Picking a material is really about trade-offs: strength, stiffness, weight, cost, corrosion, temperature and manufacturability all pull in different directions. A structured pass beats reaching for "aluminium because it's light" every time.
Start with the function, not the material
Ask what the part actually has to do. Resist bending without flexing? You want stiffness (Young's modulus E), and geometry matters more than material. Carry load without breaking? You want strength (yield). Survive millions of cycles? Fatigue strength. Not corrode? That may override everything.
Strength-to-weight and stiffness-to-weight
For anything that moves or flies, what matters isn't absolute strength but strength per unit weight (specific strength) and stiffness per unit weight. This is why titanium and carbon-fibre win in aerospace despite the cost, and why a material-property chart plotted as strength vs density is the classic selection tool — you want the upper-left corner.
Then layer in the constraints
Cost (raw + processing) usually decides between candidates that both work. Corrosion: stainless, coatings, or design out galvanic couples (dissimilar metals). Temperature: modulus and strength fall with heat; plastics soften early. Manufacturability: can you actually cast/machine/weld it? Compare candidates on real numbers with the material property tool and weight calculator.
A workable method
List the must-haves (hard constraints), rank the nice-to-haves, shortlist 3–4 materials that pass the constraints, then compare them on the ranked criteria with real values. It's faster than agonising and it leaves a paper trail for the design review.