SEDL / STP / STP1125-EB / STP17866S

The Structure and Hardening Mechanism of Copper Precipitation in Thermally Aged or Irradiated Fe-Cu and Fe-Cu-Ni Model Alloys

Phythian, WJ
Section leader, principal scientific officer, and group leader, Harwell Laboratory, Didcot, Oxfordshire

Foreman, AJE
Section leader, principal scientific officer, and group leader, Harwell Laboratory, Didcot, Oxfordshire

English, CA
Section leader, principal scientific officer, and group leader, Harwell Laboratory, Didcot, Oxfordshire

Buswell, JT
Research officer, Nuclear Electric plc, Berkeley Nuclear Laboratories, Berkeley, Glos

Hetherington, M
Research fellow, Oxford University,

Roberts, K
Lecturer and postgraduate student, Strathclyde University, Glasgow,

Pizzini, S
Lecturer and postgraduate student, Strathclyde University, Glasgow,

Pages: 20    Published: Jan 1992

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Abstract

The reactor pressure vessel of a pressurized water reactor can be embrittled by neutron irradiation from the core It has been recognized for many years that copper plays a significant role in the embrittlement process and more recently that copper precipitates pin dislocations In developing fundamental understanding and mechanistic models to predict embrittlement, it is important to identify the microscopic mechanisms underlying this phenomena Although considerable progress has been made recently on characterizing the irradiation-produced microstructure, relatively little is known about the nature, morphology, and composition of the copper precipitates or the atomic processes whereby they pin dislocations The purpose of this paper is to discuss recent results from experimental studies and computer modelling of the properties of the copper precipitates formed after irradiating or thermally aging model Fe-Cu and Fe-Cu-Ni alloys.

Results will be presented from transmission electron microscopy (TEM), atom probe field-ion microscopy (AP/FIM), small angle neutron scattering (SANS), and extended X-ray absorption fine structure analysis (EXAFS) on the size dependence of the nature, composition, and internal structure of these precipitates A particularly interesting feature of this is the transformation in thermally aged material from a body-centered cubic (bcc) lattice to a face-centered cubic (fee) lattice at sizes above approximately 5 nm Considerable insight into the properties of bcc copper has been obtained from a molecular dynamics simulation of a block of copper atoms lying on a (metastable) bcc lattice The lattice mismatch with the host iron lattice, the misfit pressure, the elastic moduli, in particular the shear modulus, and the transformation from bcc to fee have been simulated The interaction of dislocations with such a metastable precipitate will also be described Finally, the insight these studies give into the observed macroscopic hardness increase after irradiation or thermal aging will be discussed.