Magnesium AZ91D vs. Stainless Steel 304

Compare Magnesium AZ91D vs. Stainless Steel 304 strength, stiffness, weight and thermal properties.

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Mechanical
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Yield strength	150MPasourcetypical, 0.2% tensile yield, ambient temperature	241MPasourcetypical, annealed, 70 °F (21 °C)
Ultimate tensile strength	230MPasourcetypical, ambient temperature	586MPasourcetypical, annealed, 70 °F (21 °C)
Elongation at break	3%sourcetypical, ambient temperature	55%sourcetypical, annealed, in 2 in. (50.8 mm)
Young's modulus (stiffness)	45GPasourcetypical	193GPasourcetypical, in tension
Density	1,810kg/m³sourcetypical4.44x lighter	8,030kg/m³sourcetypical
Strength to weight	82.9kN·m/kg2.76x higher	30kN·m/kg
Stiffness to weight	24.9MN·m/kg3% higher	24MN·m/kg
Poisson's ratio	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)	0.3sourcedesign value (structural stainless steels)
Shear modulus	16.7GPa	74.2GPa4.45x stiffer in shear
Bulk modulus	50GPa	161GPa
Speed of sound	4,986m/s	4,903m/s
Thermal
Thermal conductivity	72W/m·Ksourcetypical	16.2W/m·Ksourceat 100 °C
Thermal expansion	25µm/m·Ksourcetypical, temperature range not stated	16.9µm/m·Ksourcemean, 0–100 °C
100 mm part over a 50 °C swing	125µm growth	84.5µm growth48% less
Specific heat	1,050J/kg·Ksourcetypical	500J/kg·Ksourcemean, 0–100 °C
Heats up and cools (diffusivity)	37.9mm²/s9.39x faster	4.03mm²/s
Thermal shock resistance	6,240W/m7.45x more resistant	838W/m
Melting point	470–595°Csourcemelting range	1,399–1,454°Csourcemelting range
Max service temperature	not sourced	899°Csourcecontinuous, oxidation (scaling) limit in air
Values for	ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.	Type 304, annealed flat-rolled (Cleveland-Cliffs 304/304L Product Data Bulletin, May 2021); Poisson's ratio from the Nickel Institute/SCI structural stainless design manual

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 Magnesium AZ91D stronger than Stainless Steel 304?

Stainless Steel 304 is stronger: its yield strength is 241 MPa against 150 MPa for Magnesium AZ91D (1.61x).

Which is lighter, Magnesium AZ91D or Stainless Steel 304?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 8,030 kg/m³ for Stainless Steel 304.

Which is stiffer, Magnesium AZ91D or Stainless Steel 304?

Stainless Steel 304 is stiffer: Young's modulus 193 GPa against 45 GPa for Magnesium AZ91D, so the same part in Stainless Steel 304 deflects less under the same load.

Which is lighter for the same job, Magnesium AZ91D or Stainless Steel 304?

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	Magnesium AZ91D	Stainless Steel 304
Stiff rod or tie (tension)	lighter	3% heavier
Stiff beam (bending)	lighter	2.14x heavier
Stiff panel or plate	lighter	2.73x heavier
Strong rod or tie	lighter	2.76x heavier
Strong beam	lighter	3.23x heavier
Strong panel or plate	lighter	3.5x heavier

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, Magnesium AZ91D or Stainless Steel 304?

Magnesium AZ91D conducts heat better: 72 W/m·K against 16.2 W/m·K for Stainless Steel 304.

Which expands less with temperature?

Stainless Steel 304 expands less: 16.9 µm/m·K against 25 µm/m·K for Magnesium AZ91D.

Among 72 materials

Stiffness against density

Strength against density