|The effects of high energy mechanical milling on the kinetics of solid state reactions in two binary metallic systems, Al-Ni and Cu-Al, and two metal-oxide systems, Al-TiO₂ and Cu(Al)-CuO, have been investigated by using X-ray diffractometry (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
A quantitative analysis of the Ni/Al system was performed to gain an understanding of the reaction behaviour during heating of the high energy mechanically milled powders. Results showed that nucleation and lateral growth along the interface region occurred at different temperatures from the diffusion controlled transverse growth. The amount of new phase formed through nucleation and lateral growth along the interface region increased in proportion to the interface area.
A two-step model for describing new phase nucleation and growth along the interface and transverse growth perpendicular to the interface was developed from the principles of classic heterogeneous nucleation and growth theories and the one-dimensional diffusion equation. The model was verified against the experimental results of Ni/Al system. It was found that the interface area, activation energy and pre-exponential factor are the major parameters affecting the kinetics of the solid state reactions of the powders.
It was found that, in the Cu/Al system, Cu(Al) solid solution or γ-Cu₉Al₄ intermetallic compound could form after the powder mixture was milled for up to 8 hours. Upon heating the powder mixtures milled for shorter times, γ-Cu₉Al₄ formed first in the powder with a nominal composition of Cu-14at%Al, and then, γ-Cu₉A₄ dissolved into Cu forming Cu(Al) solid solution. Whereas, θ-CuAl₂ formed first for the powder with a nominal composition of Cu-37at%Al, and then, θ-CuAl₂ reacted with Cu forming γ-Cu₉Al₄ intermetallic compound. The solid solution or intermetallic compound formed by heating the powders milled for shorter times is the same as that produced by mechanical alloying.
In the Al/TiO₂ system, the effects of high energy milling on the phase formation and reaction kinetics was studied. It was found that the reaction between Al and TiO₂ also followed a two step reaction mechanism: interfacial reaction and bulk diffusional reaction. The first phase formed by the interfacial reaction was always Al₃Ti irrespective of the initial composition of the powder mixture. The reaction product for the subsequent reaction depends on the composition of the powder mixture. α-Ti phase could form at a higher temperature. The formation of α-Al₂O₃ phase is highly dependent on the reaction kinetics. It requires higher temperature or longer time to form detectable amount.
In the case of Cu(Al)-CuO system, two reactions occurred sequentially: Reaction 1, forming Cu₂O from the reaction between CuO and Cu-Al powders and Reaction 2, forming Cu from the reaction between Cu₂O and Cu-Al powders. High energy mechanical milling enhanced both reactions leading to a substantial decrease in the reaction temperatures.
Overall, high energy mechanical milling enhances the solid state reactions by establishing and refining composite microstructures of the powder particles. The reaction temperature could be substantially decreased. Prolonged mechanical alloying process can be replaced by a short time mechanical milling plus a well controlled heat treatment process.