Liquid-Solid Phase DiagramsSolid and liquid phases can be present below the boiling point (e.g.,immiscible pair of metals right up to their melting points (As and Bi)2-component liquid at temperature a1:(1) a1 6 a2System enters “Liquid+B”pure solid B comes out ofsolution, remaining liquidricher in A(2) a2 6 a3More solid B forms, equalamounts from lever rule,liquid even richer in A(3) a3 6 a4Less liquid than at a3,composition given by e,liquid now freezes into atwo component system ofA and BComposition at e is known as the eutectic composition (easily melted),and the horizontal line at e is known as Te, the eutectic temperature.EutecticsLiquid with eutectic composition freezes at a single temperature, Te,without depositing A or B in advance of the freezing pointSolid with eutectic composition melts, without any composition change, atthe lowest temperature of any mixtureSolutions to the left of e deposit A asthey cool.Liquid + ASolutions to the right of e deposit B asthey cool.Liquid + BOnly the eutectic solidifies at a singletemperature (FN = 0 when C = 2 and P =3), no other components unloadedExamples:• Solder, 67% tin and 33% lead, m.p. 183oC• 23% NaCl, 77% H2O m.p. -21.1oC; saltadded to ice on a road (isothermal)mixture melts at T > -21.1oCReacting SystemsMany binary systems react to produce different compounds - oneimportant example is the formation of GaAs (gallium arsenide) which isvery important for the manufacturing of III/V semiconductors:Ga + As W GaAsSystem prepared with A (i.e., Ga) andexcess of B (i.e., As) consists of C (i.e.,GaAs) and unreacted B (i.e., As). The binary B,C system forms a eutecticThe important part of the phase diagramare the compositions of equal amounts ofA and B (x = 0.5), pure A and pure BSolid deposited along the cooling isopleth“a” is compound CBelow a4 there are two solid phases, withsome C and some BIncongruent MeltingSometimes component C is not stable as a liquid (e.g., alloy Na2K)Peritectic point(1) a1 6 a2Some Na deposited, liquidricher in K(2) a2 6 a3Just below a3, solid sample,with solid Na and solid Na2K(1) b1 6 b2No change until Na begins todeposit at b2(2) b2 6 b3Solid Na deposits, but reactionhappens to make Na2K (Katoms diffuse into solid Na)Here, liquid Na/K in eqb. with Na2K solid(3) b3 6 b4Amount of solid increases untilb4, liquid hits eutectic point,now two phase solid is formedIncongruent melting: The temperature at which one solid phasetransforms into another solid phase plus a liquid phase both of differentchemical compositions than the original substance (i.e., the peritectictemperature,