FRACTURE PLANES AND TOUGHNESS OF STOICHIOMETRIC FEAL SINGLE-CRYSTALS

Stoichiometric FeAl single crystals were grown containing a small amou nt of Al4C3-precipitates due to a residual concentration of carbon int roduced by the high purity iron slug. First K-IC-measurements of these crystals in 4-point-bending tests resulted in high room temperature ( RT) fracture toughness values between 16 and 25 MPa root m depending o n environment. Samples tested in water vapor atmosphere were found to have {100} fracture planes after stable crack growth. This (100) cleav age fracture is caused by hydrogen embrittlement, which is assumed to be related to a weakening of the phase boundaries between the Al4C3-pr ecipitates and the FeAl matrix. Samples tested in vacuum or liquid nit rogen showed - in contrast to the literature - {111} fracture facettes associated with unstable crack growth. The {111} facettes are suppose d to be the cleavage planes of the FeAl matrix.