Electron Microscopy

Qualitative SEM Shape Analysis. The most significant feature of the scanning electron microscope, in addition to its high magnification capability (useful magnifications beyond 10,000*), is its ability to produce images with a three-dimensional appearance. This ability derives from the fact that the depth of field for the scanning electron microscope is over 100x that of the optical microscope. This increased depth of field (ranging from 1 /' m at 10.000/ to 2 mm at 10/) also accounts for the extensive particulate analysis performed on the scanning electron microscope at magnifications that are within the capabilities of optical microscopy.

The imaging capabilities of the scanning electron microscope make it a useful tool for P/M applications concerned with all phases of powder production and processing. Scanning electron microscopy has been used to study all aspects of particle morphology, including size, shape, surface topography, surface structure (crystal, grain, and dendrite), coating or thin film characteristics (oxides), inclusion, void, and agglomeration characteristics, and satellite formation. The scanning electron microscope has also been used to study surface topography, effect of oxides or other coatings, porosity, inclusions, and other contaminants on P/M materials.

One of the more useful applications of the scanning electron microscope in powder metallurgy is qualitative particle characterization in terms of visual appearance. Despite the development of automated instruments for quantitative particle characterization, no substitute has been found for the interpretive capability of man. The use of the scanning electron microscope extends this capability into the microscopic domain.

Inspection of powder particles to be used in P/M processing is essential, because individual and agglomerate characteristics can have significant effects on final material properties. Figure 23 shows representative alloy powder particles used in the direct rolling of aluminum P/M strip. The irregular shape promotes interlocking of particles, which provides sufficient green strength for strip processing.

Fig. 23 Scanning electron micrographs of 7091 aluminum alloy particles used in direct powder rolling of strip. (a) 70x. (b) 700x

Quantitative Analysis of Projected Images. In general, microscopists encounter two types of projected images. The first type of image results from a transmitted beam through the specimen, representing the features located within the three-dimensional space (such as by thin-foil transmission electron microscopy). In the second type, the projected image is generated by a reflected beam from the external surface of the specimen (such as by scanning electron microscopy).

Only the most rudimentary quantitative calculations can be made on images projected by the reflection techniques (Ref 18). In rough surfaces, the intensity levels depend on topography, and some features may be masked by others. Three-dimensional characterization is based on the photogrammetric analysis of stereopairs, for which automatic image-analyzing techniques are not yet available (Ref 19).

Quantitative statistical treatment of transmitted-beam images, however, has matured to a much greater extent. These analyses (Ref 20, 21) are too lengthy and complex to be treated here, but are described in the literature (see Ref 5).

One final topic will be included, because of its importance to the analysis of particulate systems. Figure 24 provides interrelated general equations of convex particles that express the important spatial parameters in terms of measurements made on the polish and projection planes. Application of the equations to specific particles is summarized in Table 2 for the sphere, the truncated octahedron (or tetrakaidecahedron), and convex particles in general. Tabulations of the type presented in Table 2 permit the microscopist to approximate microstructures with particles of known shape when other techniques are not feasible.

Table 2 Properties of a sphere, truncated octahedron, and convex particles

Property

Sphere,

Truncated octahedron,

General equations

D = 2r

edge length = a

for convex particles

V

4r: r3/3

11.314a3

V=AL3 = AH

S

4rr r2

26.785a2

S = 4A' = 4V/L3

A'

6.696a2

A = S/4 = V/L3

H'

2r

3.0a

H = V/A = AL3/A

A

2r r2/3

3.77a2

A = V/H = AL3/H

¿3

47-/3

1.69a

L3 = 4V/S = ANA/N

r = 2Nl/:: Na

a = 0.45Nl/Na

! ■'= H/2 = NaA/2Nl = A/2L2

Nr

TT N\j4NL

0.744N\/NL

NV = Na/H = Nl/A

General equation: VV = NVV + NaA = NlL3

Source: Ref 23

Source: Ref 23

(a) P = half of mean tangent distance.

Projection Spatial Section

Projection Spatial Section

= JVj^iVA'H1 V= A O= A' Ls Lj = 4VM A = L, d Nt = NA ■ d

Li = nA'/ti L'£ = d S = 44' D - hf Lj = *AfL„ - Nv O

H' = L'r/it H' = A'/d' A = Ij ■ L'j L3 = Lt d - LJIT f4y -

= JVj^iVA'H1 V= A O= A' Ls Lj = 4VM A = L, d Nt = NA ■ d

Li = nA'/ti L'£ = d S = 44' D - hf Lj = *AfL„ - Nv O

H' = L'r/it H' = A'/d' A = Ij ■ L'j L3 = Lt d - LJIT f4y -

Fig. 24 Relationships among convex particles in space, their sections, and their projections. Projected quantities are indicated by primes. Source: ASM Handbook, Volume 9, p 134

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