UNITED STATES, WASHINGTON (OBSERVATORY) — In a new study, scientists not only significantly exceeded the previous strength record for this precious metal, but also exceeded its theoretical limit. To do this, they added a little copper to silver, but they did it in a special way.
It is known that silver is a very soft material. In an effort to make it stronger, metallurgists add impurities to the precious metal. But at the same time, another important parameter of the material is worsening – electrical conductivity.
Such a conflict is characteristic not only of silver, but also of many other metals used in modern technology. New research could put an end to this search for compromise.
“We have discovered a new mechanism of working at the nanoscale, allowing us to produce metals, which are much stronger than [received] ever before, without losing their conductivity” – argues the head of the research group Frederic Sansoz from the University of Vermont.
Recall that metals are polycrystals (that is, the material is, as it were, sewn from many rags-crystals of various shapes). Flaps are complexes of atoms (grains). Moreover, the atoms inside the grain are bound much more strongly than the grains with each other. Weak bonding between grains is one of the main reasons making material fragile.
In this regard, there is the so-called Hall-Petch ratio : the smaller the grain, the stronger the metal. For a long time, almost 70 years, researchers used this rule to create more durable materials.
However, when the grain size of metals began to reach tens of nanometers, material scientists discovered a new problem: the grain boundaries became unstable and began to move.
Having understood the nature of this process, scientists learned to sew rather small flaps with the help of “very strong threads” called coherent twin boundaries. The latter represent the boundary between two grains, which in structure are similar to each other like mirror reflections.
Such borders are extremely difficult to deform, so they acted as stitches with which scientists fastened the “rags” of metal.
However, if the gap between grains in such a coherent twin boundary became less than seven nanometers, then they could no longer keep the metal structure from sprawling. In this case, a theoretical tensile strength is achieved (the so-called Hall-Petch limit).File Illustration by Frederic Sansoz, UVM
But researchers at the University of Vermont have found a way around this problem. Using computer models of the motion of atoms, and then moving on to experiments with real metals, scientists eventually got ultra-strong silver. To do this, they added a little copper to silver (less than 1% by weight).
In their article in the journal Nature Materials, the authors conclude that copper atoms, which are slightly smaller than silver atoms, fill the boundaries between grains and prevent the grains from moving relative to each other. This makes the metal 42% stronger than the previous record and, moreover, stronger than the Hall-Petch limit allows .
“We broke the world record and the Hall-Petch limit, too, and not just once, but several times during this study in carefully controlled experiments,” says Sanzos.
At the same time, such a small admixture of copper practically does not affect the electrical conductivity of silver.
“The impurities of the copper atoms are located along the interfaces [between the grains], but not sticking between them,” Sansoz explains. “Thus, they do not interfere with the electrons that move through [the substance].”
The approach used can be applied to harden not only silver, but also other metals. As the studies of material scientists show, in a similar way it is possible to increase the strength of many metals with twin boundaries that are separated by a distance of less than seven nanometers (that is, where the grains are separated from each other by a distance of several atoms).
Researchers hope that the innovation will find many applications, because more durable current-conducting materials can be used longer and in smaller quantities. The knowledge gained in the course of this work can give humanity more efficient solar panels , lighter aircraft and even safer nuclear power plants.
At the same time, chemists do not exclude that surprises are still waiting for them along the way.
“This is a new class of materials, and we are just starting to understand how they work,” Sandoz admits.
This article is written and prepared by our foreign editors writing for OBSERVATORY NEWS from different countries around the world – material edited and published by OBSERVATORY staff in our newsroom.
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