Nanoindentation plastic zone size in irradiated ODS

dolph_nanoindentation

Nanoindentation is widely utilized to assess the evolution of mechanical properties in shallow, near-surface ion irradiated layers.  During each indentation, a plastic zone develops, the size of which increases during load application.  This plastic zone is generally believed to be 5-10 times the depth of the indent for most metals, but it precise size and morphology had not previously been quantified.  Further, the effect of irradiation on the extent of this plastic zone had never been studied.  Of greatest concern is that the plastic zone is so large – both in the direction of the indent and radially around the indent – that nanoindentation on the shallow near-surface ion-irradiated layer will oversample the unirradiated bulk substrate.

In this project, we consider a model Fe-9%Cr oxide dispersion strengthened (ODS) alloy in three conditions:  as-extruded, neutron irradiated, and Fe ion irradiated (which produces a ~1.2 μm deep irradiation damage layer at the surface).  Nanoindentation to various depths is used to probe irradiation hardening. The normalized plastic zone size from nanoindentation is calculated analytically based upon the expanding spherical cavity model, and also using finite element modeling. TEM analysis of cross-sections through a nanoindent, coupled with TEM orientation imaging, is used as a confirmatory step.

We determine that irradiation reduces the plastic zone size as compared to the as-extruded, unirradiated specimen.  Neutron irradiation produces hemispherical plastic zones surrounding nanoindents, with extensive plastic flow below the indent and in the radial direction.  In the as-extruded and ion irradiated specimens, the plastic zone is elongated, extending further below the indent than radially; but radial plastic flow is nevertheless non-negligible.  Hence, care must be taken to consider the plastic zone morphology when conducting nanoindentation experiments on shallow ion-irradiated layers; non-uniform irradiation damage levels can be sampled in a single nanoindent.

Collaborators:

  • Y.Q. Wu, Center for Advanced Energy Studies
  • P. Davis, Boise State University

Support:

  • DOE Nuclear Science User Facilities projects 14-485, 14-486, 15-540, and 15-569
  • US Nuclear Regulatory Commission award NRC-HQ-84-14-G-0056
  • US Nuclear Regulatory Commission award NRC-38-08-955

Products:

  1. C.K. Dolph, D.J. da Silva, M.J. Swenson and J.P. Wharry.  Plastic zone size for nanoindentation of irradiated Fe–9%Cr ODS.  Journal of Nuclear Materials 481 (2016) 33-45.  doi: 10.1016/j.jnucmat.2016.08.033
  2. M.J. Swenson, C.K. Dolph, and J.P. Wharry. The effects of oxide evolution on mechanical properties in irradiated 9wt% Cr ODS alloy. Journal of Nuclear Materials 479 (2016) 426.  doi: 10.1016/j.jnucmat.2016.07.022
  3. C.K. Dolph.  Plastic Deformation and Effective Strain Hardening Coefficient of Irradiated Fe-9wt%Cr ODS Alloy by Nano-Indentation and TEM.  M.S. Thesis, Boise State University, 2015.
  4. J.P. Wharry, M.J. Swenson, C.K. Dolph, and K.H. Yano. Evolution of yield strength of Fe-9%Cr ODS under neutron and ion irradiation. Transactions of the American Nuclear Society – 2016 Annual Meeting and Embedded Topical Meeting: Nuclear Fuels and Structural Materials, 114 (2016) 1255-1257.
  5. J.P. Wharry, M.J. Swenson and C.K. Dolph. Influence of irradiation particle and dose rate on strengthening mechanisms of model ODS alloy. International Conference on Plasticity, Kailua-Kona HI, January 2016.
  6. C.K. Dolph, D.J. Da Silva, and J.P. Wharry. Effective strain hardening coefficient for irradiated 9wt% Cr ODS alloy by nano-indentation and TEM. Materials Research Society Fall Meeting, Boston MA, December 2015.
  7. M.J. Swenson and J.P. Wharry. The strengthening mechanism transition in nanofeatured ferritic-martensitic alloys. The Minerals, Metals & Materials Society Annual Meeting, Orlando FL, March 2015.
  8. J.P. Wharry, M.J. Swenson, and C.K. Dolph. Microstructure-mechanical property relationship in self-ion irradiated ODS and F/M alloys. European Materials Research Society, Warsaw, Poland, September 2014.
  9. J.P. Wharry, M.J. Swenson, and C.K. Dolph. On the relationship between sink strength and irradiation hardening in an ODS steel. XXIII International Materials Research Congress (IMRC 2014), Cancún, Mexico, August 2014.
  10. M.J. Swenson, C. Dolph, J.P. Wharry. Correlation between the microstructure and mechanical properties of irradiated Fe-9Cr ODS. Transactions of the American Nuclear Society – 2014 Annual Meeting, 110 (2014) 421-424.
  11. J.P. Wharry, C. Dolph, J. Nielsen, and P. Davis. Irradiation-induced microstructure and mechanical property evolution in an Fe-9Cr ODS alloy. 8th International Conference on Processing & Manufacturing of Advanced Materials (THERMEC), Las Vegas NV, December 2013.
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