Fabricating materials with strengths that approach theoretically predicted values is no easy task, and most techniques to make stronger materials rely on controlling defects so that dislocations can no longer move as freely. Industrial single-phase nanocrystalline alloys and single-phase metallic glasses, for example, can be very strong, but they usually soften at relatively low strains of less than 2%. A team of researchers at the City University of Hong Kong has now developed a new technique that combines the strengthening benefits of nanocrystallinity with those of amorphization to produce a dual-phase material with near-ideal strength at room temperature. The alloy, which is based on nanocrystalline magnesium cores embedded in magnesium-enriched amorphous glassy shells, has a strength of 3.3 gigapascals, making this the strongest magnesium thin film ever. It might be employed in a wide variety of applications, such biocompatible and biodegradable medical implants with excellent wear resistance and as a lightweight material in consumer electronics, as well as in aerospace and automotive parts.
The main ways to strengthen crystalline materials today are: grain (or interphase) boundary strengthening; twin boundary strengthening; solid solution strengthening; and precipitate (or dispersed reinforcement particle) strengthening. These methods rely on controlling defects, such as dislocations, so they can no longer move so easily in a material. However, these effects cannot be increased indefinitely. Indeed, industrial single-phase nanocrystalline alloys, although strong, soften at relatively low strains because of the Hall–Petch effect, which comes about thanks to grain boundary sliding and softening.
Amorphization is another good way of increasing a material’s strength since an amorphous structure does not contain any grain boundaries or dislocations. Single-phase metallic glasses are one type of amorphous material, and while they are stronger than nanocrystalline alloys they do suffer from shear-band formation with low applied strain.
New class of supra-nanometre-sized dual-phase glass crystal
Researchers expect that if a material were to contain a crystalline core with a grain size of less than 10 nm as well as an expanded amorphous shell with a grain boundary-like zone of several nanometres, the amorphous shells might help stop the grains from sliding. They might also slow down the movement of dislocations. This type of structure, known as a “supra-nanometre-sized dual-phase glass crystal” (SNDP-GC), would not suffer from the reverse Hall–Petch effect and the nanometre-sized amorphous shells would thus have near ideal strength.
A team led by Jian Lu has now succeeded in making such a supra-nanostructure for the first time using a scalable technique called magnetron sputtering. Thanks to high-resolution transmission electron micrography (HRTEM), the researchers were able to observe that their magnesium-based SNDP-GC has an amorphous/nanocrystalline dual-phase structure comprising MgCu2 grains of around 6 nm in diameter uniformly embedded in magnesium-enriched amorphous shells around 2 nm thick. The structure has a nearly ideal strength of 3.3 GPa and a low Young’s modulus of 65 GPa.
10 times stronger than conventional crystalline magnesium alloys
“Our new advanced magnesium-based material is 10 times stronger than conventional crystalline magnesium alloys and has a super deformation capacity that is twice as high as magnesium-based metallic glasses,” Lu tells nanotechweb.org. “The fact that all the phases are less than 10 nm in size is important because if they were bigger than this, the material would not be almost dislocation free and would thus not have near-ideal strength.”
As for applications, the SNDP-GC alloy could be used in hard-wearing biodegradable medial implants, he adds. “It might also be used to extend the life of different microelectromechanical systems in consumer electronics, such as smart phones, tablets and laptops. The low density of the magnesium alloy means that it could be a desirable lightweight material here as well as in aerospace and automotive parts.”
The team, reporting its research in Nature doi:10.1038/nature21691, says that it is now busy looking for other supra-nanostructured materials. “We believe that this new class of nanomaterials will have unique mechanical, physical and chemical properties,” says Lu.
About the author
Belle Dumé is contributing editor at nanotechweb.org