I've found the following suggestions around the web, but I found no source which I could trust completely:

* Tinned Steel
* [Kovar](http://en.wikipedia.org/wiki/Kovar)
* A copper alloy

As suggested, I'll edit a little bit of background info here, which I have also posted in the comment section. Im currently learning about soldering, in the context of research. Its a lateral entry for me, and since I have a background in materials I wanted to know exactly what alloys can be involved. It is background research. I realise that normally, an electronics engineer doesn't care that much about it since there are standardized techniques for soldering in the industry.