Proton Irradiation-induced Disordering Reactions, Ductility and Strengthening of Ni 3 Al
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PROTON IRRADIATION-INDUCED DISORDERING REACTIONS, DUCTILITY AND STRENGTHENING OF Ni3 A1 J. CHENG, C.-S. LEE, C. N. J. WAGNER AND A. J. ARDELL Department of Materials Science and Engineering, University of California, Los Angeles, CA 90024 ABSTRACT Ordered Ni3 A1 containing 24 at.% Al was irradiated by 2 MeV protons at -45 'C to a fluence of 1.34 x 1016 H+/mm 2. The induced microstructural changes were investigated by transmission electron microscopy, X-ray diffraction, scanning electron fractography and microhardness measurements. Proton irradiation produced a large volume fraction of small disordered zones residing in the ordered matrix. The morphology of the disordered zones bears some striking similarities to "tweed" microstructures. X-ray diffraction reveals distinct splitting of the higher order fundamental peaks toward low angles, and broadening of the displaced peaks, indicating that the zones observed are small clusters of tetragonally distorted disordered Ni 3AI. The microhardness measurements suggest that the disordered zones strengthen the remaining ordered matrix considerably, most likely through a precipitation strengthening mechanism. The fractographic observations suggest that an increase in ductility accompanies disordering, as evidenced by the appearance of dimples, slip markings and river patterns on the fracture surface of the irradiated region of the sample. INTRODUCTION Dilute additions of boron to hypostoichiometric Ni3 A1 have a profound effect on the ductility of this intermetallic compound [1-4]. It is known that boron ameliorates the tendency of Ni 3AI to fracture in a brittle, intergranular manner, but the precise mechanism is still a matter of some controversy and intensive investigation. One promising suggestion is that boron segregates to grain boundaries and locally disorders the matrix immediately adjacent [5]. If this suggestion is correct, it follows that any process which can bring about a reduction in the long-range order
parameter, S, in Ni3AI should improve its ductility. We have already demonstrated [6] that irradiation by 2 MeV protons at low temperatures provides an effective means of disordering the intermetallic compound Zr3AI, which is isostructural with Ni 3 Al. It was shown that irradiation at -170 'C to a total proton fluence of 1.9 x 1016 H+ per mm 2 produced disordering and amorphization, the extent of each depending on the dose (which varies with distance from the surface of the irradiated samples). Although mechanical tests were not performed on the irradiated samples, it was possible to infer information about their ductility indirectly through the technique of fractography. Specifically, the unirradiated samples exhibited the fractographic characteristics of intergranular brittle fracture, but the fracture surfaces of the irradiation-damaged zones contained dimples which are characteristic of ductile failure. Some of the other intermetallic compounds examined, notably NiTi and CuZr, behaved in a similar manner, although the extent of disordering and amorphizatio
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