English
Menu

Nonferrous metals and alloys

Suppose that adding lithium to aluminum lowers the density by about 3% and raises Young's modulus by about 6% per mass% of Li. About how many times that of pure aluminum is the specific stiffness (Young's modulus/density) of an Al-Li alloy containing 2.0 mass% Li? Assume the changes are proportional to the amount added.

  1. AAbout 1.06 times
  2. BAbout 1.19 times
  3. CAbout 1.12 times
Show answer and explanation

Answer: B. About 1.19 times

At 2.0% Li, the density becomes 1 − 0.03 × 2.0 = 0.94 times and Young's modulus 1 + 0.06 × 2.0 = 1.12 times. The specific stiffness is their ratio, 1.12/0.94 ≈ 1.19 times, an improvement of about 19%. About 1.06 times subtracts the 6% fall in density from the 12% rise in modulus, and about 1.12 times compares only the change in modulus. Most alloying elements hardly change Young's modulus or density, so the specific stiffness of aluminum alloys is nearly constant, but Li is an exceptional element: its atoms are very light, and it forms the ordered phase Al₃Li (δ') that also raises stiffness. 2.0 mass% Li corresponds to about 7 at%. Al-Cu-Li alloys, improved to overcome problems of anisotropy and toughness, are used in aircraft structures.

Report an error

Practice this topic Back to topic

More Nonferrous metals and alloys questions

  1. Tough-pitch copper (copper containing about 0.02 to 0.05% oxygen, mostly dispersed as Cu₂O) can become brittle when it is heated in a reducing atmosphere containing hydrogen or is gas welded or brazed. Which gives the correct cause and remedy?
  2. Among copper alloys for springs and connectors, which is the correct pairing of the strengthening mechanisms of phosphor bronze and beryllium copper?
  3. The corrosion resistance of magnesium alloys depends strongly on the amounts of the impurities Fe, Ni and Cu, and in high-purity alloys such as AZ91D the upper limits for Fe and Ni are set very low, at about 0.005% or less. Which is the correct reason?
  4. A flat panel whose thickness is chosen to give a fixed bending stiffness is changed from an aluminum alloy (Young's modulus 70 GPa, density 2.70 g/cm³) to a magnesium alloy (Young's modulus 45 GPa, density 1.74 g/cm³). If the area of the panel is the same and the bending stiffness is proportional to E × (thickness)³, about how many times the mass of the aluminum alloy panel is the mass of the magnesium alloy panel?
  5. In α+β titanium alloys such as Ti-6Al-4V, cooling from above the β transus gives a lamellar structure of colonies of plate-like α, while working below the β transus and annealing gives a structure containing equiaxed α. Which statement about the difference in properties between these two structures is correct?