Nickel based Super Alloys Properties and Applications

Nickel based Super Alloys

 Properties and Applications

 

 

Nickel-Based Superalloys

The nickel-based superlloys are often the material of choice for high-temperature structural applications, particularly when resistance to creep and/or fatigue is needed and the risk of degradation due to oxidation and/or corrosion is severe.

Nickel-based superalloys were developed and have been improved over the past 50 years for applications involving stringent elevated-temperature operating conditions, such as those experienced by components of gas turbine engines. Due to their good properties at elevated temperatures, nickel base superalloys have been widely used for aerospace, power generation, and automotive high temperature applications since their emergence in the 1950s.

Nickel-base superalloys are corrosion resistant high temperature alloys typically used at service temperatures above 500°C. They usually contain significant amounts of up to 10 alloying elements including light elements like boron or carbon and heavy refractory elements like tantalum, tungsten, or rhenium . Superalloys display excellent resistance against creep, sulfidation, and oxidation even at temperatures close to their melting points.

Superalloys exhibit excellent mechanical strength and creep resistance at elevated temperatures. Hence, they have been used in several high temperature applications, particularly when in sheet form as front panels for the thermal protection systems. Other notable applications of these alloys include space components, blades and vanes in turbine engines, critical part of rocket engines, submarines, nuclear reactors, chemical processing vessels and heat exchanger. Nickel base alloys are the most complex, the most fascinating of all superalloys. Their use extends to the highest homologous temperature of any common alloy system, and they currently comprise over 50% of the weight of advanced aircraft engines

 

Chemical Composition

The nickel base superalloys mainly consist of three different classes of elements. The first class consists of elements that prefer and make up the face centered cubic (FCC) austenite matrix. These are from group V, VI and VII and include nickel, cobalt, iron, chromium, molybdenum, tungsten, and vanadium. The second class of elements partition to and make up the  precipitate Ni₃Al . Most nickel-based alloys contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B, Zr, and C. Other common additions are Mo, W, Ta, Hf, and Nb. In broad terms, the elemental additions in Ni-base superalloys can be categorized as being (i) g ' formers (elements that tend to partition to the matrix), (ii) g  'formers  (elements that partition to g' precipitate), (iii) carbide formers and (iv) elements that segregate to the grain boundaries.

Nominal chemistry of selected nickel-based superalloys-

 

 

 

Microstructure

Nickel is a versatile element and will alloy with most metals. The wide solubility ranges between iron, chromium and nickel make many alloy combinations possible. The face-centred cubic structure of the nickel matrix can be strengthened by solid solution hardening, carbide precipitation or precipitation hardening. The excellent mechanical properties of nickel-based superalloys are inherently related to the precipitation of intermetallic phases in the disordered face-centred cubic γ matrix and to their interactions with the grain growth and recrystallization mechanisms. In the case of solid solution hardening, cobalt, iron, chromium, molybdenum, tungsten, vanadium, titanium and aluminium are all solid solution hardeners in nickel. At temperatures above 0.6 Tm (where Tm is the melting temperature in kelvin), which is the range of high-temperature creep, the strengthening is diffusion-dependent, and large, slowly diffusing elements such as molybdenum and tungsten are the most effective hardeners. Carbide strengthening relies mainly on MC, M 6 C, M 7 C 3 and M 23 C 6 (where M is a metallic carbide-forming element). MC usually occurs in the form of a primary large blocky carbide, random in distribution, and is generally not desired . Microstructure of MC 2 single crystal nickel super alloy

 

 

M 6 C carbides are also blocky; when formed in grain boundaries, they can be used to control grain size, but when precipitated in a Widmanstätten pattern throughout the grains, these carbides can impair the ductility and rupture life. M 7 C 3 carbides form intergranularly and are beneficial if precipitated as discrete particles. They can cause embrittlement if they agglomerate, forming continuous grain boundary films. This condition will occur over an extended period of time at high temperatures. M 23 C 6 carbides show a propensity for grain boundary precipitation and are influential in determining the mechanical properties of nickel-based alloys. Discrete grain boundary particles enhance rupture properties. Long-time exposure at 760–980°C will cause precipitation of angular intragranular carbides, and particles along twin bands and twin ends. Heat treatment provides the alloy designer with a means of creating the desired carbide structure and morphology before placing the material in service. The chemistry of the alloy, its prior processing history and the heat treatment given to the material influence carbide precipitation and ultimately the performance of the alloy. Each new alloy must be thoroughly examined to determine its response to heat treatment or high temperature. The topologically close-packed phases are generally undesired, since they are very brittle owing to their limited number of slip planes. The most common representatives of this family are the σ , the Laves and the μ phases. Their chemistries and crystallographic structures are rather complex.

                  

Physical and Mechanical Properties of Ni-base Superalloys

Density –

The density of superalloys falls within the range 7.75 to 9.25 gm/cm3 . As a class, the iron base alloys have the lowest density, owing to the density advantage of iron over nickel and cobalt. Nickel base alloys cover a wider range of densities, as they vary widely in alloy content. For example, the density of IN-100(60% Ni) is 7.75 gm/cm3 , because of the large amount of low-density aluminum and titanium present, whereas some experimental nickel base alloys containing large amounts of high density tungsten and tantalum have densities as high as 9.0 gm/cm3 .

 

Thermal expansion-

Nickel and cobalt base alloys have similar thermal expansion coefficients, which are lower than those of austenitic iron base alloys. Gas turbines are designed to operate most efficiently with close tolerances; therefore, thermal expansion is an important design factor. In some cases it is necessary to match coefficients in mating components, whereas in others a low coefficient is preferred to minimize thermal stresses, which causes buckling and thermal fatigue cracking.

 

Thermal conductivity-

Thermal conductivity of superalloys is only 10 to 30% that of pure iron, nickel, or cobalt, owing to the effect of extensive alloying. In general, iron base alloys are slightly more conductive than cobalt base alloys, and nickel base alloys span the range of each.

Oxidation resistance-

Good oxidation resistance is achieved by formation of a tight continuous surface scale that acts as a diffusion barrier and does not spall off during thermal cycling. In general, nickel-chromium alloys with high aluminium, such as 713C and B-1900, are considered to have excellent resistance to oxidation due to their ability to form the protective oxides Cr2O3 and Al2O3

Hot corrosion-

In gas turbine industry, hot corrosion (sulfidation) refers to a particularly aggressive attack resulting from the combined effects of normal oxidation and reaction with sulfur and other contaminants ingested with inlet air and these contained in Fuel. Hot corrosion resistance is related to the chromium content in both nickel and cobalt base alloys and is also a function of the sulfide properties of these systems.

Ultimate tensile strength of selected nickel-based superalloys

 

Applications

Nickel-based superalloys usually combine high strength and corrosion resistance during service at elevated temperatures. Initially, their development was encouraged and driven by the insight that the efficiency of thermal power generation machines can be increased by increasing the combustion temperature and/or pressure. Consequently, they have been widely used in high-performance combustion engines, such as gas turbines in aircraft, and for power generation in thermal, nuclear and fossil fuel power plants. The typical nickel-based components in this energy sector are rotors, turbine discs, blades, shafts, bearings, spindles and bolts, as well as casings for stationary gas and steam turbines. In the aircraft industry, most of the rotating turbine parts and also the casings, links and some of the engine mounts are typically made of high-performance nickel-based superalloys. Some similar applications where nickel-based superalloys are also used are in turbocharger discs for large diesel engines and in high-performance racing car engines.

                      

 

Furthermore, the chemical industry uses these alloys for applications in highly corrosive environments, containing brines, carbonates, phosphates, sulphates, chlorides, nitrates or just seawater. In this respect, the oil and gas industry should be especially mentioned, where these materials are used in gas and oil exploration, and in refining and transport. The typical components are downhole equipment, wellheads, pipes, valves and pump wheels. Owing to their excellent high temperature corrosion resistance, nickel-based alloys are also often used for valves in large ship diesel engines.

The metal processing industry uses nickel-based superalloys in several components of extrusion and forging machines, especially tools for forging, shaping and deep drawing, where wear resistance plays an important role.

 

                                              

Typical forged and machined Nimonic 80A ship diesel valve

(courtesy of Böhler Schmiedetechnik GmbH & Co KG, Austria).

Future trends

The environmental effect of stationary and flying gas turbines is currently the major driver for further research and development regarding nickel-based alloys. The demand for higher efficiency combined with lower air pollution due to exhaust of CO and NO x leads to the need for an increase in the working temperature. This consequently means the development of new alloys with higher-temperature capability, as well as the development of new technologies and modification of old technologies to produce parts out of these alloys. As an example, in the case of large rotors for turbines in the field of energy production, producing ingots for remelting with the same quality as those currently used, and stronger forging equipment, will be two challenges for the near future.

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Rutvik Dagadkhair – 11

Neel Doifode – 21

Omkar Gandhal – 25

Sarthak Shelke – 60