RAPID COMMUNICATIONS The purpose of this Rapid Communications section is to provide accelerated publication of important new results in the fields regularly covered by Journal of Materials Research. Rapid Communications cannot exceed four printed pages in length, including space allowed for title, figures, tables, references, and an abstract limited to about 100 words. Face-centered-cubic to hexagonal-close-packed transformation in nanocrystalline Ni(Si) by mechanical alloying M.K. Datta, S.K. Pabi, and B.S. Murty a) Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur-721 302, India (Received 7 August 1999; accepted 27 March 2000) An allotropic transition from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) Ni(Si) solid solution in Ni 95 Si 5 and Ni 90 Si 10 during nanocrystallization by mechanical alloying is reported. The transformation was identified as a defect-induced melting accompanied by a volume expansion of 8.6% and was observed when fcc Ni(Si) reached a critical crystallite size of 10 nm. Calculation based on equation of state showed that a 37% reduction in tetragonal shear modulus and a negative pressure of about 8.7 GPa were generated at the onset of transformation. Crystal to amorphous transformation can be induced in a wide range of materials by various solid-state tech- niques such as high-energy particle irradiation, ion-beam mixing, annealing of diffusion couples, hydrogen charg- ing, and high-energy ball milling. 1,2 Despite this variety of techniques, there appears a common observation that atoms are displaced from their equilibrium lattice sites, causing lattice strain and softening of shear elastic con- stants during the progress of amorphization. 2 According to Lindemann’s melting criterion and the unified ap- proach of melting and amorphization, 3 amorphization would occur when the root-mean-square static atomic displacement reaches a critical value identical at the melting point of the crystal. On the other hand, Tallon 4 has pointed out that shear elastic constant falls rapidly with an increase in crystal volume and reaches zero at a volume equal to that of the liquid at the melting point. The experimental results 5 have identified that amorphiza- tion occurs when the crystal is strained to a material- dependent critical value, and accompanies a large decrease (∼40 to 50%) in the shear elastic constant irre- spective of the material. The prerequisities of this phase transformation, namely, the static atomic displacement and shear softening, can be obtained by the presence of static disorder in the parent crystal that can be achieved either by accumulation of defects or by forming a super- saturated solid solution by various solid-state techniques. Mechanical deformation during mechanical alloying (MA) involves the creation and annihilation of a high density of dislocation in the material, resulting in nanocrystalline grains of a special type of grain bound- ary. 6 As the atomic displacement in the nanocrystalline grain boundary is higher than the interior of the core, it is expected that the grain boundaries have significant effect on shear softening to induce phase transformation in the nanocrystalline state. Therefore, it becomes possible that the nanocrystalline grain surrounded by a disordered layer may transform to amorphous or other crystal struc- ture below a critical crystallite size. Experiments have shown that nanoparticles of a number of elements such as Nb, Mo, Co, W, Ta, 7 and less common metals such as Y, Gd, Tb, Dy, Ho, Er, and Tm 7 have shown structures other than their equilibrium ones. It is well known that face- centered-cubic (fcc) Ni does not show any polymorphism in the bulk state. However, ion irradiation studies 8,9 demonstrated that Ni can have a metastable hexagonal- close-packed (hcp) structure. The phase transitions from loose-packed structures to close-packed structures during nanocrystallization can be understood by thermodynamic consideration. 7 However, the transitions between close- packed structures are less studied. 10 The present study reports the formation of hexagonal phase on nanocrys- tallization during mechanical alloying of Ni 95 Si 5 and Ni 90 Si 10 and a possible mechanism for this phase trans- formation is suggested. Pure Ni and elemental blends of Ni and Si powders of nominal composition Ni 95 Si 5 and Ni 90 Si 10 were sub- jected to high-energy ball milling using a planetary ball mill (Fritsch Pulverisette P-5, Fritsch GmbH, Idar- Oberstein, Germany). The milling was carried out in toluene at 300 rpm up to 50 h in a tungsten carbide vial using 10-mm-diameter tungsten carbide balls with a ball- to-powder weight ratio of 10:1. The milling was inter- rupted at regular intervals of 5 h to analyze the milled a) Present address: National Research Institute for Metals, Tsukuba 305-0047, Japan. J. Mater. Res., Vol. 15, No. 7, Jul 2000 © 2000 Materials Research Society 1429