In-Situ Quantification of Strength and Plastic Deformation Mechanisms

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The world’s total consumption of metals keeps growing year by year with many applications such as steel structures in buildings, bridges, cars, planes etc. With their increasing use, the focus of researchers has shifted to understand the plastic deformation mechanisms of these metals at nano-micro scale to come up with strategies to increase the strength of these materials. The idea is to develop advanced materials with both high strength and ductility to avoid unexpected failures. The key question in this endeavor is “How can we quantitatively characterize different crystalline materials for plastic deformation and strengthening mechanisms?”

In order to make stronger materials, we need to understand what is happening during deformation of these materials. The first step is to understand two kinds of events that happen in these materials.

1. Plastic deformation mechanisms such as slip (dislocation motion), twinning, phase transformation, creep etc.

2. Strengthening mechanisms such as work hardening, solid solution strengthening, precipitation hardening, grain boundary strengthening etc.

Insights into these mechanisms are needed to be able to make stronger materials and develop processes for these materials. We need to understand the physics behind these mechanisms and then we need to understand the effect of these mechanisms on the mechanical properties of materials. Since these events are occurring at nano-micro scale, the reasonable way to understand them is by conducting experiments at nano-micro scale.

Scanning Electron Microscopy (SEM) is known among researchers as a very powerful technique to observe events at small scales. SEMs also have various accessories to observe different phenomenon such as EBSD (Electron Backscatter Diffraction) to look at the crystal orientations and STEM (Scanning Transmission Electron Microscopy) to look at nanometer scale features such as dislocation evolutions. We can utilize these techniques to look at grains, grain boundaries, phases, crystal orientations. If we can combine these powerful SEM techniques with nanomechanical testing, we can find answers to some of the most important questions in material science.

Until a few years ago, researchers would obtain EBSD maps and then perform tensile tests on macroscale samples. After failure, the samples were subjected to postmortem analysis to obtain EBSD maps and predictions were made on what could have happened. With the technological advancements in nano-micro mechanical testing, these changes in the microstructure can be observed and quantified while applying the load on the sample. This is a huge advantage of in-situ experiments.

One of the key things to note is that these plastic deformation and strengthening events are occurring at really small scales. So we also need to perform experiments on small samples to be able to observe each of these events individually. The smaller the sample, the better the chance is to isolate each mechanism and observe it in the stress-strain curve as shown in the picture below. We can then have a direct measurement of how these events are either strengthening or deforming the material. We can also measure how much energy is transferred when the grain boundaries are interacting or the dislocations are moving across them.

We cannot observe these events in large-scale tests because all these events are happening so fast that macro scale tests show an average response of all mechanisms. The whole idea is to directly observe these events in the stress-strain curve individually. The sample is loaded and the effects such as dislocations, twinning, phase transformations can be related to grain type, orientations etc. With the combined quantitative information of stress-strain and the correlative change in the microstructure, we can predict when and how a material will deform as well as develop strategies to develop materials of next generation.

You can find more details on this topic in my webinar - In-situ nanomechanical testing.

Hopefully, this article served as an introduction to the FemtoTools Nanomechanical Testing System. I will be publishing more articles on the understanding of advancements in mechanical testing at nano-micro scales in the near future. Please feel free to contact me with any questions.