Phase Diagrams Showing Solid-State Phase Transformations | Metallurgy

In this article we will discuss about the phase diagrams that show solid-state phase transformations by using eutectoid and peritectoid reaction.

1. Eutectoid Reaction:

Eutectoid reaction is best known for its occurrence in iron-carbon alloys, where it occurs because of the polymorphic transformation in iron, and is mainly responsible for the various types of heat treatments in steels. This reaction is quite common in solid state and has been seen to appear in many alloy systems such as Ag-Cd, Ag-Ga, Al-Mn, Au-Zn, Be-Cu, Cu-Zn, Cu-Sn, Cu-Si, Cu-Sb, etc.

Eutectoid reaction is similar to the eutectic reaction, but involves only solids. Here, a solid solution, upon cooling to some critical temperature, called eutectoid temperature, is seen to transform completely through alternate precipitation of two solid phases, both different from the parent solid solution. Occurring in solid state, and as the word ends in “- oid” indicates that the parent phase was a solid solution (and not a liquid solution).

In general the eutectoid reaction can be written as:

The product mixture-an extremely fine and intimate mixture of two solids (S₂ and S₃) is called eutectoid mixture. As it forms by alternate precipitation of two solid phases from the same parent solid solution, it resembles the solid eutectic mixture.

Put as it forms in the solid state, eutectoid mixture is typically a much finer mixture. Except that the parent phase is a solid solution, the interpretation of the phase changes illustrated by the V-shaped portion of the ϒ-field, Fig. 3.37, is identical with that of an eutectic reaction in which solid phases formed have limited solubilities.

The greater details of interpretation of Fe-Fe₃C diagram is drawn here to understand only the eutectoid reaction in that diagram, as an example. Point B is the eutectoid point, and 727°C is the eutectoid temperature, the invariant temperature.

Alloy of composition of point B is called eutectoid alloy (0.77% carbon steel is eutectoid steel). Alloys having compositions on the left of the point B (< 0.77% C) are called hypoeutectoid alloys, and alloys having compositions on the right of the point B are called hypereutectoid alloys (but as steels are iron-carbon alloys having a maximum of 2.11%C, hyper-eutectoid steels have carbon between 0.77% to 2.11%).

Eutectoid steel (0.77% C) has 100% ϒ (austenite) when cooled up to eutectoid temperature (727°C).

This alloy undergoes the eutectoid reaction at this temperature:

The eutectoid mixture of ferrite and cementite is called pearlite, with alternate lamellae of phases, Fig. 3.37 (f).

Just after the eutectoid reaction has been completed, i.e., at slightly below 727°C, Lever rule could be used to calculate the amount of these phases in the mixture (pearlite) by using FBD as ‘tie-line’:

As the alloy cools to room temperature, solubility of carbon in ferrite decreases as given by curve ‘FN’. The rejected carbon forms cementite and precipitates. This is called tertiary cementite. As its amount is very small, it may be assumed, that the microstructure, Fig. 3.37 (f) remains unchanged to room temperature. Eutectoid reaction occurs in all those alloys which fall in the composition range of eutectoid horizontal.

The cooling of hypoeutectoid alloy (Fe-0.4% C) is illustrated in Fig. 3.37. On cooling up to point A, the alloy has 100% austenite (0.4% C) as illustrated in Fig. 3.37 (b). At A, precipitation of ferrite phase begins.

As the carbon content of ferrite is much less than the alloy, the composition of the remaining untransformed austenite varies down along the line from A to B with the fall of temperature to 727°C.

Thus, as the alloy cools from A to E, more and more grains of ferrite form. Ferrite invariably forms at the high energy areas of grain boundaries. Lever rule can be used to calculate the amount of ferrite and austenite at just above the eutectoid temperature (727°C) by using ‘tie-line FEB’-

This amount of ferrite is called proeutectoid ferrite. The two phases are almost in equal amounts. As the austenite has the right composition and right eutectoid temperature just attained, it undergoes the eutectoid reaction. Thus, 50.67% of the alloy as austenite transforms at the constant eutectoid temperature to 50.67% of eutectoid mixture of ferrite and cementite (in proportion of 88.7: 11.3 respectively).

Thus, the microstructure after the eutectoid reaction consists of approximately 50% proeutectoid ferrite and 50% pearlite, Fig. 3.37 (d). This is also the microstructure at room temperature.

The cooling of a hypereutectoid alloy (Fe-1.2% C steel) is also illustrated in Fig. 3.37. This alloy on cooling up to M has 100% austenite. As the solid solubility of carbon in austenite decreases with the fall of temperature, the rejected carbon separates as proeutectoid cementite at the grain boundaries of austenite.

As the temperature drops below M to C, more cementite forms, the carbon content of remaining austenite varies down along M to B.

Just before the eutectoid temperature (727°C) is attained, the amount of proeutectoid cementite (6.67 % C) and austenite (0.77%C) are:

The phase Fe₃C is a compound and forms as network around austenite, Fig. 3.37 (h). Now this austenite has the right composition of 0.77% C (eutectoid point) and is at the right eutectoid temperature for the eutectoid reaction to occur. Thus, 92.71% weight of alloy as austenite transforms to 92.71% of eutectoid mixture, i.e., pearlite.

The microstructure consists of 92.71% of pearlite and 7.29% of proeutectoid cementite as network along the pearlite grains, Fig. 3.37 (f). There is little change as this alloy cools to room temperature.

As eutectoid reaction involves only solid phases, the eutectoid reaction is typically sluggish, and it can be undercooled. In extreme cases (Cd based alloys), fast cooling can suppress the reaction.

In steels, as the cooling rate increases, the formation of proeutectoid phase (ferrite or Fe₃C depending whether hypo, or hyper eutectoid steel respectively) can be suppressed, and the eutectoid reaction is delayed to a lower temperature, where the eutectoid mixture is much finer, which is stronger, harder and tougher than that produced under equilibrium conditions at 727°C. Further faster cooling may result in avoiding the formation of pearlite to produce a metastable phase, martensite.

2. Peritectoid Reaction:

This reaction is relatively uncommon, but has been seen to occur in Ni-Zn, Fe-Cb, Cu-Sb, Cu-Si, Cu-Sn, Ni-Mo, Ni-Si, Fe-Au systems. Peritectoid reaction is related to the peritectic reaction as does a eutectoid reaction to a eutectic. It occurs in the solid state (only solids are involved in it) due to thermal instability of a specific phase.

The peritectoid reaction in general is given by:

The product solid could be an intermediate phase, or a terminal solid solution. For example, intermediate phase β (secondary solid solution) is the product of peritectoid reaction in Ni-Zn system. The cooling curve of the peritectoid alloy (Ni- 48% Zn) is illustrated in Fig. 3.38 (a). This alloy on cooling, remains a liquid solution till it reaches its liquidus point ‘a’, where freezing of nuclei of α (a solid solution of Zn in Ni) starts.

As the temperature drops, more alpha solidifies as dendrites till it reaches the peritectic temperature, 1038°C. As the amount of α, in this alloy, at this temperature is more than needed for completely reacting with liquid (55% Zn) by the peritectic reaction, a part of unreacted α is present along with the solid peritectic product, ϒ, after the peritectic reaction is over at 1038°C. As the temperature of the alloy drops from b (1038°C) to c (810°C), some α changes to ϒ.

Lever rule could be used to calculate the amount of α and ϒ present just before the peritectoid temperature, 810°C is reached:

Thus, in Ni-Zn system, 14.29 wt % of α (36 % Zn) reacts completely with 85.71 wt. % of ϒ (50% Zn) to result in 100% β (48% Zn):

Thus, the microstructure consists of grains of β only under equilibrium conditions. This alloy on further cooling to room temperature does not undergo any change, i.e., shall show grains only of β in the microstructure at room temperature. Peritectoid reaction occurs in alloys of compositions which fall in the- composition range of the peritectoid horizontal.

The iron-columbium system shows the product phase of peritectoid reaction as a terminal solid solution. As is true of most solid-state reactions, a peritectoid reaction is usually quite easy to undercool drastically as the reaction is very sluggish. In many cases, the reaction could be avoided by faster cooling.

The invariant reactions considered so far here, are tabulated in short in table 3.2.