Self healing steel

In recent positron annihilation spectroscopy studies we have monitored the time-dependent precipitation process responsible for self healing in deformed Fe-Cu and Fe-Cu-B-N alloys during aging at 550 C [1]. Positrons are a uniquely suited for these in-situ studies as they have a strong affinity for open volume defects when they are implanted in a material. During their life time of 100-500 ps they are generally trapped at defect sites until they annihilate with an electron generating two 511 keV gamma particles. Measurements of the Doppler broadening, reflecting the momentum distribution of the electrons involved in the annihilation, gives a detailed insight in the concentration and chemical composition of the defect sites. The Doppler broadening gives a finger print of the chemical environment at the annihilation site. In its simplest form, two parameters S and W are derived from the Doppler broadened 511 keV gamma's, where parameter S reflects the spectral weight of the low-momentum electrons and W for the high-momentum electrons. A clear difference in response is observerd for as-quenched (self healing) and annealed (non-self healing) samples.

Complementary time-resolved small-angle neutron scattering (SANS) experiments were performed on the precipitation behaviour of deformed Fe-Cu and Fe-Cu-B-N alloys during aging at 550 C [2]. SANS measurements give a direct probe of the particle size distribution of the nanoscale copper precipitates within the bulk of the material. Two types of copper precipitation are observed: (1) spherical precipitates inside the matrix and (2) precipitation at dislocations at interfaces. The copper precipitation at defects is strongly affected by both deformation and microalloying with B and N,