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Shape memory alloy is a kind of alloy which can automatically restore its plastic deformation to its original shape at a certain temperature.
In addition to its unique shape memory function, memory alloy also has excellent characteristics of wear resistance, corrosion resistance, high damping and super elasticity. The expansion rate is more than 20%. The fatigue life is 107 times, the damping property is 10 times higher than the common spring, its corrosion resistance is better than the current better medical stainless steel, so it can meet the application needs of various engineering and medicine, and it is a very excellent functional material.
It can produce high quality memory alloy wire which can be used in the fields of eyeglass legs, women's brassiere, mobile phone antenna, women's birth control ring, dental arch wire, instruments and meters, etc.
In 1963, the U.S. Naval Ordnance Research Institute needed some nitinol alloy wires in an experiment. The alloy wires they brought back were all crooked. For the convenience of use, we will straighten these winding filaments one by one and then use them.
In the follow-up test, a strange phenomenon appeared: when the temperature rose to a certain value, these straight alloy wires suddenly and magically quickly recovered to the original shape, which was no different from the original shape. After repeated tests, the results are identical. As long as the straightened alloy wire reaches a certain temperature, it will immediately return to the original curved shape.
It's as if people changed their shape when they lost consciousness because of the "freezing". When the temperature rose to a certain value, they suddenly "woke up" and "remembered" their original appearance, so they recklessly recovered their "original appearance".
The crystal structure of Ni Ti alloy is different between the temperature above 40oC and below 40oC, but when the temperature changes above 40oC, the alloy will shrink or expand, making its morphology change. Here, 40oC is the "abnormal temperature" of Ni Ti memory alloy. All kinds of alloys have their own abnormal temperature.
Memory alloy has been used in pipeline combination and automatic control. The sleeve made of memory alloy can replace welding. The method is to expand the whole inside of the tube end by about 4% at low temperature. When assembling, the sleeve is connected together. Once heated, the sleeve shrinks back to its original shape, forming a tight joint.
The hydraulic system of the U.S. Navy aircraft uses 100000 such joints, which have not been leaking or damaged for many years. It is very convenient to repair the damaged ships and submarine oil pipelines with memory alloy fittings. In some parts with inconvenient construction, the pin is made of memory alloy, which is installed into the hole for heating, and its tail end is automatically separated and curled to form a single-sided assembly.
Memory alloy is especially suitable for thermal machinery and constant temperature automatic control. It has been made into room temperature automatic opening and closing arm, which can open the ventilation window in the daytime when the sun shines, and automatically close when the room temperature drops at night.
There are many design schemes of memory alloy heat engine, which can work between two kinds of medium 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. At present, the common problem is that the efficiency is not high, only 4% - 6%, which needs further improvement.
The application of memory alloy in medical treatment is also very attractive.
For example, the bone plate used for bone grafting can not only fix the two broken bones, but also produce compression force in the process of restoring the original shape, forcing the broken bones to join together. Tooth straightening wire for dentistry, long clip for ligation of cerebral aneurysm and vas deferens, and support plate for spine straightening are all started by body temperature after being implanted into human body. Thrombus filter is also a new product of memory alloy. After the stretched filter is implanted into the vein, it will gradually return to the network, thus preventing 95% of the clots from flowing to the heart and lungs.
The artificial heart is a kind of organ with more complex structure. The muscle fiber made of memory alloy and the ventricle of elastomer film can imitate the contraction of the ventricle. Now pumping water has been successful. Because memory alloy is a kind of "living alloy", we can design a variety of automatic control devices by using its shape change under a certain temperature, and its application is expanding.
