S.K. Pabi

Mechanism of mechanical alloying in NiAl and CuZn systems

Abstract Nanocrystalline Al 3 Ni, NiAl and Ni 3 Al phases in the Ni Al system and the α, β, γ and e phases in the Cu Zn system were synthesized by mechanical alloying of elemental blends in a planetary mill. In the as-milled state, Al 3 Ni and AlNi were always ordered, while Ni 3 Al was disordered. MA results in a large extension of the NiAl and Ni 3 Al phase fields particularly towards Al-rich compositions. The crystallite size was finest ( ∼ 6 nm) when NiAl and Ni 3 Al phases coexist after prolonged milling. In contrast, in all Cu Zn blends containing 15–85 at.% Zn, the Zn-rich phases were first to form and final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In the case of NiAl and Al 3 Ni, where the ball milled product is ordered and the heat of formation is large ( ΔH f > 120 kJ mol −1 ), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases irrespective of the magnitude of ΔH f , gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.

Structure and thermal stability of nanocrystalline materials

Nanocrystalline materials, which are expected to play a key role in the next generation of human civilization, are assembled with nanometre-sized “building blocks” consisting of the crystalline and large volume fractions of intercrystalline components. In order to predict the unique properties of nanocrystalline materials, which are a combination of the properties of the crystalline and intercrystalline regions, it is essential to understand precisely how the structures of crystalline and intercrystalline regions vary with decrease in crystallite size. In addition, study of the thermal stability of nanocrystalline materials against significant grain growth is both scientific and technological interest. A sharp increase in grain size (to micron levels) during consolidation of nanocrystalline powders to obtain fully dense materials may consequently result in the loss of some unique properties of nanocrystalline materials. Therefore, extensive interest has been generated in exploring the size effects on the structure of crystalline and intercrystalline region of nanocrystalline materials, and the thermal stability of nanocrystalline materials against significant grain growth. The present article is aimed at understanding the structure and stability of nanocrystalline materials.

An allotropic transformation induced by mechanical alloying

This study concerns a hitherto unknown bcc→fcc allotropic transformation in Nb induced by the mechanical alloying of Nb80Al20. This metastable transformation is preceded by a gradual increase in the lattice parameter of bcc–Nb. The stored excess energy in nanocrystalline bcc–Nb may be responsible for the bcc→fcc phase transition.

Thermal stability of nanocrystalline Ni silicides synthesized by mechanical alloying

Solid state reactions induced by mechanical alloying followed by isothermal annealing in elemental blends of Ni and Si have been studied over the entire composition range of the Ni-Si system. Above a critical crystallite size, the congruent melting phases, γ-Ni 31 Si 12 and NiSi, become unstable and react with second phase available to form equilibrium non-congruent, β 1 -Ni 3 Si and α-NiSi 2 , after isothermal annealing at 723 and 1023 K, respectively. Formation of equilibrium e-Ni 3 Si 2 has not been observed in the composition range 33-50at.% Si even after annealing at 1073 K. The results also indicate that the crystallite size of non-congruent melting phases is well above 100 nm even at the stage of their evolution, suggesting that they are probably stable only in the bulk state. The grain growth is faster for the non-congruent phases when compared to that of congruent melting compounds. The interfacial energy of the compounds plays an important role in controlling their stability and grain growth in the nanocrystalline state.

Phase fields of nickel silicides obtained by mechanical alloying in the nanocrystalline state

Solid state reactions induced by mechanical alloying (MA) of elemental blends of Ni and Si have been studied over the entire composition range of the Ni–Si system. A monotonous increase of the lattice parameter of the Ni rich solid solution, Ni(Si), is observed with refinement of crystallite size. Nanocrystalline phase/phase mixtures of Ni(Si), Ni(Si)+Ni31Si12, Ni31Si12+Ni2Si, Ni2Si+NiSi and NiSi+Si, have been obtained during MA, over the composition ranges of 0–10, 10–28, 28–33, 33–50, and >50 at. % Si, respectively. The results clearly suggest that only congruent melting phases, Ni31Si12, Ni2Si, and NiSi form, while the formation of noncongruent melting phases, Ni3Si, Ni3Si2, and NiSi2, is bypassed in the nanocrystalline state. The phase formation during MA has been discussed based on thermodynamic arguments. The predicted phase fields obtained from effective free energy calculations are quite consistent with those obtained during MA.

Formation of nanocrystalline phases in the Cu-Zn system during mechanical alloying

The evolution of various nanocrystalline phases in the Cu-Zn system during the course of mechanical alloying of elemental powder blends, manifests a similar sequence of phase formation in all the compositions. Zinc-rich phases were always first to form, which can be attributed to the diversities of diffusivities and diffusion distances in the constituents. The extent of zinc in α-phase becomes significant only after α turns nanocrystalline. The crystallite size of α reached a minimum (∼18 nm) near the α-β phase boundary, while the β and ɛ phases showed quite coarse crystallite size due to their low melting points. Alloying was sluggish during dry milling compared to wet milling, or at lower milling speed, which demonstrates the effects of oxidation during milling and lower milling energy.

Solid state synthesis of Al-based amorphous and nanocrystalline Al–Cu–Nb alloys

Abstract An attempt was made to synthesize amorphous and/or nanocrystalline Al-based alloys from elemental powder blends with the stoichiometry Al 65 Cu 35− x Nb x ( x =5–25 at.% Nb) by high energy planetary ball milling. Microstructure of the milled product at appropriated stages of milling was characterized by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The Al 65 Cu 20 Nb 15 powder blend seems most amenable to solid state amorphization. This amorphous alloy was subjected to controlled heat treatment to develop a two-phase or composite microstructure of nano-aluminide dispersion in amorphous matrix. The results indicate that the present ternary system is akin to our previously reported results on Al–Cu–Ti system in developing completely/partially amorphous and/or nano-aluminide dispersed Al-rich nanocrystalline or amorphous matrix composites by controlled mechanical alloying and/or subsequent annealing.

Nanocrystalline phases in CuNi, CuZn and NiAl systems by mechanical alloying

Nanocrystalline solid solutions and intermetallic phases in Cu---Ni (enthalpy of formation, ΔH f ≈2kJ/mol), Cu---Zn(ΔH f ≈-8kJ/mol) and disordered Ni 3 Al (ΔH f ≈-42kJ/mol) in Ni---Al system formed during mechanical alloying (MA) through conventional continuous diffusive mixing mechanism. In contrast, nanocrystalline ordered NiAl (ΔH f =-72kJ/mol) and NiAl3 (ΔH f =-39kJ/mol) formed during MA by a discontinuous additive mixing mechanism. Ternary additions of Fe and Cr made the as-milled NiAl phase disordered, and the alloying mechanism changed over to the continuous diffusive mode.

On the enhancement of diffusion kinetics in nanocrystalline materials

It is well known that volume diffusivity in nanocrystalline materials is orders of magnitude greater than that in coarse grained materials of identical chemistry. The present study offers a theoretical explanation of this enhancement by attempting to correlate the diffusivity with the excess free volume of the grain boundary atoms in the nanocrystalline materials. Assuming a simplified geometry of the nanocrystalline aggregate, the excess free volume of the grain boundary atoms has been expressed as a function of grain size. Applying the concept of isothermal equation of state, it is shown that the effect of negative hydrostatic pressure on the diffusion kinetics in nanocrystalline materials at a given temperature is equivalent to the same obtained for coarse grained materials at an effective elevated temperature. The predicted results have been compared with the earlier published relevant experimental data from several Cu-based systems.

Polymorphic transformation and lattice expansion in nanocrystalline niobium revealed by positron annihilation at grain boundaries

Positron lifetimes in nanocrystalline niobium were measured over the grain size range from 35 nm down to 5 nm. Two lifetimes were obtained indicating distinct annihilation sites at the grain surfaces and in the intercrystalline region. X-ray diffraction studies had indicated a bcc to fcc transformation and a concomitant lattice expansion. Positron lifetimes drastically varied below a grain size of 10 nm, coinciding with a sharp increase in the excess free volume of atoms at the grain boundaries as revealed in a model analysis. Doppler broadening measurements also indicated a redistribution of electron momentum, implying a restructuring of the grain surfaces. The occurrence of bcc to fcc transformation is also observed in binary Nb-Al and ternary Al-Nb-Cu alloys.