JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV SUPER ALLOYS Previous Question Paper (September - 2020 - R16)

Superalloys are a class of high-performance materials designed to operate at high fractions of their melting point, typically above 0.6 T_m, where they exhibit exceptional resistance to mechanical and chemical degradation. Their properties are fundamentally distinct from conventional steels due to their complex microstructural design and alloying strategy.

Physical Properties: Superalloys possess high density, typically ranging from 7.5 to 9.0 g/cm³, due to the presence of heavy elements like tungsten, tantalum, and rhenium. They exhibit high thermal conductivity compared to many ceramics but lower than pure copper, and possess a low coefficient of thermal expansion to maintain dimensional stability under thermal cycling. They are also characterized by high oxidation and hot corrosion resistance, facilitated by the formation of protective Al₂O₃ or Cr₂O₃ surface scales.

Mechanical Properties: The defining characteristic is superior creep resistance, which is the ability to withstand long-term stress at elevated temperatures without significant deformation. They maintain high yield strength, tensile strength, and fatigue resistance at temperatures exceeding 700°C. These properties are achieved through solid-solution strengthening, carbide precipitation, and, most importantly, the coherent precipitation of the L1₂ ordered gamma-prime (γ') phase (Ni₃(Al, Ti)) in a face-centered cubic (FCC) gamma (γ) matrix.

Differences from Steels:

• Temperature Capability: Steels rely on iron-carbon martensite or carbides which soften rapidly above 500°C due to tempering and diffusion-controlled processes. Superalloys retain structural integrity up to 1100°C or higher.
• Microstructural Stability: In steels, the microstructure is often metastable (e.g., martensite), whereas superalloys are designed for long-term thermodynamic stability of the γ/γ' microstructure.
• Alloying Complexity: While steels are primarily Fe-C based, superalloys are highly complex, often containing 10 or more elements (Ni, Co, Cr, Mo, W, Al, Ti, Ta, Nb, Re) to simultaneously optimize creep, oxidation, and phase stability.
• Strengthening Mechanisms: Steels are primarily strengthened by interstitial carbon and grain boundary pinning. Superalloys utilize coherent precipitate hardening (γ') and refractory element solid-solution strengthening, which are far more effective at high temperatures where interstitial diffusion would render steel ineffective.

Inconel 718 is a widely used precipitation-hardenable nickel-chromium-based superalloy. Its properties are primarily derived from the addition of niobium, which reacts with nickel to form the metastable body-centered tetragonal gamma double prime (γ'') phase, providing exceptional strength at elevated temperatures.

Key Properties:

• High Tensile and Creep Strength: It maintains excellent mechanical properties up to approximately 650°C, making it suitable for high-stress environments.
• Corrosion and Oxidation Resistance: The high chromium content provides superior resistance to oxidation and corrosion in both high-temperature gas environments and aqueous media.
• Fabricability: Unlike many other high-strength superalloys, Inconel 718 exhibits good weldability and can be easily formed or machined in the annealed condition before final heat treatment.
• Fatigue Resistance: It possesses high fatigue strength, which is critical for components subjected to cyclic loading.

Applications:

• Aerospace Industry: It is extensively used for gas turbine engine components, including turbine blades, discs, casings, and high-temperature bolts and fasteners.
• Nuclear Power: Due to its resistance to radiation-induced swelling and corrosion, it is utilized for reactor core components and springs.
• Oil and Gas Industry: It is employed in downhole drilling tools, valves, and wellhead components where high strength and resistance to sour gas (H2S) environments are required.
• Chemical Processing: Used in equipment handling aggressive chemicals at elevated temperatures, such as heat exchangers and pressure vessels.