The following is an excerpt.
Christian Oen Paulsen went by way of gold to bring our understanding of steel one step further.
When you quest for local strains in the microstructures of steel in a scanning electron microscope (SEM), contrast is essential. That is where the gold comes in. Simply, because if you sputter a layer of the precious metal on the surface of a steel specimen, contrast and resolution improve by one hundred times compared to spray paint.
The experimental technique of gold sputtering plays a central role in Christian Oen Paulsen´s fresh thesis on micromechanical modelling of steel.
A Golden Recipe
In 2017, the PhD candidate at CASA spent one month at the University of Manchester. There, he learnt the technique that helps him to see and measure the deformation and damaging mechanisms of different steels.
«The research community in Manchester worked with other materials, mainly nickel, magnesium and titanium. They were accommodating and shared their knowledge, tips and tricks with us. Thanks to them, we returned to NTNU with a recipe that we could use on steel», Christian Oen Paulsen tells.
More than 2 years after his stay in Britain, he has developed the Manchester-method further.
Challenge: From Layer to Particles
«The challenge of the technique is to get the gold layer to form particles that enable us to see what happens», Oen Paulsen explains.
He takes us on a guided tour through the Electron Microscopy lab in the 3rd floor of Department of Materials Science and Engineering at NTNU. All his tests have been performed here. In one of the corners, there is a water tap, a heater plate, glass tubes and containers. They are placed on a small table. The researcher has spent countless hours in these labs and demonstrates with enthusiasm how the workflow goes.
More than 100 tests
He has performed more than 100 tests with 6 different qualities of steel. It has been time-consuming work. First, he polishes each specimen to a mirror-like surface. Then, he coats them with a continuous layer of gold in the Gold Sputter Coater (left).
The next step is to remodel the layer into particles by placing the specimen on the hot plate.
For stainless steels, the specimens may be exposed to water vapour at the temperature of 300 degrees Celsius for one hour. This will decompound the gold layer and form particles. This is the simplest and fastest method. But for other types of steel, this vapour-process will cause corrosion.
Chasing for contrast
There is another way, too, to transform the gold layer to gold particles. Here, the specimen is kept at 180 degrees Celsius in a O2-free chamber. For 96 hours a mixture of argon and styrene is flowing across the surface.
«Different materials require different techniques», Oen Paulsen says.
Part of his research is comparing what you actually see in the SEM when different types of steel have been exposed to different techniques. Some methods do not provide sufficient contrast for DIC. Others, as the gold remodelling method, provides a finely dispersed gold speckle pattern on the surface, giving excellent contrast.
The image shows a remodeled surface showing the gold particles. These are random and finely dispersed throughout the specimen surface.
Improves our Understanding
He has studied different qualities of steel at micrometre-level. Throughout an in-situ SEM tensile test, he recorded representative areas of the patterned specimen. SEMs are designed to make images of the surfaces of tiny objects. In situ is an investigatory technique using the microscope to watch and collect the data of the sample’s response to a stimulus in real-time.
Oen Paulsen explains: «The microscope fires a beam of electrons onto the specimen which bounces back with an intensity based on the atomic weight. Alternatively, you can capture signals with an intensity based on the surface topography. This is utterly different from a Transmission Electron Microscope. In a TEM, the beam of electrons goes right through the sample».
The PhD candidate combines tensile testing with digital image correlation (DIC). DIC is an optical technique for measuring strain and displacement. Digital photographs of a component or test piece are compared at different stages of deformation. By tracking blocks of pixels, the system can measure surface displacement and build full-field 2D and 3D deformation vector fields and strain maps. The result is a strain map (below) with a spatial resolution capable of resolving strains at a sub-grain level.
Help the Design of Better Steel
Oen Paulsens goal is to link the local behaviour in the microstructure of different steels with the global performance of the material. Naturally, to help the design of better steel. Combining SEM and DIC the PhD candidate and his gold particles have paved the way for studying how steels behave under strain at a lower scale than was possible before.
And we are talking small. He is looking at particles, cracks, fractures and patterns down to the sub-micrometre level – less than 0,001 millimetres, that is.
His supervisor, Associate professor Ida Westermann, declares that Christian Oen Paulsen improves the understanding of the initiation of deformation in the steel.
His work leads to better methods and a step forward for CASA´s material models.
Microstructural level: A strain map of a steel specimen with gold particles.. The map is calculated using the DIC software of researcher Egil Fagerholt, based on the images recorded with an SEM during a tensile test).
A joint effort
Now, he stores the specimens from different experiments in small plastic containers. They lay in stacks in one corner of his desk. He defends his thesis «Experimental Characterization of Two-Phase Steels» on 21 October 2019. Then he returns to USA where he now lives with his wife.
His work is part of a joint effort between CASA and affiliated researchers looking for better methods to improve the analysis of materials and structures.
«The starting point is the sputtering technique from Manchester. When combining my work with CASA-researcher Egil Fagerholt`s DIC software ECorr, and SINTEF Research Scientist Afaf Saai´s Finite Element simulations, this is starting to become very interesting», Christian Oen Paulsen says.
Throughout his work, he has undertaken several studies to verify the technique and methods used.
«DIC functions like an answer book, our joint effort enables us to improve and verify the numerical models. It took a lot of time and effort to bring us here. Hopefully, this could be investigated further».
Different use of SEM
His supervisor, Ida Westermann, says the steel industry should pay attention to the new knowledge coming up. «Modern steels behave differently compared to what they did 40 years ago. More accurate processing methods and other types of alloys demands for new and better fracture mechanic models for steel», she says.
Christian Oen Paulsens PhD journey in steels started out with his Master´s thesis on aluminium, spring 2015: «Recrystallisation Behaviour in Extruded Profiles of Non-Dispersoid Containing Al-Mg-Si Alloys».
«Working on the MSc thesis gave me valuable training on the SEM. Now, I use the microscope differently on different materials. Several people are working in this field, and I enjoy being part of it».
Christian Oen Paulsens supervisors are associate professor Ida Westermann (main) and Professor Tore Børvik.