Irradiation-Induced Mo 2 C Precipitation in Ni-Mo
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I.
INTRODUCTION
D U R I N G a transmission electron microscope (TEM) study of the effect of irradiation on chemical order in single phase high purity Ni-20 at. pct Mo (Ni4Mo),2'2 precipitates were observed in specimens subjected to nickel ion irradiation at elevated temperatures. The precipitates could have formed as a result of either a radiation-induced chemit~al ~e.~re.~ation in the alloy or a radiation-induced change in the reqative thermodynamic stability of phases in the Ni-Mo binary system. For several up-to-date reviews discussing these mechanisms, see the proceedings of a workshop on solute tively, the formation of the precipitates could simply have been due to some chemical contaminant acquired during the elevated-temperature irradiation experiments. The identity of the precipitate and its probable mechanism of formation were thus of considerable interest since the NiaMo specimen material was of high purity, and since it is known that minor impurity concentrations as well as major alloy variations have large effects on the response to radiation of metals and alloys. 4'5 In this paper we report results of an experimental investigation of the structure and composition of the precipitate using analytical electron microscopy methods, and its crystallographic and morphological habits in the fcc matrix. Though Ni4Mo is an ordered body-centered tetragonal (bct) phase, ion irradiation disorders the matrix, leaving a short-range ordered fcc structure with af~ ~ 0.63 a~t. The probable reason for the formation' of these precipitates is discussed.
II.
EXPERIMENTAL
The chemical composition of the Ni-20 at. pct Mo specimen material is given in Table I. The carbon concentration which is of importance for this investigation has been determined by gas chromatographic analysis, which typically has an accuracy of ---50 pct. Ion or high voltage electron microscope irradiations were performed on specimens quenched from 1473 K and on specimens given additional aging treatments at either 923 or 1023 K. 2,2 Anneals were performed in sealed quartz capsules under a partial pressure of purified W. KESTERNICHis with Kernforschungsanlage, J(ilich, 517 Jiilich, Germany;R. W. CARPENTER, formerlywith Oak Ridge National Laboratory, Oak Ridge, TN, is now with ArizonaState University, Tempe, AZ 85281; and E.A. KENIK is with Oak Ridge National Laboratory, Oak Ridge, TN 37830. Manuscript submitted May 19, 1981. METALLURGICALTRANSACTIONSA
Table I. Chemical Composition of Ni-20 At. Pct Mo Specimen Material Including Major Impurity Concentrations in At. Pct
Ni 79.9
Mo
C
H
N
O
20.0
0.0046
0.053
0.011
0.0058
argon with tamaium getter material. I ne specimen material was either short-range ordered or long-range ordered with a small domain size ( < 1 0 nm diameter). Except for a low dislocation density ( < 1 0 +11 m-2), no other crystal defects such as stackinz faults or orecioitates were observed in the unirradiated controls. Irradiations using 4 MeV nickel ions were performed in a vacuum of 40/xPa (3 x 10 -7 tOll'). The specimen
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