Liquidus

Uquidus curve The freezing point of a molten mixture of substances varies with the composition of the mixture. If the freezing points are plotted as a function of the composition, the line joining the points is called a liquidus curve. Such mixtures usually freeze over a range of temperature. If the temperature at which the last traces of liquid just solidify (assuming that sufficient time has been allowed for equilibrium to be established) are plotted against composition the resulting line is called a solidus curve. 

The liquidus consists of the curves AC, CDE and EB the solidus comprises the horizontal hnes FCG and IlEJ as well as the vertical... 

It is clear from the figure that the presence of a stable compound is characterised by (i) a maximum point on the liquidus, (ii) a meeting... 

Liquidus Temperature. The Hquidus temperature determines the susceptibiUty of a glass to devitrification and therefore influences its forming limitations and often its heat-treating requirements. 

Fig. 2. Liquidus of isotherms of gold-copper-silver alloys and phase diagrams of the binary constituents (83).

Fig. 3. The viscosity and liquidus curves for molten sodium silicates, where the numbers on the dashed lines ate viscosity ia Pa-s (1).

Fig. 2. Typical binary phase diagram for host and impurity, showing a constant distribution coefficient if impurity content is low. L = liquid composition after some solidification, a = B and small amount of A, /5 = A and small amount of B, = liquidus, and = solidus.

Fig. 4.5. Schematic of top left corner of the "silicon-impurity" phase diagram. To make things simple, we assume that the liquidus and solidus lines ore straight. The impurity concentration in the solid is then always less than that in the liquid by the factor k (called the distribution coefficient).

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

DEF. The phase boundary which limits the bottom of the liquid field is called the liquidus line. The other boundary of the two-phase liquid-solid field is called the solidus line. 

The liquidus lines start from the melting points of the pure components. Almost always, alloying lowers the melting point, so the liquidus lines deseend from the melting points of the pure components, forming a shallow V. 

DEF. The bottom point of the V formed by two liquidus lines is the euteetie point. 

Most alloy systems are more complicated than the lead-tin system, and show intermediate phases compounds which form between components, like CuAlj, or AljNi, or FojC. Their melting points are, usually, lowered by alloying also, so that eutectics can form between CuAlj and A1 (for example), or between AljNi and Al. The eutectic point is always the apex of the more or less shallow V formed by the liquidus lines. 

From 245°C to 183°C. The liquidus is reached at 245°C, and solid (a lead-rich solid solution) first appears. The composition of the liquid moves along the liquidus line, that of the solid along the solidus line. This regime ends when the temperature reaches 183°C. Note that the alloy composition in weight % (64) is roughly half way between that of the solid (81 wt%) and liquid (38 wt%) so the alloy is about half liquid, half solid, by weight. 

C. Consider the cooling of an Al-6% Si casting alloy. The liquidus is reached at about 635°C, when solid (Al) starts to separate out (top of Fig. A1.32). As the temperature falls further the liquid composition moves along the liquidus line, and the amount of solid (Al) increases. When the eutectic temperature (577°C) is reached, about half the liquid has solidified (middle of Fig. A1.32). The solid that appears in this way is called primary solid, primary (Al) in this case. 

When a metal is cast, heat is conducted out of it through the walls of the mould. The mould walls are the coldest part of the system, so solidification starts there. In the Al-Si casting alloy, for example, primary (Al) crystals form on the mould wall and grow inwards. Their composition differs from that of the liquid it is purer, and contains less silicon. This means that silicon is rejected at the surface of the growing crystals, and the liquid grows richer in silicon that is why the liquid composition moves along the liquidus line. 

The phase-diagram (temperature vs concentration) for a eutectic two-component alloy shows at low temperatures a central two-phase region and two solid one-phase regions at low and high relative concentrations. At the eutectic temperature the liquid phase at an intermediate concentration can all of a sudden coexist with the two solid phases. Upon further increase of temperature, the liquidus lines open up a V-shaped liquid... 

The following discussion concerns the thermal liquidus ranges available in different ionic liquids, as functions of cation and anion structure and composition. In particular, those structural features of cation and anion that promote these properties (while providing other desirable, and sometimes conflicting characteristics of the liquid, such as low viscosity, chemical stability, etc.) and variations in liquidus ranges and stabilities are the focus of this chapter. 

Alloy Density (g/cm ) Melting pt. (liquidus, °C) Coefficient of expansion X lO Electrical conductivity % I.A.C.S. Thermal conductivity (W/m°K) Tensile strength (MN/m ) Elongation % Vickers hardness... 

Silver-copper-palladium alloys with liquidus temperatures of 800-1 000°C have very low vapour pressures combined with good wetting and flow characteristics and are widely employed in vacuum work. They exhibit a lower tendency to stress corrosion than silver-copper, and do not form brittle alloys with other metals. 

Silver-palladium-manganese brazes possess excellent creep characteristics and have been developed for high-temperature applications involving the use of cobalt or nickel-based alloys, heat-resistant steels, molybdenum and tungsten. Their liquidus temperatures lie in the range 1 100-1 250°C. 

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