Strengthening mechanism transition in irradiated ODS steels

solid_solution_ods

Oxide dispersion strengthened (ODS) steels are generally believed to gain their strength from the fine dispersion of Y-Ti-O oxide nanoclusters throughout the Fe-Cr alloy matrix.  However, we (and many others) have observed significant changes to the oxide nanocluster morphologies with irradiation – specifically, dissolution of oxide nanoclusters evidenced by a reduction in their number density.  However, the irradiation does not change the total Y, Ti, and O concentrations in the bulk material.  Irradiation simply moves these species from nanoclusters, to somewhere else.

“Somewhere else” is substitutional (Y, Ti atoms) and interstitial (O atoms) solute positions in the Fe-Cr alloy matrix.  We observe an increase in matrix concentration of Y, Ti, and O after irradiation.

This leads us to surmise that the strengthening mechanisms change with irradiation.  Prior to irradiation, there are negligible solute concentrations of Y, Ti, and O in the metal matrix, so one need not account for their contribution to solid solution strengthening.  The majority of strengthening can be accounted for using the Orowan dispersed barrier hardening mechanism.  But after irradiation, solute concentrations increase such that solid solution strengthening can no longer be negated.  Our work has demonstrated that in addition to dispersed barrier hardening, solid solution strengthening must be considered especially for interstitial solutes, in order for the irradiated microstructure in ODS alloys to reasonably predict irradiation-induced hardening.

Collaborators:

  • P. Davis, Boise State University

Support:

  • DOE Nuclear Science User Facilities projects 14-485, 14-486, 15-540, 15-569, and 16-710
  • US Nuclear Regulatory Commission award NRC-HQ-84-14-G-0056
  • US Nuclear Regulatory Commission award NRC-38-08-955

Products:

  1. 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
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. 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.
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