Molybdenum alloys
Solid solutions of molybdenum and other metals. Molybdenum is classified as a refractory metal by virtue of its high melting point (2623°C or 4750°F), and many of its applications result from its strength at high temperatures. A number of other physical and mechanical properties make it attractive for use in a wide variety of applications. Molybdenum is used extensively as electrodes in electric-boost furnaces because it erodes very slowly and does not contaminate the glass bath. Its high-temperature strength allows it to support significant structural loads imposed during operation of the furnaces. See also Molybdenum; Refractory.
Four main classes of commercial molybdenum-base alloys exist. 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.
Tungsten and rhenium are the two primary solid-solution alloys. The most common compositions are 30% tungsten (Mo-30W), 5% rhenium (Mo-5Re), 41% rhenium (Mo-41Re), and 47.5% rhenium (Mo-50Re). With the exception of the Mo-30W 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, while the 41% and 47.5% alloys are used in structural aerospace applications. See also Rhenium; Tungsten.
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.
Dispersion-strengthened alloys rely exclusively on powder metallurgy manufacturing techniques. This allows the production of fine stable dispersions of second phases that stabilize the wrought structure against recrystallization, resulting in a material having improved high-temperature creep strength as compared to pure molybdenum. Once recrystallization occurs, the dispersoids also stabilize the interlocked recrystallized grain structure. This latter effect produces significant improvements in the ductility of the recrystallized material.
The potassium- and silicon-doped alloys such as MH (150 ppm potassium, 300 ppm silicon) and KW (200 ppm potassium, 300 ppm silicon, 100 ppm aluminum) are the oldest of this category; they are analogs to the doped tungsten alloys in common use for tungsten lamp filament.
