Effect of Minor Elements on the Deformation Behavior of Nickel-Base Superalloys

Abstract

Alloying with the minor elements carbon, boron, zirconium, and hafnium has played a very critical role in the development and application of nickel-base superalloys. The elements are known to improve strength and grain boundary cohesion, but a clear understanding of the underlying mechanisms has been lacking. To systematically address this, a series of 15 polycrystalline alloys were prepared with various minor element additions using Alloy 454 (alloy chemistry of PWA 1480) as the base alloy. Intermediate temperature (1400°F) creep testing was used as a means for evaluating the effectiveness of the minor elements. Typically, a fifty-fold improvement in creep-rupture life was attained with -0.5 atom percent minor element additions. Although the increase in elongation to failure, in an engineering sense, was insignificant, the synergistic effect of adding combinations of the elements resulted in even greater enhancements. It was not possible to rationalize the results with any consistent grain boundary microstructural changes. To further understand the effects of minor element additions, eight single crystal alloys were cast with optimum additions of minor elements using Mar-M200 as the base alloy. Intermediate temperature (1400°F) creep behavior was evaluated in the three major orientations , , and . In general, the addition of the minor elements did not alter the behavior of the or the orientations, but decreased the creep resistance of the orientation. Again, the synergistic effect of combinations of the minor elements was very significant. The decrease in the creep rate of the polycrystalline material is believed to be a manifestation of the mechanism that leads to the decrease in the creep rate of oriented single crystals. This in turn is attributed to an intrinsic strengthening of the cube slip system which is geometrically favored near the orientation. It is proposed that the suppression of cube slip, with lower multiplicity than octahedral slip, enhances strain compatibility in the grain boundary regions. These areas are enriched in the minor elements, due to preferential segregation of minor elements to the grain boundaries. Superalloys Edited by S. Reichman, D.N. Duhl, G. Maurer, S. Antolovich and C. Lund The Metallurgical Society, 1988