MS-4-P-1675 Mechanical alloying of Fe–X (X=Al, Mo, Ni) powders studied by analytical electron microscopy

It is well-known that materials of the same nominal composition processed in a different way yield different microstructure and phase constitution, which result in diverse physical properties. Mechanical alloying (MA) via ball milling represents a relatively simple way of preparing materials with a micro- to nanograin crystalline and/or amorphous metastable structure. MA is a high-energy process where the energy developed during high speed rotation of grinding vial and balls is transferred to the powder particles which are brought to contact and exposed to severe mechanical deformation. This leads finally to formation of an alloy of the required mostly thermodynamically unstable composition.
In this work we have studied three binary systems in various states of MA by means of analytical electron microscopy (SEM and TEM). The Fe–Al system is frequently exploited for its low cost, high temperature corrosion resistance and good mechanical properties. The studied composition Fe₈₂Al₁₈ is close to the phase boundary between bcc Fe–Al solid solution and Fe₃Al phase. Poor mutual solubility of Fe and Mo restricts the fabrication of Fe–Mo alloys by conventional technologies. MA helps substantially as shown here for Fe₈₀Mo₂₀ composition. Fe₂₀Ni₈₀ is studied as a representative of Fe–Ni alloys with extraordinary magnetic, mechanical and electrical properties. For details of materials preparation see e.g. Jirásková et al, J. Alloys and Comp. 568 (2013) 106-111.
A TESCAN LYRA 3XMU FEG/SEM scanning electron microscope, a Philips CM12 STEM and a JEOL JEM-2010F transmission electron microscopes (all equipped with an XMax80 Oxford Instruments detector for energy dispersive X-ray (EDX) analyses) were used for microstructural studies. The comparison of evolution of the three systems during milling has shown how the rate of mixing visualized by powder  morphology and chemistry depends on the properties of constituents and on pertinent binary phase diagrams.
The alloying of the Fe-Al is observed already after 5 h of milling yielding bcc-Fe(Al) coexisting with α-Fe (Fig. 1). After 30 h EDX analyses have shown the dominant peak close to nominal composition. The alloying of Fe and Mo proceeds more slowly. Mo starts to dissolve in bcc-Fe and vice versa after 10 h of milling and bcc-FeMo and bcc-MoFe phases are formed (Fig. 2). Details observed in TEM after 250 h of milling (Fig. 2c) show the microstructure consisting of dense packed nanoparticle cores (< 10 nm). TEM yields a similar morphology also for Ni-Fe (Fig. 3) after 15 h of milling and the diffraction pattern confirms the formation of Ni₃Fe phase. The SEM micrographs of all samples document similar final morphologies of the powders formed by small particles (< 500 nm) and the larger agglomerates up to tens of micrometers.