3-D models of multilayered structures on engineering scale from nanoscale damage profiles

von Mises stresses (MPs) for the three SS316 model at varying depths (a cutaway view is shown to outline the through-thickness stress contours). Credit: Australian Nuclear Science and Technology Organisation (ANSTO)

Computer modelling of nano-indentation studies performed on ion-irradiated steels has generated 3-D stress-field maps on an engineering scale that agree well with experimental results.

The material studied is annealed stainless steel 316, the most commonly used structural alloy in marine, chemical, petrochemical, transport, manufacturing and the nuclear industries.

In the study published in the International Journal of Plasticity, researchers from ANSTO and the University of New South Wales used ion irradiation, nano-indentation and electron microscopy to gain an insight into the relationship between peak damage depth and the corresponding peak hardness depth caused by irradiation.

“What we gain from the model is something you can’t observe experimentally, especially in a visual manner, that is, a three dimensional stress state in a multilayered structure as generated by ion irradiation,” said lead author Michael Saleh, an ANSTO materials researcher, who develops simulations of advanced materials in extreme environments.

“There have been models created with a hard and soft layer, but in these simulations we were looking at multiple layers at a nanometre scale where the gradient was high and the calculation of stresses was complex.”

In the simulations, at the position of peak hardness, the plastic strain contours exhibited a double dished plastic zone profile.

“This was nothing less than a revelation as the plastic zone was expected to be continuous spherical stress,” said co-author Dr Dhriti Bhattacharyya, a senior materials engineering researcher, who carried out the nano-indentation and analytical calculations.

The investigators also found a simple linear relationship between hardness peak depth and damage peak depth, which has implications more broadly for layered materials.

The goal of the research was to understand the effect of radiation on the mechanical properties of irradiated material. Ion irradiation provides a fast and non-active method of achieving high damage doses. However, this irradiation procedure causes different amounts of atomic displacements or damage to occur in the material at different depths, creating a thin layer of material with a large gradient in hardness (as a series of nanometre scale layers of varying strength). The study of the mechanical property changes through the thickness of the affected layer is highly challenging. Nano-indentation provides a relatively easy way of probing the modified surface; however, the interpretation of the results is complicated by the layered…

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