Development and Applications of Advanced High-Temperature Nickel Alloys

For many years, nickel-based alloys have been the material of choice for the ‘heart’ of aviation and industrial gas turbine engines. As a structural material specifically designed for extreme operating conditions exceeding 538°C (1000°F), superalloys strike a near-perfect balance between high-temperature strength, fatigue resistance, oxidation resistance and coating performance. They are the ‘only choice’ for manufacturing core hot-end components such as turbine blades, guide vanes, integral rotors and combustion chambers.

In the pursuit of greater power and longer service life, its manufacturing processes have undergone decades of rigorous evolution: from the initial equiaxed casting (EQ), to directional solidification (DS) which eliminates the weakness of transverse grain boundaries, and finally to single-crystal casting (SX), which represents the pinnacle of the technology by completely eliminating grain boundaries. Today, these precision forming techniques—which border on artistry—are no longer confined to the realm of large aero-engines. Thanks to their unrivalled comprehensive performance, advanced nickel-based high-temperature alloys are making a strong inroads into the highly demanding fields of small and micro-turbines, turbochargers and missile engines!

Today, we’ll take you on an in-depth exploration of the evolution of investment casting for nickel-based high-temperature alloys, offering a comprehensive insight into the technical intricacies of equiaxed, directional solidification and single-crystal technologies, whilst providing an in-depth review of the ‘star’ materials that have made a significant impact in small, high-performance power systems.

1. Casting of high-temperature alloys

The initial applications for turbine blades and guide vanes were traditionally cast equiaxed (EQ) alloys. Equiaxial castings are used in the majority of applications, including static and rotating components, integral impellers and structural components. In addition to maintaining high strength at operating temperatures approaching 85% of the melting point, these materials exhibit the excellent thermal corrosion and oxidation resistance required in gas turbine environments. Performance requirements also include high-temperature creep and fatigue strength, as well as the ductility and weldability necessary for manufacturing and repair.

Fig. 1. Nickel Alloy Plates and Round Bars

Although equiaxial casting is the most traditional process, it continues to excel in static structural components and integral impellers through the fine-tuning of chemical composition.

Representative alloys include: UNS N07939, CM 939 Weldable, UNS N07738, UNS N07713, UNS N07247, the low-carbon modified version CM 247 LC, and CM 681 LC (a proprietary rhenium-containing alloy).

2. Directional Solidification (DS)

Metal solidified using the equiaxial casting (EQ) process exhibits non-uniform grain structure. Under the extreme temperatures and centrifugal forces of tens of thousands of revolutions per minute in gas turbines, the transverse grain boundaries between these grains become the weakest points, making them highly susceptible to fracture. The advent of Directional Solidification (DS) technology has resolved this issue. Its core principle involves controlling the temperature gradient with extreme precision during the casting of the alloy, allowing the molten metal to crystallise slowly from the bottom upwards in a single direction.

Thanks to this unique ‘columnar’ microstructure, these castings have achieved significant improvements in creep fracture strength and low-cycle fatigue life. Currently, directionally solidified alloys are typically used in rotating component applications, such as the second and third-stage turbine blades in gas turbines, which are subjected to extremely high stresses.

Fig 2.  Representative alloys: Directional-grade CM 247 LC (derived from UNS N07247), CM 186 LC (rhenium-containing directional alloy)

3. Directional Solidification of Single Crystals

Although directional solidification (DS) eliminates brittle transverse grain boundaries, parallel longitudinal seams still exist between these ‘columnar grains’. Under extreme temperatures and stresses, these seams remain potential weak points. In the pursuit of ultimate material performance, the precision casting industry has developed the currently leading single-crystal casting (SX) technology. By completely eliminating all grain boundaries, single-crystal alloys no longer require the addition of elements such as carbon and boron to ‘bond’ the grain boundaries. With these ‘dead weights’which lower the material’s overall melting point removed, the temperature resistance of single-crystal alloys is fully unleashed. Their high-temperature creep strength and fatigue life have undergone a qualitative leap, making them the undisputed first choice for the most critical hot-end components in today’s micro-turbines and missile engines.

Fig 3. Representative alloys: UNS N31233 (the industry-renowned CMSX-4), CMSX-4 [La + Y]