Hi, as a rookie in powder metallurgy field, I've heard that for sintering CP Ti Powder at 1200C in tube furnace, niobium tube or molybdenum crucible would be used as protective casing instead of ordinary alumina crucible. Is it true? I've read several paper but none of them have mentioned what the container was for their sample inside the tube furnace.
Molybdenum foil is indeed used as a diffusion barrier when hot pressing titanium alloys and titanium aluminides. But it is not about simple sintering, but about rolling titanium products in a steel casing at approx. 1200°C. A molybdenum foil prevents the diffusion between the titanium alloy and the steel shell.
Vadim Verlotski Thanks for reply. I also think the same since several papers reports using molybdenum crucible instead of Niobium.
Thank you for your answer Vit Jan . I still give it a try since now I am using alumina crucible and my Ti sample is still getting oxidized. The other reason may be that vacuum level is not good enough for sintering titanium. I am now thinking to install turbo pump to improve the vacuum level of the tube furnace.
If you have just a roughing pump, then it is pretty sure you have enough oxygen there to oxidize the Ti.
Even under turbo vacuum, it is very difficult to prevent Ti or Al oxidation.. you still have some atmosphere, even at very low pressure, and even the partial pressure of oxygen in that may be reacting with your Ti. I'm skeptical about vacuum actually, try to consider some highly reducing atmosphere.. (CO maybe?) Very pure hydrogen (not just 5N from a bottle, but dried) would be the best choice I guess.
Any chances of using the SPS technique? that would work
What temperatures are you using?
Vit Jan I am using 1200C to sinter my cold pressed Ti powder. I tested my rotary pump today. It only reached slightly below 10^-1 mbar and took a whole afternoon to reach 4*10^-2 mbar. I don’t know whether it is the problem of my own furnace or it is the limit of tube furnace.
I’ve read some essay about cold pressing and sintering of powder metallurgy and it is said that usually commercial vacuum sintering required vacuum level of 10^-3 mbar. The vacuum performance of mine is way below this standard. One thing is kind of interesting that during heating stage the pressure went up and got reduced when the heating temperature was stable. I don’t know whether it was the outgassing of my tube or there were leakages in the tube furnace.
I have tried purging argon with flow rate much greater than 160sccm. The result was promising since only part of the titanium was colored. But it is not good enough for research purpose. next time I want to try using much larger argon gas flow rate (my tube is large, 65mm inner diameter) or just store some argon gas to certain pressure to act as protecting gas.
My guess would be outgassing.. It depends on how much the pressure changed.
Be careful about pressurizing the tube, I blew one fused silica tube some time ago (big bang).
How about wrapping the compacted Ti piece in Ti foil?.. or Nb foil (possibly too expensive) but wrapping it in titanium could help..
you are fighting oxygen, who will diffuse in your furnace even if there is overpressure of argon - if there is titanium to react with. so either you
1. remove oxygen - a) high vacuum=difficult or b) another atmosphere - b1)argon of b2) hydrogen, CO and such.. reactive gas that reacts with oxygen
2.minimize the volume of the atmosphere and offer it some more sacrifice titanium., that would be the foil wrap.
Vit Jan Thank you for your suggestion. I consulted the technician today and they could offer Ar/H2 mixture. This could probably help to remove the oxygen. I’ve bought titanium sponge to act as purifier for incoming gas and Mo crucible to replace my alumina crucible. Perhaps I should buy Nb tube to wrap my sample including Mo crucible.
Vit Jan i have checked that there is no SPS in our university. Actually our university doesn‘t have any proper equipment for powder metallurgy.
That´s tough. I would still suggest the approach of minimising the atmosphere volume by wrapping your sample in titanium foil or in austenite foil with titanium sponge inside with your sample. Surely, your sample will oxidise the same as th sponge, but at least part of the oxygen will be consumed by the sponge.
On the other hand.. "if you cant beat them, join them" look for ODS materials. oxides are not totally bad when you manage to disperse them in ductile matrix. Sometimes, when you limit the amount of oxygen by wrapping the sample, what happens is that there is no full layer of oxides on the surface, but that a population of oxides is created inside the sample. These would be too big to call the material effectively a ODS, but still it might be an interesting feature for some application directions.
Vit Jan I tried 3%H2/Ar mixture today. Titanium still got oxidized but much better than that of pure vacuum environment. I am thinking that may be the oxygen leak is from alumina tube due to its porous structure. I am going to use quartz tube with lower sintering temp., e.g. 1100C to try.
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