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Formation of Advanced Metal Alloys via Ultrafast Laser-Driven Extreme States 
Date/Time:
November 28, 2016   1:15pm - 2:00pm
 
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Developing classes of metal alloys, such as stabilized nano-crystalline alloys and multicomponent High Entropy Alloys (HEA), exhibit extraordinary mechanical and chemical properties. The structure of conventional alloys based on a single host element derive primarily from the chemical interactions of the components and the free energy as depicted in the equilibrium phase diagram. In nano-crystalline alloys or HEAs minimizing strain, minimizing surface energy, or increasing configurational entropy become primary factors controlling the crystal structure and microstructure often with surprising results. A multitude of alloys can be made using various combinations of elements known to be compatible, and many have already been created. However, for some alloys, chemical incompatibility leads to separation of elements in the liquid phase and makes production by conventional casting or splat quenching difficult[1]. We demonstrate a new method to form alloys from the liquid phase via irradiation of periodic thin films with a femtosecond laser.

We will present Transmission Electron Microscopy (TEM) of nano-crystalline NiW alloy, similar to those produced by electrodeposition[2], that was produced by irradiating a 23 nm thick film composed of 12 alternating layers of 1.4 nm W and 2.4 nm Ni. Femtosecond laser pulses are absorbed in the near surface heating the top 40 nm layer to extreme temperatures and pressures on the order of 6000 C and 50 GPa within a few picoseconds so the layer remains at solid density. The thermodynamic relaxation of the film passes into the “vapor dome” in the Temperature-Density phase diagram, a region of liquid-vapor coexistence where the bonding energy between atoms is low and the kinetic energy is very high. We propose under these extreme conditions Ni and W are allowed to mix thoroughly, then thermal transport into the substrate quenches the mixed layer within a few nanoseconds. It should be noted metals irradiated with a single ultrashort pulse do not resolidify nano-crystalline, but instead regrow epitaxially from the substrate. The thermal and mechanical relaxation of femtosecond laser irradiated multilayer films includes unique, extreme thermodynamic states and thereby provides a new route to synthesize stable nano-crystalline alloys or multicomponent HEAs from the liquid phase.
 


 
 
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