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Peritectic Reaction in Binary Phase Diagrams

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Peritectic Reaction in Binary Phase Diagrams Equilibrium freezing of Peritectic Alloys

Natural Freezing of Peritectic Alloys

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Natural Freezing of Peritectic Alloys

During rapid solidification of a cast alloy through the Peritectic temperature , a non equilibrium phenomenon called surrounding envelopment or encasement occurs.

L+During Peritectic reaction, phase created by Peritectic reaction surround or encase the primary .Since beta phase a solid phase and solid state diffusion is vey slow, the beta creates a diffusion barrier and reaction proceeds even at slow rate To sum up, When Peritectic alloy is cast rapidly, coring occurs during formation of primary alpha and encasement of the cored alpha by beta occurs during peritectic reaction.6Peritectic Alloys have mechanical properties that may be associated with microstructure in accordance with principles stated for eutectic alloys.

There are two special characteristics of peritectic alloys

i-The individual phases are ordinarily far removed from their respective states of homogenous equilibrium

i.e. Regular change in properties across the range of two phase alloy is not to be expected unless special steps such as working or heat-treatment has been taken to establish a state of equilibrium.

ii-Particle size of the cast peritectic alloy is rarely fineMechanical Properties of Peritectic AlloysIntermediate Phases or Compounds in Binary Phase DiagramsSo far in diagrams, we studied Terminal solid solution The eutectic coppersilver and leadtin phase diagrams have only two solid phases &These are called TERMINAL SOLID SOLUTION

Intermediate Phases or Compounds in Binary Phase Diagrams

For other alloy systems, intermediate solid solutions (or intermediate phases) may be found at other than the two composition extremes. Example: copperzinc system.Six different solid solutionstwo terminal ( and ) and four intermediate (,, and )(The phase is termed an ordered solid solution, where copper and zinc atoms are situated in a specific and ordered arrangement within each unit cell.)Intermetallic compound (or intermediate solid solution) : made up of two or more components, producing a new phase with its own composition, crystal structure and properties.Intermetallic Compounds in Binary Phase DiagramsFor some systems, discrete intermediate compounds rather than solid solutions may be found on the phase diagram.Example, magnesiumlead system. The compound Mg2Pb has a composition of 19 wt% Mg81 wt% Pb, and is represented as a vertical line on the diagram, rather than as a phase region of finite width; hence, Mg2Pb can exist by itself only at this precise composition.

Intermetallic Compounds in Binary Phase Diagrams

Intermetallic Compounds in Binary Phase DiagramsPhase transformations may be classified according to whether or not there is any change in composition for the phases involved. Those for which there are no compositional alterations are said to be congruent transformations.Example: melting of pure materialsConversely, for incongruent transformations, at least one of the phases will experience a change in composition.Example: Eutectic and eutectoid reactions, as well as the melting of an alloy that belongs to an isomorphous system, all represent incongruent transformations.CONGRUENT PHASE TRANSFORMATIONSBinary SystemsIn binary systems, compounds are composed of various ratios of the two end members (A & B), or the basic components of the system.These end members are assumed to melt congruently.The intermediate compound AB2 melts congruently, because at some temperature (the top of the AB2 phase boundary line) it coexists with a liquid of the same composition.

CONGRUENT AND INCONGRUENT MELTINGBinary SystemsThe end components in this binary phase diagram also melt congruently. The intermediate compound in this diagram (XY2) however is incongruently melting.Incongruent melting is the temperature at which one solid phase transforms to another solid phase and a liquid phase both of different chemical compositions than the original composition.This can be seen in this diagram as XY2 melts to Y and liquid.

CONGRUENT AND INCONGRUENT MELTING

CONGRUENT AND INCONGRUENT MELTINGASSIGNMENT-2Consider the titanium Nickel (Ti-Ni)Phase diagram. This phase diagram indicates points where three phases co-exist. For each of these three-phase points:List the coordinates of composition (wt %) and temperature for each point.Write the invariant reaction that occurs during slow cooling of the Ti-Ni alloy through each point.Name the type of invariant reaction that takes place at each point.

ASSIGNMENT-2Describe the equilibrium cooling process of an alloy of Ti-60 wt% Ni alloy as it slowly cools from 1400 deg C to room temperature, and sketch the expected microstructure at each temperature.TypeTemperatureCompositionPure Metal Melting Point (INVARIENT)1670C0wt%NiPure Metal Crystal Structure Change (INVARIENT)882C0wt%NiEutectoid786C5wt%NiEutectic942C28wt%NiPeritectic984C38wt%NiEutectoid630C55wt%NiCongruent Melting Point1310C55wt%NiEutectic1118C67wt%NiCongruent Melting Point1380C78wt%NiEutectic1300C85wt%NiPure Metal Melting Point (INVARIENT)1455C100wt%NiSolution Assignment -2

Solution Assignment -2

The alloy starts out in the single phase liquid region, with 100% mass fraction of liquid of composition 60wt%Ni.About 1300C the alloy crosses into the two-phase liquid + TiNi solid phase regionAbout 1100C, the alloy moves into the single phase TiNi solid phase region, with 100% mass fraction of TiNi of composition 60wt%NiAbout 800C the alloy moves into the two-phase TiNi + TiNi3 regionAt 630C, the alloy goes through a eutectoid decomposition into the two phase Ti2Ni + TiNi3 region (The TiNi disappears).