Electron Microscopy Observations on The Influence of Boron Additions on Stoichiometric NiAl
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ELECTRON MICROSCOPY OBSERVATIONS ON THE INFLUENCE OF BORON ADDITIONS ON STOICHIOMETRIC NiA! T.-C. WU and S. L. SASS Cornell University, Department of Materials Science and Engineering, Ithaca, NY 14853
ABSTRACT The microstructure and local chemistry in single crystal and polycrystalline stoichiometric NiAl, with and without boron additions, were investigated using transmission and analytical electron microscopy. Plate-like precipitates were present in the boron-doped NiAl. Since these precipitates were not observed in the boron-free material, they must be due to the formation of borides. After irradiation with 400kV electrons, fine scale changes in the image contrast and the appearance of diffuse scattering in the diffraction pattern indicate that a phase transformation has been induced by the electron beam.
INTRODUCTION The B2-structured intermetallic, NiAl, has received considerable attention because of its potential for high temperature structural applications[1]. However, lack of ductility at room temperature is still its major drawback and restricts its use for high performance applications. A great deal of effort has been directed to overcome the intrinsic brittleness of grain boundaries in polycrystalline NiAl by microalloying with boron [2]. Unlike the Ni 3Al system, even though boron strengthens the boundaries and suppresses intergranular fracture in NiAl alloys, its low temperature ductility is still not greatly improved [3]. One suggestion to explain the continuing poor ductility is that boron causes solid solution hardening in the NiAl and in this manner raises the yield stress to above the transgranular fracture stress. Recently, Jayaram and Miller [4] reported the presence of a low number density of (Ti,V,Cr)Bi precipitates in stoichiometric boron-doped NiAl alloys examined by Atom Probe Field Ion Microscopy (APFIM). These particles are believed to cause the increase in the yield stress by precipitation hardening. In this paper, the microstructures of polycrystalline and single crystal NiAl alloys were studied using conventional transmission electron microscope (CTEM) and high resolution transmission electron microscope (HRTEM). The local chemistry of precipitate particles observed in boron-doped NiAl was investigated by analytical electron microscope. During the course of this research microstructural changes induced in the NiAl by the high energy electron beam were observed, and these will also be described.
EXPERIMENTAL PROCEDURE Polycrystalline alloys with composition of Ni-50 at%Al, both doped with 300 wppm boron and boron-free, were supplied by Oak Ridge National Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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Laboratory [2]. The alloys were arc-melted and homogenized as drop-casted ingots in vacuum at 1100 'C for 24 hrs, and then hot-extruded at 900 'C. NiAl single crystals with composition of Ni-50 at%Al, both doped with 500 wppm boron and boron-free, were supplied by GE Aircraft Engines. They were grown by the Bridgman method and homogenized at 1316 'C for 50 h
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