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Fracture of a multiphase fusible element.
The scanning electron microscopy on this page was performed
superbly by Micron, Inc. of Wilmington, Delaware at our direction.
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We were trying to make sense of the apparent failure of a
fusible element at a temperature below its usual limit. What path
would the fracture follow ? We started by peeling an exemplar
element as shown at left. The two images below are of the two
mating surfaces of the fracture - about in the middle of the upper edge
in the image at left. They are interchanged (Side 2 on the left) so as to facilitate visual correlation with the image at left. We numbered the distinctive areas 1 through 4. |
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In this set of three images we have made a cross section of
another, similar fusible element in order to identify the appearance
and composistion of its microconstituent phases. We have numbered
these Phases 1 through 4 in the high-magnification image below.
These numerals in no way are meant to match up with Areas 1 through 4
on the fracture surfaces. Those were identified and matched
across the fracture surface by comparing the macroscopic appearances of
the four distinctive surfaces along which the fracture proceeded. |
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| Energy-dispersive histogram obtained from the different
phases seen at 3500X above. The 20kV electrons in the SEM
generate X-rays which have characteristic energies corresponding to the
chemical elements in the phase. |
Elemental composition of the phases from the EDX data. |
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Phase 1: Tin plate over nickel body. |
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Phase 2: 94% tin - 4% bismuth (grey). |
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Phase 3: 92% bismuth- 8% lead matrix. |
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Phase 4: 57% bismuth, 34% lead, 9% tin (dark). |
| # |
Fracture appearance - Side 1, Area (#) |
Fracture appearance - Side 2, Area (#) |
| 1 |
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| 2 |
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| 3 |
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| 4 |
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| The
energy-dispersive
histograms below were obtained from the different areas seen at 2000X
on the
fracture surfaces in the second row of images at the top of this
page. The calculated elemental compositions are given above each histogram. |
| Fracture side 1 |
# |
Fracture side 2 |
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Area 1 Side 1: 9% tin, 13% lead,
44% bismuth, 15% cadmium 10% nickel.
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1 |
Area 1 Side 2: 19% tin, 13%
lead, 60% bismuth, 9% cadmium 4% nickel.
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|
Area 2 Side 1: 21% tin, 0% lead,
11% bismuth, 3% cadmium 66% nickel.
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2 |
Area 2 Side 2: 42% tin, 1% lead,
9% bismuth, 2% cadmium 46% nickel.
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Area 3 Side 1: 6% tin, 50% lead,
35% bismuth, 10% cadmium 0% nickel.
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3 |
Area 3 Side 2: 40% tin, 10%
lead, 4% bismuth, 0% cadmium 35% nickel.
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Area 4 Side 1: 47% tin, 1% lead,
6% bismuth, 1% cadmium 44% nickel.
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4 |
Area 4 Side 2: 21% tin, 2% lead,
12% bismuth, 2% cadmium 63% nickel.
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| SUMMARY: The
four areas of the fracture surface of the peeled-apart fusible element
failed along three different fracture paths. In general, the
fracture path tended to follow a particular inter-phase boundary rather
than travelling through the homogeneous matrix. Area 3 failed at
the interface between the nickel frame and Phase 4 for example, and so
the two fracture surfaces within Area 3 look entirely different, both
at low magnification and at high magnification. Area 1 failed
within the Phase 3/Phase 4 mixture and so both fracture surfaces within
Area 1 look quite similar. Area 2 failed at the nickel - tin
boundary, but the tin was well bonded to the nickel, so the elemental
compositions are qualitatively the same but quantitatively
different. Area 4 is much like Area 2, except that the fracture
followed the nickel - tin interface on the opposite side of the layer
of fusible metal. |