Formation of Ni3Nb and influence of Y2O3 on mechanical properties of nano-grained Ni20Cr20Fe5Nb1Y2O3 alloy

The effects of adding Y2O3, and the precipitation of Ni3Nb by heat treatment, on the mechanical properties of mechanically alloyed Ni20Cr20Fe5Nb alloy were studied. The addition of Y2O3 caused an increase in the tensile strength at room temperature, 400 degrees C and 600 degrees C. The difference in the tensile strength between the Ni20Cr20Fe5Nb and Ni20Cr20Fe5Nb1Y(2)O(3) alloys decreased gradually with increasing test temperature. The tensile strength of the Ni20Cr20Fe5Nb1Y(2)O(3) alloy at relatively low temperature was increased by the addition of Y2O3, but decreased abruptly at temperature above 600 degrees C. This seems to result from a change in the deformation mechanism due to the ultra-fine grain size, that is, grain boundary sliding is predominant at temperatures above 600 degrees C while internal dislocation movement is predominant at temperatures below 600 degrees C. Following the conventional heat treatment of the solution and subsequent aging, only a small amount of delta(Ni3Nb) phase was formed in the Ni20Cr20Fe5Nb alloy, whereas in a previous report it was indicated that a large amount of gamma"(Ni3Nb) was formed in IN 718 alloy. The small amount of 5(Ni3Nb) phase formed in the present case is due to the exhaustion of the Nb content resulting from the formation of NbC during consolidation.