A manufacturer seeks to develop an ultralight aerospace alloy based on a magnesium…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A manufacturer seeks to develop an ultralight aerospace alloy based on a magnesium matrix (atomic radius = 160 pm). Three candidate elements—aluminum (143 pm), lithium (152 pm), and boron (88 pm)—are under consideration as small-concentration additives.
The design objectives are:
• Maintain a single-phase solid solution whenever possible.
• Achieve at least 3 atomic % additive content without forming a separate phase.
(a) For each candidate additive, compute the percent radius difference relative to the Mg host lattice.
(b) Using the size criterion for alloy formation, identify which additive(s) are expected to dissolve substitutionally in Mg at the 3 atomic % level. Justify your answer quantitatively.
(c) Boron is considerably smaller than Mg. Discuss whether it is feasible for boron atoms to occupy interstitial sites in the Mg lattice in amounts approaching 3 atomic %, taking into account both atomic size and the volume fraction of interstitial sites in a close-packed metal.
(d) Explain how the presence of a small concentration of an interstitial element such as boron could influence the mechanical properties (e.g., hardness, ductility) of the Mg alloy compared with substitutional additives.