In abstract:

Against the Grain: Identifying the cause of deformation twinning in titanium alloys

Deformation in metallic materials with low crystal symmetry, such as titanium, zirconium and magnesium alloys, typically results in deformation twinning that greatly affects the microstructure generated during processing of the material. Twinning happens by reshuffling of atoms and results in a local reorientation of the crystal structure combined with a shear strain.

To date, there is no clear knowledge of twin nucleation criteria and therefore scientists have been unable to develop physically based models to predict microstructure evolution during processing such materials. One of the critical aspects of plasticity in general, and twinning in particular, is the effect of neighbouring grains, i.e. how much they promote or hinder certain deformation mechanisms. 

Researchers at The University of Manchester have carried out 3D Diffraction Contrast Tomography (DCT) studies at the European Synchrotron Radiation Facility (ESRF) on a deformed Ti alloy, which has allowed them to characterise twinning in context of the immediate neighbourhood. The 3D nature of this method has shown for the first time the existence of twin networks that are related to the orientation of the parent grains. The results of this research will be used in validating future plasticity models that attempt to incorporate twinning.

Click here to read the full article: https://doi.org/10.1016/j.actamat.2015.12.032

Q&A

  • What challenges did the researchers face in identifying the differences between the families of twinned and non-twinned grains?

The greatest challenge was to identify the twins, which are comparatively small to the parent grains and developing analysis tools to analyse grain neighbourhoods of individual grains or chains of grains.

  • How does 3D DCT work?

It is a combination of x-ray tomography and x-ray diffraction from individual grains. A small sample is rotated very slowly while absorption and diffraction images are recorded. This information enables one to reconstruct the microstructure in 3D with all the orientation information of each individual grain.

  • How may these findings benefit future researchers in this field?

The experimental findings can be used to validate future plasticity modelling activity that incorporates the twinning mechanism.

  • In which industries could this research be applied and how?

The Aerospace, Nuclear and Automotive industries all use materials that show twinning during deformation and processing. These materials are Ti-alloys (mainly used by the aerospace sector), Zr alloys (used to encapsulate nuclear fuel), and Mg alloys (used as lightweight material in cars).

Footnotes

The project was jointly funded by the EPSRC, UK (EP/F020910/1) and the European Synchrotron Radiation Facility, Grenoble, France.

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