Material
Molybdenum Mo
A refractory metal with high temperature strength and very low thermal expansion at roughly half tungsten's density. The practical choice for hot structure and tooling where tungsten is too heavy, too brittle, or too expensive.
Overview
Molybdenum sits between the ordinary structural metals and tungsten. It holds strength far above where nickel superalloys give out, expands very little with temperature, and conducts heat well, but it oxidizes catastrophically in air above roughly 600 degrees Celsius, which confines it to vacuum, inert atmosphere or coated service. Its largest use by tonnage is as an alloying addition to steel and superalloys rather than as a metal in its own right. As mill product it appears in furnace hot zones, isothermal forging dies, glass melting electrodes, rocket nozzle hardware and electronic packaging where its expansion coefficient matches silicon and ceramics.
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Defense applications
High temperature tooling and dies
Isothermal and hot die forging of titanium and nickel superalloys is done against molybdenum alloy dies, because they retain strength at the workpiece temperature. This is a quiet dependency behind a large share of aerospace forging.
Isothermal forging dies, Hot die forging tooling, Extrusion tooling, Die inserts
Furnace and vacuum hot zones
Heating elements, heat shields and support structure inside vacuum and inert atmosphere furnaces, including the furnaces used to heat treat everything else in this atlas.
Vacuum furnace hot zones, Heating elements, Heat shields, Sintering boats and fixtures
Propulsion and hot gas hardware
Rocket nozzle components, thrust chamber hardware and hot gas valves that see temperatures above the nickel superalloy ceiling for short durations.
Nozzle throat hardware, Hot gas valve components, Thrust chamber inserts
Electronic packaging
Molybdenum and molybdenum-copper heat spreaders match the thermal expansion of silicon and ceramic substrates closely enough to avoid cracking through thermal cycles, which matters in high power radar and electronic warfare hardware.
Heat spreaders and heat sinks, Microwave package bases, Power semiconductor carriers
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Pure Molybdenum | Unalloyed refractory metal | Sintered or arc cast unalloyed molybdenum, worked to sheet, plate, bar and wire. The baseline grade for furnace and vacuum hardware. | Sheet, Plate, Bar and Rod, Wire, Powder | |
| TZM | Titanium-zirconium-molybdenum alloy | Small titanium, zirconium and carbon additions raise recrystallization temperature and roughly double high temperature strength over pure molybdenum. The standard alloy for isothermal forging dies. | Bar and Rod, Plate, Forgings | |
| MoLa (Lanthanum Oxide Doped) | Oxide dispersion strengthened molybdenum | Lanthanum oxide dispersion raises recrystallization temperature substantially and preserves ductility after high temperature exposure, addressing pure molybdenum's main failure mode. | Sheet, Wire, Bar and Rod | |
| Molybdenum-Copper | Metal matrix composite | Copper infiltrated molybdenum, tuned by composition to match the thermal expansion of a specific semiconductor or ceramic while conducting heat far better than molybdenum alone. | Plate, Sheet, Bar and Rod |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 10.22 | g/cm3 | 0.369 lb/in3; roughly half tungsten and about 30 percent more than steel |
| Melting point | 2623 | degrees C | 4753 degrees F; second only to tungsten and tantalum among practical structural metals |
| Elastic modulus | 320 to 330 | GPa | Substantially stiffer than steel |
| Tensile strength, stress relieved sheet | 550 to 800 | MPa | Depends heavily on the amount of retained cold work |
| Coefficient of thermal expansion | 4.8 | micrometre/m-K | Very low, and close to silicon and alumina, which drives electronic packaging use |
| Thermal conductivity | 138 | W/m-K | Roughly triple that of steel |
| Oxidation behavior | Catastrophic above ~600 C in air | Forms volatile MoO3 that offers no protection; service above this needs vacuum, inert gas or a coating | |
| Recrystallization temperature | approximately 900 to 1200 | degrees C | Above this the worked structure is lost and the metal becomes brittle |
Advantages
- Retains useful strength far above the nickel superalloy ceiling
- About half the density of tungsten with much of the temperature capability
- Very low thermal expansion, closely matching silicon and ceramic substrates
- High thermal conductivity, roughly triple that of steel
- More machinable and more ductile at room temperature than tungsten
- Excellent resistance to molten glass and many molten metals
Limitations
- Oxidizes catastrophically above roughly 600 degrees Celsius in air, forming a volatile oxide that offers no protection
- Becomes brittle after recrystallization, so service and joining temperatures must stay below it
- Difficult to weld, with fusion welds generally brittle
- Ductile to brittle transition can sit above room temperature depending on condition and purity
- Melting point too high for conventional casting; mill product goes through press and sinter or arc casting
- Small number of domestic mill product producers, so lead times and second sourcing are real constraints
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM B386 | ASTM International | Molybdenum and molybdenum alloy plate, sheet, strip and foil |
| ASTM B387 | ASTM International | Molybdenum and molybdenum alloy bar, rod and wire |
| AMS 7817 | SAE International | Molybdenum alloy TZM bar and rod |
| ASTM B655 | ASTM International | Molybdenum and molybdenum alloy foil, sheet, strip and plate for electronic applications |