What is shape memory alloy?
志联
2025-04-23
Shape memory alloys are alloys that can automatically recover their plastic deformation to their original shape at a specific temperature.
In addition to its unique shape memory function, memory alloy also has excellent characteristics such as wear resistance, corrosion resistance, high damping, and super elasticity. The elongation is over 20%. The fatigue life reaches 107 times, and the damping characteristic is 10 times higher than that of ordinary springs. Its corrosion resistance is superior to that of currently better medical stainless steel, so it can meet the application needs of various engineering and medical fields, and is a very excellent functional material.
Production of high-quality memory alloy wires that can be used in the fields of glasses legs, women's bras, mobile phone antennas, women's IUDs, orthodontic wires, instruments and meters, etc.
In 1963, the US Navy Ordnance Research Laboratory needed some NiTinol wires in an experiment. The alloy wires they received were all bent and curved. For ease of use, these bent and curved thin wires were straightened one by one before use.
In subsequent experiments, a strange phenomenon occurred: when the temperature rose to a certain value, these alloy wires, which had been straightened, suddenly magically and quickly returned to their original bent and curved shape, and were exactly the same as the original shape. After repeated experiments many times, the results were exactly the same each time. The straightened alloy wires, as long as they reached a certain temperature, immediately returned to their original bent and curved appearance.
It's as if they were 'frozen' and lost consciousness in the past, and their shape was changed by people. When the temperature rises to a certain value, they suddenly 'wake up' and 'remember' their original appearance, so they desperately restore their 'original appearance'.
The crystal structure of NiTinol is different above and below 40oC, but when the temperature changes around 40oC, the alloy will contract or expand, causing its shape to change. Here, 40oC is the "transformation temperature" of NiTinol-titanium memory alloy. Various alloys have their own transformation temperatures.
Memory alloys have been used in pipeline joining and automation control. Sleeves made of memory alloys can replace welding. The method is to fully expand the pipe ends by about 4% at low temperature. When assembling, they are connected together. Once heated, the sleeve contracts and recovers its original shape, forming a tight joint.
The hydraulic system of US Navy aircraft uses 100,000 such joints, and no oil leakage or damage has occurred for many years. For damaged ships and submarine oilfield pipelines, it is very convenient to repair them with memory alloy fittings. In some places where construction is inconvenient, pins made of memory alloy are installed in the holes and heated, and their tail ends automatically separate and curl, forming a single-sided assembly.
Memory alloys are particularly suitable for thermomechanical and constant temperature automatic control. Room temperature automatic opening and closing arms have been made, which can open ventilation windows during the day when the sun shines and automatically close them at night when the room temperature drops.
There are also many design schemes for memory alloy heat engines, which can work between two media with low temperature difference, thus opening up new ways 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 application of memory alloys in medicine is also eye-catching.
For example, bone plates used for bone setting can not only fix the two broken bones, but also generate compressive force in the process of restoring their original shape, forcing the broken bones to join together. Orthodontic wires used in dentistry, long clips for ligating cerebral aneurysms and vas deferens, and support plates for spinal straightening are all activated by body temperature after being implanted in the human body. The thrombus filter is also a new memory alloy product. After the straightened filter is implanted into the vein, it will gradually recover into a net shape, thus preventing 95% of blood clots from flowing to the heart and lungs.
An artificial heart is a more complex organ. The combination of muscle fibers made of memory alloy and elastic film ventricles can simulate ventricular contraction. Pumping water has been successful now. Because memory alloy is a "living alloy", using its shape change at a certain temperature, various self-control devices can be designed, and its uses are constantly expanding.
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