Microstructure
Lead and bismuth are practically insoluble in solid copper, and they solidify last as almost pure metals at the grain boundaries. Lead is distributed as globules which is the desired morphol- ogy. Bismuth is also present as globules, but it can also form a film along the grain boundaries. If this film is continuous, it can have a detrimental effect on material properties. This can lead to severe alloy embrittlement. Plewes et al.4
performed tests on
possible third-element additives to Cu-1% Bi alloys to combat this phenomenon. They found that one of the best additives to combat the effect of bismuth was tin. Since tin is added to these alloys to provide solid solution strengthening, it should also help to achieve a more uniform distribution of bismuth.
tion was used on the samples, and micrographs from some selected samples are presented in Fig. 13. All the micrographs show a structure typical for cast tin bronze. Due to the nature of copper-tin alloys, considerable segregation occurs during freezing. This causes high composition gradients within the grains, their central portions being rich in copper, surrounded by zones increasingly rich in tin. This phenomenon, known as coring, commonly occurs in the as-cast structure. The last liquid to solidify is enriched with tin upon cooling and forms alpha and delta phases. The alpha and delta phases fill in the areas between the dendrite arms; this results in the eventual
Samples for microscopic examination and microhardness testing were cut from the threaded grip section of the tested tensile bars. Micrographs taken of as-polished samples cut from Alloys 1, 2 and 3 are shown in Figs. 10, 11 and 12, respectively, after testing at room temperature. These mi- crographs show that the bismuth – containing alloys have particles of pure bismuth (gray particles) and particles of an intermetallic copper-tin phase (light blue particles). The as- polished micrographs taken of the leaded alloy show a similar structure, except the particles are lead, not bismuth. The mor- phology of the lead particles seems to be more globular than the bismuth particles. A ferric chloride (FeCl3
) etching solu-
formation of a hard, brittle phase, occurring as bluish lakes of alpha-delta eutectoid at the alloy grain boundaries. Since these alloys have a long freezing range (160ºC) and are cast in sand, some samples have massive grains – as large as 8 mm – while some grain sizes are in the 100-200 µm range.
Alloy 2 (Fig. 11) has much more shrinkage porosity than Alloy 1 (Fig. 10). The shrinkage porosity shown in Fig. 11 is quite significant and would affect the ultimate tensile strength, elongation and fatigue strength, but not the yield strength. Referring back to Fig. 7(a), the differences in yield strength between Alloys 1, 2 and 3 are not as dramatic as those of the ultimate tensile strength and elongation. The micrographs presented in Figs. 10 and 11 show that bismuth has an elongated smooth morphology. The micrographs in Fig. 13 show considerable bismuth along the grain boundar- ies. Despite bismuth segregation, in some cases, the proper- ties are still encouraging. The high tin content must serve to partially combat the adverse effect of the bismuth. Alloy 3 has much less shrinkage porosity than Alloys 1 and 2. It is possible that the design of the mold poured for the test bars can be optimized for the bismuth-containing alloys.
The Scanning Electron Microscope (SEM) was used to examine as-cast microstructures and the fracture surfaces of the tensile bars both after room temperature and 250ºC (482°F) testing. Figure 14 shows a back-scattered electron (BSE) image and results from energy-dispersive spectros- copy (EDS) of the phases present in as-cast Alloy 1. In BSE images, the higher the atomic number the lighter the phase will appear in the image. Therefore lead and bis- muth, which have higher atomic numbers than copper, will appear as white while phases having lower atomic number elements will appear darker. The light phase in Fig. 14(a) is the bismuth; Fig. 14(b) is the EDS for the copper-tin rich intermetallic. These copper-tin particles are the alpha and delta lamellar phase mentioned previously. The matrix contains copper and tin.
Figure 9. Fatigue limit of Alloys 1, 2 and 3. International Journal of Metalcasting/Winter 10 25
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