Recently, a young PhD student from the Materials Research Division at Risø DTU took research a step further by discovering a new phenomenon. The new discovery could speed up the practical application of strong nanometals and has been published in the highly esteemed journal ”Proceedings of the Royal Society” in London in the form of a paper of approx. 30 pages written by three authors from Risø DTU.
Recovery by triple junction motion in aluminium deformed to ultrahigh strains
Microscopic metal grains of nanostructured metals are not stable - a problem of which Tianbo Yu’s discovery now provides an explanation.
Tianbo Yu’s has now shown that the boundaries of the grains can be locked, when small particles are present and that the solution is technologically feasible. This has paved the way for car components to be made of nanometals.
Nanometals contain very small metal grains - from 10 to 1,000 nanometers. One nanometer is a millionth of a millimetre. The smaller the metal grains become, the stronger the metal becomes. The metal becomes twice as strong, for example, if the individual metal grains are made four times smaller. That is why the materials scientists work to reduce the size of the individual metal grains. In steel and aluminium, the particles have been reduced to below 1 micrometre, which is one thousandth of a millimetre. There is a great interest in nanometals worldwide. Nanometals are super strong and their super strength can be combined with other desired properties, too.
Commercial purity aluminium at true strains ε=2∼5.5 was annealed in a wide temperature range (from room temperature to 220°C), and the evolution of microstructure was characterized using transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) techniques. Triple junctions in an ultrafine lamellar structure are classified into three categories based on the structural morphology, and a relationship is formulated between the density (length per unit volume) of triple junctions and the boundary spacing. The triple junction density increases with increasing strain during plastic deformation and decreases during isochronal and isothermal annealing. Based on TEM and EBSD observations, thermally activated triple junction motion is identified as the key process during the recovery of highly strained aluminium, leading to the removal of thin lamellae with small dihedral angles at the ends and structural coarsening. A mechanism for recovery by triple junction motion is proposed, which can underpin the general observation that a lamellar structure formed by plastic deformation during annealing can evolve into an equiaxed structure, preceding further structural coarsening and recrystallization. Within this framework, the grain boundary surface tension on triple junctions is discussed based on the structural parameters characterizing the deformed and annealed microstructure.
Nanometals bend the laws of nature (translated from Danish)
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