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Homework Help: Problems with pliability of nitinol at room temperature

  1. Jan 30, 2013 #1
    1. The problem statement, all variables and given/known data
    I'm working on a science fair project which involves nitinol, a shape-memory alloy, and have a few questions regarding it that I cannot find the answers to online.

    The nitinol I purchased is 0.009"⌀, 56% Nickel, 44% Titanium (link to product). In the numerous videos I've seen of nitinol, it appears to be pliable at room temperature and will only revert to its trained shape when sufficient heat is applied. The nitinol I'm working with seems much less responsive to temperature changes, and at room temperature it is not pliable at all, always returning to a straight wire immediately after being bent.

    I successfully trained my nitinol using a propane torch and some steel clamps. I was careful to ensure that the entire strand that I wanted to heat was touching the metal to prevent it from simply being singed, and I heated it with the inner blueish cone of the flame for 1-2 minutes. After being unclamped, it held its new shape, springing back after bending it with my hands. The problem is I want it to be pliable at room temperature and only spring back when heated.

    I tried submerging a new length of wire in water at 0ºC and 100ºC, and there is a barely noticeable contraction and expansion respectively. While holding it under the cold water, I tried crumpling it up into a ball, but it would still spring back, though less energetically than in air. I tried making a loop, submerging it in cold water, then immediately in hot water, and there is a noticeable but not significant difference.

    I have read that the more heat applied to the nitinol during the training process, the higher the transformation temperature becomes. I have not been able to test this yet as each time I try to heat it extensively, the wire becomes brittle and easily snaps.

    2. Relevant equations

    3. The attempt at a solution
    My best hypothesis as to why it isn't behaving as expected is that the wire is too thin. I can't say for sure though as I have no experience with this material.

    If anyone has any experience with nitinol that could give me some tips or point me towards some good resources, it would be much appreciated.
  2. jcsd
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