Findings on the development  of voids in the material

It leads to cable breakage in mobile phones and defects in aircraft turbines: the Kirkendall effect. This causes composite materials to become porous and unstable. Prof Dr Bettina Camin wants to find out how porosity develops so that the defects can be better controlled. In a research project at Bremerhaven University of Applied Sciences, she is the first scientist to observe the voids in the material in real time as they develop. She recently left her laboratory to carry out experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The project is funded by the German Research Foundation (DFG).

The Kirkendall effect can always be observed where two different materials meet and are exposed to high temperatures. This results in diffusion currents in which atoms of the materials migrate to the other side. However, this does not happen evenly. "Let's take pure nickel on one side and a nickel alloy with aluminium on the other, for example. Pure nickel migrates much more slowly than aluminium. This initially creates tiny voids where the materials meet. Over time, these become larger and larger until the material is so porous that it fails,’ says Prof Camin. The interesting thing is that although this effect has already been well researched, it is currently not possible to predict exactly how these holes will develop. Their shape can also be very different: ‘At the beginning, the pores are spherical, but over time they change and then look like a drop, anchor or branched cactus, for example.’

To find out why this is the case, Prof Camin's research team is looking at the formation of the pores in real time. To do this, they are using synchrotron X-ray tomography. This method allows structures to be visualised in 3D with high resolution. The material to be analysed is heated up to 1,300 °C. 
The first pores become visible after just 24 hours."No research group has ever done this before.We hope that this will allow us to predict how the number and geometry of the pores will develop under certain conditions.This would make it easier to better control the defects in practice,’ says Prof Camin. 
Some of the complex experiments did not take place in the university's laboratories. Prof Camin successfully applied for a two-day research period at the European Synchrotron Radiation Facility (ESRF) in Grenoble. This is a multinational large-scale research facility whose measuring stations can be used by research groups from public institutes and private companies.‘Our research question was the development of damage in a composite of pure nickel and a nickel-based superalloy, as used in turbines, as a function of time, temperature and the crystallographic orientation of the material,’ says the professor.As the demand for research time is very high, this is only approved in rare cases.In order to use the time as efficiently as possible, the professor travelled to Grenoble with her four student assistants."I have an incredibly good group that works very independently and with great dedication.That's why we were able to split up for the experiments at the ESRF and work on different devices at the same time.I was even told how great the students were,’ says Prof Camin. 

Analysing the measurement data from the tests in Grenoble will still take some time:The team brought around 20 terabytes of data with them to Bremerhaven."We will certainly need six to twelve months before we have analysed everything.From an initial overview, the result so far is that the three varied parameters actually each have an influence on the damage to the material.We have also been able to carry out an experiment that has shown us that we can achieve the goal of our project,’ says Prof Bettina Camin. 
The project ‘Kirkendall porosity in diffusion pairs:Investigation of formation and morphologically unstable growth using synchrotron X-ray tomography and phase field simulation’ is a collaborative project with the Ruhr University Bochum.

Contact person
Prof. Dr.-Ing. Bettina Camin
Phone.+49 471 4823 402
Mail: bcamin@hs-bremerhaven.de