Direct observation of zirconium segregation to dislocations and grain boundaries in NiAl

Abstract

Segregation of zirconium to dislocations in microalloyed NiAl has been directly observed in the atom probe field ion microscopy (APFIM). Approximately two thirds of the dislocations in the NiAl matrix exhibited zirconium segregation. Zirconium-rich regions with ribbon-like morphology were also detected in the NiAl matrix. It is suggested that these regions are associated with dislocations. These results support the suggestion that the substantial increase in ductile-to-brittle transition temperature (DBTT) of the zirconium-doped alloy is due to pinning of dislocations by zirconium. Since the mechanism underlying the DBTT in binary NiAl is not understood, it would be of interest to examine the possibility of a similar pinning of dislocations by trace interstitial and other impurities in the undoped alloy. Segregation of zirconium to grain boundaries has also been observed in the APFIM. This result indicates the possible strengthening of grain boundaries has also been observed in the APFIM. This result indicates the possible strengthening of grain boundaries due to zirconium segregation and is consistent with the change in the fracture mode from intergranular in the undoped NiAl to a mixture of intergranular and transgranular mode in zirconium-doped NiAl. The NiAl matrix was heavily depleted of both solutes molybdenum and zirconium.more » Small Mo-rich precipitates observed in the matrix and at grain boundaries and the ribbon-like Zr-rich regions are likely to contributed to the increase in yield stress through precipitation hardening.« less