Glass vs. Magnesium AZ91D

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

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Mechanical
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Yield strength	not given (tensile used)	150MPasourcetypical, 0.2% tensile yield, ambient temperature
Ultimate tensile strength	41MPasourcetypical mean surface tensile stress at fracture (MOR), 60 s load, weathered, 50% breakage probability	230MPasourcetypical, ambient temperature
Flexural strength	41MPasourcemean MOR (same value as tensile)	not sourced
Elongation at break	not sourced	3%sourcetypical, ambient temperature
Young's modulus (stiffness)	72GPasourcetypical1.6x stiffer	45GPasourcetypical
Density	2,500kg/m³sourcetypical	1,810kg/m³sourcetypical38% lighter
Strength to weight	16.4kN·m/kg	82.9kN·m/kg5.05x higher
Stiffness to weight	28.8MN·m/kg16% higher	24.9MN·m/kg
Poisson's ratio	0.22sourcetypical	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)
Shear modulus	29.5GPa1.77x stiffer in shear	16.7GPa
Bulk modulus	42.9GPa	50GPa
Speed of sound	5,367m/s	4,986m/s
Thermal
Thermal conductivity	0.937W/m·Ksourcetypical at 75 F	72W/m·Ksourcetypical
Thermal expansion	8.3µm/m·Ksourcetypical, 75-575 F	25µm/m·Ksourcetypical, temperature range not stated
100 mm part over a 50 °C swing	41.5µm growth3.01x less	125µm growth
Specific heat	880J/kg·Ksourcetypical at 75 F	1,050J/kg·Ksourcetypical
Heats up and cools (diffusivity)	0.426mm²/s	37.9mm²/s89x faster
Thermal shock resistance	50.1W/m	6,240W/m124x more resistant
Melting point	not sourced	470–595°Csourcemelting range
Max service temperature	511°Csourcestrain point (ASTM C336) - stress-relaxation onset, not a rated service limit	not sourced
Values for	Annealed soda-lime-silica float glass, Pilkington North America Technical Bulletin ATS-129 (04/2025)	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 Glass stronger than Magnesium AZ91D?

Magnesium AZ91D is stronger: its yield strength is 150 MPa against tensile strength 41 MPa for Glass (3.66x).

Which is lighter, Glass or Magnesium AZ91D?

Magnesium AZ91D is lighter: 1,810 kg/m³ against 2,500 kg/m³ for Glass.

Which is stiffer, Glass or Magnesium AZ91D?

Glass is stiffer: Young's modulus 72 GPa against 45 GPa for Magnesium AZ91D, so the same part in Glass deflects less under the same load.

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

For the same stiffness or strength: Glass 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	Glass	Magnesium AZ91D
Stiff rod or tie (tension)	lighter	16% heavier
Stiff beam (bending)	9% heavier	lighter
Stiff panel or plate	18% heavier	lighter
Strong rod or tie	5.05x heavier	lighter
Strong beam	3.28x heavier	lighter
Strong panel or plate	2.64x 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, Glass or Magnesium AZ91D?

Magnesium AZ91D conducts heat better: 72 W/m·K against 0.937 W/m·K for Glass.

Which expands less with temperature?

Glass expands less: 8.3 µm/m·K against 25 µm/m·K for Magnesium AZ91D.

Among 72 materials

Stiffness against density

Strength against density