Crystallographic aspects of fracture in martensite

Abstract

The fracture of an iron-0.3 wt. % carbon alloy was investigated by transmission electron microscopy. As-quenched martensite, a number of tempered martensitic structures and a ferrite-pearlite mixture, were investigated. The crystallography of the fracture process was studied by means of electron diffraction patterns obtained from transparent areas adjacent to cracks in foils 5–10 μ thick. The fracture path was observed to be transgranular with respect to the martensitic crystallites, and inside each crystallite the crack was found to be broken up into a large number of segments. Histograms were constructed for the angular distribution of fracture segments, and fracture was shown to follow certain crystallographic planes preferentially. Room temperature fracture in as-quenched martensite took place on multiple planes with some preference for {100}. Fracture on {100} planes was also observed after tempering at 260°C and 340°C, but fractures on {211} and {321} planes were also found. In the case of martensite tempered at 480°C, and in the case of ferrite, fracture on {110} planes seemed to dominate for the room temperature tests. In untempered martensite fractured at—196°C, multiple fracture planes were still observed, but in ferrite {100} fracture was present almost exclusively. The latter observation is consistent with the cleavage fracture planes observed in bulk ferrite at low temperatures and served as an external check on the statistical procedures used in identifying the fracture planes. An estimate based on lattice registry requirements showed that the shear strains around ε-carbide precipitates do not favor any particular fracture plane. In the case of cementite particles, {211} M and {321} M have higher resolved shear strains than {110} M, and this is consistent with the observed fracture plane distribution for martensite tempered at 260°C and 340°C.