William J. Baxter

The strength of metal matrix composites

There is intensive interest in metal matrix composites (MMCs) for automotive components, and the first production applications in Japan use discontinuous fibers as the reinforcements. These fibers are randomly oriented, resulting in an MMC with isotropic properties. However, there are conflicting reports on the tensile strengths attainable. In some cases, the strength increases with increasing volume fraction(Vf) of fibers, while in other cases, there is little or no benefit. A simple method is proposed to calculate the strength of this type of MMC. It is shown that the fibers oriented perpendicular to the stress direction play a key role, and the strength depends upon the strength of the interfacial bond. Upper and lower limits of the composite strength are calculated. If the bond strength is larger than the matrix strength, the composite strength has a maximum value which increases withVf. If the bond strength is weaker than the matrix, the composite strength has a minimum value which is either weakly dependent or even independent ofVf. These calculations are in good agreement with examples taken from the literature of aluminum composites reinforced with either A12O3, graphite, or SiC. The strength of the matrix alloy is shown to be a very important parameter: weak alloys are easily strengthened, while in certain cases, strong alloys may be weakened.

The detection of fatigue damage by exoelectron emission

Plastic deformation of metal enhances the photoelectron emission producing so‐called exoelectrons In our experiments the metal surface is scanned by a small spot of ultraviolet light and the electron emission is recorded as a function of position of the light spot. Results for aluminum and steel demonstrate that fatigue deformation produces exoelectron emission after less than 1% of the fatigue life. The regions of emission are very localized; some are beyond the resolution of the apparatus (15 μm). The localized emission increases throughout the fatigue life, and failure finally occurs in the region of most intense emission. The regions of intense emission are attributed to the development and propagation of fatigue cracks.

Photostimulated exoelectron emission from slip lines: A new microscopy of metal deformation

In this paper we show that photostimulated exoelectron emission resulting from the plastic deformation of a metal originates solely from the slip lines produced on the surface. Conclusive evidence is provided by micrographs formed by direct electron optical imaging of the exoelectrons. The exoelectrons are emitted only from bare metal revealed by a localized rupturing of the natural surface oxide during the formation of the slip step. This contrast mechanism is unique and offers a new form of electron microscopy for the study of plastic deformation of metals.

Photoemission electron microscopy of oxide fracture at slip steps on metals

The surface deformation of metals and the concomitant mechanical behavior of the surface oxide film may be observed simultaneously by photoemission microscopy. This is illustrated by photoemission micrographs of steel and aluminum plastically deformed in either tension or compression. During tensile deformation of both metals the ’’natural’’ thin surface oxide is ruptured by the emerging slip steps. This reveals the bare metal surface of the step itself, which appears on the micrographs as a line of enhanced photoemission (exoelectrons). On the other hand, compressive deformation of ?4% rarely enhances the photoemission, the majority of the slip lines exhibiting only topographical contrast. This difference is related to the geometry of the slip steps as well as to the behavior of the surface oxide. In compression the oxide on aluminum does not fracture at the slip steps, and no exoelectron emission is observed. The oxide on steel clearly does fracture, but few slip steps emit exoelectrons because they are...

The sequence of precipitation in 339 aluminum castings

The precipitation sequences in direct-quenched from the die (DQD) and solution-treated (SOL) 339 aluminum have been determined by a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). DSC scans for the alloy in both conditions exhibit two distinct exothermic peaks, each associated with a unique precipitate. The peak temperatures for precipitation in the DQD and SOL alloys differ by only a few degrees. TEM of samples heated to the lower temperature peak shows that the first precipitate to form in the DQD alloy is S′ (Al2CuMg), whereas in the SOL alloy it is β′ (Mg2Si). The principal precipitate associated with the higher temperature peak in both DQD and SOL alloys is Si. The DSC peak temperature identifies the specific precipitate in 339 Al, but the peak area is not a reliable measure of precipitate density. Nano-indentation of the dendrites shows that the strength provided by the precipitates increases in the sequence Si < S′ < β′. However, their thermal stability increases in the reverse order.

Measurement of surface fatigue damage by exoelectron emission

Plastic deformation enhances the photoelectron emission from a metal-an effect known as exoelectron emission. Previous results demonstrated that fatigue cycling produces exoelectron emission from localized regions, and that by scanning the surface with a small spot of ultraviolet radiation these fatigued regions may be detected very early in life. This paper demonstrates that the intensity of the localized emission in steel is a true measure of the accumulated damage, and can be correlated with changes in the surface topography. The development of persistent slip bands produces microcracks in the surface oxide revealing bare metal from which the electrons are emitted. The growth of these bands and the formation and propagation of fatigue cracks increases the area of fresh metal surface and hence the emission. The increase in exoelectron emission is very systematic and hence provides a calibration for the prediction of the remaining fatigue life.

Photostimulated exoelectron emission and slip‐step geometry during tensile and compressive deformation

Photostimulated exoelectron emission produced by tensile deformation is much more intense than that produced by compressive deformation. This result is demonstrated for a number of metals and apparently is quite general. The strain required for the onset of emission is not appreciably different for either mode of deformation. However the intensity of emission increases rapidly with increasing tensile strain, whereas additional compressive strain has little further effect. In both cases the natural surface oxide cracks to reveal a fresh metal surface of lower work function, thereby increasing the photoelectric yield. The differing effects of tensile and compressive deformation can be correlated with the geometry of the slip steps as observed by scanning electron microscopy.

A photoemission electron microscope using an electron multiplier array

A Philips thermionic emission electron microscope has been converted into a photoemission electron microscope by the addition of a simple ultraviolet illumination system and an electron multiplier array for image intensification. Photoemission images are formed without heating or activation of the surface. One example includes the first electron optical image of exoelectron emission from an abraded metal surface.

Growth of slip bands during fatigue of 6061-T6 aluminum

The growth of persistent slip bands (psb), the initial surface manifestation of metal fatigue, was measured with a photoelectron microscope equipped with a fatigue stage. Once the psb had been identified and located, the specimen was subjected to a final detailed examination by scanning electron microscopy. In 6061-T6 aluminum psb initiated within a grain, usually at the site of an inclusion. The psb appeared as a small extrusion and elongated across the grain by the sequential addition of new extrusions. As the psb elongated, the initial extrusions became more pronounced and eventually microcracks developed. Under constant amplitude loading, the rate of elongation of a psb in polycrystalline material varied inversely as the length, whereas in a large grain specimen the rate remained constant. This difference is attributed to the constraints imposed upon a small grain by the surrounding material. The growth laws can be accounted for in terms of a simple two phase model in which the psb has a much lower yield stress than the matrix of the grain.

The effect of oxide thickness on photostimulated exoelectron emission from aluminum

The development of photostimulated exoelectron emission (PSEE) was observed directly in a photoelectron microscope during plastic deformation of anodized aluminum. The PSEE defines the mode of fracture of the oxide as well as the tensile strain required to initiate fracture. Thick oxides (280 nm) develop cracks perpendicular to the stress direction which propagate independently of the underlying microstructure of the metal. Oxides ?70 nm thick fracture at the slip steps in the same manner as the very thin natural oxide. Contrary to the results of previous observers of PSEE, the tensile strain required for the onset of emission actually decreases slightly with increasing oxide thickness.