I just started my PhD this year and am doing some work on high entropy alloy synthesis via spark plasma sintering. My alloy composition has Nb and Ta amongst all the other elements, and I'm observing heavy oxide contamination from these two elements (likely because of their oxide formation energies) after I make my sample. I start from elemental powders and have tried the following:
1. HEBM in both SS and WC jars:-
Unfortunately we don't have space in our glovebox for a mill/attritor , so what we do is seal the jar inside our glovebox with some teflon tape around the threads. This works reasonably well for short milling times, but the longer times required for alloying our powders introduce contamination and make this infeasible.
2. Wet Milling with PCA (ethanol) using a planetary ball mill:-
This works better but still results in considerable oxide contamination. Going by literature, wet milling produces more oxide contamination than dry milling. The powders are dried and handled in inert atmosphere after milling.
Strangely enough, I tried hand mixing the raw powders and sintering them to see whether there was contamination and sure enough there was. This leads me to believe the powders themselves are contaminated (XRD spectra however, don't support this and the powders are always handled and stored in a glovebox), or something is happening during the SPS process.
Does anyone have experience making clean alloy samples through SPS with minimal/no secondary phases? What else can I do to stop this?
Try the experiment in ambient of Argon, or calcinate the alloy powder after mixing with carbon in vacuum furnace with suitable temperature as a post treatment.
I dont think there is any way to completely impede the formation of oxide during milling but you should be able to reduce the oxide by using forming gas (5% H2, 95% N2) at medium temperature in a tube furnace.
Thanks for the inputs everyone! Our SPS chamber is pumped down to about a millitorr after backfilling with Argon a few times. Since Tantalum and Niobium are hydride formers, using forming gas is tricky....but heat treatment options and using carbon sounds like a good direction.
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