Plastic Deformation and Effective Strain Hardening Coefficient of Irradiated Fe-9wt%Cr ODS Alloy by Nano-Indentation and TEM

The objective of this study is to characterize changes in the yielding, and effective strain hardening coefficient of an oxide dispersion strengthened (ODS) alloy upon exposure to irradiation. It is well known that irradiation produces a supersaturation of defects, which alters the mechanical properties of a material. In order to engineer materials for use in advanced nuclear reactors, the long-term effects of neutron irradiation on mechanical performance must be understood. However, high-dose neutron exposure is often simulated using ion bombardment. Unfortunately, ion irradiation results in a shallow damage layer that prevents traditional bulk mechanical characterization methods from being utilized. A technique with the ability to examine the thin film of irradiated damage is required to provide insight into the changes in yield stress, elastic modulus, and hardness. Nano-indentation experiments have thus become a powerful tool to analyze ion irradiated materials, but a thorough understanding of the plastic deformation that occurs during nano-indention is required to accurately interpret the results. In this work, a coupled experimental and modeling approach resulted in an understanding of the effects of irradiation on strain hardening in a model Fe-9wt%Cr ODS alloy. Nano-indentation was performed on the alloy before and after irradiation, either with 5.0 MeV Fe++ ions to 100 displacements per atom (dpa) at 400°C or with a fast neutron spectrum to 3 dpa at 500° C. Nano-hardness measurements reported similar hardening between the two conditions, which is supported by investigation of the microstructure. The size and shape of the residual plastic zone beneath nano-indents was characterized using transmission electron microscopy coupled with Automated Crystal Orientation Mapping (ACOM-TEM) techniques. A model developed from finite element analysis, using the spherical indenter approximation, was combined with the experimental results to calculate the effective strain hardening coefficient that resulted from irradiation induced defects. Results indicate a 39.2%, and 49.5% increase in strain hardening resulting from respective ion and neutron irradiation conditions, and a 10.9% between the two irradiations. The similar hardening yet slight variation in the effective strain hardening coefficient is thought to be due to the slight difference in the nature of the damage cascades developed under ion and neutron irradiation