文档介绍：Chapter 2
Solidification of pure metals
Phenomena of metal solidification
Thermodynamic conditions of
solidification
Formation of crystalline nuclei
Crystal growth
Microstructures of metal ingots
Defects of metal ingots
Contents
Phenomena of metal solidification
Macroscopic phenomena
The cooling curve of pure metal solidification
Time
∆T
Temperature
Tm
Tr
Liquid-L
Solid-S
Liquid+Solid
Undercooling (Thermal undercooling)
The solidification process is initiated and proceeds
only when the metal is cooled below equilibrium
temperature of solidification Tm.
The degree of undercooling
The difference between the temperature Tm and
temperature Tr at which solidification proceeds
under a given condition, or ΔT= Tm – Tr
Factors influencing the undercooling degree
External factor: cooling rate R
R↑, Tr↓, ΔT ↑.
Internal factors: metal nature and purity
The purer the liquid metal, the more its tendency to
be undercooled. .
Latent heat of solidification
Appearance of the constant temperature plateau on the
cooling curve is associated with latent heat of
solidification.
Incubation period: solidification does not begin at once
when liquid metal was cooled to real solidification
temperature Tr, the undercooled liquid metal will keep a
time. The lasting time before solidification?
Microscopic processes of solidification
Grain
Grain boundary
Grain
Nucleation and growth of grains
Nucleation and growth are alternate for the whole
crystalline metal and single for a grain.
Thermodynamic conditions of solidification
Gibbs free energy per mole is defined as
G = H - TS = U + PV - TS
where H- enthalpy, S- entropy, U-internal energy,
P- pressure, V-volume, T- absolute temperature
Therefore dG=dU+PdV+VdP-TdS-SdT
Due to dU=TdS - PdV
Therefore dG=VdP - SdT
Under a constant pressure condition, or dP=0
Thus dG/dT=-S