Molecular Dynamics Studies of Tweed and ω -Phase Instabilities in B2 NI62.5AL37.5 Alloys

Abstract

AbstractDiffuse TEM and x-ray data has been reported by several research groups that has been interpreted in terms of both tweed and (ω -phase instabilities. Computer simulations of 1,024 atom arrays were run at various temperatures using the Voter-Chen Embedded Atom Method (EAM) interactions. If the extra Ni atoms were placed at random on the Al sub-lattice of the B2 structure, at low temperatures both <110>* and <112>* diffuse streaking were observed in reciprocal lattice plots, indicative of tweed and ω - phase instabilities, respectively. The atomic displacements causing the <110> diffuse streaking could generally be classified as {110}<1-10> local shears, in agreement with accepted models of tweed. However, the displacements producing the <112>* streaking were best described as coordinated atom row motions in <111> directions, which does not agree with traditional ω -phase models based on (111) interplanar collapse. Both types of diffuse streaking decreased with increasing temperature, but the <110> type was stronger at all temperatures and persisted to higher temperatures. Furthermore, the atomic displacements producing the streaking were static at low temperatures ( persistence times greater than 1000 vibrational periods) but became dynamic at higher temperatures ( persistence times of the order of a few vibrational periods). Another interesting result was that by merely rearranging the extra Ni atoms into an ordered superlattice pattern, virtually no diffuse streaking of any type was observed at any temperature, strongly suggesting that compositional disorder ( or other kinds of point defects) are essential to induce tweed instabilities.*The asterisk represents a quantity in reciprocal space.