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The microstructure of a material has a large effect on its properties. In order to understand the properties of a material it is necessary to consider many things including the chemical compositions, proportions, shapes and types of phase present as well as the defect distributions and crystal structure. For a given micrograph and associated phase diagram it is often possible to ‘plot’ a cooling route, suggesting which phases would precipitate out first and connecting this to what is seen in the micrograph.
Steel has very widely ranging applications, and a number of different commonly used forms. Many of these forms can be seen on the Fe-Fe3C phase diagram showing the equilibrium between iron and the cementite (Fe3C) phase. Many steels are described by the carbon content, they have less than 1.5% carbon, but most contain other additions. The interactive phase diagram below shows different regions with varying carbon concentrations, hover over them for more information.
The phase diagram above shows a very important example of a solid-solid transformation: the eutectoid transformation. The type of microstucture that forms is dependant on the cooling rate, due to diffusion limitations in the lattice.
One of these microstructures is the eutectoid lamellae structure. As a eutectoid steel cools below the eutectoid temperature pearlite lamellae form. Due to kinetic limitations carbon rejected from the ferrite phase cannot diffuse far away from the boundaries. This results in cooperative growth of the lower and higher carbon phases: ferrite and cementite respectively.
Increasing the cooling rate, taking the alloy further from thermodynamic equilibrium can result in shear transformations rather than diffusive. The transformation to martensite is an example of this.
Additions such as Cr, Si, Ni or Mn stabilise different phases. The diagrams below show how increasing concentrations of additions can change the stable phase at low and high temperatures. This is how we can have austenitic steel at room temperature, where ferrite and cementite are more stable and a diffusionless martensitic transformation would take place if the system (without additions) was quenched.
These show temperature vs concentration of stabilisers. The diagram on the left shows ferrite is stable at low and high temperatures for high concentrations of stabilisers. The one on the right shows that austenite can be stable at lower temperatures with C, Ni or Mn additions.
The steel phase diagram above is not actually the equilibrium phase diagram for the iron-carbon system, but due to kinetics Fe3C usually forms. For ferrous alloys with higher carbon contents graphite often (although not always) forms. These high C content alloys are referred to as cast irons, the three common types of which are ‘grey’, ‘spheroidal’ and ‘white’.
Cast irons usually contain some Si or other alloying additions, which often stabilise the graphite phase, such that it precipitates out even when the wt% present is less than 4.3% (the eutectic composition for the Fe-C phase diagram).
Usually contain more C or Si than white cast irons, and require a lower cooling rate. They are called ‘grey’ cast irons not because of their colour, but due to the appearance of a fractured surface. Grey cast irons are quite ductile and have unreflective fracture surfaces.
Steps on cooling:
These are similar to grey cast irons, but they contain ‘inoculants’ – alloying additions that change the form of the graphite precipitates. These inoculants are usually Mg or Ce (~0.1wt%) and they cause the graphite to grow in spheres rather than flakes.
There are two theories offering an explanation for this. The first describes the Mg or Ce impurities “poisoning” the graphite growth sites, attaching to them and slowing growth in that direction. The second suggests an increase in interfacial energy – the surface energy between the melt and the graphite, such that surface area per volume is minimised.
The cooling steps follow the same route as the grey cast irons with the graphite precipitates growing in spherical shapes.
These contain less Si or C than grey cast irons and undergo faster cooling. This results in cementite forming in favour of graphite. Again the name ‘white’ has little to do with the ordinary appearance of the alloy, but rather refers to the fracture surface. White cast irons are much more brittle than grey cast irons, and so their fracture surfaces are reflective, leading to their classification as ‘white’.
The cooling route depends on the composition of the melt, whether it is hyper – or hypo – eutectic (eutectic composition is at 4.3wt.%C). A hypereutectic composition leads to the cementite precipitating out first; a hypoeutectic composition leads to γ – austenite precipitating out first.
Note – “hypereutectic” has a higher carbon content than the eutectic composition.
“hypoeutectic” has a lower carbon content than the eutectic composition.
The first phase to precipitate out forms dendrites due to non-equilibrium effects; the cooling melt does not always follow the predicted composition on the phase diagram (see the page on the lever rule in the Phase Diagrams and Solidification TLP and the dendritic growth page in the Solidification of Alloys TLP).
When the eutectic is crossed the remaining melt solidifies as an austenite, cementite eutectic (ledeburite). The carbon continues to be ejected from the austenite as the alloy cools, diffusing to the cementite. At the eutectoid temperature the final transformation takes place from austenite to pearlite. In some very quickly cooled white cast irons the austenite may transform to martensite.
Brasses are copper alloys with zinc, see the Cu-Zn phase diagram.
From the copper-zinc phase diagram we can see the solid solubility of zinc in copper, for concentrations of zinc upto about 30 at.%, at equilibrium the alloy should be of a single phase.
Alpha brasses are often seen with a single phase, however this usually arises due to annealing. As the alloy cools α phase copper precipitates out first, changing the composition of the remaining melt. This may result in coring and dendritic growth as well as the formation of other phases such as the β phase when the zinc concentration in the remaining liquid is sufficiently high. Annealing the sample to aid diffusion means the composition becomes more uniform as zinc diffuses down the concentration gradient and a single phase predominates.
Another very common form of brass is α-β brass. α-β brasses have zinc concentrations of between about 30at.% and 45at.% and are two phase alloys.
The α phase precipitates out first and may form a Widmanstatten structure (see micrograph below), solidifying in plates along preferred growth directions.
Cu-Sn alloys are sometimes called bronzes, although this includes other kinds of copper alloys (e.g. with silicon and aluminium).
The peritectic reaction (see diagram above) is an important example of a microstructural transformation. Sn – 21wt.%Cu exhibits this transformation from a solid phase and a liquid phase to a different, solid phase.
Before the transformation begins the system is comprised of the ε phase and liquid. Below 415°C the equilibrium solid phase is η The peritectic transformation begins to take place at 415°C; the new phase precipitates heterogeneously on the surface of ε precipitates. The growing layer of η on the surface of the epsilon precipitates prevents the copper diffusing out to remove inhomogeneities, so some of the copper is trapped within the ε precipitates and the liquid has a lower Cu concentration than the bulk composition. This means the peritectic reaction never goes to completion (i.e. all liquid and solid going to the second solid). In this example the liquid continues to cool until it reaches the eutectic temperature, 227°C, when it transforms.
By considering thermodynamics (phase diagrams) and some kinetics it is possible to gain an understanding of how a microstructure comes about. Phase diagrams are very powerful tools in interpreting micrographs, but there also exist many microstructures that can only be explained by considering things like diffusion limitation.
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You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!
Where α and β are solid phases, which of the following describes a eutectic transformation?
Where α and β are solid phases, which of the following describes a peritectic transformation?
Which of the following alloying additions stabilises the austenite phase of steel? (You may select more than one)
What differences in appearance might you expect between annealing and deformation twins?
Why is spheroidal cast iron tougher than grey cast iron?
Porter, D. A. and Easterling K, Phase Transformations in Metals and Alloys, 2nd edition, Routledge, 1992.
G.A. Chadwick, Metallography of phase transformations, Butterworth & Co (Publishers) Ltd, 1972.
Academic consultant:Zoe Barber (University of Cambridge)
Content development: Catriona Yeoh
Photography and video: Brian Barber
Web development: Lianne Sallows and David Brook