Simultaneous magnetic investigations of Cu precipitation and recovery in thermally aged Fe-Cu alloy by first-order-reversal-curves

Satoru Kobayashi,Riko Kawagoe,Hiroaki Murakami

doi:10.1063/1.5007349

Abstract

We have measured first-order reversal curves (FORCs) for Fe-1wt%Cu alloy thermally aged at 753 K up to 20000 min. While hardness exhibits a maximum at around 1000 min, reflecting the formation and growth of Cu precipitates, major-loop coercivity monotonically decreases and becomes almost constant above 100 min.; an increase of coercivity associated with Cu precipitation is masked by a large decrease due to recovery. On the other hand, FORC diagrams exhibit two distribution peaks at low and high switching fields after aging. While the former shifts towards a lower switching field after aging, reflecting recovery, the latter shows up after aging up to ∼1000 min, possibly due to the formation of Cu precipitates. These observations demonstrate that FORCs are useful to separately evaluate competing microstructural changes in thermally aged Fe-Cu alloy where recovery and Cu precipitation take place simultaneously.

Highlights

Non-destructive evaluation (NDE) of neutron irradiation embrittlement in nuclear reactor pressure vessel (NRPV) steels has been one of important issues for long-term operation of nuclear power plants
As one of NDE methods, we have focused on a magnetic hysteresis method, because nanoscale defects act as pinning center for Bloch wall motion through the magneto-elastic interaction, resulting in a change in a shape of magnetic hysteresis loops
Fe alloy containing a small amount of Cu (0 1.0wt%) has been extensively investigated,[5,6,7] because thermal aging at high temperatures induces Cu precipitation and this situation is regarded as simulated irradiation embrittlement

Summary

INTRODUCTION

Non-destructive evaluation (NDE) of neutron irradiation embrittlement in nuclear reactor pressure vessel (NRPV) steels has been one of important issues for long-term operation of nuclear power plants. There are many works reporting irradiation-induced magnetic changes for NRPV steels,[2,3,4] the mechanism was not fully understood, primarily due to the lack of a direct comparison of magnetic results with microstructural observations, and competing mechanisms of magnetic property changes i.e. the formation of nanoscale precipitates, rearrangement of dislocations, strain relief in a matrix etc. 2) Coercivity continuously decreases with aging time for samples with and without 10% rolling reduction. This can be due to recovery, rearrangement of dislocations, stress relief of lattice strain in a matrix associated with precipitation. Though earlier work reported the existence of multiple FORC peaks,[11] their origin was not discussed in detail

EXPERIMENTAL PROCEDURE

RESULTS AND DISCUSSION

CONCLUSION