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BigDon
2015-Oct-22, 08:43 PM
Sublime in a hard vacuum?

I've been wondering that since my Navy days.

Swift
2015-Oct-22, 09:04 PM
It does? I've never heard of this.

BigDon
2015-Oct-22, 09:07 PM
I heard that back in the service Swift. Even being told I heard wrong would be positive information. :)

Swift
2015-Oct-22, 09:23 PM
Titanium does love to form an oxide passivation film. There is a device used in very high vacuum systems that is essentially just a titanium filament that you put current through to get hot; it reacts with any remaining oxygen in the vacuum and pulls it out of the vacuum. Some of titanium does sublimate in the process and you eventually replace the filament. But you are talking about trace quantities and a very thin filament and so hot it glows.

I suspect a piece of titanium metal in a high vacuum at any reasonable temperature would have close to zero vapor pressure.

cjameshuff
2015-Oct-22, 09:56 PM
Titanium does love to form an oxide passivation film. There is a device used in very high vacuum systems that is essentially just a titanium filament that you put current through to get hot; it reacts with any remaining oxygen in the vacuum and pulls it out of the vacuum. Some of titanium does sublimate in the process and you eventually replace the filament. But you are talking about trace quantities and a very thin filament and so hot it glows.

I suspect a piece of titanium metal in a high vacuum at any reasonable temperature would have close to zero vapor pressure.

An odd device: https://en.wikipedia.org/wiki/Titanium_sublimation_pump
The sublimation (at high temperature) is why it works...the sublimed titanium is deposited on the chamber walls, without an oxide film. I suppose titanium is used because it's easier to store and handle than most more-reactive metals.

https://en.wikipedia.org/wiki/Vapor_pressures_of_the_elements_(data_page):

Looks like most common metals (including, for example, iron) have higher vapor pressures than titanium.

BigDon
2015-Oct-22, 10:51 PM
I recall more detail now.

I was told this was the reason it wasn't used pure in spacecraft and satellites.

profloater
2015-Oct-22, 11:34 PM
I have some experience with titanium including welding it in electron beam welder at high vacuum, so any vapour coming off is pretty low value because otherwise it would short the electron gun. One unique property of ti is you write with it on glass. Being highly reactive it forms an instant oxide which has a strangely high friction with glass, the oxide reforms instantly in air but if you did it in vacuum the bare metal would be exposed. It then acts as a getter, that is it will get any stray oxygen or even nitrogen molecules and thus take you to even higher vacuum. In order to sputter ti in high vac you would have to heat it pretty hot, and you would need very high vac or you just get oxide etc.

profloater
2015-Oct-22, 11:39 PM
I think ti would be fine in space but its main advantage of high stiffness to weight may not be enough to justify the cost and it's hard to weld, but electron or tig will weld it. It was used a lot on Concorde. The even better metal is beryllium but that is so toxic it is rarely used.

swampyankee
2015-Oct-23, 12:16 AM
Pure titanium has fairly mediocre mechanical properties, so nobody uses it unalloyed. Beryllium is used on spacecraft and missiles, because it has a greater Young's modulus than steel, but a much lower density: E/\rho is much greater than that for steel, aluminum, or titanium.

John Mendenhall
2015-Oct-23, 12:54 AM
The source of a possile misinterpretation may be here:

https://en.wikipedia.org/wiki/Titanium_sublimation_pump

John Mendenhall
2015-Oct-23, 01:06 AM
And since some unreferenced, and perhaps wrong. assertions have crept into this thread, the general Wiki article is here:

https://en.wikipedia.org/wiki/Titanium

John Mendenhall
2015-Oct-23, 01:15 AM
BigD, the fire hazard info is interesting in the Wiki article. (Burning metals are a carrier nightmare).

profloater
2015-Oct-23, 11:56 AM
Pure titanium has fairly mediocre mechanical properties, so nobody uses it unalloyed. Beryllium is used on spacecraft and missiles, because it has a greater Young's modulus than steel, but a much lower density: E/\rho is much greater than that for steel, aluminum, or titanium.
Indeed that's right for mechanical properties you would not select pure ti nor pure al or pure iron. Alloys always seem to improve and ti alloys are chosen for high temperature low creep and I believe high fatigue resistance such as turbine blades. At lower temperatures al alloys are so much cheaper. But just to finish off aluminium always fatigues eventually while steel has a "fatigue limit" and IIRC ti alloys also.

Jetlack
2015-Oct-23, 01:09 PM
Sort of OP but my old dentist told me how accidentally they found titanium fuses with human bone without rejection. Apparently by mistake a piece was left inside someone´s body during an operation, and they realised it could be used for implants etc...So without titanium dental implants would not be possible or at least not fused with the jawbone.

swampyankee
2015-Oct-23, 01:44 PM
BigD, the fire hazard info is interesting in the Wiki article. (Burning metals are a carrier nightmare).

Titanium does burn! When I was a test engineer, we had a compressor rub on an engine. Lots of TiO 2 dust got spewed out (also MgO, and some ThO2).

cjameshuff
2015-Oct-23, 09:30 PM
Titanium does burn! When I was a test engineer, we had a compressor rub on an engine. Lots of TiO 2 dust got spewed out (also MgO, and some ThO2).

The MgO makes sense, but what was thorium doing in that mix?

swampyankee
2015-Oct-24, 01:15 AM
Indeed that's right for mechanical properties you would not select pure ti nor pure al or pure iron. Alloys always seem to improve and ti alloys are chosen for high temperature low creep and I believe high fatigue resistance such as turbine blades. At lower temperatures al alloys are so much cheaper. But just to finish off aluminium always fatigues eventually while steel has a "fatigue limit" and IIRC ti alloys also.

Actually, it's only mild steel that is thought to have a fatigue limit1. The sort of high strength steels used in the aerospace industry (I managed to move from structural test to aerodynamic analysis at Sikorsky) don't have some threshold below which fatigue doesn't happen, nor do titanium-based alloys or aluminum-based alloys. Even composites will degrade from repeated loadings. Titanium-based alloys are useful, but there is a lot of ** surrounding them. As an aside, most of the steel in a 787 (it's about 10% of the aircraft's weight: http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_06/article_04_2.html) is in the landing gear -- possibly one of the most highly stressed parts of the airframe.


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1 It's also something that savvy designers don't trust: that "fatigue limit" won't hold if the steel is operating in a corrosive environment or if it's in a structural member which is subject to fretting (see http://www.epi-eng.com/mechanical_engineering_basics/fretting_corrosion.htm) or corrosion.

profloater
2015-Oct-24, 03:36 AM
Fair enough I had some qualms when a colleague told me they were considering fully flexural helicopter blades ie the blade being twisted every revolution to the required angle using a high alloy steel. They reckoned it would be more reliable than the usual bearings! As another aside I tested some composite grp car leaf springs and could not get them to fail at millions of reverse cycles. It's off topic to get deep into fatigue but for those interested, fatigue requires reverse loading, not just high fluctuations but you have to consider stress raisers like holes where stress can reverse even when the overall load seems to be in one direction.

swampyankee
2015-Oct-24, 03:56 AM
The MgO makes sense, but what was thorium doing in that mix?

Thorium was added to magnesium to improve its high temperature properties. See, for example, http://www.google.com/url?sa=t&rct=j&q=thorium+magnesium+alloy&source=web&cd=11&ved=0CBsQFjAAOApqFQoTCOr73a-b2sgCFYg6PgodsjUBhw&url=http%3A%2F%2Fwww.ewp.rpi.edu%2Fhartford%2F~fle mis2%2FEP%2FAn_Overview_of_Magnesium_based_Alloys_ for_Aerospace_and_Automotive_Applications.pdf&usg=AFQjCNGLj1b53DPCAcyf_3noh7sQW0N-1w&bvm=bv.105841590,d.cWw

swampyankee
2015-Oct-24, 10:52 AM
The MgO makes sense, but what was thorium doing in that mix?

Thorium was (is?) added to magnesium alloys to improve its properties at high temperature. See https://www.orau.org/ptp/collection/consumer%20products/magthor.htm

swampyankee
2015-Oct-25, 02:21 PM
Fair enough I had some qualms when a colleague told me they were considering fully flexural helicopter blades ie the blade being twisted every revolution to the required angle using a high alloy steel. They reckoned it would be more reliable than the usual bearings! As another aside I tested some composite grp car leaf springs and could not get them to fail at millions of reverse cycles. It's off topic to get deep into fatigue but for those interested, fatigue requires reverse loading, not just high fluctuations but you have to consider stress raisers like holes where stress can reverse even when the overall load seems to be in one direction.

MBB pioneered hingeless rotors; they don't have flapping hinges on their blades. Kaman didn't use pitch change bearings on their blades; they controlled collective and cyclic pitch by warping the blade. Removing flapping and lead-lag hinges and pitch change bearings is probably a bit of a leap, but surely workable.