C. E. Krill

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

Highly supersaturated nanocrystalline FexCu100−x alloys (10≤x≤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 fcc alloys with x≤60 and single‐phase bcc alloys with x≥80 are formed even though the Fe‐Cu system exhibits vanishingly small solid solubilities under equilibrium conditions. For 60≤x≤80, fcc and bcc 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 cryst...

Reversible grain size changes in ball-milled nanocrystalline Fe-Cu alloys

Nanocrystalline Fe{sub {ital x}}Cu{sub 100{minus}{ital x}} solid solutions ({ital x}{lt}60) with single-phase fcc structure have been prepared by mechanical alloying. The average grain size of the powders (6--20 nm) depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by the underlying mechanism of plastic deformation and solution hardening during mechanical alloying coupled with the recovery behavior of the material.

Static mean-square displacement as an indicator of the crystal-to-amorphous transformation

Abstract Catastrophic amorphization of crystalline alloys supersaturated beyond a critical concentration has been postulated by Fecht, Desre and Johnson [Philos. Mag. B59 (1989) 577]. NbPd is a good candidate system for testing their ideas because of its plunging T 0 line on the Nb-rich side of the phase diagram. Signs of instability in bcc NbPd solutions were looked for by elastic neutron diffraction measurements of atomic mean-square displacements (MSDs) between 12 K and room temperature. Thermally induced MSD values have been estimated from low-temperature heat capacity measurements and subtracted from the total MSD. The remaining static MSD increases rapidly with Pd concentration and is much larger than expected from the size mismatch of Nb and Pd atoms. At 42 at.% Pd, the rms static disorder reaches approximately half the value at which the Lindemann criterion predicts that the lattice should melt. Supersaturation also causes the Debye temperature to decrease, suggesting that an elastic modulus softens. In light of these results, the usefulness of static atomic disorder as a measure of the stability of a crystal against amorphization is discussed.

Relaxation and Grain Growth Behavior of Nanocrystalline Iron

Nanocrystalline Fe has been prepared by inert gas condensation and ball milling. The kinetics of relaxation and grain growth are investigated by differential scanning calorimetry. The development of the microstructure is monitored by x-ray powder diffraction and transmission electron microscopy. Emphasis is placed on the differences observed for samples prepared by the two different techniques. We find that the kinetics of relaxation and grain growth are very sensitive to the sample preparation method. Samples with the same initial average grain size, as determined by the peak broadening in x-ray diffraction, show very different recovery behavior. The differences are discussed in terms of the estimated grain boundary energies and the initial grain size distribution obtained by the two preparation techniques.

Rapid-Quenching Investigation of the Nb-Pd-Ge System

The structure of rapidly quenched Nb 100-x-y Pd x Ge y alloys has been investigated using x-ray diffraction. Niobium concentrations were varied between 100 and 45 at.% the remainder at each Nb concentration was composed of Pd and up to y = 15 at.% Ge. Germanium was found to suppress the nucleation rate of the fcc α-NbPd phase relative to that of the bcc α-Nb phase, thereby extending the single-phase bcc solubility range by ≈ 2 at.% Nb. High Ge content ( y > 6) also induced quenching of the amorphous phase. These results can be understood from the standpoint of classical nucleation theory and from a consideration of the polymorphic phase diagram of Nb-Pd. The two approaches are consistent with Ge addition depressing the T o line of the fcc phase more rapidly than it depresses the T o line of the bcc phase.