82 Aerospace Materials Handbook Although oxide-dispersion-strengthened (ODS)nickel-based superalloys generally are not as strong as precipitation-strengthened nickel-based superalloys,they have a much flatter rate of creep-rup- ture strength reduction with time than the precipitation-strengthened alloys [5]. The modern superalloys are made and used as single crystals(monocrystals).They are extra alloyed,especially with ruthenium,and can operate up to 1100C.The introduction of improved casting methods,such as processing by directional solidification,enabled significant improvements to be made.In directionally solidified castings,the grain runs only unidirectionally,as along the length of the turbine blades.Eliminating transverse grains improves stress-rupture properties and fatigue resistance.Single-crystal alloys,which are in fact grain boundary-free,have also been cast, further improving high-temperature creep resistance.The grain boundaries are completely removed such that monocrystalline (single-crystal)superalloys are produced.Single-crystal directionally solidified casting technology,pioneered by Pratt Whitney mainly for aircraft-engine turbine blades,has extended the useful temperature range of alloys.Single-crystal directionally solidified alloys are in wide use in aircraft gas turbines at the present time and are expected to see significant use in industrial gas turbines [4]. 2.2 MACHINABILITY OF SUPERALLOYS It is important to select cutting tools and cutting conditions for optimum economic and technological machining performance.Machinability is used to indicate the ease or difficulty with which a mate- rial can be machined to the size,shape,and desired surface finish relative to the cost [2].Although it has been used for many years,the term machinability is in fact an ambiguous one,which may have a variety of different meanings.There is no standard or universally accepted method of mea- suring machinability.In general,low energy consumption,short(broken)chips,smooth finish and long tool life are aspects of good machinability.Some of these aspects are directly related to the continuum mechanical and thermal conditions of the machining process [13-16]. The machinability of a material is affected by many factors,such as the composition,microstructure, and strength level;the feeds,speeds,and depth of cut;and the choice of cutting fluid and cutting tool material.These machining characteristics,in turn,affect the cost of producing superalloy parts, particularly when the cost of machining represents a major part of the cost of the finished part. During the machining operation,some factors can be used to evaluate the machinability,such as cut- ting force,tool wear,temperature,tool-workpiece vibration,and machined surface integrity. Superalloys have an austenitic structure which imparts properties of high ductility and work hardening.Work hardening occurs when the metal ahead of the cutting tool is plastically deformed. This hardened layer is very hard to penetrate in subsequent passes or following operations.The superalloys designed for high-temperature applications remain strong at the temperatures of chip formation during machining,and thermal conductivity is much less than that of steel and many other materials.The age hardening nickel alloys also contain abrasive titanium and aluminum particles. These factors make nickel alloys more difficult to machine than steel,and it is an understanding of the extent to which each nickel alloy is affected by these factors which is the key to their successful machining [17]. Machinability ratings can be measured based on cutting speed or metal removal rate.On the other hand,there are also ratings that permit estimations of machining costs and shop load for production scheduling which are more useful.The evaluation and judgments of machinability have been historically based on one or more of the following major machining performance criteria: Tool life:Measured by the amount of material that can be removed by a standard cutting tool under standard cutting conditions before tool performance becomes unacceptable or tool wear reaches a specified amount. Cutting speed:Measured by the maximum speed at which a standard tool under standard conditions can continue to provide satisfactory performance for a specified period.82 Aerospace Materials Handbook Although oxide-dispersion-strengthened (ODS) nickel-based superalloys generally are not as strong as precipitation-strengthened nickel-based superalloys, they have a much flatter rate of creep-rup￾ture strength reduction with time than the precipitation-strengthened alloys [5]. The modern superalloys are made and used as single crystals (monocrystals). They are extra alloyed, especially with ruthenium, and can operate up to 1100°C. The introduction of improved casting methods, such as processing by directional solidification, enabled significant improvements to be made. In directionally solidified castings, the grain runs only unidirectionally, as along the length of the turbine blades. Eliminating transverse grains improves stress-rupture properties and fatigue resistance. Single-crystal alloys, which are in fact grain boundary-free, have also been cast, further improving high-temperature creep resistance. The grain boundaries are completely removed such that monocrystalline (single-crystal) superalloys are produced. Single-crystal directionally solidified casting technology, pioneered by Pratt & Whitney mainly for aircraft-engine turbine blades, has extended the useful temperature range of alloys. Single-crystal directionally solidified alloys are in wide use in aircraft gas turbines at the present time and are expected to see significant use in industrial gas turbines [4]. 2.2  MACHINABILITY OF SUPERALLOYS It is important to select cutting tools and cutting conditions for optimum economic and technological machining performance. Machinability is used to indicate the ease or difficulty with which a mate￾rial can be machined to the size, shape, and desired surface finish relative to the cost [2]. Although it has been used for many years, the term machinability is in fact an ambiguous one, which may have a variety of different meanings. There is no standard or universally accepted method of mea￾suring machinability. In general, low energy consumption, short (broken) chips, smooth finish and long tool life are aspects of good machinability. Some of these aspects are directly related to the continuum mechanical and thermal conditions of the machining process [13–16]. The machinability of a material is affected by many factors, such as the composition, microstructure, and strength level; the feeds, speeds, and depth of cut; and the choice of cutting fluid and cutting tool material. These machining characteristics, in turn, affect the cost of producing superalloy parts, particularly when the cost of machining represents a major part of the cost of the finished part. During the machining operation, some factors can be used to evaluate the machinability, such as cut￾ting force, tool wear, temperature, tool-workpiece vibration, and machined surface integrity. Superalloys have an austenitic structure which imparts properties of high ductility and work hardening. Work hardening occurs when the metal ahead of the cutting tool is plastically deformed. This hardened layer is very hard to penetrate in subsequent passes or following operations. The superalloys designed for high-temperature applications remain strong at the temperatures of chip formation during machining, and thermal conductivity is much less than that of steel and many other materials. The age hardening nickel alloys also contain abrasive titanium and aluminum particles. These factors make nickel alloys more difficult to machine than steel, and it is an understanding of the extent to which each nickel alloy is affected by these factors which is the key to their successful machining [17]. Machinability ratings can be measured based on cutting speed or metal removal rate. On the other hand, there are also ratings that permit estimations of machining costs and shop load for production scheduling which are more useful. The evaluation and judgments of machinability have been historically based on one or more of the following major machining performance criteria: • Tool life: Measured by the amount of material that can be removed by a standard cutting tool under standard cutting conditions before tool performance becomes unacceptable or tool wear reaches a specified amount. • Cutting speed: Measured by the maximum speed at which a standard tool under standard conditions can continue to provide satisfactory performance for a specified period