Phase transformations and microstructures of alloys in the Nb-Cr-Al system / Transformações de fase e microestruturas de ligas no sistema Nb-Cr-Al

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

The interest for new structural materiaIs for application in high temperature has created a large volume of publications concerning refractory metal-based alloys that contain intermetallic compounds. The low fracture toughness of those alloys leads to the search of combination of phases that guarantees high strength and ductility. In such a case, the best choice has been the use of composite materiaIs, mainly the in-situ composites, where the formation of the solid phases processes under thermodynamic equilibrium, as in the eutectic transformations. In terms of high temperature applications, the Nb-Cr-AI system is very promising, as it presents several eutectic reactions, that result in the formation of intermetallic compounds. This work deals with obtaining, processing and characterization of alloys in the Nb-Cr-AI system. Samples in such a system were obtained in an arc furnace. Their microstructures were analysed in as-cast condition and after heat-treatments. An investigation conceming eutectic microstructure was carried out through the application of directional solidification using Bridgman equipment at several growth rates. Characterization of samples was performed by employing optical and scanning electron microscopy, X-ray diffraction and thermal analysis techniques. The results obtained indicated the eutectic transformation temperature is near to 1,347.9°C with formation of Ab(Nb,Cr) and Cr(AI,Nb) phases. The ductile phase of eutectic, Cr(AI,Nb) phase, during the decrease of temperature undergoes a polymorphic transformation (-892.3°C) forming the AI(Nb)Cr2 compound, followed by an eutectoid decomposition Cr(AI,Nb)~ AI(Nb)Cr2+AIgCrs.The eutectic microstructure presents is of regular complex type and the eutectic composition is near 49.3%AI- 39.7%Cr-l1.0%Nb (at.%). Through studying the phase transformations and microstructures of the samples obtained, it was possible to establish the liquidus surface in the region around the eutectic point.

ASSUNTO(S)

eutetic alloys materiais materials intermetallic compounds solidification solidificação ligas euteticas compostos intermetalicos

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