Formation of supersaturated solid solutions in the immiscible Ni–Ag system by mechanical alloying

Abstract

The mechanical alloying process in the immiscible Ni-Ag system with a positive heat of mixing was investigated by x-ray diffraction and differential scanning calorimetry. High energy ball milling of mixed elemental powders, with nominal composition Ni x Ag 100Ϫx (x ϭ 95, 90, 70, 50, and 30͒, results in the formation of mixtures of supersaturated, nanocrystalline Ni-rich and Ag-rich solid solutions. The solubilities and final grain sizes of these phases depend on the nominal composition of the powder. The maximum solubilities were determined using Vegard's law to be 4.3 at. % Ni in Ag and 6.6 at. % Ag in Ni for samples milled at room temperature. The effect of milling temperature on mechanical alloying was examined in the range Ϫ195 to 250°C. Lower temperature milling leads to a larger solubility of Ni in the Ag-rich samples, up to 7.1 at. % for the Ni 30 Ag 70 composition. Indications for the existence of a concentrated solid solution (Ni 36 Ag 64-Ni 44 Ag 56) were also found. Milling at higher temperatures leads to lower solubilities. A study of the thermal stability of supersaturated Ag-rich and Ni-rich phases shows that milling at high temperature can be understood in terms of a competition between mechanical mixing and thermal decomposition. At room temperature, nonequilibrium vacancies are responsible for decomposition. The results give new insight into the general characteristics of the mechanical alloying process in thermodynamically unstable systems.

Deformation‐Induced Supersaturation in Immiscible Material Systems during High‐Pressure Torsion

“…[4] In contrast, in systems with high positive heat of mixing, such as Ni-Ag or Ag-Fe, only extended solubilities could be obtained. [7,12,40] Besides thermodynamic considerations, it is proposed that large differences in strength of the individual phases of the composites can lead to strong strain localization, thereby, impeding to achieve full supersaturation during deformation. [41] Since dislocations play a key role in the kinetic roughening model and the dislocation shuffling process, which are proposed to be mechanisms for deformation-induced mixing, the crystal structures of the present phases might additionally influence the amount of mixing.…”

“…Extended solubilities or complete supersaturation was achieved in many material systems, such as Cu-Cr, [1] Cu-W, [2] Cu-Co, [3] Cu-Ag, [4][5][6] Ni-Ag, [7] Cu-Fe, [8][9][10][11] or Ag-Fe. [12] A review on mechanical alloying can be found in ref.…”

Effect of the Noble Metals Addition on the Oxidation Behavior of Ni3Al

“…Since the solidification cooling rate is inversely proportional to the space between the secondary arms of the dendrites, from It is observed that the addition of Ag favors an increase in the ratio γ-γ′/β due to the reduction in the number and volume of the interdendritic phase (Figure 3(b)). Because of the low solubility of Ag in Ni [56], it is possible to observe the presence of Ag-rich phases. According to Zhou and Guo [57,58], the presence of Ag at grain boundaries begins during the solidification process since this element segregates at the solid-liquid interface, and at the same time, the Ag solubility decreases causing their precipitation into the grains.…”

Phase Differentiation and Characterization of Nanostructured Composites by Synchrotron Radiation Techniques

“…Several efforts to produce such a solid solution by creating metastable states under extreme conditions, such as liquid quenching, did not lead to a reproducible fabrication procedure. The formation of the solid solution was only reported on thin films using laser ablation deposition [ 10,11 ], co-evaporation method [ 12], and recently mechanical alloying [13]. Taking advantage of the high surface-to-bulk energy ratio, which is characteristic of nanocrystals, we were able to prepare samples [7] containing significant amounts of AgNi alloy phase with a wide range of compositions using a co-evaporation of Ag and Ni.…”