A crystallographic examination of fracture mechanism in low cycle fatigue of pure iron at low temperature

Low-cycle fatigue tests were performed at-140°C using commercial grade pure iron having average ferrite grain size of 400 μ. A crystallographic examination of deformation twins around the fracture origination site has been carried out based on detailed observation of fracture surface using scanning electron microscope. The following two conclusions have been obtained. (1) For all the observed internal fracture modes in which internal cracks initiate at (i) the intersection of a deformation twin and a grain boundary (T-B type), (ii) the intersection of deformation twins (T-T type) and (iii) the intersection of a deformation twin and an inclusion (T-I type), the deformation twins which result in the final fracture of specimens belong to the {112} twinning group defined in this paper. ( 2 ) In the case of T-T type, an intersection of definite kinds of deformation twins among the {112} twinning group intersecting along {021} direction causes a cleavage fracture on {001} plane.