Do metals have "memory"?

Let’s watch a magic show first.

The magician placed a straight spoon in 0 degrees Celsius water and bent it. Then, the magician held the spoon in his hand, blew three times mysteriously into his hand, spread his fingers, and a miracle happened: the spoon became straight!

Why is this so? The secret is definitely not in those three breaths. So where is it?

It turns out that the spoon is made of a metal with a "memory" function. It "remembers" its original straight shape, and when it is heated by the hand and the temperature rises, its "memory" is awakened, so when the hand opens, it becomes straight.

Ha, metal actually has "memory"!

People call these metals that can "remember" their own shapes shape memory metals. Because they are composed of two or more elements, they are also called shape memory alloys.

You must have heard the story of "Chang'e flying to the moon"! Landing on the moon successfully has always been the dream of Chinese aerospace workers. With the successful launch of Chang'e spacecraft time and time again, landing on the lunar surface is no longer just a dream.

However, successfully reaching the lunar surface is only the first step. We also need to "pack" the lunar "specialty" - lunar soil and bring it back to Earth, which is very important for scientists' research.

However, without manual operation on the moon, how can we put the collected lunar "local specialties" into bags, pack them, and seal them to prevent them from spilling out?

Scientists thought of shape memory alloys. They made a sealer like a twist out of the shape memory alloy wire, bent it into a circle and sewed it to the mouth of the cloth bag containing the lunar soil before the launch of Chang'e 5, and then put the cloth bag on the outside of the metal tube for collecting lunar soil.

After the drilling and collection begins, the automatic device slowly pulls the cloth bag into the metal tube through the drawstring at the top of the cloth bag, and the collected lunar soil slowly enters the cloth bag inside the tube. When the collection task is completed, the cloth bags on the outside of the metal tube are all turned over and put into the tube. At this time, the memory alloy sealer sewn on the bag mouth will automatically return from a round shape to a twisted shape under the lunar temperature conditions, tightly sealing the bag mouth. Packing is complete!

Finally, the "package" returned to Earth safely on the return capsule.

On December 17, 2020, the Chang'e-5 lunar probe returned to Earth with lunar soil samples, successfully completing my country's first unmanned lunar sampling mission. Shape memory alloys contributed to the completion of this mission.

It seems simple and smooth, but in fact, from thinking to doing, it is full of twists and turns and challenges. Accurate temperature control is one of the major problems to be solved.

On the moon, the temperature is very high during the day and very low at night. The temperature difference between day and night is more than 300 degrees Celsius, which is very different from the earth! In this case, ordinary shape memory alloys cannot "show their abilities" at all. What should we do?

The shape memory alloy research team of Beijing University of Aeronautics and Astronautics was determined to "fight to the death" against this difficulty. They found that adding some "materials", that is, other alloying elements, to ordinary shape memory alloys and improving the processing methods of alloy wires can help shape memory alloys "upgrade"!

But what kind of alloy elements should be added, and how much should be added? This is not something that can be decided on a whim. In order to find the correct answer, they repeatedly experimented and improved the formula, and finally obtained a shape memory alloy that can meet the requirements of the lunar environment, and processed it into a sealer. The aerospace technology personnel then repeatedly tested it in various complex environments, and finally selected two sealers to carry out the space mission and successfully completed the lunar soil collection.

The widespread application of shape memory alloys in the aerospace field today is inseparable from the silent dedication of senior scientists. Academician Zhao Liancheng was one of the people who noticed and realized that it would play an important role in the development of my country's aerospace industry.

That year, Zhao Liancheng, who had just graduated from university, resolutely left Shanghai because of the needs of the country and came to the "Ice City" Harbin, and began his exploration in the "world of materials".

During his research, he discovered that certain metals have special "talents" similar to memory. Ha, how interesting!

Which metals have such "talents"? And how can we make good use of such amazing "talents"?

There were no answers to these questions, and no previous experience, so he eagerly consulted domestic and foreign data, and combined a large number of complex experiments. Finally, he concluded that the memory alloys that can be used in engineering applications can basically be locked in the range of nickel-titanium-based memory alloys. This inference has brought my country's memory alloy research to a new level.

Next is the application of memory alloys, including aviation hydraulic pipe joints.

This pipe joint is an important component that affects the safety and reliability of aircraft. In the past, pipe joints were basically made of ordinary metals, with nuts and bolts screwed together. When encountering vibration and impact, they are easy to loosen, causing oil and gas pipelines to leak and cause aircraft failure. If memory alloys are applied to this kind of pipe joint, its performance can be greatly improved and the failure rate of aircraft can be greatly reduced.

In order to ensure that memory alloys are foolproof during their service in aerospace, Zhao Liancheng not only repeatedly tests each memory alloy device he develops, but also sometimes has to solve various interdisciplinary and cross-field problems.

A major problem faced at the time was to find a suitable low-temperature lubricant that could help the memory alloy pipe joints to achieve "diameter expansion" at low temperatures, that is, to expand the pipe joints without cracking so that they can be put on the corresponding pipelines.

But, it was not found.

Curiosity is really an essential element for scientists to succeed! Just when they were at their wit's end, Zhao Liancheng's team received a letter from abroad.

While opening the letter, they found that the envelope felt unusually smooth. Could it be made of some special material? They were struggling to find a lubricant, so they quickly took the envelope to do a low-temperature lubrication test. The result was a ray of hope - with this lubricant made of envelope paper, the pipe joint was successfully expanded!

In this way, under the leadership of Academician Zhao Liancheng, the scientific research team successfully developed one shape memory alloy device after another, which were successfully applied to more than 10 spacecraft including our country's Shenzhou IV spacecraft and Shijian V satellite.

Nowadays, shape memory alloys are not only used in the aerospace field, but can also be seen in common dental orthodontic wires and eyeglass frames.

Students, have you ever encountered a problem that you couldn't solve for a while? Don't be discouraged. As long as you observe more and think more, you will definitely be able to see the light at the end of the tunnel.

Source: Chongqing Industry-University-Research Cooperation Promotion Association

Author: Pan Fusheng, Academician of the Chinese Academy of Engineering, Professor of Chongqing University, Chairman of Chongqing Association for Science and Technology

Review expert: Mao Yi, chief expert of Chongqing Science Communication Expert Group, professor of Chongqing Electronic Engineering Vocational College

Image sources: Photo Network, CCTV News, Palace Museum, Jiangxi Provincial Museum, Shanghai Museum, Shanxi Museum

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