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 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 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 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 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 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 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 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 1200°F (649°C)