Void Morphology In NiAl

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Void Morphology In NiAl

M. Zakaria and P.R. Munroe Electron Microscope Unit University of New South Wales Sydney NSW 2052, Australia ABSTRACT

Void formation in stoichiometric NiAl was studied through controlled heat treatments and transmission electron microscopy. Voids formed at temperatures as low as 400°C, but dissolved during annealing at 900°C. Both cuboidal and rhombic dodecahedral voids were observed, often at the same annealing temperature. At higher annealing temperatures (•ƒ& H[WHQVLYH dislocation climb was noted. The relative incidence of void formation and dislocation climb can be related to the mobility of vacancies at each annealing temperature. Further, differences in void shape can be described in terms of their relative surface energy and mode of nucleation. INTRODUCTION

Large supersaturations of vacancies can be quenched into NiAl following high temperature (>1000°C) heat treatment. During subsequent annealing at lower temperatures the excess vacancies are removed from the lattice through either dislocation climb or void formation. A number of workers have studied voids in NiAl, but the behaviour of these defects has not been unambiguously defined [1-5]. That is, the conditions under which voids form, their shape, size and range of stability have not been clearly defined. Two distinct void shapes, cuboidal and rhombic dodecahedral, are observed, but their incidence cannot be unambiguously related to particular heat treatment conditions or alloy composition. In some cases, dislocation loops, rather than voids, are formed [6-8]. We have examined the structure of voids in stoichiometric NiAl as a function of annealing temperature over a range from 400°C to 900°C. Detailed descriptions of the observed microstructures can be found elsewhere [9]. In this paper experimental observations will be summarised more briefly, the principal aim of this paper is to discuss the relative incidence of void formation and dislocation climb, and the variations in void shape. EXPERIMENTAL METHODS

Nominally stoichiometric NiAl was prepared by arc melting under an argon atmosphere. The material was remelted several times to improve homogeneity. Chemical analysis indicated that composition was close to stoichiometry. Samples were annealed, in air, at 1300°C for 2 hours and cooled to room temperature by air-cooling. Subsequent annealing was performed at temperatures ranging from 400°C to 900°C for 1, 5 or 24 hours at each temperature. Thin foils for transmission electron microscopy (TEM) study were prepared and examined in a JEOL 2000FX TEM operating at 200kV. N5.6.1

RESULTS AND DISCUSSION

The microstructure of NiAl prior to annealing was examined. As expected, the microstructure was single phase with equaixed grains about 200µm in diameter. Following homogenization at 1300°C the microstructure contained fine (~ 20nm diameter) dislocation loops, with Burgers vectors. A very small number of fine (~20nm diameter) voids were also noted. These voids were presumably formed during cooling, where there is presumably suf

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