Superalloy powders

Vacuum atomization is a commercial batch process)180 The development of vacuum atomization started in the mid 1960 s, concurrent with the development of inert gas atomization. In 1970, a patent for the vacuum atomization method was issued to Homogeneous Metals, Inc. Using vacuum atomization, this company routinely produces superalloy powders of fine size without great consumption of argon, giving powders free of inert gas filled porosity. Wentzell1 801 has made detailed description of this proprietary process. 

The arrangement of the melting and vacuum spray chambers is critical for guiding the liquid metal to eject into the vacuum chamber. Difficulties exist in precisely controlling the expulsion of the liquid metal into the vacuum chamber. Therefore, flaky droplets may be formed in vacuum atomization. Although vacuum atomization was developed mainly for the production of high-purity nickel and cobalt based superalloy powders, it is also applied to atomize the alloys of aluminum, copper and iron. 

This process, originally designated as RSR (rapid solidification rate), was developed by Pratt and Whitney Aircraft Group and first operated in the late 1975 for the production of rapidly solidified nickel-base superalloy powders.[185][186] The major objective of the process is to achieve extremely high cooling rates in the atomized droplets via convective cooling in helium gas jets (dynamic helium quenching effects). Over the past decade, this technique has also been applied to the production of specialty aluminum alloy, steel, copper alloy, beryllium alloy, molybdenum, titanium alloy and sili-cide powders. The reactive metals (molybdenum and titanium) and... 

Table 4. Chemical Composition of Wrought and Powder Nickel-Base Superalloys, wt %...

G. H. Gessiager, Powder Metallurgy of Superalloys, Butterworths, London, 1984, p. 282. 

Using rapid solidification technology molten metal is quench cast at a cooling rate up to 10 °C/s as a continuous ribbon. This ribbon is subsequently pulverized to an amorphous powder. RST powders include aluminum alloys, nickel-based superalloys, and nanoscale powders. RST conditions can also exist in powder atomization. 

When the structure of a metal changes, it is because there is a driving force for the change. When iron goes from b.c.c. to f.c.c. as it is heated, or when a boron dopant diffuses into a silicon semiconductor, or when a powdered superalloy sinters together, it is because each process is pushed along by a driving force. 

Calorised Coatings The nickel- and cobalt-base superalloys of gas turbine blades, which operate at high temperatures, have been protected by coatings produced by cementation. Without such protection, the presence of sulphur and vanadium from the fuel and chloride from flying over the sea promotes conditions that remove the protective oxides from these superalloys. Pack cementation with powdered aluminium produces nickel or cobalt aluminides on the surfaces of the blade aerofoils. The need for overlay coatings containing yttrium have been necessary in recent times to deal with more aggressive hot corrosion conditions. 

Die compaction of simple shapes can be carried out at elevated temperature using carbide, superalloy, refractory metal, or graphite dies. An inert gas atmosphere or vacuum is often used to protect the die and/or the powder. For example, beryllium powder is compacted at about 1350°C in a graphite die under vacuum with pressures of 2-4 MPa. 

When the powder is isostatically compacted at elevated temperatnres, the process is called hot isostatic pressing (HIP). In this case, the flexible dies are often made of thin metals, and high-pressnre gases snch as argon are nsed to heat the part rapidly and rednce thermal losses. Pressnre np to 100 MPa and temperatnres in excess of 2000°C are possible nsing HIP, and parts up to 600 kg can be fabricated. A schematic diagram of a typical HIP apparatus is shown in Figure 7.18. Metals that are processed commercially by HIP include various specialty steels, superalloys, hard metals, refractory alloys, and beryllium. We will see in Section 7.2 that HIP is also particularly useful for the densification of ceramic components. 

Complex carbides containing boron, occurring frequently in boron-alloyed steels and superalloys, are also named carboborides. Metal borocarbides (see Table 1) are synthesized by powder metallurgical methods or are extracted from a metal matrix. There are pseudoternary or -quaternary borocarbides, such as Mn23(B, C) or (Cr, Mn, Fe)23 (B, C)g (t phases) although boron-carbon substitution in borocarbides is less pronounced than nitrogen-carbon substitution in metal carbonitrides. 

Superalloys are produced via master alloys. In case of nickel-based alloys they include NiW and more complex nickel-based alloys containing Cr, Ta, and Mo. Some examples are given in Table 8.5. All raw materials have to be of high purity. Melting (performed in induction furnaces), casting and cooling takes place in vacuum. Master alloys are supplied in small lumps or as powder. 

The most important applications of superalloys are in aircraft engines, marine vehicles, and stationary powder imits as turbine blades and vanes, and in sheet form for exhaust case assemblies and burner liners. Moreover, they are used in furnace combustion tubes, muffles, cracking and reformer tubes. 

Huang, Z. F., Iwashita, C., Chou, 1, and Wei, R. P., Environmentally Assisted, Sustained-Load Crack Growth in Powder Metallurgy Nickel-Based Superalloys, Metallurgical and Materials Trans A, 33A (2002), 1681-000. 

Table 1 is a partial listing of the comp>ositions of the common p>orous sintered P/M materials. Selected compositions of P/M materials possessing near theoretical full density (>99 %), sometimes called full dense or high-performance P/M materials, are shown in Table 2 [2,9]. Full dense P/M parts are made by liquid phase sintering, powder injection molding, extrusion, or hot isostatic pressing. Important full dense P/M materials include P/M superalloys, P/M tool steels, several P/M aluminum alloys, and many P/M specialty alloys. Owing to their refined microstructure and greater homogeneity, full dense P/M materials offer superior mechanical properties with equal or superior corrosion resistance, compared to their wrought or cast counterparts.

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