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SOLID-LIQUID INTERACTIONS . Zero-order reactions have a constant rate. This rate is independent of the concentration of the reactants. A first order reaction has a rate proportional to the concentration of one of the reactants. - PowerPoint PPT Presentation
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SOLID-LIQUID INTERACTIONS
• Zero-order reactions have a constant rate. • This rate is independent of the concentration
of the reactants. • A first order reaction has a rate proportional
to the concentration of one of the reactants. • A common example of a first-order reaction is
the phenomenon of radioactive decay.
Zero-order reactions
• A 0-order reaction has a rate which is independent of the concentration of the reactant(s).
• Increasing the concentration of the reacting species will not speed up the rate of the reaction.
• In zero-order reactions a material that is required for the reaction to proceed,
• Such as a surface or a catalyst, is saturated by the reactants
• A reaction is zero order if concentration data are plotted versus time and the result is a straight line
First-order reactions
• A first-order reaction depends on the concentration of only one reactant
• (a unimolecular reaction). Other reactants can be present, but each will be zero-order.
• Solidification and melting are transformations between crystallographic and non-crystallographic states of a metal or alloy.
• These transformations are of course basic to such technological applications as
• Ingot casting, foundry casting, continuous casting, single-crystal growth for semiconductors,
• More recently directionally solidified composite alloys
• Another important and complex solidification and melting process, often neglected concerns the process of fusion welding.
• An understanding of the mechanism of solidification and how it is affected by
• Such parameters as temperature distribution, cooling rate and alloying,
• Is important in the control of mechanical properties of cast metals and fusion welds.
• The objective our lecture is to develop some of the basic concepts of solidification,
• Apply these to the practical processes of ingot casting, continuous casting and fusion welding.
• We then consider a few practical examples illustrating the casting of engineering alloys in the light of the theoreticalbackground.
THE NUCLEATION OF SOLIDS IN LIQUIDS
• The liquid to solid transformation occurring by a first order process is discontinuous:
• The liquid does not change gradually into solid as it does in the glass transition.
• Before a first order solidification process can begin,• A nucleus of the solid phase has to exist in the
liquid, • Freezing then takes place by the growth of the
nucleus into the melt.
• Solidification in this sense requires the simultaneous presence of both the solid and the liquid phases
• An important feature of crystalline solids is that
• At constant pressure they have a characteristic equilibrium freezing/melting temperature, Tm.
• The solid and the liquid phases can only be in thermodynamic equilibrium at this unique temperature.
• we can also say that at the equilibrium freezing temperature
• The free energies of the solid and liquid phases are equal.• At temperatures above Tm , the liquid has a lower free
energy than the solid, • And the liquid is therefore the stable phase.• At temperatures below Tm the solid has the lower free
energy and is therefore the stable phase.
• when a liquid reached to a temperature below its thermodynamic freezing temperature
• It not immediately and spontaneously crystallize• A certain degree of “undercooling’ or ‘super
cooling’ of the liquid is usually necessary before the freezing process starts.
• If the undercooling is actually measured, a time— temperature graph such as is shown in Figure 3.2 is obtained.
• The liquid first under cools an amount T, • At this temperature the solid nucleates: • Growth then proceeds with the evolution of
latent heat, • The temperature rises to equilibrium freezing
temperature• ( more accurately, to slightly below
equilibrium freezing temperature,
• Since a small under cooling is required to drive the process in the direction of solidification.
• When solidification is complete and no more latent heat is evolved,
• The temperature of the solid continues to fall.• From this description of the freezing process, two questions immediately arises.
• (1) what are crystal nuclei? and • (2) why do they appear only at temperatures
significantly less than the equilibrium freezing temperature?
• To answer these questions we again have to consider the
• i) Atomic structure of liquids and solids, • Ii) Thermodynamic energy requirements for the
formation of a small crystal of the solid phase in the bulk liquid phase.
Homogeneous Nucleation• Consider a certain volume of liquid being progressively cooled. • The atoms of the liquid are in a state of constant agitation,• Therefore, the liquid exhibits no permanent long range order, • It is probable that some atoms will cluster together momentarily
into small regions having the crystal structure of the solid phase (embryos).
• These small clusters, called embryos, are potential nuclei for the solidification process.
• We can determine if the embryos will become actual nuclei by considering the free energy changes involved in their formation.
• If GL and Gs are the free-energies per unit volume of the liquid and solid phases respectively,
• the difference between GL and Gs ( Delta Gv) provides a negative energy contribution at temperatures less than TE (see Figure 3.1).
• There is, however, a positive energy term due to the creation of the solid—liquid interface.
• Let gammaSL represent the (positive) energy per unit area of the surface dividing the solid embryo from the liquid.
• The change in free energy corresponding to the formation of a sphere of solid of radius r is then given by
• The important thing to notice about this equation is that the surface energy term varies as the square of the radius,
• whereas the volume free energy term varies as the cube.
• The change in free energy varies with the size of solid sphere in a manner shown in Figure 3.3.
• When the solid clusters are small the surface energy term predominates;
• when they are large the negative volume energy term predominates.
• At the temperature to which the diagram refers, i.e. some temperature less than Tm,
• A cluster is stable only when its radius is greater than r, the critical radius;
• The free energy of sub-critical clusters would have to increase if they were to grow,
• This is thermodynamically impossible.• Consequently, embryos of radius less than r*
disappear, • Nuclei of radius greater than r grow larger.• The critical radius can be found from the equation 4.4• By differentiating G with respect to r and setting the
derivative equal to zero.
• It can easily be shown by differentiation of Equation 4.4 that
• The above formulation tells us what size embryo is required to form a critical nucleus at any temperature,
• But it does not say whether embryos of that size actually exist in the liquid at that temperature.
• Intuitively, we would expect a range of embryo sizes in the liquid at any temperature, and
• We would further anticipate that at the higher the temperature the smaller would be the largest-size embryos
• Because of the increased amplitude of atomic vibrations at the higher temperatures.
• It is also reasonable to assume that the larger the volume of liquid, the greater is the probability of an embryo of critcal size existing.
• Figure 3.4 the larger the supper cooling the smaller the critical nucleus size.
• Figure 3.5 shows the expected form of the ‘relationship between the maximum embryo size and temperature,
• A relationship we could confirm if we were to follow the argument through mathematically
• By superimposing Figure 3.5 onto Figure 3.4, • The point of intersection of the curves represents the
temperature at which embryos become viable crystal nuclei.
• Figure 3.6 is, therefore, the temperature at which solid will nucleate spontaneously in the liquid,
• And is called the homogeneous nucleation temperature. • It has been found empirically that for simple liquids, such as liquid
metals, • The amount of undercooling required for homogeneous nucleation • Is an approximately constant fraction of the melting point
(expressed in K )• For each material, T – O.2O Tm. The basic reason for this large
barrier t:
• The basic reason for this large barrier nucleation is the great difference in structure between the solid and liquid phases,
• The solid consisting of a regular arrangement of atoms on a crystal lattice
• whereas the liquid is best described as amorphous. • DeltaT(critical) is obviously the maximum undercooling
attainable• Since by definition a liquid cannot be (slowly)
supercooled to a temperature less than T*
• For the high temperature metals Delta T amounts to several hundred degrees centigrade:
• The homogeneous nucleation temperature of pure iron, for instance, is approximately 300 K below its equilibrium freezing temperature.
• From Equation (3.3) it can be calculated that the critical size nucleus for homogeneous nucleation contains in the region of 300 atoms
• It is known that the presence of foreign solids in the liquid, or
• Even the walls of the container in which the liquid is held,
• may serve to nucleate the solid heterogeneously at a temperature considerably above the homogeneous nucleation temperature.
• Using the example of commercial steels cast into ingot moulds,
• we know from experience that • The solid nucleates in the vicinity of the mould walls at
a temperature of only a few degrees less than the equilibrium freezing temperature
• The mould walls serve to chill the liquid locally and• Thus permit small solid particles in suspension in the
liquid to act as heterogeneous nucleation centres for the freezing process.
• Solidification begins at the outside of the ingot and progresses inwards with time.
• The purest commercial metals and alloys always contain sufficient foreign bodies, or inclusions,
• To serve as heterogeneous nucleation catalysts even if the mould walls are themselves inert in this respect.
• Very rarely, and then only under the most careful experimental conditions, will a liquid supercool to its homogeneous nucleation temperature.