Experimental and theoretical investigation of the order-disorder transformation in Ni 3 Al
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The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L l 2 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni 3 Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.
I. INTRODUCTION The study of intermetallic compounds has been extensively pursued both at a fundamental and at a technological level, for their remarkable high-temperature, high strength, and corrosion resistance properties. However, these compounds would be of even higher practical interest if their attractive high-temperature behavior could be combined with good room temperature formability. A possible route for the obtainment of this feature might consist of the introduction of a substantial amount of disorder in their crystal lattices.1 In the case of Ni3Al several techniques for the obtainment of a disordered state have been proposed like laser-quenching,2 ion irradiation,3 and mechanical milling.4 In particular, the effect of mechanical milling has been the object of several structural and calorimetric studies4"7 that have shown that disordered Ni3Al is a crystalline solid solution with little, if any, tendency toward amorphization, thus ruling out the possibility that all LI2 structures are easy to amorphize.5 Lesser attention has been devoted to the characterization of the phase of the same composition obtained by mechanical alloying (MA) of elemental powders. In this case, what appears conclusively established8'9 is that MA leads to a disordered fee phase having the same crystal structure of the powder obtained by mechanical milling of the ordered compound. Recently, several LI2 compounds have been studied by atomistic simulations. In the case of Ni 3 Al, bulk thermodynamical properties and point defects formation energies have been evaluated10 while, for Cu3Au, the thermodynamics of the order-disorder transformation has been investigated by a free energy calculation.11 2504
J. Mater. Res., Vol. 8, No. 10, Oct 1993
http://journals.cambridge.org
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In order to characterize the alloying process and for a better understanding of the ordering mechanism, we have decided to perform an experimental study of the phase obtained by MA of elemental Ni and Al powders. Disordered Ni3Al has also been studied by means of
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