Mechanically driven alloying and grain size changes in nanocrystalline Fe-Cu powders

Abstract

Highly supersaturated nanocrystalline Fe,Culcc-X alloys (lO<n<95) have been prepared by mechanical alloying of elemental crystalline powders. The development of the microstructure is investigated by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The results are compared with data for ball-milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films. The deformation during milling reduces the grain size of the alloys to 6-20 nm. The final grain size of the powders depends on the composition of the material. Single-phase fee alloys with x<60 and single-phase bee alloys with ~80 are formed even though the Fe-Cu system exhibits vanishingly small solid solubilities under equilibrium conditions. For 6O<x<80, fee and bee solid solutions coexist. The alloy formation is discussed with respect to the thermodynamic conditions of the material. The role of the large volume fraction of grain boundaries between the nanometer-sized crystals, as well as the intluence of internal strains and stored enthalpies introduced by ball milling, is critically assessed.

Nanocrystalline Metal Hydrides Obtained by Severe Plastic Deformations

“…The coexistence of FCC and BCC has been seen in the system Fe-Cu and was explained by an enhanced solubility due to the high dislocation density [45] but this explanation may not be applicable in the present case. From Rietveld refinement it was determined that the crystallite size of the BCC phase goes from 44 nm before milling to 2 nm after 80 h of milling.…”

Dissolving Equilibrium-Immiscible Elements via Severe Plastic Deformation

“…34) Due to this kinetic advantage, a filament with sharp tips would sphereodize rather than dissolve, as observed in many experiments. Schwarz 35) (and Eckert et al, 3,4,7) to some extent) has suggested mechanisms based on interactions between the dislocation strain fields and the solutes. They hypothesized preferential segregation of solute atoms into core regions of dislocations, which are abundant in SPD materials.…”

“…In other words, the externally imposed randomizing action forces the issue despite the fact that the two species dislike each other. The typical deformation routes used were ball milling of powder mixtures, [3][4][5][6][7][8][9][10][11][12][13] cold rolling of stacked multilayers, 14) extrusion of in situ composites, 15) and high pressure torsion. 16) Studies of deformation-driven alloying overcoming thermodynamic driving forces are motivated not only by the science appeal of this fundamental subject, but also by the unusual properties resulting from the nonequilibrium alloys created.…”

Synthesis and Characterization of Nanocrystalline Al-20 at. % Cu Powders Produced by Mechanical Alloying

“…It is commonly known that during MA, powders undergo a severe plastic deformation, which introduces a number of defects into the material, and it is worth noting that this causes a gradual change in the state of the powder mixtures and hence their properties [8,9]. Further, Eckert et al [10] found that the final grain size is determined by the competition between the deformation produced by a milling process, and the dynamic recovery in the milled material. On the other hand, it has been suggested that the stacking fault energy (SFE) has a strong influence on the evolution of the dislocation structure, which precedes and results in the nanocrystalline structure formation [11].…”