19 May 2017
Materials & Manufacturing
A Look at the World of Widmanstätten Ferrite
This is one of the articles that was a finalist in the Ingenious Writers Contest organized by SARA and Substance ÉTS. It summaries an article co-written by Mohammad Jahazi, Professor in the Department of Mechanical Engineering at the École de technologie supérieure (ÉTS), entitled: Formation of Widmanstätten ferrite at very high temperatures in the austenite phase field.
Since ancient times, humanity harnessed the power of metals to manufacture tools, ensuring evolution and success in conquering nature … and other humans. Studies show that the first finely polished stone tools were used as far as 10,000 years BC. Later came gold, copper, silver, lead, and then the Bronze Age. By 1500 BC, smelted iron came into play and changed our perception of the world forever.
Tools and weapons made from iron, and later steel, gave a distinct advantage to those civilizations brave enough to explore this. Additionally, they received “God’s favor” with iron fallen from the skies. Space debris, or meteorites, have been revered throughout history as sacred stones with presumably magic powers imbued by the gods as their gift to man.
Meteorites have been around for many, many, many years. There are billions of them in fact. If you get to touch one, it will probably be the oldest thing you’ll ever touch in your life. Interestingly, the most ancient iron artifacts, dated 3200 BC, are beads made from meteorite iron. Amounting to about 5% of all meteorites recovered, iron meteorites are mostly made of an alloy of our beloved iron plus the lesser revered nickel. It is believed that these meteorites originated from the molten core of large space bodies such as planetoids.
Imagine two planets similar to Earth colliding and literally breaking apart. The molten iron core, now free from the earthly outer shell, would end up travelling in space for billions of years. Interestingly, the outer layer of a piece of core will cool down quickly, while the inner parts shall cool down at an extremely slow rate: somewhere between 100 – 10,000 °C over a period of a million years (a myr). Finally, the piece of core reaches our Earth for us to study and pick our scientific curiosity.
These extreme conditions lead to the formation of some interesting geometric patterns inside the solid piece of iron meteorite. These patterns are commonly known as Widmanstätten patterns/structures. Unfortunately, we cannot replicate this exact phenomenon here on Earth, under laboratory conditions, but this geometry appears in many other metal alloys, especially steel, titanium and zirconium alloys.
When present in steel with low carbon concentration, these structures are called Widmanstätten ferrites and can appear at very high temperatures within one of the many crystalline phases of steel. See Figure 3.
Figure 3 Iron-carbon (Fe-C) phase diagram. This diagram shows the required conditions to form different atomic arrangements (phases) of steel based on the temperature of carbon concentration and alloy formation. Steel is between 0% and 2.06% of carbon mass. Cast iron is between 2.06% and 6.67%. Source.
Why is Detection of Widmanstätten Ferrite Important?
As mentioned before, the use of iron, and in recent times steel, has a significant importance in our society. Depending on the desired application, a piece of steel can be ductile and flexible or, on the contrary, present higher strength and brittleness. The presence of Widmanstätten structures is an indicator of important structural changes within the material and, therefore, the extent to which its physical properties will be affected.
This is the objective of a recent study presented by a team of materials and mechanical engineers at McGill University and École de technologie supérieure in Montreal, Quebec. In this study, their goal is to characterize the dynamic transformations that take place within one of many steel’s crystalline phases, called austenite, where Widmanstätten ferrite is formed at high temperatures. See Figure 3.
In material mechanics, one can induce transformations in the atomic structure in two ways:
- Reconstructive transformation: You basically break all the bonds inside a sample and rearrange all the atoms into a new crystalline structure. In the case of a Fe-C alloy, this allows the carbon atoms to diffuse within the iron material, and reorganization favors the best possible arrangement. Internal strains within the material are minimized since every component falls neatly into place.
- Displacive transformation: This takes place when the crystalline structure changes through physical deformation of the periodic atomic arrangement, the so-called lattice. In this transformation, the diffusion of materials is not necessary but, depending on the type of stress, it can be present nonetheless. The price to pay here is the internal strain created within the atoms of the sample. This internal strain may yield unstable crystalline phases.
The tests performed by the team of researchers were displacive transformations that combined compression and strain stress applied to the samples. Measuring the material response, at temperatures ranging from 1000–1350 °C, they could detect the dynamic transformations that produced ferrite micro-plates whose geometrical patterns emulate the Widmanstätten pattern. At the highest temperature, they obtained clear perpendicular ferrite plates that matched the pattern perfectly, as can be seen in Figure 4.
Finally, the team concluded with a model of the critical values of temperature and strain that trigger the recrystallization and yield the appearance of Widmanstätten ferrite within the austenite phase. The presence of this ferrite carries significant weight in the mechanical properties of the steel sample.
Large sections of steel used in plates for structural support are usually austenite steel types, displaying coarse grain sizes to its structure. When right conditions of material stress are met, such as the ones present with heavy loads in construction, coarse austenite favors the formation of Widmanstätten ferrite, changing its characteristic tensile stress-strain behavior. It has been shown that depending on the volume fraction of Widmanstätten ferrite present, changes in material strength can be detected. A low volume fraction yields lower material strength. Conversely, a relatively large volume fraction leads to higher resistance to deformation.
Even of greater concern is the effect of Widmanstätten ferrite to the potential detriment of material toughness. These Widmanstätten structures do not only occur when cooling steel under certain conditions, but also in those parts of welded seams or cast pieces that are subject to rapid cooling. Once again, they may not be particularly welcome because of their tendency to be harder and display higher brittleness than the regular ferrite/pearlite structures.
For example, low carbon steel, less than 0.4% in carbon concentration, possesses a natural ductility compared to that of steel of higher carbon concentrations. The appearance of Widmanstätten ferrite in low carbon steel makes it more brittle and defeats the purpose of acquiring a ductile steel. There is, however, an advantage to this increased brittleness: it is easier to machine carve the piece using a lathe or a drill press. Once the machining is finished, a proper annealing (heating) and posterior slow cooling process can facilitate the diffusion of carbon and bring back the initial ductility of the material.
For more information on this research, see the following reference article: Grewal, R.; Aranas, C.; Chadha, K.; Shahrian, D.; Jahazi, M.; Jonas J.J. 2016. “Formation of Widmanstätten ferrite at very high temperatures in the austenite phase field”. ScienceDirect, Volume 109, 1 May 2016, p. 23-31.
Luis Felipe Gerlein Reyes
Luis Felipe Gerlein R. is a Ph.D. candidate at ÉTS. His research interests include nanofabrication and characterization of optoelectronic devices based on lead chalcogenides, carbon-based nanostructures and perovskite materials.
Program : Electrical Engineering