Is there scar tissue associated with achilles tendonitis?

Possibly yes: Signs of inflammation such as swelling, warmth, and pain occur in a person with achilles' tendonitis. Damaged tendons can become calcified (get deposits of calcium), thicker than normal, less elastic (not as stretchable), and fibrotic (form scar tissue fibers). Tendon rupture occurs when a sudden shear stress force is applied to an already inflamed or damaged tendon.

Depends: Is really depends. There is a gradation of issues with the tendon from simple tendonitis/inflammation to tendinosis/degeneration. The tendonitis can improve with little to no scar tissue. The tendinosis is less likely to do so.