Shape memory alloy is an alloy that can automatically restore its plastic deformation to its original shape at a certain temperature. In addition to the unique shape memory function, memory alloys also have excellent characteristics such as wear resistance, corrosion resistance, high damping and super elasticity. The expansion rate is more than 20%. The fatigue life reaches 107 times, the damping characteristic is 10 times higher than that of ordinary springs, and its corrosion resistance is better than the best medical stainless steel at present. Therefore, it can meet the requirements of various engineering and medical applications, and is a very excellent functional material. Produce high-quality memory alloy wires that can be used in such fields as eyewear legs, women's bras, mobile phone antennas, women's birth control rings, weak wire instruments and meters.
In 1963, the US Naval Ordnance Research Office needed some nickel titanium alloy wires in an experiment, and the alloy wires they brought back were all crooked. For the convenience of use, these twisted filaments are straightened one by one before use. In the subsequent experiment, a strange phenomenon appeared: when the temperature rose to a certain value, these alloy wires, which had been pulled straight, suddenly and magically quickly returned to their original crooked shape, which was no different from the original shape. After repeated tests, each time the results are completely consistent. As long as the straightened alloy wire reaches a certain temperature, it will immediately return to the original zigzag appearance. It's like that people changed their shapes when they were "frozen" out of consciousness in the past. When the temperature rose to a constant value, they suddenly "woke up" and "remembered" their original appearance, so they recklessly restored their "true colors". The crystal structure of nickel titanium alloy above 400 ℃ and below 400 ℃ is different, but the temperature is 400 ℃. When the temperature changes up and down, the alloy will shrink or expand, Make its shape change.
Here, 400 ℃ is the "abnormal temperature" of nickel titanium memory alloy. Each alloy has its own abnormal temperature.
In terms of the combination and automatic control of Xu pipes for memory alloy, the sleeve made of memory alloy can replace welding. The method is to expand the inside of the pipe end by about 4% at low temperature. During assembly, the sleeve is sleeved together. Once heated, the sleeve shrinks back to its original shape, forming a tight joint. The hydraulic system of the US Navy aircraft has used 100000 such joints, and no oil leakage or damage has occurred for many years. It is very convenient to repair damaged pipelines of ships and submarine oil fields with memory alloy fittings. In some places where construction is inconvenient, memory alloy is used to make pins, which are put into the holes for heating, and their ends are automatically separated and curled to form single-sided assemblies. Memory alloy is especially suitable for thermal machinery and automatic control of constant temperature. It has been made into an automatic opening and closing arm at room temperature, which can open the ventilation window in the sunny day and automatically close when the room temperature drops at night.
There are also many design schemes for memory alloy heat engines, which can work between two mediums with low temperature difference, thus opening up a new way to utilize industrial cooling water, nuclear reactor waste heat, ocean temperature difference and solar energy. The common problem now is that the efficiency is not high, only 4%~6%, which needs further improvement. The medical application of memory alloy is also very attractive.
For example, the bone plate used for bone connection can not only fix two broken bones, but also produce compressive force in the process of restoring the original shape, forcing the broken bones to join together. Dental orthodontic wire, long clip for ligating cerebral aneurysm and vas deferens, and support plate for spinal straightening are all started by temperature after implantation into human body. Thrombus filter is also a new product of memory alloy. Straightened filters will gradually return to a network after being implanted into veins, preventing 95% of clots from flowing to the heart and lungs. Artificial heart is a kind of organ with more complex structure. The muscle fiber made of memory alloy can simulate the contraction of ventricle by matching with the elastic membrane ventricle. Now the pumping of water has been successful. Since memory is a "living alloy", we can design various automatic control devices by using its shape changes at a certain temperature, and its use is expanding.
Amanda
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