Page 19 - Materials Australia - April 2019

INDUSTRY NEWS
Nanoindentation + Modelling = Innovation for Industry
Source: Sally Wood
In a series of three publications with collaborators at the University of New South Wales, ANSTO materials researchers Dr Dhriti Bhattacharyya and Michael Saleh have demonstrated that two different approaches to using nanoindentation for an assessment of the mechanical properties
of ion-irradiated steel provide different advantages.
Ion irradiation is a faster and safer way to simulate radiation damage to steel components caused in reactor environments, as compared to neutron irradiation. However, it causes the formation of a very shallow damage region with a steep gradient, the mechanical properties of which are difficult to probe.
Nanoindentation is a quick and effective technique used widely to perform such assessments.
The research suggested that a ‘top down’ indentation method, in which nanoindentation is performed directly on top of the irradiated surface, was better suited to samples in which the ion beam energy was low, resulting in a shallow peak damage depth.
Hardness profiles that were determined by ‘top down’ nanoindentation provided better spatial resolution for samples irradiated with ions at a single energy of 1 MeV, as the hardness peak is sharper for these samples. The hardness peak due to irradiation at 1 MeV is closer to
the surface.
As reported in a second paper in the International Journal of Plasticity, the oblique cross section method was better suited for samples in which the ion beam energy was higher at 2 MeV and 3 MeV.
In this method, the sample is mounted on an epoxy base at
an angle of 15 degrees from the horizontal, which exposes a large cross section of irradiated layers.
The investigators made a series of indents along a row which was rotated from the edge of the sample by about 4 degrees. Each indent was made to a depth
of 250 nanometers.
The indent depth and the plastic zone are much smaller in the oblique cross section approach.
In this orientation, the hardness peak was closer to the damage peak as the plastic field is much smaller and more sensitive to local hardness values.
In experiments for the first paper, Dr Bhattacharyya and collaborators found that the peak of hardness occurred at a depth that correlated directly with peak irradiation damage.
Shear stress at d=2000nm for the top down method (left) and at d=250nm for the OCS method.
TEM Bright Field (left) and TEM Weak-beam Dark Field (right) showing primary and secondary plastic zones.
Displacement damage profile and peak hardness values for both top down and OCS method for (a) 1, (b) 2, (c) 3 MeV samples.
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“The top down method is not
sensitive to changes in depth because the damage layer is very shallow and has a steep gradient,” said Dr Bhattacharyya.
“A prediction of the amount of hardening, based on a damage profile using this approach, relies on averaging, as the plastic zone intersects a multitude of layers containing different doses of radiation.”
“In contrast to the top down method, using the oblique cross section measurements you get a better correlation of hardness with dose, since the indents are much smaller, and the averaging is performed over fewer layers. The indents on the
cross section are sensing the layers with a
greater sensitivity due to a much smaller plastic zone,” said Dr Bhattacharyya.
The most recent work also provided experimental evidence using
Transmission Electron Microscopy (TEM) that nanoindentation of single energy
ion irradiated material caused plastic deformation with a ‘double dished’ primary and secondary plastic zone, which the authors had hypothesised about in their first paper.
“The modelling is very useful for looking at hardness differences in stratified structures because it doesn’t rely
on absolute values. The incremental increase in hardness difference due to radiation damage, whether in a single crystal or polycrystalline material, is commensurate,” said Michael Saleh, who carried out modelling to illuminate the complex deformation process in both approaches.
“We were able to apply the same modelling framework, Kinetic Monte Carlo, coupled to a simplified radiation- dependent hardening model and then implement it in finite element analysis,” said Saleh.