The earliest blacksmiths of the Bronze and Iron Age found that when metal is deformed by bending or hammering, it becomes stronger.
This process, called work hardening or strain hardening, is still widely used in metallurgical manufacturing to improve the strength of facilities such as car frames to overhead wires.
But so far, material scientists have not been able to observe this important process in real time.
(photo source: nature.
com) according to foreign media reports, researchers at Harvard University’s John Paulson School of Engineering and Applied Sciences (SEAS) have observed for the first time the specific mechanisms that drive the basic process of work hardening.
The study, conducted at Harvard University’s Center for Materials Research Science and Engineering (MRSEC), is expected to have a broad impact on material design and manufacturing through a deeper understanding of material strength.
The related papers are published in the journal Nature.
“work hardening is used in many industrial deformation processes,” said researcher Frans Spaepen.
At present, people use large computer programs to model work hardening, but in order to make these models more effective, we need to further understand the underlying mechanisms that control this process.
This work provides us with a real-time window to understand the general process of work hardening.
” Work hardening in metals cannot be observed in real time because the atomic structure can only be observed through an electron microscope.
Researchers can compare structures before and after deformation, but only have a limited understanding of what happens in the process.
Previous studies have shown that defects in the structure (called dislocation) form a defect network, which leads to work hardening.
Researcher Ilya Svetlizky said: “what is not clear is the overall complexity of the interaction between the defects in these atomic crystals that cause hardening.
” To understand the key parts of the process, the team turned to colloidal crystals, which are about 10,000 times larger than atoms and can spontaneously form crystal structures at high concentrations.
These crystals can be used to simulate atomic systems because they have the same structure, undergo the same phase transition process, and have similar defects.
However, colloidal crystals are very soft, 100000 times softer than Jell-O, a transparent dessert in the United States.
The researchers developed colloidal crystals made up of millions of particles and used a confocal optical microscope to observe each particle.
When strain is applied to these crystals, they can measure the motion of each particle.
Surprisingly, these colloidal crystals undergo significant work hardening and are even stronger than any other material.
In fact, taking into account differences in particle size, these ultra-soft materials become stronger than other metals.
“We didn’t expect that these hard spherical colloidal crystals would undergo work hardening,” said researcher Seongsoo Kim.
Compared with ordinary metals, the interaction between these particles is very simple.
In fact, we found that these soft materials show unusually obvious work hardening, even more than most copper and aluminum metals.
” This is the first time that work hardening has been observed in colloidal crystals.
The results show that the process is mainly controlled by the geometric shape and defects of the particles.
These crystals become stronger because of dislocation defects and the way they interact and entangle each other.
These observations reveal the general mechanism of work hardening, which will be more generally applicable to all materials, even those that cannot be studied with an optical microscope.
These soft colloidal crystals exhibit such excellent work hardening because they can accommodate very high density defects.
Researcher David A.
Weitz said: “this study shows some basic and universal mechanisms that cause materials to become stronger.
These materials are attractive and very soft, but work hardening makes them the strongest materials known.
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