Cleveland-Cliffs / AK Steel ARMCO Pure Iron Product Data Bulletin (07/2022): Table 1 typical physical properties, Table 2 typical mechanical properties after SRA/recrystallization anneal.
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 Iron stronger than Magnesium AZ91D?
Iron is stronger: its yield strength is 186 MPa against 150 MPa for Magnesium AZ91D (24%).
Which is lighter, Iron or Magnesium AZ91D?
Magnesium AZ91D is lighter: 1,810 kg/m³ against 7,860 kg/m³ for Iron.
Which is stiffer, Iron or Magnesium AZ91D?
Iron is stiffer: Young's modulus 207 GPa against 45 GPa for Magnesium AZ91D, so the same part in Iron deflects less under the same load.
Which is lighter for the same job, Iron or Magnesium AZ91D?
For the same stiffness or strength: Iron is lighter for a stiff rod or tie (tension); Magnesium AZ91D is lighter for a stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate.
Part that must be
Iron
Magnesium AZ91D
Stiff rod or tie (tension)
lighter
6% heavier
Stiff beam (bending)
2.02x heavier
lighter
Stiff panel or plate
2.61x heavier
lighter
Strong rod or tie
3.5x heavier
lighter
Strong beam
3.76x heavier
lighter
Strong panel or plate
3.9x 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, Iron or Magnesium AZ91D?
Iron conducts heat better: 73.2 W/m·K against 72 W/m·K for Magnesium AZ91D.
Which expands less with temperature?
Iron expands less: 13.7 µm/m·K against 25 µm/m·K for Magnesium AZ91D.