THE EFFECT OF CHROMIUM ADDITIONS ON THE MECHANICAL-PROPERTIES OF NI-25AL-27.5FE-BASED INTERMETALLIC COMPOUNDS

To further improve room temperature ductility of alloys based on Ni-25 Al-27.5Fe, alloys containing additional chromium were prepared by are- melting and drop-casting into ingots. The mechanical properties of tho se alloys were characterized by means of hardness testing and tensile testing. The microstructural evolution of these alloys was examined by X-ray diffraction, scanning electron microscopy and transmission elec tron microscopy. The results reveal that two phases, gamma' and beta' phases, exist in these casting alloys. The dendritic beta' phase mixed with the interdendritic of the beta' and gamma' phases in as-cast all oys. After annealing at 1000 degrees C for 4h, the ordered gamma' phas e transformed to a disordered gamma phase, and a large number of gamma phase particles were precipitated from the beta' phase. These precipi tates have almost the same chemical composition and lattice constant a s the interdendritic gamma phase. The gamma phase contained higher iro n and chromium concentrations when compared with the beta' phase. The chemical compositions of the beta' phases in both dendrite and interde ndrite appear to be almost the same. Increasing chromium content in th is alloy system results in increasing the hardness and elongation, but decreasing the yield strength; for example an alloy with 1 at.% chrom ium had 1230 MPa of ultimate strength, 484 MPa of yield strength and 1 1.3% of elongation.