Cr-Mo solid solutions forced by high-energy ball milling
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2/12/04
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Cr-Mo Solid Solutions Forced by High-Energy Ball Milling J.D. HAHN, FANG WU, and P. BELLON Mixtures of Cr and Mo elemental powders, with the nominal compositions Cr25Mo75, Cr50Mo50, and Cr75Mo25, are processed by high-energy ball milling at ambient temperature. Milling is observed to force the mixing of the immiscible bcc elements Cr and Mo into solid solutions. The lattice parameter of these solid solutions, measured by X-ray diffraction (XRD), displays the expected positive deviation from Vegard’s law. These deviations are compared to the ones predicted by Eshelby’s inclusion model for dilute alloys. The conventional Williamson–Hall approach is shown to fail to determine the grain size in as-milled samples, probably due to the high density of dislocations. Annealing at 700 °C for 10 hours under argon leads to a large reduction in structural defect density, without inducing any significant decomposition. The mixing measured in Cr-Mo is discussed in the broader context of the mechanical mixing forced by ball milling in moderately immiscible systems.
I. INTRODUCTION
OVER the last three decades, powder processing by highenergy ball milling has gained wide practical interest, as it offers a simple yet powerful way to synthesize nonequilibrium phases and microstructures, from nanograin materials to extended solid solutions, amorphous phases, chemically disordered compounds, and nanocomposites.[1–5] In particular, it is well documented that ball milling can lead to the forced mixing of elements that are immiscible at equilibrium. In these immiscible systems, as the forced mixing competes against thermally activated decomposition, the maximum solubility is obtained by performing the milling at a low homologous temperature. The amplitude of the (positive) heat of mixing appears to play a significant role in the maximum solubilities that can be achieved by ball milling. In the case of systems with moderate heats of mixing (0 Hm 11 KJ/g. atom), complete solubility has generally been reported in isomorphous systems, e.g., Cu-Ni (Hm 2 kJ/g. atom),[6] Co-Cu (Hm 6 KJ/g.atom),[7,8,9] Ag-Cu (Hm 7 kJ/g.atom),[10,11] and Cr-Fe (Hm 6 kJ/g. atom),[12] whereas very large solubilities are reported in Cu-Fe (Hm 11 kJ/g. atom).[9, 13–17] This is to be contrasted with systems with large positive heats of mixing, e.g., Cu-W (Hm 35 kJ/g. atom),[18] Ag-Fe (Hm 25 kJ/g. atom),[19,20,13] and Ag-Ni (Hm 15 kJ/g. atom),[21] for which only partial solid solutions can be obtained, even when milling is performed at low homologous temperatures. Various explanations have been put forward to account for the forced mixing produced by ball milling, namely, based on dislocation glide[22] or stress-stimulated diffusion.[8] However, the origin of the correlation between the amplitude of the heat of mixing and the amount of solubility that can be forced by ball milling remains an open question. In order to test the generality of this correlation, it is important to study alloy systems with different
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