Fracture behaviour of crystalline Al₃Ni intermetallic fibres

Abstract

Fracture surface morphologies have been examined in detail in fibres of the intermetallic phase Al₃Ni using scanning electron microscopy. Relative crack propagation velocities were also obtained from Wallner line markings on fracture surfaces. Fibres with different crystallographic axes were identified and the nature of their brittle fracture surface morphologies related to their different fracture strengths.

The fracture strengths were found to increase with decreasing fibre cross-sectional area and this was a consequence of the existence of surface defects of varying severity.

In all cases the fracture surfaces were comprised of mirror, mist and hackle regions of various proportions. The proportion of mirror decreased with increasing fracture strength. The nature of the mirror surfaces indicated that these were not the result of crystallographic cleavage. Bifurcation of the propagating crack was observed in the hackle region with the interbranch angle consistenty in the range 60–80° and this was attributed both to the attainment of a critical stress intensity produced by crack advancement during mirror formation, and to the influence of the crack propagation velocity on the stress field near the crack tip. Critical velocities of 0·6 times the shear wave velocity were measured near the transition from mirror to hackle using Wallner markings.

It is concluded that the fracture behaviour of crystalline Al₃Ni intermetallic fibres has much in common with the fracture behaviour of brittle amorphous materials.