THE MICROSTRUCTURES AND MECHANICAL-PROPERTIES OF NIOBIUM-MODIFIED NI-25AL-27-CENTER-DOT-5FE INTERMETALLICS

The Ni-25Al-27.5Fe alloy was selected from the Ni-25Al-xFe alloy syste m (x is from 20-40 at%) and macroalloyed with niobium to further impro ve the mechanical properties. The results reveal that three phases, fc c (gamma); ordered fee (gamma'); and ordered bcc (beta') phases, exist in these alloys. The alloys show dendritic microstructures, and the i nterdendritic regions get thinner and fewer after adding niobium. Afte r annealing at 1000 degrees C for 4h a large amount of fee-structured particles (gamma-Fe) was precipitated within the matrix of beta'-phase d dendrite. This gamma-Fe contained higher iron-and niobium-concentrat ion when compared to the beta' phase. In addition, increasing the niob ium content to 1 at% results in increasing all the room-temperature te nsile properties. However, the tensile properties will decrease sharpl y with the addition of niobium beyond 1 at%. This phenomenon could be caused by decreasing the interdendritic regions after adding niobium o f more than 1 at%. Moreover, the temperature effect on the tensile pro perties is very significant. Both the yield strength and ultimate tens ile strength decrease with increasing temperature, and the elongation increases sharply when the temperature is beyond 600 degrees C. (C) 19 97 Elsevier Science Limited.