A BRIEF STUDY ON NICKEL BASED SUPERALLOYS PRESENTED BY , i) D.PREM MADHUR (14/MM/63) ii) VANDAN RAJ (14/MM/64) iii) MAINAK SAHA (14/MM/65) iv) POOJA MAURYA (14/MM/66) v) RAJDEEP BANIK (14/MM/67) vi) SOURAJIT PRAMANIK (14/MM/68) SUPERVISED BY : DR. MANAB MALLIK DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY DURGAPUR

CONTENTS INTRODUCTION TO SUPERALLOYS NICKEL BASED SUPERALLOYS PROCESSING OF SUPERALLOYS PROPERTIES & MICROSTRUCTURE STRENGTHENING MECHANISMS APPLICATIONS

INTRODUCTION : A Superalloy is an alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature (Tm). A Superalloy or high-performance alloy exhibits several key characteristics such as: 1. Excellent mechanical strength 2. Resistance to thermal creep deformation 3. Good surface stability 4. Resistance to corrosion or oxidation 5. High fracture toughness 6. High fatigue resistance. Examples: Haste alloy, Inconel, Waspalloy, Rene alloys, Haynes alloys, Incoloy, etc.

Fe-Ni-Based Superalloys Why Superalloys? Initially Austenitic Stainless Steels were used for high temperature applications. But working in the range of 450ºC-850ºC , sensitisation occurs due to depletion of Chromium in the grain boundary region which makes it susceptible to cracking. So Superalloys were developed. There are three types of Superalloys on the basis of the predominant elements present in the alloy. Ni-Based Superalloys Fe-Ni-Based Superalloys Co-Based Superalloys

General Composition: Ni-BASED SUPERALLOYS : Composition : 38-76% Ni, up to 27% Cr, up to 20% Co. Example : Nimonic, Inconel, Waspalloy. Fe-Ni-BASED SUPERALLOYS: Composition : 32-67% Fe, 9-38% Ni, 15-22% Cr . Example : Incoloy series Co-BASED SUPERALLOYS: Composition : 30-65% Co, 19-30% Cr, up to 35% Ni .

Ni-BASED SUPERALLOYS : Ni-based superalloys are more strong and have more corrosion resistance. They are the most commonly used superalloys generally used above 500ºC in oxidising and corrosive environment. Example: Turbine Blades. Why is Ni used as a matrix for superalloys ? Due to FCC structure it provides good ductility It has low linear thermal coefficient of expansion (13x10-6 /ºC at 20ºC) Low volume thermal coefficient of expansion (39x10-6 /ºC at 20ºC) Ni-matrix does not undergo phase transformation up to Tm

Composition: 15% Cr - 6.75% Fe - 2.5% Ti - 0.8% Al - 0.85% Co The Ni-based superalloys contain high Cr with Ti, Al to form precipitates and additions of Mo, Co, Fe, C etc. On the basis of variation of additional alloying elements Ni-based superalloys are generally of three types : i) Inconel Composition: 15% Cr - 6.75% Fe - 2.5% Ti - 0.8% Al - 0.85% Co ii) Nimonic Composition: 20% Cr - 18% Co - 2.5% Ti - 1.5% Al - 0.05% C iii) Waspalloy Composition: 19.5% Cr - 13.5% Co - 2% Fe - 4.25% Mo - 1.3% Al - 3% Ti - 0.1% C

Directional Solidification Single Crystal Growth PROCESSING OF SUPERALLOYS There are many forms of superalloy present within a gas turbine engine, and the processing methods vary widely depending on the necessary properties of each specific part. Some of them are: Casting and Forging Powder Metallurgy Directional Solidification Single Crystal Growth

Conventional Casting: Initially the Superalloys were produced through conventional casting process. But superalloys having high alloying elements, ingot casting caused a high amount of shrinkage along with blowholes, porosities etc. and also had dendritic structure. Vacuum melting process is used to remove the blowholes and gas porosities, specifically oxygen. Dendritic structure and shrinkage is eliminated by deformation by rolling or forging. Here thermomechanical treatment gives uniform deformation of phases but still causes segregation in weaker regions. So they have short service life. Powder Metallurgy: This is a class of modern processing techniques in which metals are first converted into a powder form, compacted and then formed into the desired shape by sintering below the melting point. This is in contrast to casting, which occurs with molten metal. Superalloy manufacturing often employs powder metallurgy because of its material efficiency - typically much less waste metal must be machined away from the final product—and its ability to facilitate mechanical alloying. But it also facilitates higher cost of production.

Directional solidification: This process uses a thermal gradient to promote nucleation of metal grains on a low temperature surface, as well as to promote their growth along the temperature gradient. This leads to grains elongated along the temperature gradient, and significantly greater creep resistance parallel to the long grain direction. Single Crystal Growth: In this process, a single crystal is manufactured by removing all the grain boundaries resulting in a sharp increase in strength and creep properties. Single crystal growth starts with a seed crystal which is used to template growth of a larger crystal. This involves highly controlled and therefore relatively slow crystallization. They have long service life.

Fig: Showing variation of Creep rate with Time for differently cast Superalloys. Single Crystal gives more creep life, hence can work for prolonged time compared to Directionally Solidified and Conventionally Cast Superalloys

γ-PHASE γ'-PHASE PROPERTIES & MICROSTRUCTURE : The continuous matrix is a FCC Ni-based austenitic phase (γ-phase) that usually contains a high percentage of solid-solution elements such as Co, Cr, Mo and W. γ'-PHASE The primary strengthening phase in Ni-based superalloys is Ni3(Al, Ti) or the γ'-phase. It is a coherently precipitating phase (i.e., the crystal planes of the precipitate are in registry with the γ-matrix) with an ordered FCC crystal structure.

Ni-9.7 Al-1.7 Ti-17.1 Cr-6.3 Co-2.3 W (TEM Imaging) Transmission Electron micrograph showing cuboidal γ' (Ni3(Al,Ti)) (FCC-L12) precipitate in γ (FCC) (A1)matrix. There are also the carbide formers (Cr, W, and Ti) present. The carbides tend to precipitate at grain boundaries and hence reduce the tendency for grain boundary sliding. Cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium and aluminium are also solid-solution strengtheners, both in γ and γ'. Cr, Co, here, partitions into γ whereas Ti partitions into γ'. W may partition both into γ and γ'. Dislocations in γ find it difficult to penetrate into atomically ordered γ' and leads to strengthening.

Ni-9.7 Al-1.7 Ti-17.1 Cr-6.3 Co-2.3 W (SADP) (3 1 1) (1 1 0) (1 3 1) [1 1 4] Fig: SADP of [1 1 4] zone axis with (110) and (110) – superlattice (γ‘) reflections and {131}, {220}- γ reflections

STRENGTHENING MECHANISMS : There are three prior strengthening mechanisms for Ni-based superalloys ;  Solid Solution Hardening  Coherent Precipitate Hardening  Hardening by Carbide precipitation on grain boundaries

Solid Solution Strengthening: Cr, Mo, Al, Nb, Ti and other alloying elements go to the solution and replace the solvent atoms in their lattice positions. So local stress fields are formed that interact with those of the dislocations, impeding their motion and causing an increase in the yield stress of the material, which means an increase in strength of the material. Precipitation Strengthening: It happens mostly due to Al & Ti, when they form γ' or Ni3(Al,Ti) with the solvent atoms. This γ' phase impede the movement of dislocations and defects in a crystal lattice which leads to increase of strength. Hardening by Carbide Precipitates: Presence of carbide precipitates (like M23C6, M6C or MC) which are hard and brittle in nature causes strengthening of superalloys. Oxide dispersion strengthening is another method of strengthening in which Yttrium oxides are dispersed on the matrix making the material suitable for high temperature applications.

Fig: Alloying element effects in Nickel based superalloys. Effect of Alloying Elements : Fig: Alloying element effects in Nickel based superalloys.

Fig: Variation of Yield Stress with Temperature for different Ni-based superalloys

Superalloys are used in, Aerospace Marine Industry  Submarines APPLICTIONS : Ni-based superalloys are widely used in load bearing structures to the highest homologous temperature 0.9 Tm Superalloys are used in, Aerospace  Turbine Blades and Jet/Rocket engines Marine Industry  Submarines Nuclear Reactors Heat Exchanger Tubing Industrial Gas Turbines and Combustion Engine Valves Petrochemical Equipments Hot working Tooling and Dies

Fig. Jet Engines Fig. Turbine Blades Fig. Rocket Engine

Fig. Gas Turbine for marine propulsion Fig. Gas Turbine for Thermal power plant