I don’t think anything can fly mach 6 in the lower atmosphere without melting. There’s no material strong enough.
Ablative materials will allow it to last long enough to reach its target. They poor conductors of heat, keeping the airframe cool, and the ablation process takes a majority of the aeroheating away from the vehicle.
Mach 8.5 at Holloman Air Force Base in 2003.
Worked on the X-30 National Aerospace Plane from 88-92. Yep. There are plenty of materials that will already take Mach 10.
See my book, “The Quest for the Orbital Jet.” (https://www.amazon.com/Hypersonic-Revolution-Studies-History-Technology/dp/1478146176/ref=sr_1_fkmr0_1?ie=UTF8&qid=1496250410&sr=8-1-fkmr0&keywords=The+quest+for+the+orbital+jet+Larry+Schweikart)
Note this edition says the author is the “US Government” Nope. (https://www.amazon.com/Hypersonic-Revolution-Technology-Aero-Space-1983-1995-ebook/dp/B00T0ZT214/ref=sr_1_1?ie=UTF8&qid=1496250248&sr=8-1&keywords=The+quest+for+the+orbital+jet)