Pure aluminium is a very soft metal, not suitable for any kind of structural use. To make it stronger, some kind of alloying elements are always used.
Magnesium and silicon are the main components of the popular AA6000 series of alloys or magnesium. Zinc and copper are used in the AA7000 series, which is the primary aerospace series of alloys.
The alloying elements are first dissolved in the aluminium, then by controlling the materials temperature one may make them precipitate into numerous very small particles, in a process that is very similar to e.g. formation of fog in the atmosphere or salt crystals in an oversaturated brine. These precipitate particles reinforce the material, making it much stronger and more durable.
Metals are usually polycrystalline, i.e. they consist of a lot of very small crystals bundled together. The crystals are separated from each other by grain boundaries. When an alloy is heat treated to produce the reinforcing precipitate particles, these grain boundaries tend to suck the alloying elements out of the surrounding crystal, leaving no material to create the precipitates. As a result, a narrow zone around all the grain boundaries has no reinforcement in it and consists of more pure aluminium than the rest of the crystal. And as we remember – pure aluminium is soft.
The zone is called, rather self-explanatory, precipitate-free zone (PFZ). It is very narrow (tens to hundreds nanometres). It was discovered in the middle of the 20th century and has been studied quite a lot since then. When you have a soft part in your structure, you can expect the failure to happen in it first. This is what was often observed – the fracture in alloys with PFZ is usually tearing the grains apart from each other across the grain boundary without breaking the grains. The PFZ is first to yield and at some point under load small cavities start to nucleate in it and grow (due to its low strength) leading to the so called intergranular fracture. Yet not everything was clear. Sometimes even though the alloy contained PFZ, its fracture would look differently, some grains will not be separated from each other but rather split in two halves themselves. What kind of a process could lead to a stronger material breaking before the softer one?
To answer this question we need to have a good understanding of what exactly is going on in a crystal with PFZ under loading. This is a difficult task. The PFZs are too small to apply the standard mechanical tests to them. Separating them from the crystal and studying them as e.g. very thin films, is also not an option; the material behaviour depends a lot on the kind of surrounding we put it in. Even electron microscopy observations are rather difficult and only reveal a thin static slice of PFZ on the surface of the specimen, while in reality they are complex evolving 3D objects. Over the years a lot of observations were made and the experimental techniques were also evolving. So we can make some educated guesses, propose hypotheses and test them. We can hardly give final answers, but we can try to ask the right kind of questions.
In between scales
In the field of mechanics we have different levels or scales of modelling. Macroscopic objects are represented by macroscale phenomenological models, which the engineers use to design everything from bridges to soft drink cans. When we go deeper in the material and consider its microstructure, we start using the models of the next scale – the mesoscale and the appropriate equations, which deal with crystals, precipitates and grain boundaries etc. From the other side of the spectrum, on the microscale, we have the models that utilize the universal physical principles of quantum mechanics and may describe the behaviour of small groups of atoms – the First Principles models. The Molecular Dynamics models larger groups of atoms or molecules and the Dislocation Dynamics goes even higher in its scale and abandons the atomistic approach. The main objects for the Dislocation Dynamics are, as the name implies, the dislocations – the defects of the crystalline structure that include numerous atoms but are too small for the mesoscale methods. The PFZ lies in between these two worlds. It is too small for the macro or mesoscale models, it is just a very thin film that occupies a tiny proportion of the materials volume. But it is too big for the microscale models; it consists of trillions of atoms and dislocations.
Therefore the details of the plastic deformation evolution in and around PFZs and their role in the fracture initiation remain unclear.
At SIMLab, we have carried out experiments and reviewed available experimental and theoretical data on the dislocation arrangements in the PFZ. We propose an alternative approach, where PFZ are flowing with virtually no work-hardening. The combination of inter and intra-granular fracture is then achieved by a different mechanism than in the standard approach, in addition to reproducing other characteristic features of the plastic deformation, observed around the PFZ.
The goal of the project is not as ambitious as to “create an exact model of a PFZ”, but rather to try and model different aspects of PFZ on different levels, try to make a connection between the levels, try to understand the PFZ better.
A thin layer of crystal
My level is the crystal plasticity theory, i.e. the mesoscale. In the work we have done, we tried to represent the PFZ as a thin layer of crystal, but with very special properties in and around it. These properties arise from the level below – the microscale of dislocation dynamics. By connecting these two levels we created a model of a crystal with PFZ which may in principle explain why the fracture may happen both in the softer PFZ and a stronger grain interior. It is a very simplified and coarse model, but it shows some facets of the crystal behaviour we have seen in the microscope but haven’t seen in the computer model yet. So the next step will be – more observations, more models and, hopefully, better understanding of how and why the aluminium alloys deform and fracture.
The article was originally posted on SFI CASA’s Newsletter.
Mikhail Khadyko is a postdoctoral fellow at the Department of Structural Engineering, NTNU, working on the project “Closing the gaps in multiscale materials modeling of precipitation free zones in alloys”.