Apparently Alcoa recently unveiled a new alloy for use in truck wheels that is 16.5% stronger than 6061. This may be an interesting alloy for rockets when they let it into the market. There wasn't any real detail in the press release that I saw, and they didn't mention (and I didn't look up) what heat treatment of 6061 they are comparing with. Here is the press release http://www.asminternational.org/portal/site/www/NewsItem/?vgnextoid=3a23ccc1c26e1410VgnVCM100000621e010aRCRD