Answer:
Most cars manufactured in the last 30 years do not have a chassis frame, they have a monocoque or unibody design. The materials here are mostly steel, but in exotics can include aluminium, carbon fiber and even some elements are made from titanium.
For bigger cars and trucks that do have a chassis frame, it is always steel. These cars and trucks range from the earliest Ford (the Model T) through to the 1980's Range Rover and the 2010 Ford and GM trucks and big SUV's that use a frame rail chassis.
The types of steel vary, but it is always steel. Heavy and strong, although not very rigid when you look at the chassis from a transverse torsion point of view.