Voids in solder joint under high environmental pressure

Solder, for instance. ;)

You can make indium wet metal or glass by rubbing it on.

Cheers

Phil Hobbs

Hmm, that seems like cheating. Force is being applied.

I wouldn't think so.

Metal migration? Still seems like cheating the terminology.

Indium isn't quite as low-melting as that.

Cheers

Phil Hobbs

Are you sure the friction is not causing it to heat and melt?

How about surfaces that are machined and polished so flat that they bond when in contact?

durability.asp)

Not an old wives tale, although the one about glass being a supercooled liquid certainly is. A good example is clamping a thin foil of gold between two clean and flat blocks of silver. The gold will completely diffuse into the silver in a few years at room temperature, apparently vanishing into what becomes a solid block of silver, although all of the gold will still be there not too far from the original position of the foil. It happens faster at higher temperatures of course. Silver plate is used on the bonnet seal rings of many high pressure main steam valves, diffusing into the steel valve body and bonnet, creating a hermetic seal.

Most metals will undergo some creep with zero externally applied stress at room temperature if you look close enough, due to internal stresses. For this reason it is necessary to let the hardened tool steel for micro- inch accurate gage blocks age for a few years before producing the gage block. The atoms in solids move around a lot more than most people realize. Tin and zinc whiskers are good examples. Stress free materials are virtually nonexistent, with the exception of dislocation free single crystals, which are not too common.

The structure of latex paint, which contains no latex, is produced by microscopic beads of acrylic diffusing into each other and the surface they are applied to, a process which starts when the carrier evaporates, allowing the beads to make contact. Latex paint which is not labeled

100% acrylic contains clay fillers which significantly weaken it and reduce the life of the paint dramatically in many cases, definitely not worth saving a few bucks for paint that won't hold up.

Sounds like metal migration which is a completely different kettle than wetting.

Yes, quite like corn starch and water. ...and the shape of the "beads" changes the characteristics of the paint (directionality of the shear).

I'm still not buying the "solid" part.

Yes. See also: weighted wire drawn through ice. In this case, the pressure causes the ice to liquify under the wire, and freeze above it, leaving the block otherwise intact. This can be done at any arbitrary speed, so friction is not causing it to melt.

Solid state metallurgical reactions have been known since antiquity: one can "smelt" mercury from cinnabar (its sulfide) by pounding it in a mortar and pestle with another metal more reactive towards sulfur (I think the story uses a silver mortar, though I'd think the mercury metal would stick to that rather well given its propensity for amalgamation).

These days, reactive ball milling is a common laboratory practice (though I don't know if it's seen much industrial use). Especially "high energy" ball milling probably does proceed through contact frictional heating, but solid state diffusion is nonetheless a necessary mechanism.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Hmm, seems like a potentially bad idea -- diffusion can leave porosity:

Maybe not an issue for hermetic seals.

Similar stuff goes on as normal industry practice in silicon, especially MEMS stuff: take two slabs of commercial silicon -- high purity and single crystal, a materials theoretician's dream -- and press them together in the middle and heat. Usually a charge is applied, especially for bonding glass. The charge causes ions to diffuse to the surface, effectively reducing the melting point locally. The perfectly planar surfaces begin to bond by diffusion, and the bonding wave can be observed by infrared interferometry.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Not likely important for the OP, but what we were taught in first year...

.. does not seem to be 100% true..

--sp

Best regards, Spehro Pefhany

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"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Ice is a weird material. That doesn't happen with metal, at least not at ordinary temperatures.

Cheers

Phil Hobbs

Metal migration is a surface phenomenon, not a bulk phenomenon like diffusion. The term "wetting" does imply a liquid being involved, when solid metals are involved it is generally referred to as forming a metallurgical bond, as is the case in the silver-bonded seal rings.

I don't blame you for not buying the "solids wetting" part, but there are lots of examples of solid to solid metallurgical bonds made with no melting, including fabrication of bi-metal sheet by pressure bonding between rollers, explosive bonding of heavy bi-metal plates, and the fabrication of aluminum evaporators for refrigerators, where a pattern for the integral tubing is printed on one clean aluminum sheet, two clean sheets are pressure bonded, and then the unbonded tube pattern is hydraulically expanded.

Interesting, thanks. Filed for future reference!

Cheers

Phil Hobbs

Interesting link, thanks. I never heard of it before. This does not seem to be an issue with the silver plated steel seal rings however. When they are broken apart with a large hammer what you see is a fracture of a steel-silver alloy, mostly steel (no silver visible, although it must still be there), with no significant voids, where a leak would be required for it to be significant. The only leak I ever saw in one of these was caused by faulty surface preparation.

By eye alone? Seems to be a microscopic phenomenon, probably not visible on that scale.

Hmm... I find no phase diagram on Ag-Fe in my list, so I have no idea what intermetallics form (if any). If it's like Cu-Fe, then there would be only a little solid state solubility, and that's it, which wouldn't diffuse much. Offhand I'd expect behavior near nickel or palladium (which do form solid solutions with iron), but not so, wholeheartedly shall we say.

If the seals tend to fracture on the seam, then there's certainly something going on, such as voids and brittle intermetallics. Fracture would seem to suggest a brittle rather than malleable phase (such as Fe-Ni or Fe-Pd analogs would probably give).

Oh... also on topic, "3000 A.D.":

He made some other interesting things too.

Also...is that girl in the "Weathering Wood" pic topless... :-?

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Weird indeed!

Although, albeit at much higher pressures, bismuth and a couple of other things ought to pull the same trick, no? Expanding on freezing can only exert so much pressure, thermodynamically speaking.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

I use to make low temperature (helium leak tight) seals by taking two flat pieces of metal (mostly copper alloys) and pressing indium onto both sides, and then a light smear of vacuum grease... with a several screws around the perimeter.

George H.

Interesting thanks. Hmm I make little flexure mounts from aluminum (7075) for diffraction gratings. Vibrated at 10 Hz to 1 kHz, should I worry?

George H.

That depends on the peak stress and the required life expectancy.

Considering only bending moment (neglecting shear, probably safe)

stress = My/I (lbf/inch^2)

M = bending moment (lbf*inch) y = maximum distance to the neutral axis, 1/2 thickness for a rectangular beam. (inch) I - rectangular moment of inertia (inch^4) = 1/12 width x thickness^3 for a rectangular beam bent across the thickness.

The peak bending moment is a bit tricky (meaning I don't recall how) for a resonant beam, which is not simply loaded, but if you can estimate the minimum radius of curvature r (inches) then:

M = EI/r

E = longitudinal modulus of elasticity (Young's modulus) (lbf/inch^2) = approx 10E6 for aluminum

Look up your expected fatigue life for the estimated peak stress on the above wikipedia graph and see if it meets your needs.

It occurred to me after posting that if the mass of the grating assembly is reasonably larger than the the mass of the flexure, then you could approximate as simply loaded on the end of the flexure and calculate stress based on peak deflection, which is probably easier to measure than radius:

M = 3EId/l^2

where d is peak deflection from free state (1/2 total deflection at the end of the beam) and l is the length of the beam.