2 Tool Condition Monitoring in Machining Superalloys H.Z.Li and X.Q.Chen CONTENTS 2.1 Introduction97 2.1.1 Microstructure… .79 2.1.2 Classifications. 81 2.13 Strengthening Mechanisms..... 8 2.2 Machinability of Superalloys...... .82 2.3 Machining Techniques for Superalloys............................ .83 2.3.1 Cutting Tool Materials. .84 2.3.1.1 High-Speed Steel........... .84 2.3.1.2 Cemented Tungsten Carbide.84 2.3.1.3 Ceramic Cutting Tools............. .85 2.3.14 Cermet Cutting Tools..85 2.3.1.5 Cubic Boron Nitride.............. 85 2.3.1.6 Whisker-Reinforced Materials. .85 2.3.2 Turning… .86 2.3.3 Milling .87 2.3.4 Drilling… 87 2.3.5 Grinding. .88 2.3.6 Nontraditional Machining........... .88 2.4 Case Study:Process and Tool Wear in Cutting Inconel 718............................................89 2.4.1 Tool Wear Monitoring.............. 89 2.4.2 Experimental Study................ .91 2.4.3 Development of a Tool Wear Model...... .98 2.4.4 Adaptive Machining Using Outer Feedback Loop Control..........104 2.1 INTRODUCTION Superalloys are important enabling materials for modern aircraft.Although there have been different definitions over the years,superalloys are generally regarded as a class of high-temperature,high- strength alloys based on Group VIIIA-base elements of the periodic table,which achieve high strength and corrosion resistance for elevated-temperature service [1].According to Ref.[2].super- alloys are heat-resisting alloys based on nickel,nickel-iron,or cobalt.They exhibit a combination of mechanical strength and resistance to surface degradation,including high strength at elevated temperature;resistance to environmental attack;excellent creep resistance,stress-rupture strength, toughness,and metallurgical stability;and resistance to thermal fatigue and corrosion.Superalloys 77
77 Tool Condition Monitoring in Machining Superalloys H. Z. Li and X. Q. Chen 2.1 INTRODUCTION Superalloys are important enabling materials for modern aircraft. Although there have been different definitions over the years, superalloys are generally regarded as a class of high-temperature, highstrength alloys based on Group VIIIA-base elements of the periodic table, which achieve high strength and corrosion resistance for elevated-temperature service [1]. According to Ref. [2], superalloys are heat-resisting alloys based on nickel, nickel-iron, or cobalt. They exhibit a combination of mechanical strength and resistance to surface degradation, including high strength at elevated temperature; resistance to environmental attack; excellent creep resistance, stress-rupture strength, toughness, and metallurgical stability; and resistance to thermal fatigue and corrosion. Superalloys 2 CONTENTS 2.1 Introduction ............................................................................................................................77 2.1.1 Microstructure ............................................................................................................79 2.1.2 Classifications............................................................................................................. 81 2.1.3 Strengthening Mechanisms ........................................................................................ 81 2.2 Machinability of Superalloys..................................................................................................82 2.3 Machining Techniques for Superalloys..................................................................................83 2.3.1 Cutting Tool Materials................................................................................................84 2.3.1.1 High-Speed Steel .........................................................................................84 2.3.1.2 Cemented Tungsten Carbide ........................................................................84 2.3.1.3 Ceramic Cutting Tools.................................................................................85 2.3.1.4 Cermet Cutting Tools...................................................................................85 2.3.1.5 Cubic Boron Nitride.....................................................................................85 2.3.1.6 Whisker-Reinforced Materials.....................................................................85 2.3.2 Turning........................................................................................................................86 2.3.3 Milling ........................................................................................................................87 2.3.4 Drilling .......................................................................................................................87 2.3.5 Grinding......................................................................................................................88 2.3.6 Nontraditional Machining ..........................................................................................88 2.4 Case Study: Process and Tool Wear in Cutting Inconel 718 ..................................................89 2.4.1 Tool Wear Monitoring ................................................................................................89 2.4.2 Experimental Study .................................................................................................... 91 2.4.3 Development of a Tool Wear Model...........................................................................98 2.4.4 Adaptive Machining Using Outer Feedback Loop Control...................................... 104 2.5 Concluding Remarks ............................................................................................................ 105 References...................................................................................................................................... 106
78 Aerospace Materials Handbook are primarily used in gas turbines,coal conversion plants,and chemical process industries,and for other specialized applications requiring heat and/or corrosion resistance. There are a great variety of superalloys.Nickel-based superalloys,which are based on Ni,Cr,Al, Ti,Mo,and C.are precipitation hardened by precipitation of a Ni Al-like y'phase that is rich in Al and Ti [3].Most of these alloys contain substantial chromium for oxidation resistance;refractory metals for solid-solution strengthening;small amounts of grain-boundary-strengthening elements, such as carbon,boron,hafnium,and/or zirconium;and aluminum and titanium for strengthening by precipitation of an Ni(Al,Ti)compound known as gamma prime during age-hardening.A great variety of cast and wrought alloys are available.Among the well-known wrought alloys are D-979; GMR-235-D:N102:Inconel625,700,706,718,722,X750,and751;MAR-M200and412:Rene 41,95,and 100;Udimet 500 and 700;and Waspaloy.Cast alloys include B-1900;GMR-235-D;IN 100,162,738,and792:M252:MAR-M200,246,and42l;Nicrotung:Rene41,77,80,and100: and Udimet 500 and 700.Some wrought alloys are also suitable for casting,primarily investment casting.In recent years,considerable attention has been focused on the use of powder metallurgy techniques as a means of attaining greater compositional uniformity and finer grain size [4]. Nickel-based superalloys,primarily developed for high-temperature structural applications,are widely used in aerospace industry,especially for the gas turbine aeroengine.To meet the require- ments of harsh and corrosive working environments in aircraft engines and gas turbines,new engineering materials such as nickel-based superalloys or titanium alloys have been increasingly introduced for high-strength and high-temperature components,such as turbine blades or turbine rotors [5]. The operating temperatures of gas turbine aeroengine have kept rising steadily ever since its invention,and the trend is set to continue.This is driven by the need to increase the thrust,decrease the fuel consumption,and reduce the emissions.The modern jet engine relies very heavily upon superalloys to withstand the significant loads and temperatures developed during operation.Nickel- based superalloys are the materials of choice in the engine with high operating temperatures.For example,the turbine rotor is a set of massive disks of nickel-based superalloy that is especially composed to survive that environment.It is noted that the turbine disks represent about 20%of its total weight and their cost accounts for about 10%of the engine's value upon entry into service. Aeroengine manufacturers are under increasing pressure to develop and adopt new machining technologies to reduce cost and increase profit margin.Machining plays a vital role in superalloy part manufacture.Figure 2.1 shows an example of machined aircraft engine blades and Figure 2.2 shows an aircraft turbine engine blade being machined.Nickel-based superalloys possess excellent creep-rupture strength up to very high temperatures of 700C as well as high resistance against cor- rosion and fatigue.Unfortunately,nickel-based alloys tend toward work-hardening and adhesion, and are very hard-to-machine compared with conventional steels.Hence,the machining costs are considerably high. FIGURE 2.1 Example for aircraft engine turbine blades
78 Aerospace Materials Handbook are primarily used in gas turbines, coal conversion plants, and chemical process industries, and for other specialized applications requiring heat and/or corrosion resistance. There are a great variety of superalloys. Nickel-based superalloys, which are based on Ni, Cr, Al, Ti, Mo, and C, are precipitation hardened by precipitation of a Ni3Al-like γ ′ phase that is rich in Al and Ti [3]. Most of these alloys contain substantial chromium for oxidation resistance; refractory metals for solid-solution strengthening; small amounts of grain-boundary-strengthening elements, such as carbon, boron, hafnium, and/or zirconium; and aluminum and titanium for strengthening by precipitation of an Ni3(Al,Ti) compound known as gamma prime during age-hardening. A great variety of cast and wrought alloys are available. Among the well-known wrought alloys are D-979; GMR-235-D; IN 102; Inconel 625, 700, 706, 718, 722, X750, and 751; MAR-M 200 and 412; Rene 41, 95, and 100; Udimet 500 and 700; and Waspaloy. Cast alloys include B-1900; GMR-235-D; IN 100, 162, 738, and 792; M252; MAR-M 200, 246, and 421; Nicrotung; Rene 41, 77, 80, and 100; and Udimet 500 and 700. Some wrought alloys are also suitable for casting, primarily investment casting. In recent years, considerable attention has been focused on the use of powder metallurgy techniques as a means of attaining greater compositional uniformity and finer grain size [4]. Nickel-based superalloys, primarily developed for high-temperature structural applications, are widely used in aerospace industry, especially for the gas turbine aeroengine. To meet the requirements of harsh and corrosive working environments in aircraft engines and gas turbines, new engineering materials such as nickel-based superalloys or titanium alloys have been increasingly introduced for high-strength and high-temperature components, such as turbine blades or turbine rotors [5]. The operating temperatures of gas turbine aeroengine have kept rising steadily ever since its invention, and the trend is set to continue. This is driven by the need to increase the thrust, decrease the fuel consumption, and reduce the emissions. The modern jet engine relies very heavily upon superalloys to withstand the significant loads and temperatures developed during operation. Nickelbased superalloys are the materials of choice in the engine with high operating temperatures. For example, the turbine rotor is a set of massive disks of nickel-based superalloy that is especially composed to survive that environment. It is noted that the turbine disks represent about 20% of its total weight and their cost accounts for about 10% of the engine’s value upon entry into service. Aeroengine manufacturers are under increasing pressure to develop and adopt new machining technologies to reduce cost and increase profit margin. Machining plays a vital role in superalloy part manufacture. Figure 2.1 shows an example of machined aircraft engine blades and Figure 2.2 shows an aircraft turbine engine blade being machined. Nickel-based superalloys possess excellent creep-rupture strength up to very high temperatures of 700°C as well as high resistance against corrosion and fatigue. Unfortunately, nickel-based alloys tend toward work-hardening and adhesion, and are very hard-to-machine compared with conventional steels. Hence, the machining costs are considerably high. FIGURE 2.1 Example for aircraft engine turbine blades
Tool Condition Monitoring in Machining Superalloys 79 FIGURE 2.2 An aircraft turbine engine blade under machining. In machining Inconel material as well as other nickel-based alloys,high tool wear rates occur due to very high time-varying mechanical and thermal loads exerted on the cutting edges of the cutter.As tool wear deteriorates accuracy and surface finish of the parts being machined,the maxi- mum length that can be machined is thus very short,and tools have to be replaced sooner.If cut- ting tools are not replaced in time,tool breakage may occur.The workpiece being machined may become a scrap then.In addition,tool breakage may also cause severe damage on the machine tool itself.Repairing a machine tool or removing the causes for process interruption costs production time and money. 2.1.1 MICROSTRUCTURE The most important characteristics of nickel as an alloy base are the high phase stability of the face- centered cubic (fcc)nickel matrix,and the capability to be strengthened by different approaches. The surface stability of nickel is readily improved by alloying with chromium and/or aluminum [2]. The atomic number of nickel is 28,with an atomic weight of 58.71.Its crystal structure is fcc, as shown in Figure 2.3,from ambient conditions to the melting point,1455C,which represents an absolute limit for the temperature capability of the nickel-based superalloys.The density under ambient conditions is 8907 kg/m3 [6]. FIGURE 2.3 Face-centered cubic lattice
Tool Condition Monitoring in Machining Superalloys 79 In machining Inconel material as well as other nickel-based alloys, high tool wear rates occur due to very high time-varying mechanical and thermal loads exerted on the cutting edges of the cutter. As tool wear deteriorates accuracy and surface finish of the parts being machined, the maximum length that can be machined is thus very short, and tools have to be replaced sooner. If cutting tools are not replaced in time, tool breakage may occur. The workpiece being machined may become a scrap then. In addition, tool breakage may also cause severe damage on the machine tool itself. Repairing a machine tool or removing the causes for process interruption costs production time and money. 2.1.1 Microstructure The most important characteristics of nickel as an alloy base are the high phase stability of the facecentered cubic (fcc) nickel matrix, and the capability to be strengthened by different approaches. The surface stability of nickel is readily improved by alloying with chromium and/or aluminum [2]. The atomic number of nickel is 28, with an atomic weight of 58.71. Its crystal structure is fcc, as shown in Figure 2.3, from ambient conditions to the melting point, 1455°C, which represents an absolute limit for the temperature capability of the nickel-based superalloys. The density under ambient conditions is 8907 kg/m3 [6]. FIGURE 2.2 An aircraft turbine engine blade under machining. FIGURE 2.3 Face-centered cubic lattice
80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions:aluminum,titanium,and chromium reduce density,whereas tungsten,rhenium,and tantalum increase it.The corrosion resistance of superal- loys depends primarily on the alloying elements added,particularly chromium and aluminum,and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composi- tion and the processing conditions that control the state of microstructure.The microstructure of a typical superalloy consists of different phases.The major phases that may be present in most nickel- based superalloys are as follows [2,7-9]: Gamma matrix (Y).The continuous matrix is a fcc nickel-based nonmagnetic phase that usu- ally contains a high percentage of solid-solution elements such as cobalt,iron,chromium, molybdenum,and tungsten.All nickel-based alloys contain this phase as the matrix. Gamma prime (y).This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc y'[Ni(Al,Ti)]. which precipitates coherently with the austenitic gamma matrix.Other elements,particu- larly niobium,tantalum,and chromium,also enter y.This phase is required for high-tem- perature strength and creep resistance.Gamma prime phase has an ordered Ll2 structure, which coherently precipitates in the austenitic gamma phase.The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the y'to pre- cipitate homogeneously throughout the matrix and have long-time stability.It is interesting that the flow stress of y'increases with increasing temperature up to about 650C.Other intermetallics behave in a similar way;the flow stress increases with temperature.This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime(Y").It is a body-centered tetragonal (bct)phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum.In this phase, nickel and niobium combine in the presence of iron to form bct NiaNb,which is coherent with the gamma matrix,while including large mismatch strains of the order of 2.9%.This phase provides very high strength at low to intermediate temperatures,but is unstable at temperatures above about 650C.This precipitate is found in nickel-iron alloys. Carbides.Carbon is added in an amount of about 0.02-0.2 wt%;combining with reactive elements,such as titanium,tantalum,hafnium,and niobium to form metal carbides(MC). During heat treatment and service,these MC carbides tend to decompose and generate other carbides,such as M23C6 and/or M6C,which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fec crystal structure.It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries,though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases.These are generally undesirable,brittle phases that can form during heat treatment or service.The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as o,u,and Laves that may form for some compositions and under certain conditions.These cause lowered rupture strength and ductility.The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of yly'relationship,the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance,and the introduction of a range of novel processing techniques such as directional modification,single crystal technology,powder processing,mechanical alloying,and so on [9]
80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions: aluminum, titanium, and chromium reduce density, whereas tungsten, rhenium, and tantalum increase it. The corrosion resistance of superalloys depends primarily on the alloying elements added, particularly chromium and aluminum, and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composition and the processing conditions that control the state of microstructure. The microstructure of a typical superalloy consists of different phases. The major phases that may be present in most nickelbased superalloys are as follows [2,7–9]: Gamma matrix (γ). The continuous matrix is a fcc nickel-based nonmagnetic phase that usually contains a high percentage of solid-solution elements such as cobalt, iron, chromium, molybdenum, and tungsten. All nickel-based alloys contain this phase as the matrix. Gamma prime (γ ′). This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc γ ′ [Ni3(Al,Ti)], which precipitates coherently with the austenitic gamma matrix. Other elements, particularly niobium, tantalum, and chromium, also enter γ ′. This phase is required for high-temperature strength and creep resistance. Gamma prime phase has an ordered L12 structure, which coherently precipitates in the austenitic gamma phase. The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the γ ′ to precipitate homogeneously throughout the matrix and have long-time stability. It is interesting that the flow stress of γ ′ increases with increasing temperature up to about 650°C. Other intermetallics behave in a similar way; the flow stress increases with temperature. This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime (γ″). It is a body-centered tetragonal (bct) phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum. In this phase, nickel and niobium combine in the presence of iron to form bct Ni3Nb, which is coherent with the gamma matrix, while including large mismatch strains of the order of 2.9%. This phase provides very high strength at low to intermediate temperatures, but is unstable at temperatures above about 650°C. This precipitate is found in nickel–iron alloys. Carbides. Carbon is added in an amount of about 0.02–0.2 wt%; combining with reactive elements, such as titanium, tantalum, hafnium, and niobium to form metal carbides (MC). During heat treatment and service, these MC carbides tend to decompose and generate other carbides, such as M23C6 and/or M6C, which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fcc crystal structure. It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries, though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases. These are generally undesirable, brittle phases that can form during heat treatment or service. The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as σ, μ, and Laves that may form for some compositions and under certain conditions. These cause lowered rupture strength and ductility. The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of γ/γ ′ relationship, the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance, and the introduction of a range of novel processing techniques such as directional modification, single crystal technology, powder processing, mechanical alloying, and so on [9]
Tool Condition Monitoring in Machining Superalloys 81 2.1.2 CLASSIFICATIONS Superalloys can be classified into three types as nickel-iron-(or iron-nickel-),nickel-,and cobalt- based superalloys.They may be further subdivided into cast and wrought.The main characteristics of nickel as an alloy base are the high phase stability of the fcc nickel matrix and the capability to be strengthened by different means.Many nickel-based superalloys contain significant amounts of chromium,cobalt,aluminum,and titanium,and small amounts of boron,zirconium,hafnium, and carbon.There are also common additions like molybdenum,niobium,tantalum,rhenium,and tungsten which work as strengthening solutes and carbide formers.Certain superalloys,referred to as nickel-iron superalloys such as IN718 and IN706,contain significant proportions of iron [10,11]. Nickel-based superalloys typically consist of y'dispersed in a y matrix.The strength increases with increasing y'volume fraction.y'causes strengthening through the necessity to disorder the particles as they are shared,while y"strengthens by virtue of high coherency strain in the lattice.In Inconel 718,y"often precipitates together with y,but y"is the principal strengthening phase under such circumstances. In high-temperature service,the properties of the grain boundaries are very important.Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal car- bides;small additions of Zr and B improve the morphology and stability of these carbides.Optimum properties are developed by multistage heat treatment [12]. When nickel-based alloys are loaded under high temperature,plastic strain will accumulate over time by the process of creep.Creep strengthening in polycrystalline nickel alloys arises both from solid-solution strengthening due to the presence of solute atoms and from precipitation hardening due to phases such as y'[6]. Superalloys are available in cast or wrought forms,where wrought includes powder metallurgy processing.Wrought alloys generally are considered more ductile than cast alloys.On the other hand,castings are intrinsically stronger than forgings at elevated temperature.A principle for super- alloys selection is to choose wrought alloys for intermediate-temperature applications where homo- geneity and ductility is desired,and cast alloys for high-temperature applications.Intermediate temperatures imply a range from about 1000F up to about 1400F(540C up to 760C),while high temperature can be considered to be about 1500F(816C)and up to the melting point of an alloy [1,5].Recently the use of powder metallurgy techniques has attracted considerable attention as a means of attaining greater compositional uniformity and finer grain size. 2.1.3 STRENGTHENING MECHANISMS There are different types of strengthening in superalloys,which include solid-solution hardening (substituted atoms interfere with deformation),work hardening (energy is stored by deformation), precipitation hardening (precipitates interfere with deformation),and carbide production as well. Carbides or other ceramics act as dispersion strengthening or second phase strengthening. Typical solid-solution alloys include Hastelloy X;Inconel 600,601,604,617,615,625,783; RA333,and so on.Precipitates strengthen an alloy by impeding the deformation process that takes place under load.The precipitation-strengthened alloys are the most numerous.Inconel X-750, Inconel 718,and IN-100 are famous examples.Other precipitation-strengthened wrought alloys include Astroloy;D-979;IN 102;Inconel 706 and 751;M252;Nimonic 80A,90,95,100,105. 115,and 263;Rene 41,95,and 100;Udimet 500,520,630,700,and 710;Unitemp AF2-1DA; and Waspaloy.Other cast alloys,mainly investment-cast,include B-1900;IN-738;IN-792;Inconel 713C:M252:MAR-M200,246.247,and421:NX-188;Rene77,80,and100:Udimet500.700,and 710;Waspaloy;and WAZ-20 [4]. Generally,the creep-rupture strengths of the iron-nickel-based alloys and the nickel-based solid- solution strengthened alloys are considerably lower than those of the nickel-based precipitation strengthened and carbide-hardened cobalt-based alloys at temperatures above about 1200F(649C)
Tool Condition Monitoring in Machining Superalloys 81 2.1.2 Classifications Superalloys can be classified into three types as nickel–iron- (or iron–nickel-), nickel-, and cobaltbased superalloys. They may be further subdivided into cast and wrought. The main characteristics of nickel as an alloy base are the high phase stability of the fcc nickel matrix and the capability to be strengthened by different means. Many nickel-based superalloys contain significant amounts of chromium, cobalt, aluminum, and titanium, and small amounts of boron, zirconium, hafnium, and carbon. There are also common additions like molybdenum, niobium, tantalum, rhenium, and tungsten which work as strengthening solutes and carbide formers. Certain superalloys, referred to as nickel–iron superalloys such as IN718 and IN706, contain significant proportions of iron [10,11]. Nickel-based superalloys typically consist of γ ′ dispersed in a γ matrix. The strength increases with increasing γ ′ volume fraction. γ ′ causes strengthening through the necessity to disorder the particles as they are shared, while γ ′′ strengthens by virtue of high coherency strain in the lattice. In Inconel 718, γ ′′ often precipitates together with γ ′, but γ ′′ is the principal strengthening phase under such circumstances. In high-temperature service, the properties of the grain boundaries are very important. Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal carbides; small additions of Zr and B improve the morphology and stability of these carbides. Optimum properties are developed by multistage heat treatment [12]. When nickel-based alloys are loaded under high temperature, plastic strain will accumulate over time by the process of creep. Creep strengthening in polycrystalline nickel alloys arises both from solid-solution strengthening due to the presence of solute atoms and from precipitation hardening due to phases such as γ ′ [6]. Superalloys are available in cast or wrought forms, where wrought includes powder metallurgy processing. Wrought alloys generally are considered more ductile than cast alloys. On the other hand, castings are intrinsically stronger than forgings at elevated temperature. A principle for superalloys selection is to choose wrought alloys for intermediate-temperature applications where homogeneity and ductility is desired, and cast alloys for high-temperature applications. Intermediate temperatures imply a range from about 1000°F up to about 1400°F (540°C up to 760°C), while high temperature can be considered to be about 1500°F (816°C) and up to the melting point of an alloy [1,5]. Recently the use of powder metallurgy techniques has attracted considerable attention as a means of attaining greater compositional uniformity and finer grain size. 2.1.3 Strengthening Mechanisms There are different types of strengthening in superalloys, which include solid-solution hardening (substituted atoms interfere with deformation), work hardening (energy is stored by deformation), precipitation hardening (precipitates interfere with deformation), and carbide production as well. Carbides or other ceramics act as dispersion strengthening or second phase strengthening. Typical solid-solution alloys include Hastelloy X; Inconel 600, 601, 604, 617, 615, 625, 783; RA333, and so on. Precipitates strengthen an alloy by impeding the deformation process that takes place under load. The precipitation-strengthened alloys are the most numerous. Inconel X-750, Inconel 718, and IN-100 are famous examples. Other precipitation-strengthened wrought alloys include Astroloy; D-979; IN 102; Inconel 706 and 751; M252; Nimonic 80A, 90, 95, 100, 105, 115, and 263; René 41, 95, and 100; Udimet 500, 520, 630, 700, and 710; Unitemp AF2-1DA; and Waspaloy. Other cast alloys, mainly investment-cast, include B-1900; IN-738; IN-792; Inconel 713C; M252; MAR-M 200, 246, 247, and 421; NX-188; René 77, 80, and 100; Udimet 500, 700, and 710; Waspaloy; and WAZ-20 [4]. Generally, the creep-rupture strengths of the iron–nickel-based alloys and the nickel-based solidsolution strengthened alloys are considerably lower than those of the nickel-based precipitation strengthened and carbide-hardened cobalt-based alloys at temperatures above about 1200°F (649°C)