Question

What is the difference between a solidus line and asolvus line? When is the information given in a phase diagram no longer accurate? Give an example of whenthis can occur.

Answer 1

Solidus line and solvus line -The solidus is the locus of temperatures (a curve on a phase diagram) below which a given substance is completely solid (crystallized). The solidus is applied, among other materials, to metal alloys, ceramics, and natural rocks and minerals.
The solidus quantifies the temperature at which melting of a substance begins, but the substance is not necessarily melted completely, i.e., the solidus is not necessarily a melting point. For this distinction, the solidus may be contrasted to the liquidus. 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.

solvus is a line (binary system) or surface (ternary system) on a phase diagram which separates a homogeneous solid solution from a field of several phases which may form by exsolution or incongruent melting. The line determines a solid solubility limit which changes as a function of temperature. It is a locus of points on the equilibrium diagram. An example is the formation of perthite when an alkali feldspar is cooled down.
In the other word, The solidus is represented by a line on a phase diagram that separates a solid phase from a solid + liquid phase region. The system is not completely solid until it cools below the solidus temperature.
The solvus is represented by a line on a phase diagram that separates a solid phase from a solid1 + solid2 phase, where solid1 and solid2 are different microstructures.

Information given in phase diagram are no longer accurate when-
A hypothetical phase diagram (see Fig below) illustrates such typical violations at points A to T. Most of these problems can also be demonstrated graphically with the use of appropriate free energy curves.
A: A two-phase field cannot be extended to become part of a pure element side of a phase diagram at zero solute. In example A, the liquidus and the solidus must meet at the melting point of the pure element.
B: Two liquidus curves must meet at one composition at a eutectic temperature.
C: A tie line must terminate at a phase boundary.
D: Two solvus boundaries (or two liquidus, or two solidus, or a solidus and a solvus) of the same phase must meet (i.e., intersect) at one composition at an invariant temperature. (There should not be two solubility values for a phase boundary at one temperature.)
E: A phase boundary must extrapolate into a two-phase field after crossing an invariant point. The validity of this feature, and similar features related to invariant temperatures, is easily demonstrated by constructing hypothetical free energy diagrams slightly below and slightly above the invariant temperature and by observing the relative positions of the relevant tangent points to the free energy curves. After intersection, such boundaries can also be extrapolated into metastable regions of the phase diagram. Such extrapolations are sometimes indicated by dashed or dotted lines.
F: Two single-phase fields (alpha and beta) should not be in contact along a horizontal line. An invariant temperature line separates two-phase fields in contact.
G: A single-phase field (alpha in this case) should not be apportioned into subdivisions by a single line. Having created a horizontal (invariants) line at F (which is an error), there may be a temptation to extend this line into a single-phase field, ct, creating an additional error.
H: In a binary system, an invariant temperature line should involve equilibrium among three phases.
I: There should be a two-phase field between two single-phase fields. Two single phases cannot touch except at a point. However, second-order and higher-order transformations may be exceptions to this rule.
J: When two phase boundaries touch at a point, they should touch at an extremity of temperature.
K: A touching liquidus and solidus (or any two touching boundaries) must have a horizontal common tangent at the congruent point. In this case, the solidus at the melting point is too "sharp" and appears to be discontinuous. L: A local minimum point in the lower part of a single-phase field (in this case the liquid) cannot be drawn without an additional boundary in contact with it. In this case, a horizontal monotectic line is most likely missing.
M: A local maximum point in the lower part of a single-phase field cannot be drawn without a monotectic, monotectoid, syntec. tic, and sintectoid reaction occurring below it at a lower temperature. Alternatively, a solidus curve must be drawn to touch the liquidus at point M.
N: A local maximum point in the upper part of a single-phase field cannot be drawn without the phase boundary touching a reversed monotectic, or a monotectoid, horizontal reaction line coinciding with the temperature of the maximum. When an N-type error is introduced, a minimum may be created on either side (or on one side) of N. This introduces an additional error, which is the opposite of M, but equivalent to M in kind.
O: Phase boundary cannot terminate within a phase field. Termination due to lack of data is, of course, often shown in phase diagrams, but this is recognized to be artificial.
P: The temperature of an invariant reaction in a binary system must be constant. The reaction line must be horizontal.
Q: The liquidus should not have a discontinuous sharp peak at the melting point of a compound. This rule is not applicable if the liquid retains the molecular state of the compound, i.e., in case of an ideal association.
R: The compositions of all three phases at an invariant reaction must be different.
S: A four-phase equilibrium is not allowed in a binary system.
T. Two separate phase boundaries that create a two-phase field between two phases in equilibrium should not cross one another.