Hard Silver Solder

Yes you can use hard silver solder, but not one which contains zinc as it will outgas a lot. Harris would probably recommend a zinc free alternative suitable for your application if you ask them.

Brr. Putting a torch on anything containing zinc wouldn't be my first choice even for bicycle repair--zinc vapour is _not_ good for you.
Cheers,
Phil Hobbs

Any lead-free solder is prone to whisker growth. Whisker growth is often faster in vacuum. I wouldn't expect outgassing to be an issue, but I haven't experience with this alloy, provided you clean flux residues thoroughly.
But whiskers, that's likely the issue. ALthough people typically think "tin whiskers", zinc is notoriously prone to whisker growth also (became a problem in computing rooms with raised floors where the flooring used zinc-plated steel supporting members. Zinc whiskers would grow, break off and circulate with the forced air, shorting out digital circuits --weird failures, often intermittent, resulted.) Silver has also been shown to grow whiskers under at least some conditions. I think of all the metals NASA tested, only lead and high lead-containing alloys avoided whisker growth.
Google "tin whisker" if you haven't already been through the issues, there are some good sites with useful info out there.

Yeah, and watch the new Airbuses fall out of the sky when the Pb free solders start growing whiskers.
Al

and bismuth?