The Boeing Company (Chicago, IL) developed a process that reduces the amount of the rhenium used in high temperature applications such as rocket propulsion systems without sacrificing its high temperature and mechanical properties. Cryomilling in the presence of nitrogen is used to prepare rhenium alloys with a stable fine grain structure at high temperatures, according to inventors Jerry W. Brockmeyer and Clifford C. Bampton. The rhenium nano alloy has high temperature strength and ductility superior to conventionally processed rhenium and earned Boeing U.S. Patent 7,592,073. The rhenium alloy contains a refractory compound which may include one or more of hafnium (Hf), zirconium (Zr), tantalum (Ta), silicon (Si), vanadium (V), and titanium (Ti), and which makes up between 0.1% and 10% of the alloy by weight.
The refractory compound comprises a nano-scale dispersion that is incorporated into the conventional rhenium structure. The nano-scale dispersion acts as grain boundary pins that result in a relatively fine grained, equiaxed structure that helps to improve the mechanical properties of the alloy and helps to minimize the growth of large grains during operations at high temperatures. Carbon nanotubes, similar to the refractory compounds discussed above, may also act as nano-scale pins that help prevent the growth or the rhenium grains at higher temperatures. Carbon nanotubes typically have a lower density and weight than refractory compounds containing refractory metals. As a result, the overall weight/density of the rhenium alloy may be reduced which may result in reduced weight structures and/or enhanced density-specific properties.
Many rocket propulsion systems use either a pressure fed system or a turbopump system that transfers propellants to the combustion chamber where they are mixed and burned to produce a high velocity stream of heated gases. The stream of heated gases is then exhausted through one or more nozzles to provide the desired thrust. Typically, combustion takes place at temperatures that may be in excess of 6000 F., which may be higher than the melting point of most conventional engine materials. As a result, in the absence of active cooling, it may be necessary to line the interior of the combustion chamber with a material having a high melting point and oxidation resistance. Boeing’s nano rhenium alloys have melting temperatures as high as 6150 degree F (3400 degree C).
Iridium-coated rhenium is a material that is commonly used to line the interior of the combustion chamber. Iridium provides high temperature oxidation resistance and has an intrinsic resistance to oxidation. Rhenium has a higher melting point than iridium and excellent high temperature structural capability. Iridium and rhenium are dense materials that are prohibitively expensive. As a result, the use of iridium and rhenium may increase the overall cost and weight of the propulsion system. Boeing is able to reduce the amount of rhenium needed while improving the alloys’ heat resistance.
The processing of rhenium presents several challenges. In many applications, chemical vapor deposition (CVD) fabrication is used. Typically, high temperatures are needed to deposit rhenium using CVD. However, conventional equipment for CVD produces temperatures on the order of 1000.degree. C., which is much lower than the melting point of rhenium. CVD also typically requires relatively expensive starting materials and processing reactors that are relatively expensive to run and maintain. Other methods of processing rhenium, such as electrodeposition, may also present challenges and may result in the rhenium having an undesirable grain size. Post-processing, e.g. machining of rhenium, may also be difficult because of the high work hardening coefficient of rhenium.
The advantageous properties of rhenium may also be adversely affected, in part, by the processing conditions. For instance, in many cases rhenium properties may be dynamic when exposed to high temperatures. This dynamic behavior may result from grain growth that can occur at higher operating temperatures. Grain growth may decrease the mechanical properties of rhenium. Additionally, current methods of processing rhenium typically result in relatively large grain structures or grain structures that have an acicular grain structure. Such grain structures tend to increase the difficulty of processing rhenium and may also result in the rhenium having reduced mechanical properties, such as strength, at higher operating temperatures.
Many rhenium processing disadvantages are overcome by "cryomilling" which is the fine milling of metallic constituents at extremely low temperatures. Cryomilling takes place within a high energy mill such as an attritor with metallic or ceramic balls. During milling, the mill temperature is lowered by using liquid nitrogen or a similar compound to a temperature of between -240.degree. C. and -150.degree. C. In an attritor, energy is supplied in the form of motion to the balls within the attritor, which impinge portions of the metal alloy powder within the attritor, causing repeated comminuting and welding of the metal. As-milled grain sizes in these metal powders are on the order of 50 to 100 nm.