Non equilibrium systems Advanced material and technologies, MSc 2017

What is the meaning of „equilibrium state"? phenomenology thermodynamic consideration The water-ice system is popular example 1 atm is 0 oC, T 0:supercooled liquid (metastable) Supersaturated solution: metastable. How metastability evolves generally? Atomic rearangements do not be able to succeed the cooling rate.

Fundamental terms: - stability - unstability - metastability Unstable materials are avoided by engineering practice, metastable states are often used. Examples: - unstable: highly deformed, high purity materials - metastability: diamond, martensite

Characters of metastable states They have a role in strength-enhancing. CHARACTER OF METASTABLE STATE EXAMPLES EXCESS ENERGY (RTm) EXCESS ENERGY J/mol) COMPOSITIONAL SUPERSATURATED SOLUTIONS  1 10 STRUCTURAL OVERCOOLED MELTS, AMORPHOUS METALS AND INTERMETALLIC PHASES  0.5 5 MORPHOLOGICAL OR TOPOLOGICAL NANO SIZED DISPERSION OF PHASES WITH HIGH SURFACES (TO VOLUME)  0.1 1

Kinetic consideration for the metastability formation Basic concept: relation between the rate of the energy subtraction and time-scale necessary for the atomic rearrangements in the case of delay Gm state will be frozen

Formation of metastable structure by melt quenching:

Property modification by grain size: Hall―Petch-equation:

Morphological metastability: nanostructured materials

Morphological metastability: clusters in condensed materials Materials with high specific surfaces Cluster: a set of small number of atoms that permanently or temporarily coincide during an observation process. Binding strength depends significantly on the number of atoms that form the cluster. The “size-related” properties between the individual properties of constituent atoms and the thermodynamically stable macroscopic properties. All properties are valid (in the thermodynamic sense) only for the macroscopic material!

Sintering (example for morphological metastability and technology based on it)

Tsintering  2/3 Tmelting The driving force behind the sintering process is to reduce surface energy: For example: in the case of Al2O3 powder with particle size of1μ, the surface of 10 cm3 material ≈ 1000 m2, and the interfacial energy is approx. 1 kJ. The change of density as a function of time and temperature: a: particle size C: constant Q: activation energy

Compositional metastability: k0: compositional partition coefficient in equilibrium state v: cooling rate

How does free enthalpy change during phase transition How does free enthalpy change during phase transition? (amorphous-crystalline, liquid-crystalline, crystallization from supersaturated solid solution) Assumed free enthalpy diagram for depicting the formation conditions of amorphous and crystalline states (am – amorphous phase,  – solid solution,  – compound) [21]

Slope of T0 curves and maximum supersaturation, solidification without compositional partition, phenomenon of glass forming What are the boundary conditions? Forming of supersaturated, crystalline solid solutions. Forming of metallic amorphous states (glassy alloys)