Determination of the Degree of Homogeneity

Direct experimental techniques for the determination of the progress of homogenization or the evaluation of the state of homogeneity include microscopy (qualitative and quantitative), compositional analysis of the microstructure via electron microprobe analysis or scanning electron microscope instrumentation, magnetic, electrical, or x-ray compositional line broadening techniques (Ref 10, 11, 12, 14).

X-ray compositional line broadening is a technique for the rapid determination of the degree of homogeneity. It displays the spectrum of compositions existing in a given inhomogeneous phase from the shape of a diffraction peak broadened by a range of lattice parameters in the phase. This technique may be used for the analysis of single-phase or multiphase binary alloys. The experimental procedure involves obtaining x-ray diffractometer patterns of the specimen whose degree of homogeneity is to be determined.

The progress of homogenization is seen clearly, although qualitatively, in Fig. 13. The Ni (311) peak occurs at a diffraction angle 20 ^93°, and the Cu (311) peak occurs at 20 ^90°. Figure 13(b) indicates that the initial stages of homogenization (low temperatures and short times) proceed by forming the complete range of compositions from nickel to copper. This is the manifestation of the initial interdiffusion across the nickel-copper interparticle boundaries.

Fig. 13 X-ray diffraction peaks (311) for compacts of blended nickel and copper powders. Mean composition: 0.28 atom fraction copper. Nickel: -400 mesh. Copper: -200+270 mesh. (a) Initial condition. (b) and (c) Various partially homogenized conditions

As shown in Fig. 13(c), later stages of homogenization (high temperatures and long times) result in the formation of a single peak at a diffraction angle between those of pure nickel and pure copper. This corresponds to the approach to a single homogeneous phase having the mean composition of the system. The diffraction angle of the single peak is determined by this mean composition. Quantitative analysis of peak shapes can be used to obtain the spectrum of phase compositions within a specimen.

Microscopy is also useful to determine the degree of homogeneity, particularly for multiphase systems that are difficult to analyze using x-ray compositional line broadening techniques. Figure 14 shows the development of microstructures and the progress of homogenization in compacts made from a blend of nickel and tungsten powders. In a partially homogenized compact (Fig. 14), three distinct regions are apparent:

• Dispersed light gray particles (the undissolved tungsten-rich phase that originated as tungsten particles)

• Dark gray unetched regions surrounding the tungsten-rich particles (the nickel-rich phase that has tungsten concentrations ranging from approximately 0.05 atom fraction of tungsten at the interface between the etched and unetched regions up to 0.17 atom fraction of tungsten at the interface between the unetched region and the tungsten-rich particles)

• Etched matrix (the nickel-rich phase that has tungsten concentrations less than 0.05 atom fraction)

Fig. 14 Scanning electron micrographs showing the progress of homogenization in compacts made from a blend of nickel (-400 mesh) and tungsten (-325+400 mesh) powders. Mean composition: 0.05 atom fraction tungsten. 75x. (a) t = 0.0 h. (b) T = 1150 °C (2100 °F); t = 10.0 h. (c) T = 1150 °C (2100 °F); t = 48.0 h

Fig. 14 Scanning electron micrographs showing the progress of homogenization in compacts made from a blend of nickel (-400 mesh) and tungsten (-325+400 mesh) powders. Mean composition: 0.05 atom fraction tungsten. 75x. (a) t = 0.0 h. (b) T = 1150 °C (2100 °F); t = 10.0 h. (c) T = 1150 °C (2100 °F); t = 48.0 h

Quantitative microscopy can be used to determine the extent of homogenization in multiphase systems. This technique is compatible with x-ray compositional line broadening (XCLB) in providing data on the presence of second phases and may be performed on the same test specimens used for XCLB measurements.

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