J. C. Holzer

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...

Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys

X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.

Melting behavior of nanocrystalline aluminum powders

Abstract Nanocrystalline aluminum powders have been synthesized by mechanical attrition under different atmospheres and by gas condensation. The crystal refinement and the development of the microstructure are investigated by x-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and electron energy loss spectrometry. Both preparation techniques lead to powders with comparable grain sizes. With decreasing grain size we find a drastic reduction of the melting point, ΔTm, in comparison to the bulk value. Subsequent remelting does not recover the bulk melting point. The results are discussed in terms of the microstructure of the nanocrystalline powders, the contribution of the stored enthalphy of cold work, and the nucleation of disorder/melting at grain boundaries/particle interfaces.

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.

Gibbs free energy difference between the undercooled liquid and the beta phase of a Ti-Cr alloy

The heat of fusion and the specific heats of the solid and liquid have been experimentally determined for a Ti60Cr40 alloy. The data is used to evaluate the Gibbs free energy difference, ΔG, between the liquid and the β phase as a function of temperature to verify a reported spontaneous vitrification (SV) of the β phase in Ti‐Cr alloys. The results show that SV of an undistorted β phase in the Ti60Cr40 alloy at 873 K is not feasible because ΔG is positive at the temperature. However, ΔG may become negative with additional excess free energy to the β phase in the form of defects.

Formation And Stability Of Nanocrystalline Nb-Cu Alloys

Nanocrystalline Nb 100−x Cu x (0 ≤ x ≤ 30) alloys of 8 – 25 nm grain size are synthesized with the mechanical alloying technique. Differential scanning calorimetry reveals two separate stages of grain growth. In the first stage, the grain growth is associated with migration of solute Cu atoms to grain boundaries. Grain growth stops as the grain boundaries are saturated with Cu. The second reaction takes place either when the threshold for the nucleation of an Fcc Cu phase in the grain boundary is overcome by thermal activation, or when segregating Cu atoms in the grain boundary are driven by diffusion to the growing Cu grains, and the Cu concentration in the grain boundary drops. This increases the grain boundary energy, and initiates a second stage of rapid grain growth.

Gibbs free-energy difference between the glass and crystalline phases of a Ni-Zr alloy

The heats of eutectic melting and devitrification, and the specific heats of the crystalline, glass, and liquid phases have been measured for a Ni24Zr76 alloy. The data are used to calculate the Gibbs free-energy difference, DeltaGAC, between the real glass and the crystal on an assumption that the liquid-glass transition is second order. The result shows that DeltaGAC continuously increases as the temperature decreases in contrast to the ideal glass case where DeltaGAC is assumed to be independent of temperature.

Effects of chemistry on the grain size refinement in nanocrystalline Ru and Ru-C powders prepared by mechanical attrition

Abstract Nanocrystalline Ru and Ru80C20 powders have been prepared by high-energy ball milling of elemental powders under a reactive methane atmosphere. The powders are investigated by x-ray diffraction and transmission electron microscopy. The deformation during milling results in a decrease of the grain size to 7 nm for elemental Ru and to 4 nm for Ru80C20. The results are compared to ball milling experiments on Ru under inert argon gas atmosphere. The influence of composition and milling atmosphere on the grain size refinement during milling are discussed with respect to the underlying mechanism of plastic deformation, coupled with solution hardening during milling and the role of solute segregation at grain boundaries.

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.