Thermal conductivity against densityHigher and further left: spreads more heat for its weightThermal expansion against thermal conductivityLower and further right: warps less when heated
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Sources3 documents: Magnesium Elektron, Pilkington North America, U.S. Department of Defense
Glass
Annealed soda-lime-silica float glass, Pilkington North America Technical Bulletin ATS-129 (04/2025)
Magnesium AZ91D
ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.
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Elektron Die Casting Alloys (Datasheet 475)Magnesium Elektronarchived copyMagnesium AZ91D · Yield strength: typical, 0.2% tensile yield, ambient temperatureMagnesium AZ91D · Ultimate tensile strength: typical, ambient temperatureMagnesium AZ91D · Elongation at break: typical, ambient temperatureMagnesium AZ91D · Young's modulus (stiffness): typicalMagnesium AZ91D · Density: typicalMagnesium AZ91D · Thermal conductivity: typicalMagnesium AZ91D · Thermal expansion: typical, temperature range not statedMagnesium AZ91D · Specific heat: typicalMagnesium AZ91D · Melting point: melting range
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Technical Bulletin ATS-129: Properties of Soda-Lime Silica Float Glass (2025)Pilkington North Americaarchived copyGlass · Ultimate tensile strength: typical mean surface tensile stress at fracture (MOR), 60 s load, weathered, 50% breakage probabilityGlass · Flexural strength: mean MOR (same value as tensile)Glass · Young's modulus (stiffness): typicalGlass · Density: typicalGlass · Poisson's ratio: typicalGlass · Thermal conductivity: typical at 75 FGlass · Thermal expansion: typical, 75-575 FGlass · Specific heat: typical at 75 FGlass · Max service temperature: strain point (ASTM C336) - stress-relaxation onset, not a rated service limit
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MIL-HDBK-5J, Metallic Materials and Elements for Aerospace Vehicle Structures (2003)U.S. Department of DefenseMagnesium AZ91D · Poisson's ratio: base material: AZ91C casting (MIL-HDBK-5J handbook physical constant)
Each number points to the document that states it, with the exact basis. Values without a number are computed from the sourced ones. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.
Questions
Mechanical
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).
Thermal
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.