Why Nickel base superalloys are recommended for high stress applications

The nickel base super alloys used for jet engine structural components are subjected to high stresses and extreme environmental conditions. At the high temperatures in the jet engines, molten salts form, and thin films of salt coat the internal components. An extremely corrosive gas phase comprising of oxygen, SO2 and SO3. A combination of stress and corrosive environment results in decreased component life and eventual failure. Research on high temperature materil properties and environmental degradation is a crucial part of the technology base for engine life prediction and alloy development.

This study evaluates the effect of a molten salt condition on the high temperature creep properties of Inconel 718. Sustained-load creep tests were conducted at a temperature about 1472oF, in lab air at stress levels about 10 ksi – 30 ksi. High strength round Inconel bars are coated with 90% Na2SO3/ 10% NaCL salt mixture by spraying a heated sample with an aqueous salt solution.

Creep data is studied over a 72 hour test period. Tests conducted with salt coated samples were compared with tests  conducted with uncoated samples.

Degradation of high temperature creep properties of Inconel 718 due to the presence of molten salt. It is because of primarily to oxide penetration in metal that has been depleted of alloying elements and after cracking oxide-metal interfaces. Additionally, grain boundary sliding and void developed along the grain boundaries occurred in the alloy- depleted region of corrosion attack. The influence of stress in the corrosion process and the microstructural changes and mechanisms that occur during corrosion mechanical property interactions .

Static hot corrosion tests on samples were also conducted. Cylindrical pins were salt coated and analyzed for 72 hours at a temperature of 1652oF in lab air.

Nickel base super alloys are high temperature, heat resistant alloys retain high strength in the temperature range of 1400oF to 1000oF. These complex alloys have supreme oxidation and corrosion resistance and supreme resistance to creep and cracking at high temperatures. Nickel base super alloys are precipitation hardenable alloys. The major phase of these alloys is gamma phase and gamma prime phase and carbides. The phase is reinforced by solid solution elements like chromium, molybdenum, tungsten, cobalt, columbium, titanium and aluminium.

Iron in the austenitic matrix decreases the cost and enhanced workability and weldability, however it also significantly lowers the strength and oxidation resistance of alloy. The gamma prime phase can be precipitated in nickel-phase super alloys by precipitation hardening heat processing. Gamma prime precipitate in high nickel matrices is usually the intermetallic phase Ni3(Al,Ti). In presence of cobalt, it replaces nickel as (Ni,Co),(Al,Ti).

The gamma prime phase has outstanding long term stability and a high volume fraction of gamma prime provides the significant high temperature strength. As carbides are harder and more brittle than alloy matrix, their distribution along the grain boundaries will influence the high temperature strength, ductility and creep properties. If continuous chains of carbides extend along the grain boundaries, grain boundary sliding will be prevented. Wide stresses will develop during deformation and result to premature failure along the regular fracture paths at the carbide interfaces.