Formation of aluminum nitride during cryomilling of NiAl

Abstract

Prealloyed NiAl powders were milled in liquid nitrogen, a process known as cryomilling. The materials, formed by extrusion or hot isostatic pressing the cryomilled powders, exhibit higher creep strengths than unreinforced NiAl during slow strain rate compression testing. The increased strength was attributed to the presence of approximately 10 volume percent AlN particles which were dispersed along the NiAl prior particle boundaries. The presence of AlN particles within a NiAl matrix was not expected based on previous cryomilling work with oxide dispersion strengthened (ODS) nickel and iron alloys by Petkovic-Luton and Vallone. The experiments of Luton, et al. on ODS aluminum alloys indicated that reactions between liquid nitrogen and aluminum were possible and led to Al(O,N) particles. The hypothesis of Lutton, et al. for the formation of the aluminum oxy-nitrides and the absence of AlN x-ray diffraction peaks in the spectra from as-cryomilled NiAl powders convinced researchers that: (1) the nitrogen exists as an interstitial or solid solution element in NiAl after cryomilling, and (2) AlN precipitates during subsequent thermomechanical processing of the powders. Recent work suggests that this hypothesis is not correct. In an effort to establish the nature of nitrogen in cryomilled NiAl, cryomilled powder particles have beenmore » subject to bulk chemical analyses, x-ray diffraction analyses, x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Evidence supporting the existence of AlN formation on the surface of cryomilled NiAl powders is presented in this paper.« less