Molybdenum News Center

Four main classes of commercial molybdenum-base alloys exist. One class relies on the formation of fine metal carbides that strengthen the material by dispersion hardening, and extend the resistance of the microstructure to recrys-tallization above that of pure molybdenum.

 

The most common of the carbide-strengthened alloys is known as TZM, containing about 0.5% titanium, 0.08% zirconium, and 0.03% carbon. Other alloys in this class include TZC (1.2% titanium, 0.3% zirconium, 0.1% carbon), MHC (1.2% hafnium, 0.05% carbon), and ZHM (1.2% hafnium, 0.4% zirconium, 0.12% carbon). The high-temperature strength imparted by these alloys is their main reason for existence.

 

Both TZM and MHC have found application as metalworking tool materials. Their high-temperature strength and high thermal conductivity make them quite resistant to the collapse and thermal cracking that are common failure mechanisms for tooling materials. A particularly demanding application is the isothermal forging process used to manufacture nickel-base superalloy gas-turbine engine components. In this process the dies and workpiece are both heated to the hot working temperature, and the forging is performed in a vacuum by using large hydraulic presses.

 

A second class relies on solid-solution hardening to strengthen molybdenum. These two classes of materials are typically produced in both vacuum-arc-casting and powder metallurgy grades. In the solid solution class tungsten and rhenium are the two primary alloy additions. The most common compositions are 30% tungsten (Mo-30% W), 5% rhenium (Mo-5% Re), 41% rhenium (Mo-41% Re), and 47.5% rhenium (Mo-50% Re). With the exception of the Mo-30% W alloy, which is available as a vacuum-arc-cast product, these alloys are normally produced by powder metallurgy. The tungsten-containing alloys find application as components in systems handling molten zinc, because of their resistance to this medium. They were developed as a lower-cost, lighter-weight alternative to pure tungsten and have served these applications well over the years. The 5% rhenium alloy is used primarily as thermocouple wire, whereas the 41% and 47.5% alloys are used in structural aerospace applications.

 

The third class uses combinations of carbide formers and solution hardeners to provide improved high-temperature strength. This class of alloys is normally produced by powder metallurgy techniques, but some of the alloys are also amenable to vacuum-arc-casting processing.

 

The beneficial effects of solid-solution hardening and dispersion hardening found in the carbide-strengthened alloys have been combined in the HWM-25 alloy (25% tungsten, 1% hafnium, 0.07% carbon). This alloy offers high-temperature strength greater than that of carbide-strengthened molybdenum, but it has not found wide commercial application because of the added cost of tungsten and the expense of processing the material.

 

The final class of alloys, known as dispersion-strengthened alloys, relies on second-phase particles (usually an oxide of a ceramic material) introduced or produced during powder processing to provide resistance to recrystallization and to stabilize the re-crystallized grain structure, enhancing high-temperature strength and improving low-temperature ductility. These latter materials by their very nature must be produced by powder metallurgy techniques.