Magnesium AZ91D vs. Steel

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

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Mechanical
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Yield strength	150MPasourcetypical, 0.2% tensile yield, ambient temperature	385MPasourcetypical: mean of 20,295 A992 flange mill-test coupons (1998 SSPC survey), as reported by AISC
Ultimate tensile strength	230MPasourcetypical, ambient temperature	505MPasourcetypical: mean of 20,295 A992 flange mill-test coupons (1998 SSPC survey)
Elongation at break	3%sourcetypical, ambient temperature	not sourced
Young's modulus (stiffness)	45GPasourcetypical	200GPasourcedesign value (AISC 360-16)
Density	1,810kg/m³sourcetypical	7,850kg/m³sourcenominal (constructional data)
Strength to weight	82.9kN·m/kg1.69x higher	49kN·m/kg
Stiffness to weight	24.9MN·m/kg	25.5MN·m/kg2% higher
Poisson's ratio	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)	0.3sourcedesign value (AISC 360-16 Commentary)
Shear modulus	16.7GPa	76.9GPa4.62x stiffer in shear
Bulk modulus	50GPa	167GPa
Speed of sound	4,986m/s	5,048m/s
Thermal
Thermal conductivity	72W/m·Ksourcetypical	48W/m·Ksourcenominal at 20 °C
Thermal expansion	25µm/m·Ksourcetypical, temperature range not stated	12µm/m·Ksourcenominal (constructional data, no temperature range stated)
100 mm part over a 50 °C swing	125µm growth	60µm growth2.08x less
Specific heat	1,050J/kg·Ksourcetypical	460–480J/kg·Ksourcebase material: EN S275JR unalloyed structural steel (the EN counterpart of A36), typical, 50/100 °C
Heats up and cools (diffusivity)	37.9mm²/s2.91x faster	13mm²/s
Thermal shock resistance	6,240W/m16% more resistant	5,390W/m
Melting point	470–595°Csourcemelting range	not sourced
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.	Generic structural steel, cited as ASTM A992 (the current US structural grade): ASTM A992 W-shapes; Fy/Fu specified minimums from AISC publications (AISC 360-16 Commentary; Modern Steel Construction); elastic constants AISC 360; physical constants from the bauforumstahl structural-steel EPD, which names A992 among covered product standards.

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 Steel?

Steel is stronger: its yield strength is 385 MPa against 150 MPa for Magnesium AZ91D (2.57x).

Which is lighter, Magnesium AZ91D or Steel?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 7,850 kg/m³ for Steel.

Which is stiffer, Magnesium AZ91D or Steel?

Steel is stiffer: Young's modulus 200 GPa against 45 GPa for Magnesium AZ91D, so the same part in Steel deflects less under the same load.

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

For the same stiffness or strength: Magnesium AZ91D is lighter for a stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam, strong panel or plate; Steel is lighter for a stiff rod or tie (tension).

Part that must be	Magnesium AZ91D	Steel
Stiff rod or tie (tension)	2% heavier	lighter
Stiff beam (bending)	lighter	2.06x heavier
Stiff panel or plate	lighter	2.64x heavier
Strong rod or tie	lighter	1.69x heavier
Strong beam	lighter	2.31x heavier
Strong panel or plate	lighter	2.71x 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 Steel?

Magnesium AZ91D conducts heat better: 72 W/m·K against 48 W/m·K for Steel.

Which expands less with temperature?

Steel expands less: 12 µm/m·K against 25 µm/m·K for Magnesium AZ91D.

Among 72 materials

Stiffness against density

Strength against density