MIM of nickel-base superalloys(24.1-24.3)

Table of Contents

• 24.1 Introduction

• 24.2 MIM processing of nickel-base superalloys

• 24.2.1 Powder preparation and powder quality

• 24.2.2 Feedstock preparation and injection molding

• 24.2.3 Debinding and sintering

• 24.2.4 Post processing (HIP, heat treatment)

• 24.3 Specific nickel-base superalloys produced by MIM

• 24.3.1 Udimet 700 (U700)

• 24.3.2 Udimet 720 (U720)/Udimet 720Li (U720Li)

• 24.3.3 Hastelloy X (HX)

• 24.3.4 Nimonic 90

• 24.3.5 Inconel 625 (IN 625)

• 24.3.6 Inconel 718 (IN 718)

• 24.3.7 Inconel 713 (IN 713)

• 24.3.8 Mar-M247 and CM 247 LC

24.1 Introduction

Nickel-base superalloys are corrosion resistant high-temperature alloys typically used at service temperatures above 500℃. 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. 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.

The first nickel-base superalloys were polycrystalline wrought alloys. By introducing vacuum melting, cast nickel-base superalloys were becoming increasingly important. Starting with polycrystalline cast alloys, casting techniques and alloys were further developed and directionally solidified and single-crystalline high-strength superalloys could be manufactured. Powder metallurgy (PM) of nickel-base superalloys was introduced in the 1960s. Conventional PM manufacturing consisting of hot isostatic pressing (HIP) and subsequent forging is typically used for parts that cannot be made by ingot metallurgy, e.g., because of high amounts of certain alloying elements or when high alloy homogeneity, density and uniform grain size are required.

Metal injection molding (MIM) of nickel-base superalloys is a comparatively new manufacturing technology which traces back to the late 1980s. As highly automated near-net shape process, MIM is a promising complementary or even alternative production technology for polycrystalline superalloy parts in large quantities. Homogeneous microstructures and good mechanical properties can be achieved. Macrosegregation problems that occur during the solidification of high-strength nickel-base superalloys can be avoided. Moreover, costly and time-consuming machining steps can be reduced compared to conventional manufacturing techniques.

During the last 25 years, MIM capability of several nickel-base superalloys like Inconel 625 (IN 625), Inconel 713 (IN 713), Inconel 718 (IN 718), Nimonic 90, Udimet 720 (U720), and Mar-M247 has been investigated. According to a review, nickel-base superalloys account for about 2% of global MIM sales. MIM of nickel-base superalloys shows promising results, i.e., high densities after sintering and good mechanical properties at room temperature have been reported so far. Unfortunately, only limited information is available in literature regarding high temperature mechanical properties including creep and fatigue tests.

24.2 MIM processing of nickel-base superalloys

MIM of nickel-base superalloys requires special precautions in processing compared to other types of materials. This concerns finding the right sintering parameters, control of impurity pick-up and adjusting the microstructure during sintering and subsequent heat treatments to achieve good high temperature properties.

24.2.1 Powder preparation and powder quality

Nickel-base superalloy powders for MIM are usually produced by argon inert gas atomization to avoid oxidation of the alloy, because surface oxides would reduce the sintering activity leading to a lower density. As starting material for the atomization, a conventional high-quality ingot of the alloy is used. To obtain the desired particle size distribution, the powder is sieved after atomization. A typical nickel-base superalloy powder for MIM is shown in Fig. 24.1.

Fig. 24.1 Argon inert gas atomized nickel-base superalloy CM 247 LC MIM powder with mostly spherical powder particles

Optimum powder filling can be achieved with spherical powder morphology and particle sizes below 45μm. Powders that do not fulfill these criteria require a higher binder content, which leads to increased shrinkage and consequently a higher risk for uneven densification and distortion of the final part.

Argon atomization is known to introduce voids in powder particles filled with entrapped gas. Such powders should be avoided. As powder porosity is mostly observed for coarser particles (100μm), it is usually not an issue for the typical fine powders used for MIM. Moreover, the use of blended powders, i.e., mixtures of elemental powders or mixtures of prealloyed powders of different chemical compositions poses a great challenge in MIM of nickel-base superalloys. Risks from oxidation and uneven shrinkage must be controlled.

One threat for the strength of MIM parts are inclusions and impurities. Ceramic impurities can be released by the ceramic crucible used during atomization. Small splinters may break away from the atomization nozzle and contaminate the powder. The use of ceramic filters in front of the atomization nozzle helps to reduce and limit the size of the ceramic impurities of the crucible. These imperfections are further reduced to a defined limit by sieving.

Using the right powder that fulfills all MIM requirements is essential. Thus, a thorough powder characterization should be done in development phase as well as for serial production to guarantee that the powder has the desired particle size, particle morphology, density, and chemistry.

24.2.2 Feedstock preparation and injection molding

For nickel-base superalloys, feedstock preparation and injection molding are basically the same as for other types of metals. Comprehensive information about these two steps can be found in Chapters 4-6.

The uptake of elements like carbon, oxygen and nitrogen from the binder can have a negative impact on the microstructure and mechanical properties of nickel-base superalloys. Therefore, it is recommended to choose a suitable binder system that enables almost complete removal of the binder.

By using an oxygen-free binder system or a binder system where oxygen containing components are extracted and dissolved during solvent debinding prior to thermal debinding an oxygen pick-up can be reduced to a minimum or in best case avoided. Up to date, several different binder systems have been successfully used for MIM of nickel-base superalloys. Typical binder systems are composed of wax, typically paraffin wax, or a water soluble component like polyethylene glycol (PEG) and a backbone polymer. Commonly used backbone polymers are polyethylene, polypropylene, and ethylene-vinyl-acetate. Like for other metals, stearic acid is often added as dispersant. Acetal-based binder systems are also often used for nickelbase superalloys.

24.2.3 Debinding and sintering

In the course of MIM processing, impurities like carbon and oxygen may be picked-up from the binder or the atmosphere during thermal debinding and sintering. For nickelbase superalloys, this impurity pick-up is generally undesirable. Carbon and oxygen contents have to be controlled within narrow limits as they can be detrimental to the mechanical properties and the oxidation resistance. Thus, it is necessary to reduce the contamination by these elements to the minimum extent possible. To realize this demand, the selection of binder-components, the preparation and handling of the feedstock as well as parameter selection for thermal debinding and sintering need extra attention.

Furthermore, the alloy composition may also influence the pick-up of impurities, as some reactive elements like hafnium have a very high tendency to react with carbon and oxygen. Picked-up carbon and oxygen are usually visible in the form of additional carbides and oxides, often at prior particle boundaries (PPBs). Fig. 24.2 shows a PPB covered by carbides and oxides in IN 713.

Fig. 24.2 Prior particle boundary (PPB) in IN 713 alloy produced by MIM

Even though carbides on grain boundaries can contribute beneficially to high temperature strength, too high amounts of high-temperature stable carbides on grain boundaries and inside grains can have detrimental effects on mechanical properties by, e.g., being initiation sites for cracks.

The acceptable maximum size and concentration of carbides are determined by extensive static and dynamic strength investigations of samples and parts. Compliance with specifications is monitored by metallographic examinations. A consistent process control of the furnace atmosphere and sintering temperature helps to ensure the defined quality standard.

Besides carbon and oxygen, phosphorus, and sulfur are impurities that can be picked-up from the binder as well. Both of these elements are known for having a detrimental effect on the mechanical properties of nickel-base superalloys. One of the main reasons for this is, that they both can form a very low melting eutectic (635℃ eutectic in the binary system Ni-S and 870℃ eutectic in the binary system Ni-P), which can cause liquation cracking under load at high temperatures.

During sintering and densification, linear shrinkage in the range of approx. 15%-22% occurs, mainly depending on the powder/binder volume content. Obviously, control of the sintering process is a key success factor for final part geometries. A successful sintering process for high quality parts is characterized by a uniform temperature distribution and a well-controlled atmosphere within the sintering furnace and an optimized sintering temperature program to achieve high densities in the as-sintered condition.

Dilatometry, differential scanning calorimetry (DSC) analysis and thermodynamic calculation of phase transition temperatures like the solidus temperature can help to find a suitable sintering temperature. Usually very high temperatures close to the solidus temperature are needed for sintering of superalloys (Fig. 24.3). High contents of alloying elements like aluminum lead to oxidation and reduction of the solid-state sintering rate. If the sintering temperature is slightly above the solidus temperature of the alloy, a liquid phase is formed during sintering and the process would then be termed as supersolidus liquid phase sintering (SLPS). In general, there is only a narrow temperature range for good sintering results.

Fig. 24.3 Porosity of MIM CMSX-4 sintered at different temperatures with a higher sinteringtemperature leading to a lower porosity: (A) 1300℃, 5.8% porosity. (B) 1310℃, 4.5% porosity.(C) 1320℃, 2.7% porosity. (D) 1330℃, 1.6% porosity. (Solidus temperature of CMSX-4≈1325℃.)

During sintering the microstructure of superalloys, especially the grain size, needs to be controlled. Due to the fine powder used for MIM, the grain size is typically also very fine in the as-sintered condition, but for high temperature applications where creep resistance is required a coarse grain structure would be beneficial. Grain growth can be influenced during sintering by adjusting sintering temperature and sintering time. Moreover, both the sintering temperature and subsequent cooling rate have an impact on the size and morphology of precipitates like γ0 -particles, the main strengthening phase of most nickel-base superalloys, and carbides.

Special ceramic setters should be used in order to prevent shape deviations and distortions. Additionally, heating and cooling rates should be adjusted to restrain distortions and shrinkage stresses. These rates are typically in the range of 5-15 K/min, depending on part geometry and size. An inhomogeneous wall thickness distribution of the part is prone to cause shrinkage cracks. Overall tolerances are also influenced by shrinkage distortion. An adequate design of the setters eliminates shrinkage cracks and reduces distortion to an acceptable level. A reaction between the ceramic setter and the metal part should be avoided.

Typically, low porosities are desired for most applications. However, a certain level of porosity always remains after sintering. This can be seen in Fig. 24.3 for metal injection molded nickel-base alloy CMSX-4 for different sintering temperatures. If some of these residual pores are connected to the surface, they will not be closed by subsequent HIP. Isolated pores with a small size might be acceptable for application since the impact on mechanical properties is at an acceptable low level. Nevertheless, the detrimental effects of a cluster of such pores may be the same as those of one large imperfection.

24.2.4 Post processing (HIP, heat treatment)

Almost every microstructural feature of nickel-base superalloys can be influenced by post processing, i.e., HIP and/or heat treatments after sintering. These two post processing steps are often applied to nickel-base superalloy cast parts. The size and morphology of precipitates (carbides, γ0 -particles) can be adjusted, pores can be closed through the applied pressure during HIP and grain boundaries can move at high temperatures leading to grain coarsening. In case of cast alloys, heat treatment also leads to homogenization of the microstructure by dissolution of segregations. All these microstructural changes affect the mechanical properties.

HIP and heat treatments can also be used to improve the performance of MIM superalloy parts. However, a HIP treatment as well as a heat treatment leads to additional costs. Moreover, part distortion has to be considered.

The relative density of nickel-base superalloy parts after sintering is in the range of 96%-99%. If full densification is required for application, subsequent hot isostatically pressing can be used to close pores and small internal cracks. Fig. 24.4 shows the porosity of CM 247 LC manufactured by MIM after sintering at 1315℃ for 3 h (left) and after HIP at 1200℃ for 4 h and 103MPa (right). Typically, a relative density>99% can be achieved after HIP.

Fig. 24.4 Porosity of CM 247 LC produced by MIM in the as-sintered condition (left) and after hot isostatic pressing (right)

The overall carbide content of MIM nickel-base superalloys is depending on the powder carbon content as well as on additional carbon uptake during processing. Different types of carbides are observed for different nickel-base superalloys. MC, M6C, and M23C6 are the most common types of carbides. M is standing for the metallic component(s). Carbides can be precipitated inside the grains and at grain boundaries. Some of these carbides are very stable, others can be dissolved during heat treatment and precipitated again during cooling and aging (see Fig. 24.5). Thus, heat treatments can be used to change the carbide size and morphology, which influences the mechanical properties.

Fig. 24.5 Microstructure of as-sintered (left) and as-sintered and subsequently heat-treated (right) MIM IN 713LC (1220℃/2 h/furnace cooling and 925℃/4 h/air cooling) etched with V2A etchant.

A second more important phase in high-strength nickel-base superalloys is the γ0-phase. This phase is so important, because it retains its high strength and volume fraction even at elevated temperatures and thus makes nickel-base superalloys even useable at very high service temperatures. Like for carbides, there is an optimum size and morphology for the γ0-particles to achieve the best strengthening effect (Nathal, 1987). Therefore, solution and aging heat treatments are often applied to cast or forged superalloys and this can also be done with MIM parts. The γ-γ0-microstructure of superalloy CM 247 LC in the as-sintered condition and after heat treatment can be seen in Fig. 24.6. This more homogeneous γ-γ0-microstructure together with grain coarsening caused by the heat treatment led to improved creep properties compared to the as-sintered condition.

Fig. 24.6 γ-γ0-microstructure of CM 247 LC produced by MIM in the as-sintered condition (left) and after a solution (1260℃/2 h) and two-step aging (1080℃/4 h + 870℃/20 h) heat treatment (right).

Another important microstructural feature is the grain size. A small grain size and thus a high number of grain boundaries can improve strength and ductility of many metals at room temperature (Hall-Petch relation) and are also beneficial for high cycle fatigue resistance. At high temperature, however, they are considered to be the weak point of the material. In aero engines, the failure of equiaxed components usually occurs at the grain boundaries from a combination of creep, thermal fatigue, and oxidation. Therefore, during development of superalloy casting processes, it was important to reduce the grain boundary area perpendicular to the stress direction to improve the creep resistance, which resulted in directional solidification (DS) casting, or to even eliminate grain boundary area entirely, which resulted in single crystal (SC/ SX) casting.

MIM nickel-base superalloys usually have a very fine grain size after sintering due to the use of fine powders (Fig. 24.7, left). Heat treatments can be used for grain coarsening to reduce grain boundary area and thus to increase creep resistance (Fig. 24.7, right).

Fig. 24.7 Grain size of CM 247 LC produced by MIM in the as-sintered condition (left) and after a heat treatment (1290℃/20h) that is obviously leading to grain coarsening (right). The different colors are showing the grain orientation. It can be clearly seen, that the orientation is randomly distributed.

Other researchers also successfully applied heat treatments or HIP to improve the mechanical properties of nickel-base superalloys produced by MIM used HIP to increase the ductility and strength of several superalloys. By HIP of injection molded IN 713C, they could increase the room temperature elongation at fracture from 12% to 25%, the yield strength from 916 to 959 MPa and the ultimate tensile strength (UTS) from 1082 to 1375 MPa. Developed a heat treatment reducing the γ0-size to improve the yield strength of IN 713C over a wide range of temperatures.Could improve the strength of IN 718 by a solution and aging heat treatment increasing the yield strength from 506 to 780MPa and the UTS from 629 to 1022MPa used a solution and aging heat treatment to increase the yield strength of MIM418 superalloy from 756 to 1004MPa.

24.3 Specific nickel-base superalloys produced by MIM

Many different alloys have been processed byMIM in the last 30 years. The development of MIM parts and the investigations on metal injection molded nickel-base superalloys were mainly driven bythe aerospace andthe automotiveindustry. Inmost cases, common wrought or cast alloys like IN 718 or IN 713 are chosen and processed byMIM. However, data on production details and binder systems are rather scarce as they are often IP (Intellectual Property) controlled. According to the number of publications, the nickelbase superalloy IN 718 has been studied most intensively to date. This is understandable as IN 718 represents about half of superalloyworldtonnage. IN 718is alsothe only nickelbase superalloy for which an AMS material standard for MIM is available (AMS 5917). Further intensively studied materials are IN 625 and IN 713. For other alloys like, e.g., Udimet 700 (U700), Udimet 720 (U720), Udimet 720Li (U720Li), Hastelloy X (HX), IN 100, and Nimonic 90 only few literature is available.

The feasibility of processing the superalloy by MIM was the focus of most published studies. Further investigated topics are additional treatments like HIP and/or heat treatments in order to improve mechanical properties and the investigation of oxidation and corrosion behavior.

An overview on the most widely investigated nickel-base superalloys including mechanical data is presented in the following sections. The nominal chemical compositions of the alloys described later are given in Table 24.1. One should note that the carbon content of as-sintered MIM material may differ from the nominal composition due to carbon uptake during debinding.

Table 24.1 Nominal compositions (in wt%) of some common nickel-base superalloys

24.3.1 Udimet 700 (U700)

Investigated the producibility of Udimet 700 by MIM in the 1980s. They showed that similar properties compared to cast material can be achieved. For as-sintered and subsequently hot isostatically pressed material, high temperature tensile tests as well as creep tests have been performed. It was shown that for coarser powders with particle sizes smaller than 150μm 100 h creep strength very close to cast U700 material can be achieved.

24.3.2 Udimet 720 (U720)/Udimet 720Li (U720Li)

Udimet 720 is a nickel-base superalloy manufactured by conventional PM and subsequent forging or by forging of cast ingots. Typical Udimet 720 parts are, e.g., turbines disks.

Microstructure and elevated temperature tensile properties of metal injection molded and heat-treated (solution annealing: 1100℃/1 h, aging: 650℃/24 h + 760℃/16 h) Udimet 720 were investigated. The tensile properties at 650, 800, and 900℃ were inferior compared to previous tests on PM and cast samples. Detailed test results are listed in Table 24.2. This was explained by increased carbon content above specification values due to MIM processing and the resulting formation of crack-inducing carbides. Also the oxygen content was increased which can lead to oxide formation with negative effect on the mechanical properties. Udimet 720Li (Li: "Low interstitial") powder was used instead of Udimet 720. Moreover, the processing was optimized. A significant increase of 0.2% yield strength could be obtained due to decreased level of impurities (see Table 24.2). At temperatures up to 800℃ the difference is 72% between Udimet 720 and Udimet 720Li. The drop in yield strength above 800℃ occurs for both alloys. However, at 900℃ the difference is still about 70%. The tensile test data of MIM Udimet 720Li are in a similar range as material manufactured by alternative manufacturing routes.

24.3.3 Hastelloy X (HX)

Hastelloy X is a solid-solution-strengthened nickel-chromium-iron-molybdenum alloy that combines good oxidation resistance, high-temperature strength and exceptional stress-corrosion resistance which makes this alloy also interesting for petrochemical applications. Therefore, Hastelloy X was also the material selected for metal injection molded fuel nozzle applications. Metal injection molded HX was also using a HX powder with particle size <22μm. Via sintering at 1300℃ for 3 h under hydrogen atmosphere a relative density of 97.8% could be achieved. An additional HIP treatment (1185℃/100MPa/4 h) led to 100% densification. The tensile properties in the solution-annealed state were better than that of a cast alloy, but lower compared to wrought reference material. The tensile properties are summarized in Table 24.2.

24.3.4 Nimonic 90

Another precipitation-hardened forged nickel-base superalloy that can also be processed via MIM is Nimonic 90. Several research groups investigated the microstructure and tensile properties at room temperature in different states: As-sintered, sintered and heat-treated, hot isostatically pressed and heat-treated. By using a Nimonic 90 powder with particle size <22μm for MIM and sintering at 1325℃ for 3 h under argon atmosphere, relative densities in the range of 97.8% are reported. Applying an additional HIP treatment (1160-1185℃/100MPa/4 h) closes the remaining pores and increases the density to >99% theoretical density. This leads to an increase of UTS while the yield strength is reduced. A heat treatment consisting of solution annealing followed by and aging step (solution annealing: 1080℃/8 h, aging: 700℃/16 h) leads to an increase of both, yield strength and UTS, without any reduction of ductility. Applied solution annealing and aging as well as HIP followed by a solution annealing and an aging step. The combination of HIP and heat treatment led to the highest UTS and ductility. Taking only the data for heat-treated material into account, also the yield strength at room temperature is increased by combining HIP and heat treatment. The tensile data are summarized in Table 24.2. However, it has to be considered that tensile data for elevated temperatures would be more interesting for application.

24.3.5 Inconel 625 (IN 625)

IN 625 is a solid-solution-strengthened alloy, which is additionally strengthened by Mo/Nb-carbides. IN 625 is, e.g., used for chemical process equipment, jet engine exhaust systems and seawater equipment.

The microstructure and tensile properties of metal injection molded IN 625 in as-sintered and sintered and heat-treated state were investigated. By sintering at 1300℃ for 3 h, which is slightly above the solidus temperature of 1298℃, a relative density of 98.3% was achieved. The determined average room temperature yield strength and UTS of as-sintered material were 351MPa and 650MPa, respectively. A solution treatment led to a decrease of both yield strength (306MPa) and UTS (505MPa). Poorer tensile properties compared to cast and wrought reference material were also reported for sintered (1290℃/30min/hydrogen atmosphere; powder batch with D90 value of 16.9μm) and solution-treated (1150℃/2 h/oil quenching) MIM IN 625 alloy with a density of 99.5% of theoretical (see Table 24.2). However, by applying an aging step, 385MPa yield strength and 674MPa UTS could be achieved. The elongation at fracture was only reduced by about 4% compared to as-sintered material from 44.7% to 40.6%. The results show that for as-sintered and aged MIM IN 625 yield strengths in the range of cast alloy are possible. Also the ductility is very close to that of cast material.

Reported also tensile properties comparable to or better than those of cast and solution-treated material (see Table 24.2). It was furthermore shown, that the microstructure and especially the grain size can be significantly changed by varying the sintering temperature and time. Average grain sizes in the range of 20 μm (1288℃/24min sintering) up to >175μm (1298℃/60min sintering) were obtained.

24.3.6 Inconel 718 (IN 718)

By far the most investigated metal injection molded nickel-base superalloy is IN 718. IN 718 can be used at temperatures up to 650℃. Due to its comparably low costs and excellent mechanical properties in this temperature range forged IN 718 for instance is used for aero engine parts such as compressor vanes and turbine disks. IN 718 is hardened by γ'' (Ni3Nb) and γ' Ni3(Al, Ti) precipitates. In order to adjust the mechanical properties, the alloy is typically supplied in as heat-treated condition. Typical heat treatment for IN 718 consists of solution annealing followed by a precipitation heat treatment. Most investigations on MIM IN 718 are also made with heat-treated samples.

An evaluation of different literature data shows that as-sintered MIM IN 718 tensile strengths are lower compared to heat-treated MIM material (see Table 24.2). Therefore, according to AMS 5917, heat treatment is recommended for IN 718. The applied heat treatment in literature usually consists of a solution annealing step followed by an aging/precipitation heat treatment. In most cases, the heat treatment is based on existing specifications for cast or wrought IN 718 like AMS 5662 or AMS 5663 (process parameters: see Table 24.2). By applying this kind of heat treatment, UTSs comparable to cast and wrought materials are obtained for MIM IN 718. HIP after the sintering step and before the heat treatment usually leads to comparable tensile strengths but increased ductility. The comparison of literature data with the industry standard AMS 5917 for MIM IN 718 illustrates that most tensile data fulfill AMS 5917 requirements (see Table 24.2).

Although several publications about MIM IN 718 are available published high temperature mechanical properties are very scarce. However, it was shown that high temperature tensile, stress rupture and fatigue properties better than AMS 5596 minimum requirements can be achieved by appropriate processing. The tested samples were prepared from gasatomized IN 718 powder with a mean particle size of 15μm. Sintering was conducted at 1260℃ for 6 h in vacuum with argon partial pressure of approximately 1000μm Hg. After sintering, hot isostatically pressing was performed at 1190℃ for 4 h at 103.5MPa. The as-hipped samples were heat treated at 870℃ for 10 h, followed by a solution heat treatment at 950℃ for 1 h in argon followed by air cooling. The precipitation heat treatment was conducted at 718℃ for 8 h, followed by furnace cooling at a cooling rate of 38℃/h to 620℃, followed by a holding step for 8 h and subsequent air cooling.

24.3.7 Inconel 713 (IN 713)

IN 713is a relatively cost-effective γ'-hardened cast alloythat has already been developed in 1956 at Inco's U.S. research laboratories. Typical applications are turbocharger wheels and low pressure turbine blades due to its excellent high-temperature strength and oxidation resistance. IN 713 is usually used in the as-cast state. Typically two different types of alloys are distinguished: IN 713C (Inconel 713 Carbon) with carbon contents in the range of 0.08-0.20wt% and IN 713LC (Inconel 713 Low Carbon) containing 0.03-0.07wt% carbon. Both alloys have been used forMIM by different research groups.

For MIM IN 713C material that is made from powder with particle size <22μm and was sintered at 1280℃ for 3 h under argon atmosphere, a relative density of 99.1% is reported. Via HIP at 1200℃ and 1030bar for 4 h 100% densification can be achieved. The resulting grain size was rather small and high volume fraction of cuboidal γ'-precipitates of 0.1-0.2μm size are reported for as-sintered condition. Room temperature tensile properties for as-sintered material could be increased by HIP and were better comparedto cast reference materialin both conditions as-sintered and hot isostatically pressed. MIM of IN 713C using a water soluble binder system. Compared to the values reported, no carbon uptake is reported for the water soluble binder system. The roomtemperature yield strengthRp0.2 ¼829.5MPais somewhatlower,the UTS Rm ¼1319.4MPa and the elongation at fractureA ¼16.4% are somewhat higherthanthe values reported for as-sintered material. Besides the room temperature tests also tensile tests at 650, 850, and 1000℃ have been performed. The obtained values are in good accordance with the test results reported for as-sintered and hot isostatically pressed (1200℃/100MPa/4 h) samples. In a newer study using IN 713LC instead of IN 713C powder an increase of the 650℃ and 900℃, yield strength (843MPa instead of 800MPa and 459MPa instead of 373MPa) could be achieved due to the decrease of impurity levels both onthe powder and onthe processing side. All determinedtensile test results are summarized in Table 24.2.

The influence of heat treatment on metal injection molded IN 713 was investigated by different research groups in order to further improve the mechanical properties. The applied heat treatments led to an improved yield strength, also at elevated temperatures, due to refined γ'-phase in the different investigations. However, the increase of yield strength was accompanied with a severe reduction of ductility. A heat treatment consisting of a solution annealing step (1220℃/2 h) and an aging step (925℃/4 h/AC) lead to an improved creep resistance. No difference could be observed for rotating bending fatigue properties at 500℃.

The oxidation behavior is another important factor for h