Speaker
Description
Amorphous materials, such as amorphous alloys and metallic glasses (MGs) [1-4] are of current interest due to their peculiar internal structures and unusual properties [5-8] like high strength, excellent corrosion and wear resistance, localized deformation by shear banding and much higher hardness than crystalline alloys of comparable elastic modulus. Because glassy alloys do not exist in thermodynamic equilibrium, they undergo crystallization with the supply of thermal energy. However, despite decades of experimental and theoretical efforts, many questions regarding the details of these processes remain still open.
Due to the small space scales involved, the experimental investigation of the mechanisms underlying the phase formations caused by strain/stress as well as during nanoindentation on amorphous materials is difficult. Fortunately, the fast increase in available computation power is today allowing more and more accurate and larger size molecular dynamics (MD) simulations of almost any kind of system.
In this work, the crystallization of refractory metals (W, Nb, Ta, V, Mo) has been examined by means of molecular dynamics simulations. All these metals are more stable in their usual bcc structure, so under indentation, a growth of bcc crystals around the indenter (and occasionally in the whole sample) is observed. The velocity at which this process occurs depends on various factors like the type of metal, initial conditions on the amorphous sample, etc. The dependence on the initial density, melting point, and cohesive energy, has been investigated, as well as the structure of the resulting grain boundaries and grain size average. A simple model correlating crystal-forming ability (CFA) with thermodynamic properties will be presented.
References:
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8. Ma, E. Tuning order in disorder. Nat. Mater. 14, 547–552 (2015).
