Online Lecture by prof. Jilt Sietsma

Quantifying dislocation structures, simply from tensile tests

Materials Science and Engineering, TU Delft, The Netherlands

Title:
Quantifying dislocation structures, simply from tensile tests

Abstract:
Since it is governing to a large extent the mechanical behaviour of metals, the dislocation structure is crucial in studying and applying this important class of materials. Although the theoretical aspects of dislocations have been developed through decades and are relatively well-known, experimental observation and quantification of dislocation structures is problematic. This is a strong drawback when aiming to optimise mechanical properties by tailoring the dislocation structure. Transmission Electron Microscopy can make dislocations visible (but with severe limitations, as will be explained in the lecture), X-ray diffraction can to a certain extent quantify the dislocation density (but not more than that and with limited accuracy) and numerical simulations at the atomic or microscale can provide insight into dislocation behaviour. These are all valuable tools, but limited when regarding the complexity of the dislocation structure.

On the other hand, it is well-known that the elastic part (i.e. before yielding) of a tensile curve is not as linear as text books often claim. The deviations from the Young’s modulus are caused by (partly reversible) motion of dislocations at stresses even well below the yield stress. This means that these deviations contain information on the dislocation structure. In the presentation it is shown how this information can be quantified, yielding values for the dislocation density and the average length of dislocation segments with an appreciable accuracy, provided that the pre-yield part of the tensile curve is measured accurately. This method does not reveal the full complexity of the dislocation structure, but it is a valuable addition to the experimental tools that we have to quantify the dislocation structure.

This session was chaired by prof. Dr. Erik Offerman (TU Delft).