurs at higher temperatures for higher Ni content alloys.

Brailovski used measurement of latent heat measured by DSC and Vickers hardness to map mechanical and physical properties as a function of heat treatment. He obtained maximum fatigue performance when the combination of transformation temperature and hardness were optimized.

References:

  1. K. N. Melton, “Ni-Ti Based Shape Memory Alloys”, in Engineering Aspects of Shape Memory Alloys, ed by T. W Duerig et al., Butterworth , London, 1990, pp 28 – 34.
  2. A. R. Pelton, “Optimization of Processing and Properties of Medical Grade Nitinol wire”, Minimally Invasive Therapy and Applied Technology, Vol 9 (1), 2000, pp 107 – 118.
  3. V. Brailovski, “Optimization of Post-deformation Annealing Heat treatment for NiTi Shape Memory Alloys” in SMST 2000, Proceeding of the International Conference on Shape Memory and Superelastic Technologies, April, 2000, Edited by Russell Pelton, pp 33 – 42.
  4. N. B. Morgan and M. Broadley., “Taking the Art Out of Smart!—Forming Processes and Durability Issues for the Application of NiTi Shape Memory Alloys in Medical Devices”, in Proceedings of the Materials and Processes for Medical Devices Conference, edited by S. Shrivastava, ASM International, Sept., 2003, “, pp 247 – 252.
  5. S. A. Smith et al., “Shape Setting Nitinol”, in Proceedings of the Materials and Processes for Medical Devices Conference, edited by S. Shrivastava, ASM International, Sept., 2003, “, pp 266 – 270.
  6. M. Niashida et al., “Precipitation Processes in Near-Equiatomic TiNi Shape Memory Alloys”, Metallurgical Transactions A, Vol. 17A, Sept., 1986, pp 1505 – 1515.
  7. T. Todoroki and H Tamura, “Effect of Heat Treatment after Cold Working on the Phase Transformation in TiNi Alloys”, Trans. Japan Institute of Metals, Vol. 28, No. 2, 1987, pp 83 – 94.
  8. J. Zang et al., “Reversible Changes in Transformation Temperature of a Ti-51at% Alloy Associated with Alternating Aging”, Scripta Materialia, Vol. 41, No. 10, 1999, pp 1109 – 1113.
  9. Y. Okamoto et al., “Reversible Changes in Yield Stress and Transformation Temperature of a NiTI Alloy by Alternate Heat Treatments”, Scripta Metallurgica, Vol.22, 1988, pp 517 – 520.
  10. X. Huang and Y Liu, “Effect of Annealing on the transfromation behavior and superelasticity of NiTi Shape Memory Alloy”, Scripta Materialia, Vol.45, 2001, pp 153 160.
  11. T. Saburi et al., “Effects of Heat Treatment on the Mechanical Behavior of Ti – Ni Alloys”, Journal De Physique, Colloque C4, supplement au No 12, Tome 43, Decembre 1982, pp C4-261 – C4-266.
  12. Miyazaki, S., “Thermal and stress cycling effects and fatigue properties of Ni-Ti alloys”, in Engineering Aspects of Shape Memory Alloys, ed by T. W Duerig et al., Butterworth , London, 1990, pp 394 – 414.