While chemically pure materials have a single
melting point,
chemical mixtures often partially
melt at the solidus temperature (TS or Tsol), and fully melt at the higher liquidus temperature (TL or Tliq). The solidus is always less than or equal to the liquidus, but they need not coincide. If a gap exists between the solidus and liquidus it is called the freezing range, and within that gap, the substance consists of a mixture of solid and liquid phases (like a
slurry). Such is the case, for example, with the
olivine (
forsterite-
fayalite) system, which is common in
Earth's mantle.[1]
Definitions
In
chemistry,
materials science, and
physics, the liquidus temperature specifies the temperature above which a material is completely liquid,[2] and the maximum temperature at which
crystals can co-exist with the melt in
thermodynamic equilibrium. The solidus is the
locus of temperatures (a curve on a
phase diagram) below which a given substance is completely
solid (crystallized). The solidus temperature, specifies the temperature below which a material is completely solid,[2] and the minimum temperature at which a melt can co-exist with
crystals in
thermodynamic equilibrium.
Liquidus and solidus are mostly used for impure substances (mixtures) such as
glasses, metal
alloys,
ceramics,
rocks, and
minerals. Lines of liquidus and solidus appear in the phase diagrams of binary
solid solutions,[2] as well as in
eutectic systems away from the invariant point.[3]
When distinction is irrelevant
For pure elements or compounds, e.g. pure copper, pure water, etc. the liquidus and solidus are at the same temperature, and the term
melting point may be used.
There are also some mixtures which melt at a particular temperature, known as
congruent melting. One example is
eutectic mixture. In a eutectic system, there is particular mixing ratio where the solidus and liquidus temperatures coincide at a point known as the invariant point. At the invariant point, the mixture undergoes a eutectic reaction where both solids melt at the same temperature.[3]
Modeling and measurement
There are several models used to predict liquidus and solidus curves for various systems.[4][5][6][7]
For impure substances, e.g.
alloys,
honey,
soft drink,
ice cream, etc. the melting point broadens into a melting interval. If the temperature is within the melting interval, one may see "slurries" at equilibrium, i.e. the slurry will neither fully solidify nor melt. This is why new snow of high purity on mountain peaks either melts or stays solid, while dirty snow on the ground in cities tends to become slushy at certain temperatures. Weld melt pools containing high levels of sulfur, either from melted impurities of the base metal or from the welding electrode, typically have very broad melting intervals, which leads to increased risk of
hot cracking.
Behavior when cooling
Above the liquidus temperature, the material is
homogeneous and liquid at equilibrium. As the system is cooled below the liquidus temperature, more and more
crystals will form in the melt if one waits a sufficiently long time, depending on the material. Alternately,
homogeneous glasses can be obtained through sufficiently fast cooling, i.e., through kinetic inhibition of the
crystallization process.
The crystal phase that crystallizes first on cooling a substance to its liquidus temperature is termed primary crystalline phase or primary phase. The composition range within which the primary phase remains constant is known as primary crystalline phase field.
The liquidus temperature is important in the glass industry because crystallization can cause severe problems during the glass melting and forming processes, and it also may lead to product failure.[12]
^
abcAskeland, Donald R.; Fulay, Pradeep P. (2008-04-23). Essentials of Materials Science & Engineering (2nd ed.). Toronto: Cengage Learning. p. 305.
ISBN978-0-495-24446-2.
^
abCallister, William D.; Rethwisch, David G. (2008). Fundamentals of Materials Science and Engineering: An Integrated Approach (3rd ed.). John Wiley & Sons. pp. 356–358.
ISBN978-0-470-12537-3.
^Radomski, R.; Radomska, M. (1982). "Determination of solidus and liquidus temperatures by means of a Perkin-Elmer 1B differential scanning calorimeter". Journal of Thermal Analysis. 24 (1). Springer Science and Business Media LLC: 101–109.
doi:
10.1007/bf01914805.
ISSN0368-4466.
S2CID96845070.
^Liu, Gang; Liu, Lin; Zhao, Xinbao; Ge, Bingming; Zhang, Jun; Fu, Hengzhi (2011-03-31). "Effects of Re and Ru on the Solidification Characteristics of Nickel-Base Single-Crystal Superalloys". Metallurgical and Materials Transactions A. 42 (9). Springer Science and Business Media LLC: 2733–2741.
Bibcode:
2011MMTA...42.2733L.
doi:
10.1007/s11661-011-0673-4.
ISSN1073-5623.
S2CID135753939.
^Wallenberger, Frederick T.; Smrček, Antonín (2010-05-20). "The Liquidus Temperature; Its Critical Role in Glass Manufacturing". International Journal of Applied Glass Science. 1 (2). Wiley: 151–163.
doi:
10.1111/j.2041-1294.2010.00015.x.
ISSN2041-1286.