Phase Transformation

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Transcript of Phase Transformation

Pearlitic Transformation


Based onMasstransportPHASE TRANSFORMATIONSDiffusionaltransformationDiffusion less military transformationBased onOrderPHASE TRANSFORMATIONSIst order nucleation and growth2nd order entire volume transformsNo change in compositionChange in composition

Polymorphic Transformations: Typically exhibited by single component systems where different crystal structures are stable over different temperature ranges. E.g. bcc-fcc transformation in Fe

Major phase transformations that occur in solid phase are due to thermally activated atomic movements.The different types of phase transformation that is possible can be divided into 5 groups: Polymorphic changes Precipitation Transformation Eutectoid transformation Ordering reactions Massive transformation .

Precipitation Transformations: Generally expressed as + where is a metastable supersaturated solid solution is a stable or metastable precipitate is a more stable solid solution with the same crystal structure as but composition closer to equilibrium

Eutectoid Transformations: Generally expressed as +Metastable phase () replaced by a more stable mixture of + Precipitation and eutectoid transformations require compositional changes in the formation of the product phase and consequently require long-range diffusion

Massive Tranformations: Generally expressed as Original phase decomposes into one or more new phases which have the same composition as the parent phase but different crystal structures

Ordering Transformations: Generally expressed as (disordered) (ordered) . These do not require long range diffusion

PHASE transformation- Change in crystal structure+ Change in composition.Surface creations always hinders the process of transformation. The new phase always trys to create the surface, so energy needs to be supplied. So volume free energy will try to decrease the energy but surface free energy will try to increase the energy.

Fv=VfV= Vol of the new crystalf=free energies of the new phase Fs = ss = surface area of the new crystal = free energy per unit area

Mechanism of phase transformation :Changes of phase in the solid state involve a redistribution of the atoms in that solid and the kinetics of the change necessarily depend upon the rate of atomic migration. The transport of atoms through the crystal is more generally termed diffusion. This can occur more easily with the aid of vacancies, since the basic act of diffusion is the movement of an atom to anempty adjacent atomic site.Let us consider that during a phase change an atom is moved from an -phase lattice site to a more favorable -phase lattice site. The energy of the atom should vary with distance as shown in Figure 1, where the potential barrier which has to be overcome arises from the interatomic forces between the moving atom and the group of atoms which adjoin it and the new site. Only those atoms (n) with an energy greater than Q are able to make the jump, where Q = Hm-H barrier is given, from the MaxwellBoltzmann distribution law, as proportional to exp [Q/kT ], where k is Boltzmanns constant, T is the temperature and Q is usually expressed as the energy per atom in electron volts.

fig 1

During the transformation it is not necessary for the entire system to go from to in one jump and, in fact, if this were necessary, phase changes would practically never occur. Instead, most phase changes occur by a process of nucleation and growth. Chance thermal fluctuations provide a small number of atoms with sufficient activation energy to break away from the matrix (the old structure) and form a small nucleus of the new phase, which then grows at the expense of the matrix until the whole structure is transformed.

By this mechanism, the amount of material in the intermediate configuration of higher free energy is kept to a minimum, as it is localized into atomically thin layers at the interface between the phases. Because of this mechanism of transformation, the factors which determine the rate of phase change are: (1) the rate of nucleation, N (i.e. the numberof nuclei formed in unit volume in unit time) (2) the rate of growth, G (i.e. the rate of increase inradius with time). Both processes require activation energies, which in general are not equal, but the values are much smaller than that needed to change the whole structure from to in one operation.But Growth is more spontaneous process , because it already has surface to grow over it. With more (surface/volume) ratio it tends to go faster with less undercooling.Even with such an economical process as nucleation and growth transformation, difficulties occur and it is common to find that the transformation temperature, even under the best experimental conditions, is slightly higher on heating than on cooling.

The combined effect of (a) and (b) is shown in the curve below:

Where (a) is rate of crystal growth and (b) is rate of nucleation

This sluggishness of the transformation is known as hysteresis, and is attributed to the difficulties of nucleation, since diffusion, which controls the growth process, is usually high at temperatures near the transformation temperature and is therefore not rate controlling. Perhaps the simplest phase change to indicate this is the solidificationof a liquid metal.

The transformation temperature, as shown on the equilibrium diagram, represents the point at which the free energy of the solid phase is equal to that of the liquid phase.Thus,we may consider the transition, as given in a phase diagram, to occur when bulk or chemical free energy change,DGv is infinitesimally small and negative, i.e. when a small but positive driving force exists.However such a definition ignores the process whereby bulk liquid is transformed to bulk solid i.e. nucleation & growth.

When the nucleus is formed the atoms which make up the interface between the new and old phase occupy positions of compromise between the old and new structures, and as a result these atoms have rather higher energies than the other atoms. Thus, there will always be a positive free energy term opposing the transformation as a result of the energy required to create the surfaceof interface. Consequently, the transformation will occur only when the sum DGv + DGs becomes negative, where DGs arises from the surface energy of solid/liquid interface.Normally, for the bulk phase change, the number of atoms which form the interface is small and DGs compared with DGv can be ignored.However, during nucleation DGv is small, since it is proportional to the amount transformed, and DGs, the extra free energy of the boundary atoms, becomes important due to the large surface area-to-volume ratio of small nuclei. Therefore, before transformation can take place the negative term DGv must be greater than the positive term DGs and, since DGv is zero at the equilibrium freezing point, it follows that undercooling must result.


Undercooling: It is the gap between the temp predicted for the transformation to occur and the temp at which the transformation actually occurs.

During the cooling of a liquid, solidification (nucleation) will begin only after the temperature has been lowered below the equilibrium solidification (or melting) temperature Tm. This phenomenon is termed supercooling or (undercooling). The driving force to nucleate increases as T increases.Small supercooling slow nucleation rate - few nuclei - large crystals.Large supercooling rapid nucleation rate - many nuclei - small crystals.

Effect of degree of undercooling on the rates of nucleation and growthTammanns curve

The transition from a highly disordered liquid to an ordered solid is accompanied by a lowering in the energy state of the metal and the release of thermal energy (latent heat of solidification), forming the arrest on the cooling curve shown in the previous figure. This ordering has a marked and immediate effect upon other structure-sensitive properties of the metal; for instance, the volume typically decreases by 16%, the electrical conductivity rises and the diffusivity, or ability of the atoms to migrate, falls.Solidification is a classic example of a nucleation and growth process. In the general case of freezingwithin the bulk of pure molten metal, minute crystalline nuclei form independently at random points. After this homogeneous form of nucleation, continued removal of thermal energy from the system causes these small crystalline regions to grow independently at the expense of the surrounding melt. Throughout the freezing process, there is a tendency for bombardment by melt atoms to destroy embryonic crystals; only nuclei which exceed a critical size are able to survive.

Rapid cooling of a pure molten metal reduces the time available for nuclei formation and delays the onset of freezing by a temperature interval of dT. This thermal undercooling (or super cooling), which is depicted in previous figure , varies in extent, depending upon the metal and conditions, but can be as much as 0.10.3Tm, where Tm is the absolute melting point. However, commercial melts usually contain suspended insoluble particles of foreign matter (e.g. from the refractory crucible or hearth), which act as seeding nuclei for so-called heterogeneous nucleation. Undercooling is much less likely under these conditions; in fact, very pronounced undercooling is only obtainable when the melt is very pure and extremely small in volume. Homogeneous nucleation is not encountered in normal foundry practice.

The growing crystals steadily consume the melt and eventually impinge upon each other to form a structure of equiaxed (equal-sized) grains (in upper 2 figures).