Metal Intermetallic Compound High Temperature Alloys
Metal intermetallic compound high temperature alloys are a class of materials that can be developed to resist extreme high temperature environments. These alloys are beneficial in the applications related to missiles and aerospace, rockets and spacecraft, gas turbines, nuclear power and others. The main components in metal intermetallic compound high temperature alloys are Nb, Ta and Mo, and they are usually called as the “refractory” metals (refractory metal is defined as a metal having a melting point greater than 2000 degree Celsius). Alloying elements like Ti, Al, W and Re may be added to improve the properties of the alloys in circumstances such as melting and oxidation resistance, formability and machinability.
The intermetallic compounds in metal intermetallic compound high temperature alloys provides the ability for the alloys to withstand normal and mechanical stresses at high temperatures without undergoing any dimensional changes. On top of this, the compounds also become hard and accretive at high temperature service, therefore reducing wear and tear. Moreover, the intermetallic compound also aids in increasing the stress rupture and creep strength of the alloys, thereby facilitating their use in high temperature critical applications.
Typically, intermetallics are formed by the diffusion reaction of two or more metals. The intermetallic compounds formed can exist in solid, liquid and vapor form, but the reactivity of the compounds depend on the temperature. The bond of the intermetallic compounds are configured to either form cubic or hexagonal lattices depending on their stoichiometry. Most of the high temperature alloys derived from intermetallic compounds will have face-centered cubic or body-centered cubic lattices that are formed directly when the atoms within the alloy solidify at high temperature.
Intermetallic alloys are known for their high temperature stability and strength, allowing them to be used in high temperature environments like space vehicles, high-performance jet engines and nuclear reactors. The mechanical properties of intermetallic alloys are beneficial in the persistent operations of components like turbine blades in the high temperature turbines of the industrial setups. The high temperature stability, strength and fatigue resistance of the intermetallic alloys are utilized in these applications.
For example, the reaction of Nb and Ta leads to formation of intermetallic compounds β-NbFe and β-TaFe. These two compounds are widely used in high temperature machines due to their superior properties like excellent oxidation resistance at temperatures of up to 1000ºC and long-term strength at temperatures of 1100ºC. The strength of β-NbFe and β-TaFe can be attributed to their crystal chemical stability as a result of their low temperature stability.
The intermetallic compounds of Nb-Ta-Mo used in high temperature applications possess properties like excellent oxidation resistance, good creep resistance and reduced thermal expansion. This ensures that the high temperature alloys made from these compounds have better quasi-static and fatigue properties, which makes them suitable for different aerospace and industrial applications.
At high temperatures, the intermetallic compounds of Nb-Ta-Mo form a structure of four different crystallographic phases known as χ, γ, ε and δ. These phases are thermodynamically stable and create a stepped interface between the three elements. Different phases lead to different properties, hence depending on the required application, the intermetallic alloys can be tuned to get the desired property.
Metal intermetallics are ideal materials for high temperature applications due to their ability to resist elevated temperatures and high stresses. Their properties like creep resistance, stress rupture resistance and oxidation resistance make them suitable for application in different aerospace, missile and gas turbine systems. Intermetallic alloys are also preferable because they are relatively lightweight, corrosion resistant and have high thermal conductivity. In addition, the selection of the elements used to form the alloys depends on the desired properties and applications for which the alloy is used. The development of metal intermetallic compound high temperature alloys have been beneficial in the development of advanced materials with superior thermal and mechanical properties compared to conventional materials.
