Elaboration and Metallographic Study of M - 25Cr - based Alloys Containing Hafnium Carbides (M=Co, Ni or Fe): M icro structures in the As - cast S tate and after E xposure at 1200°C.
Résumé
Six chromium - rich M - based alloys (M=Co, Ni or Fe) with conventional carbon contents (0.25 and 0.50wt.%) and containing near 4wt.% of hafnium were elaborated in foundry. As expected , dense precipitation of hafnium carbides was achieved, with repartitions and shapes similar to what it is generally obtained with tantalum carbides. Chromium carbides were also obtained in some alloys. After exposure of samples at 120 0°C for about 48 hours the HfC carbides network was kept without significant changes of morphology and surface fractions, in the cobalt - based and nickel - based alloys, in contrast with the iron alloys which appeared less stable. Indentation performed in as - cast and aged samples showed that these HfC - containing alloys display various hardness values. The HfC - containing cobalt alloys are the hardest ones and the high temperature exposure induced a limited lowering of this property. The cast HfC - containing coba lt alloys are potentially very interesting for applications at very high temperature requiring foundry equi - axed chromium rich alloys.