High Temperature Nickel-Based Superalloys for Turbine Discs: Part One

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Introduction to Nickel-Based Superalloys

Nickel-based superalloys are an exceptional class of metallic materials combining high-temperature strength, toughness, and resistance to corrosion and oxidation. They find critical applications in aircraft turbines, power generation, rocket engines, nuclear power, and chemical processing due to their ability to maintain integrity under extreme conditions.

Significant development in alloy chemistry and manufacturing over recent decades has resulted in superalloys capable of tolerating average temperatures of 1050°C and localized hotspots approaching 1200°C—about 90% of their melting point. This article reviews key microstructural aspects, production processes, and mechanical properties of superalloys, focusing primarily on gas turbine engine discs

High-Temperature Challenges in Turbine Discs

Gas turbines, whether in aircraft propulsion or land-based power generation, involve highly demanding environments. The high-pressure turbine discs, especially their rim sections near the gas flow path, face some of the highest temperatures and stresses, reaching up to 760°C routinely and 815°C in specialized military uses. These conditions demand highly specialized nickel-based superalloys to ensure performance and reliability.

Turbine discs serve as the attachment point for turbine blades and are connected to the turbine shaft. Unlike turbine blades, discs typically operate at lower temperatures but must withstand high mechanical stresses, particularly fatigue. Discs are generally cast and then forged, and are polycrystalline in structure.

Production and Microstructure of Turbine Discs

One challenge in manufacturing turbine discs is that cast alloys often develop large columnar grain structures and significant chemical segregation, which can cause variability in mechanical properties. This segregation is not fully eliminated in the finished product, leading to potential inconsistencies.

A common approach to mitigate this is to start with fine, clean powder produced by atomization in inert gas. The chemical segregation within this powder cannot exceed the particle size. Some turbine discs are manufactured by hot isostatic pressing (HIP), extrusion, and subsequent forging of this powder, yielding improved microstructural uniformity and mechanical consistency.

Figure 1: Powder metallurgical aeroengine disc

The process demands strict contamination control, as the introduction of foreign particles (from atomization refractories or solidification impurities) can initiate fatigue cracks, risking catastrophic failure of the disc.

Table 1. Chemical Compositions of Several Nickel-Based Superalloys (wt.%)

Alloy	Cr	Ni	Co	Mo	W	Nb	Ti	AI	Fe	C	B	Other
A286	15	26	-	1.25	-	-	2	0.2	55.2	0.04	0.005	0.3 V
AF115	10.7	56	15	2.8	5.9	1.7	3.9	3.8	-	0.05	0.02	0.75 Hf; 0.05 Zr
AF2-1DA	12	59	10	3	6	-	3	4.6	<0.5	0.35	0.015	1.5 Ta, 0.1 Zr
AF2-1DA6	12	59.5	10	2.75	6.5	-	2.8	4.6	<0.5	0.04	0.015	1.5 Ta, 0.1 Zr
Alloy 706	16	41.5	-	-	-	-	1.75	0.2	37.5	0.03	-	2.9 (Nb+Ta),0.15 Cu
Alloy 718	19	52.5	-	3	-	5.1	0.9	0.5	18.5	0.08	-	0.15 Cu
APK12	18	55	15	3	1.25	-	5	2.5	-	0.03	0.035	0.035 Zr
Astroloy	15	56.5	15	5.25	-	—	3.5	4.4	<0.3	0.06	0.03	0.06 Zr
Discaloy	14	26	—	3	-	-	1.7	0.25	55	0.06	-
IN100	10	60	15	3	-	-	4.7	5.5	<0.6	0.15	0.015	0.06 Zr, 1.0 V
KM-4	12	56	18	4	-	2	4	4	-	0.03	0.03	0.03 Zr
MERL-76	12.4	54.4	18.6	3.3	-	1.4	4.3	5.1	-	0.02	0.03	0.35 Hi; 0.06 Zr
N18	11.5	57	15.7	6.5	-	-	4.35	4.35	-	0.015	0.015	0.45 Hf; 0.03 Zr
PA101	12.5	59	9	2	4	-	4	3.5	-	0.15	0.015	4.0 Ta; 1.0 Hf; 0.1 Zr
Rene 41	19	55	11	10	-	-	3.1	1.5	<0.3	0.09	0.01
Rene 88	16	56.4	13.0	4	4	0.7	3.7	2.1	-	0.03	0.015	0.03 Zr
Rene 95	14	61	$	3.5	3.5	3.5	2.5	3.5	<0.3	0.16	0.01	0.05 Zr
Udimet 500	19	52	19	4	-	-	3	3	<4.0	0.08	0.005
Udimet 520	19	57	12	6	1	-	3	2	-	0.08	0.005
Udimet 700	15	55	17	5	-	-	3.5	4	<1.0	0.07	0.02	0.02 Zr
Udimet 710	18	55	14.8	3	1.5	-	5	2.5	-	0.07	0.01
Udimet 720	18	55	14.8	3	1.25	-	5	2.5	-	0.035	0.033	0.03 Zr
Udimet 720L1	16	57	15.0	3	1.25	-	5	2.5	-	0.025	0.018	0.03 Zr
V57	14.8	27	-	1.25	-	-	3	0.25	48.6	0.08	0.01	0.5 V
Waspaloy	19.5	57	13.5	4.3	-	-	3	1.4	<2.0	0.07	0.006	0.09 Zr

Commercial Pressures and Alloy Development

Besides technical challenges, modern turbine materials must meet growing commercial demands, including reducing component acquisition, life-cycle, and maintenance costs. Efforts focus on alloys with reduced cobalt content and higher processing yields to lower acquisition expenses.

For life-cycle cost reduction, new alloys are designed for longer service lives with improved stability and very low crack-growth rates. Additionally, alloys that enable nondestructive inspection methods are increasingly favored.

Fuel efficiency and emissions regulations also influence superalloy development, pushing materials to balance performance with economic and environmental considerations. The alloys listed in Table 1 represent this ongoing optimization.