Background:

EP 742 is a nickel base wrought super alloy used in critical aero engine components such as high pressure turbine discs. The study aimed to evaluate the temperature dependent tensile properties of the alloy in different heat treatment conditions. Testing Method:

Test bars were produced by melting, forging, and heat treatment. Samples were tensile tested at room temperature and at 650oC at a strain rate of 2 mm/minute. Microstructure:

Gamma prime size and morphology varied with the heat treatment conditions. Finer size of gamma prime was observed in the solutionizing + aging (STA 16hrs) condition. Tensile Properties:

The alloy in STA 16hrs condition exhibited good tensile properties at room temperature based on power law relationships. The alloy in STA 24hrs condition showed good tensile properties at 650oC. Abbreviations:

ASTM: American Society for Testing and Materials DS: Directional Solidification Introduction to High Temperature Materials:

Certain materials retain their characteristics at high temperatures, needed in turbines, rockets, and heat exchangers. They should sustain loads at operating temperature near their melting point, exhibit resistance to mechanical degradation over time, and tolerate severe operating environments. Superalloys:

Superalloys are composed of Fe, Ni, Co, Cr, and lesser amounts of other elements, providing exceptional mechanical strength and creep resistance at higher temperatures. They are utilized in aircraft and industrial gas turbines, rocket engines, and heat exchanger tubes. Nickel Based Superalloys:

Nickel base superalloys are corrosion resistant high temperature alloys typically used at service temperatures above 500 C. They exhibit excellent resistance to creep, sulfidation, and oxidation even at temperatures close to the melting point. Aeroplane Turbine Engine:

Transfers chemical energy from fuel to mechanical energy by driving the engine and propeller with gaseous energy from the working fluid Works similar to any other engine that derives power from fuel with basic four processes: air intake, air compression, combustion, and expansion and exhaust Gas Turbo Fan Engine:

The turbine component creates useful output shaft power to push the propeller by expanding gas at high pressure, temperature, and velocity Inclusion of the fan slightly alters the fuel flow rate for the core, making the gas turbo fan engine significantly more fuel efficient Nickel Base Superalloys:

Employed in the combustion chamber and turbine portions of a gas turbine engine Chosen for their greater oxidation and corrosion resistance at high temperatures and aggressive environments Microstructure of Nickel Base Superalloys:

Contains gamma (γ) phase with matrix of FCC and gamma prime (γ′) phase with a superlattice structure of FCC Volume fractions and size distribution impact alloy strength, with some superalloys having gamma double prime (γ′′) as a substitute for the strengthening phase Melting and Casting of Nickel Base Superalloys:

Vacuum induction melting technique is commonly used for melting superalloys Vacuum arc re-melting furnace is employed for further processing and quality improvement of the ingots Material Development for Aviation Engines:

Focus on developing materials with good mechanical and raised temperature capabilities, such as nickel base, titanium base alloys, and special steels Increased Turbine Entry Temperature (TET) of nickel base superalloys has improved the efficiency of aviation engines Development of Nickel Base Superalloy EP 742:

Developed as a near-equivalent to Russian Nickel base Superalloy EP 742, developed by M/s MIDHANI in remelt Stock form with R&D inputs from DMRL, Hyderabad. Certified for airworthiness by RCMA (Materials), Hyderabad. Heat Treatment and Mechanical Behavior Study:

Conducted studies on mechanical behavior of the alloy with varying heat treatments. Thorough evaluation of all required mechanical properties. Experimental Work:

Two different heat treatment conditions were selected to study the mechanical behavior of the alloy. Details of work carried out at different stages including heat treatments, microstructural studies, and tensile properties were reported. Literature Review:

Literature review aimed to gain better understanding of the superalloys' physical metallurgy and mechanical behavior. Optimization of parameters for heat treatment and mechanical characterization. Alloy Composition:

The nickel base superalloys mainly consist of three different classes of elements. Solid solution strengthened alloys are used in applications requiring only modest strength. Chemical Composition and Role of Alloying Elements:

The nickel base superalloys mainly consist of three different classes of elements. Chromium (Cr) contributes to solid solution as well as carbide strengthening of the alloy. Main Alloying Elements:

Formers, ' formers, carbide formers, and elements that segregate to the grain boundaries are the main categories of alloying elements. Chromium and aluminum are essential for oxidation resistance. Role of Chromium:

Mainly contributes to solid solution as well as carbide strengthening of the alloy. Forms Cr23C6 type discrete carbide precipitates at grain boundaries when subjected to ageing treatment. Influence of Alloy Elements:

Cobalt addition enhances creep resistance and lowers stacking fault energy of the alloy. Molybdenum and Tungsten contribute to solid solution strengthening and retard coarsening at elevated temperatures. Role of Aluminum and Titanium:

Aluminum forms Ni3Al, an ordered intermetallic compound, strengthening the alloy through fine scale precipitation. Titanium strengthens the alloy by uniform precipitation of fine Ni3Ti throughout the matrix and also forms MC type of carbides (TiC). Contribution of Boron and Zirconium:

Boron and Zirconium occupy interstitial lattice sites on grain boundaries, preventing coarsening of discrete carbide precipitates and strengthening the grain boundaries. Heat Treatment:

Nickel base Superalloys attain their strength through heat treatments known as solutioning and ageing. Aging treatments strengthen age-hardenable alloys by causing the precipitation of one or more phases from supersaturated matrix. Properties of NI Base Superalloys:

Superalloys have outstanding strength, good ductility, impact resistance, and resistance to mechanical fatigue and thermal fatigue. The close-packed FCC lattice crystal structure contributes to their capabilities to maintain good mechanical properties. Mechanical Properties and Definitions:

Tension test is used to evaluate the resistance of a material to deformation and failure. The stress-strain curve is a representation of the performance of the specimen as the applied load is increased to fracture. Mechanical Behavior and Microstructure:

Mechanical properties of metals depend on their microstructure and mechanical behavior is studied at the macroscopic level. Understanding the relationship between mechanical behavior and microstructure is essential to well understand the mechanical properties. Stress-Strain Curve and Elastic Modulus:

The stress-strain curve represents the performance of the specimen as the applied load is increased to fracture. The young modulus is related to the binding forces between atoms and is considered microstructure insensitive. Engineering Stress Strain Curve:

The stress-strain curve is used to evaluate the properties of a material under load. The true stress increases after necking as the area decreases more than the load required. Strain Hardening:

The strain hardening behavior of metallic materials is usually described by flow curves with mathematical expressions. The strain hardening coefficient describes the susceptibility of a material to work harden and varies between 0 and 1. Fractography:

Fractography is used to analyze fracture features and relate the topography of the fracture surface to the causes of fracture. Fracture modes can be ductile or brittle, characterized by plastic deformation and rapid crack propagation, respectively. Ductile Fracture:

Ductile fracture is characterized by appreciable plastic deformation and can absorb energy before failure. FCC metals are usually ductile due to the presence of active slip systems. Brittle Fracture:

Brittle fracture is characterized by rapid crack propagation with negligible plastic deformation and occurs mostly in cold metals and ceramics. BCC and HCP metals are susceptible to brittle fracture due to the limited number of active slip planes. Nickel Base Superalloy EP 742:

The alloy EP 742 is an advanced nickel base wrought superalloy used for aero engine hot end applications. The alloy attains its strength from gamma prime precipitates and exhibits excellent creep and fatigue strengths at high temperatures. Alloy Composition and Properties:

The alloy contains low diffusing elements like W and Mo, which help retain strength at high temperatures. It also contains primary carbides of MC type and secondary carbides of M6C type, with TiC being the most stable carbide. Carbide Formation and Volume Fraction:

The secondary carbides (M6C type) form by degeneration of primary carbides TiC at a temperature of about 1080°C. The alloy contains about 65% volume fraction of gamma prime and is free from topologically close packed (TCP) phases. Experimental Procedure:

The experimental details include heat treatment, machining of tensile specimens, and the equipment used for microstructure studies. It also covers the procedure followed and the parameters maintained during the tensile tests. Production of Bar Stock:

The test bars were produced by using Vacuum induction melting (VIM) and Vacuum arc re melting (VAR). The as-cast ingots were subjected to homogenization treatment to achieve chemical homogeneity. Preparing the Samples:

Samples for heat treatment and microscopy were extracted from cogged bars using Electric Discharge Machine (EDM), and subsequently heat-treated. The EDM used for the study was at Aeronautical Material testing laboratory, M/s. MIDHANI, Hyderabad. Heat Treatment:

The test bars underwent two different heat treatments: Solutioning + Aging (Aging I condition) and Solutioning + Aging (Aging II condition). Both heat treatment conditions involved Solutioning and Aging, with specific temperature and time parameters. Microscopy and SEM:

The mounted samples were examined for optical microscopy after thorough polishing and etching, and then observed using an optical microscope and SEM. Fracture surfaces of tensile specimens were also examined using the scanning electron microscope. Tensile Testing:

Tensile testing was conducted at room temperature and 650°C using servo hydraulic INSTRON 5500 R 4507 machine. A total of five specimens were tested, and the results are reported and analyzed in detail in section 4.4. Hardness Measurement:

Hardness was measured using The BUEHLER Vickers Microhardness Tester with a load of 30Kgf and a square base diamond pyramid indenter with an included angle of 1360 between the faces. Tables were used to convert the measured diagonals to Vickers pyramid hardness number. Chemical Composition:

The chemical composition of the Nickel Base Superalloy EP 742 was analyzed, with a narrow range of compositional limits to be maintained, particularly controlling gaseous impurities below 10 ppm. The alloying elements and their weight percentages were presented in Table 4.1. Optical Microscopy:

The microstructures of the alloy in different heat treatment conditions were examined and the average grain sizes were reported. Micrographs of the alloy in different conditions, along with their corresponding magnifications, were presented. Scanning Electron Microscopy:

The distribution, size variation, and morphology of Gamma prime and carbide particles with respect to the heat treatments were studied using Scanning Electron Microscope. The strengthening precipitate and the type of carbides present were observed and analyzed. Tensile Properties:

Samples tested in Solutioning + Aging 24hrs (Aged II) heat treatment condition showed higher yield strength than those tested in other conditions. Samples tested in Solutioning + Aging 16 hrs (Aged I) heat treatment condition resulted in higher ultimate tensile strength than samples from other conditions. Flow Behaviour:

Analysis of strain hardening behaviour using the Hollomon equation showed that parameters k and n were higher for Solutionising + Aging 16 hrs (Aged I) samples at both room temperature and 650°C. Parameters n and L were inversely related, and the size of the Gamma prime (γ) in Solutionising + Aging (Aged II) condition was optimum in hindering dislocation motion. Fractography:

Scanning Electron Microscope studies revealed the mode and origin of fracture on the surfaces of tensile tested specimens. Fractured surfaces of samples in different heat treatment conditions were analyzed, showing differences in the fracture behavior. Microstructural Examination:

Microstructural examination of cast Nickel base superalloy EP 742 revealed variations in gamma prime (γ) size and morphology based on different heat treatment conditions. The finer particle size in Solutioning + Aging (STA 16hrs) condition led to higher tensile strength values compared to the STA 24hrs condition. True Stress-Strain Analysis:

True Stress-Strain data analysis showed higher values of transition strain (L), k, and n in Solutioning + Aging (STA 16hrs) condition, attributed to delayed dislocation interaction. Lower values of these parameters in Solutioning + Aging (STA 24hrs) condition indicated early dislocation interaction due to lower inter-particle spacing. Scope for Future Work:

Detailed microstructural analysis using Transmission Electron Microscope is recommended for different heat treatment conditions. LCF testing in different heat treatment conditions at higher strain amplitudes should be conducted to establish power law relationships. Hardness Measurements:

The sample in Solutionising + Ageing 24hrs (Aged II) displayed the highest average hardness value of 425 HV. Solutionised (1080 C 8 hrs) condition had an average hardness value of 380 HV. Conclusion:

The study provided insights into physical metallurgy, tensile deformation, and microstructural changes of the cast Nickel base superalloy under different heat treatment conditions. Further research is recommended to establish comprehensive correlations between microstructural features and mechanical properties.

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