C11000 ETP copper, annealed to 0.050 mm grain size (OS050), 1 in. rod, copper.org typical values
ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.
Every value links to the document that states it. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.
Questions
Is Copper C110 stronger than Magnesium AZ91D?
Magnesium AZ91D is stronger: its yield strength is 150 MPa against 69 MPa for Copper C110 (2.17x).
Which is lighter, Copper C110 or Magnesium AZ91D?
Magnesium AZ91D is lighter: 1,810 kg/m³ against 8,913 kg/m³ for Copper C110.
Which is stiffer, Copper C110 or Magnesium AZ91D?
Copper C110 is stiffer: Young's modulus 117 GPa against 45 GPa for Magnesium AZ91D, so the same part in Copper C110 deflects less under the same load.
Which is lighter for the same job, Copper C110 or Magnesium AZ91D?
For the same stiffness or strength: Magnesium AZ91D is lighter for a stiff rod or tie (tension), stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate.
Part that must be
Copper C110
Magnesium AZ91D
Stiff rod or tie (tension)
1.89x heavier
lighter
Stiff beam (bending)
3.05x heavier
lighter
Stiff panel or plate
3.58x heavier
lighter
Strong rod or tie
10.7x heavier
lighter
Strong beam
8.26x heavier
lighter
Strong panel or plate
7.26x heavier
lighter
For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).
Which conducts heat better, Copper C110 or Magnesium AZ91D?
Copper C110 conducts heat better: 391 W/m·K against 72 W/m·K for Magnesium AZ91D.
Which expands less with temperature?
Copper C110 expands less: 16.9 µm/m·K against 25 µm/m·K for Magnesium AZ91D.