Abstract
This work was focused on two particular phenomena contributing to a damage process of nodular cast iron under tensile stress: Internal destruction of graphite nodule and debonding at graphite/matrix (G-M) interface. The G-M debonding was analyzed depending on the phase characteristics of the metal matrix and with the increase in the distance of the observation field from the main crack surface. Typical morphological effects of decohesion in the graphite-matrix microregions related to an internal structure of graphite nodule were revealed and classified. The obtained results of the microscopic observations suggest that the path of both types of internal cracks in the graphite nodule passed through areas of weakened cohesion. Detailed microscopic observations allowed revealing some additional phenomena associated with G-M debonding along the G/M interface. In the most ductile of the tested alloys, with ferritic and ausferritic matrix, the G-M debonding was preceded by the formation of a layer of shifted graphene plates in the external envelope of the spheroid. In the alloys of polyphase pearlitic and ausferritic matrix, the revealed morphology of the G-M interface suggests that G-M debonding might be delayed by the interaction with some phase components as cementite lamellae and austenite plates.
Highlights
Cast iron with spheroidal graphite is considered as a perspective structural material for designers of machine parts, due to the favorable ratio of strength to plasticity as compared to that of grey cast iron.Usually, an optimization of properties of this material for specific applications was carried out by a selection of its chemical composition and by controlling the casting technology [1,2,3]
According to the mesoscopic point of view proposed by Bonora [17], the damage course is determined by a sequence of irreversible
Detailed revealed that some effects effects of the the internaldestruction, destruction, in the Detailedmicroscopic microscopicobservations observations revealed revealed that that some some Detailed microscopic observations effects of of the internal internal destruction,ininthe the form of the characteristic micro-voids, were visible in the center of the spheroid’s cross-section, even formofofthe thecharacteristic characteristicmicro‐voids, micro‐voids,were were visible visible in in the the center form center of of the thespheroids spheroidscross‐section, cross‐section,even even inin those where the deformation deformation ratewas was thosespheroids spheroidslocated locatedmore morethan than55mm mm from from the the main main fracture, fracture, where in those spheroids located more than 5 mm from the main fracture, wherethe the deformationrate rate was already the material material damage
Summary
Cast iron with spheroidal graphite is considered as a perspective structural material for designers of machine parts, due to the favorable ratio of strength to plasticity as compared to that of grey cast iron.Usually, an optimization of properties of this material for specific applications was carried out by a selection of its chemical composition and by controlling the casting technology [1,2,3]. A material having a phase composition as that of the spheroidal cast iron cannot be considered as a continuous one. The effect of second phase properties and interface interactions should be taken into account. Many of the recently elaborated models of the mechanism of damage of ductile cast iron assume that graphite spheroids are no longer just “micro-voids”, but constituents of a microstructure with specific mechanical properties [8,9,10,11]. Morphological features of graphite particles, such as shape [8,12,13,14], degree of sphericity [15,16], size and distribution [17] have been taken into account as the parameters affecting the material destruction. According to the mesoscopic point of view proposed by Bonora [17], the damage course is determined by a sequence of irreversible
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