Ahhh, I gotcha. I'm not a material scientist either, armchair analysis all the way.

I never thought ultimate tensile strength was terribly important for wheelchairs, but maybe it is. Resisting axial stretching doesn't seem like something we need. Both Stiffness and Strength to Weight, at bending points, apply to wheelchair tube stress more so than axial (compressive or tensile) forces, but the obligatory "I'm not an engineer" certainly applies. Aluminum (7075) has higher Stiffness to Weight bending ratio (by ~25%) but titanium (Grade 5 4AL) has a higher Strength to Weight (by ~10%), when measured at bending points, rather than stretching or compressing (axially tensile) points.

Stands to reason, by using less material (thereby eating into that 10% Str to Wgt bending advantage), titanium frames can certainly be made to be lighter, as raw frames, by that 10% margin. Also stands to reason, that will expand the Stiff to Weight advantage of aluminum (assuming a rigid frame is desired). The old Quickie Ti took this trade off to the extreme! It flexed like a wet noodle under load. The TiLite ZR seems to be more a conservative balance. The TR nearly negates the issue of flex entirely.