Try again...
In low-flexion applications, aluminum will do quite well. When it's stressed well below the Yield Limit (the point where plastic deformation will occur and become permanent,) it recovers nicely.
The main reason that aluminum is used in the aero industry is the same that magnesium alloys, titanium alloys, and composite alloys are used - weight. Airplanes are not "lighter than air" and expensive to operate - so anything they can do to lower the weight of the airframe translates into cargo and passengers that can be carried, or less energy used to generate sufficient lift (cf. Bernoulli's Principle.)
Also, airframes are not generally subject to impact stress or point stress - like automotive parts. If an airframe receives an impact, it's usually written off entirely, or major replacement of structural members has to take place. Cheapness of steel is not the only reason it is used in automobiles - it's also cheaper to fix (since you don't need a controlled process to weld it, and it can usually be straightened out after being bent with minimal stress risers - damn near none if you're willing to handle a little extra process.) Steel and alloys can also take a "varaible" heat treat, and that can't be done readily with aluminum.
If you'd really like to get into a discussion of the chemical and molecular properties of engineering materials, I'd be happy to accommodate you - I'm sure I can answer most questions with references to hand. However, suffice it to say, that material cost is not the only reason to use steel (it's easier to work, it's easier to fuse, and it's easier to form - though more difficult to machine, than aluminum) - process control and manufacturing cost also play a HUGE part in it's selection. That's why DeLorean used CRES for his "futuristic, gull-wing" bodies, rather than aluminum - and why Corvette body panels (beyond the unit body and frame) are fibreglas rather than aluminum. I think you'll find that most structural elements are steel as well.
Aluminum finds extensive use on ships (corrosion resistance and a greater displacement ratio than steel) and airframes (less impact stress and lighter mass per unit volume) for the reasons I've gone into - and steel alloys are used in automotive applications for reasons I've referred to as well. I'm not saying that aluminum doesn't have automotive uses (later Ford Rangers, for instance, use aluminum driveshafts...) but if the part is going to be subject to impact stresses or point stresses, you're a lot better off using steel than you are anything else. Aluminum, magnesium, titanium, and all the other "Exotic" materials will do well with uniform stresses, repeating stresses, and stresses below the plastic limit, but take them beyond that point, and you're in trouble.
Also, it's worth noting that pretty much the strongest aluminum alloy in common use - 7075-T6 - approaches (as I recall,) 10L18 steel in strength - and 10L18 is a leaded, free-machining steel that isn't very strong. It's a low-carbon steel, used in low-stress applications (and material in machinery student shops as well...) You can do a LOT better than 10L18 - I think most body panels are 1040 steel, which adds about half again the strength of 10L18/10L20/1018/1020 steels with a slight increase in carbond content (and don't even get me started on comparing steels - we'll be at it this time next month, without even getting into the exotics and "superalloys" of steel...)
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