CHARACTERIZATION OF FRACTURE IN Fe3Al-BASED ALLOY USING THE SMALL PUNCH TEST

Abstract

The small punch tests at constant deflection rate were performed on two Fe 3 Al-based alloys with additions of niobium and carbon. The purpose was to study the effect of carbides on ductility and fracture behaviour. Three main quantities were evaluated: maximum force, deflection at maximum force and fracture energy. Temperature dependence of these quantities was fully corresponding to the microscopic image of the fractured specimens. Radial cracks were observed at low temperatures, the contraction of the disc thickness was negligible and the fracture surface was characterized by cleavage facets. On the other hand, a circumferential crack clearly separated spherical cap from the rest of specimen at the highest temperatures. Reduction of the specimen thickness was observed as well as the volume analogical to necking in tensile test. Fracture surface had a dimpled appearance. It was proved that both the fracture energy and the deflection at maximum load were greater in the alloy with the greater carbon and that the ductility was not reduced by the presence of niobium carbides. lower t� tap�qP$ue slow pulling rate. Microstructure analyses showed the composition of the surface zone was formed by ferrite and pearlite. Coarser ferrite was seen in the surface zone with the higher slab pulling rate and higher cooling rate in the secondary cooling zone. The surface zone microstructure was polyedric for the lower cooling rate and sporadically nonpolyedric with needle-like, or acicular ferrite for faster cooling. Brittle fracture test pieces showed fracture surfaces with transcrystalline cleavage facets (TCF) regardless of the applied cooling rate. With lower cooling rates, smooth facets of intercrystalline decohesion (FID) were identified too, but at less than 0.1%. With faster cooling they showed up in a few isolated cases only. The occurrence of dimpled transcrystalline ductile fractures (DTDF) was generally low. It was confirmed that the morphology of forced fractures was influenced by the cooling rate via the produced microstructure. The embrittlement of the tested samples was assisted by clusters and single particles. They were identified using EDX as based on Al, or combined with Ti, Nb nitrides, or carbide and sulphide eutectics, or inclusions ordered in rows in the ferrite network. Since the occurrence of intercrystalline fractures was low with faster cooling and high slab pulling rate, distinctive suppression of segregation can be assumed for this technology, if compared to slow cooling and low slab pulling rate.