Engineering Physics of High-Temperature Materials. Nirmal K. Sinha

Figure 2.18 Chemical composition diagrams of three nickel‐base superalloys. The data are taken from Table 2.1. Whereas Nimonic 90 is weldable, IN‐738LC and René 80 are very difficult to weld as indicated in Figure 2.16.

      Source: N. K. Sinha.

      The complexity of the chemical compositions of superalloys, with a dozen or more important elements (and several “tramp” elements, such as oxygen, nitrogen, sulfur, phosphorous, and silicon, and “trace” elements, such as bismuth, lead, selenium, and thallium), may at first glance appear to be confusing (Betteridge and Heslop 1974; Ross and Sims 1987; Porter and Easterling 1992). However, the chemical compositions are carefully controlled to impart desired mechanical and service exposure requirements. Although Table 2.2 provides the data quantitatively, Figure 2.18 with the use of logarithmic scale (that may be extended to show the “trace” and “tramp” elements) for the composition brings out the differences in the amount of several elements that are used in small quantities in these alloys. Note the positions of these three alloys in the “weldability” plot (Figure 2.17). This type of compositional representation was appreciated by the colleagues of the author, and the primary objective of this communication is to share this idea with other researchers whose main background may not be metallurgy.

      Thus far, we have focused our discussion on polycrystalline material. Single crystal (SC or SX) materials, with (ideally) no grain boundaries in the entire sample, are generally prepared using a modified directional solidification technique. While the process can be time‐ and cost‐intensive, the properties of single crystal material can be quite unique. This will be explored throughout this text.

      With proper heat treatment, it is generally assumed that there are very few dislocations in cubes of γ′ as well as in the matrix of γ single crystal materials. However, the microstructure within the grain can still be optimized through the addition of microalloying elements, and several different single crystal Ni‐base superalloys are currently in use. The first generation of SX alloys was essentially based on modifications of the chemical composition of earlier successful polycrystalline nickel‐base superalloys. Rhenium additions improved certain desirable properties (Giamei and Anton 1985), and alloys containing up to about 3% rhenium were developed. These are called the second‐generation SX alloys. The rhenium content was then increased further to about 6%, and these alloys were labeled as the third‐generation SX alloys. For example, CMSX‐10 is a third‐generation SX alloy with a nominal composition provided in Table 2.2.

2.8.3 Titanium‐base Alloys

Titanium is the ninth most abundant material in Earth's crust. It is generally present in igneous rocks and sediments. Titanium is also present in many iron ores as well as minerals, such as rutile (TiO₂) and ilmenite (FeTiO₃). Ti‐base alloys can not only maintain high tensile strength and toughness even at high temperatures, but they are also lightweight making the strength‐to‐weight ratio attractive. The high cost of the base materials and processing has limited their use to specialty applications, but they have been found to be particularly attractive in the aerospace industry. Between 1950 and 2005, the Ti‐content has increased from 3 to 31% of aeroengine weight (Gogia 2005). Recently, aeroengine Ti‐content has decreased slightly with the rise of carbon‐matrix, ceramic‐matrix, and metal‐matrix composites (Gogia 2005).

      Below 1153 K at ambient pressures, titanium has an HCP structure, which limits plastic deformation due to the limited availability of slip planes and directions compared to cubic structures. This is called the α‐phase and is strong, but has low ductility. However, above 1153 K, it transforms to a BCC structure. The BCC or β‐phase has greater ductility and greater ability to be solution treated and aged to optimize its properties. Additives, such as aluminum and oxygen, can stabilize the low temperature α‐phase by raising the transition temperature while others, including chromium, iron, molybdenum, and vanadium, stabilize the β‐phase by lowering the transition temperature. Titanium alloys are thus usually classified into four different categories:

       “α‐alloys”: Contain neural alloying elements and/or α‐stabilizers and tend to be stronger, but less ductile, than β‐alloys. Examples include Ti‐5Al‐2Sn‐ELI and Ti‐8Al‐1Mo‐1V (ASM 2000).

       “Near‐α‐alloys”: Contain 1–2% β‐stabilizers that enable small volumes of ductile β‐phase. Examples include Ti‐5Al‐5Sn‐2Zr‐2Mo and IMI 685 1V (ASM 2000).

       “(α‐β) alloys”: Contain a combination of α‐ and β‐stabilizers and are generally metastable with the ability to be heat treated to improve properties. These alloys have tended to be the most widely used for high‐temperature applications. Examples include Ti‐6Al‐2Sn‐4Zr‐6Mo and Ti‐Ti‐6Al‐7Nb‐1V (ASM 2000).

       “β and near‐β alloys”: Contain sufficient β‐stabilizers to maintain the β‐phase when quenched, and thus are also generally metastable with the ability to be optimized through heat treatment. They often exhibit promising biocompatibility and are used in medical and orthodontic applications. Examples include Ti‐10V‐2Fe‐3Al and Ti‐13V‐11Cr‐3Al.

      As mentioned earlier, titanium alloys for high‐temperature applications generally contain various proportions of both α‐ and β‐stabilizing elements depending on the application and the required mechanical properties. Both the composition and heat treatment of (α‐β) alloys are used to optimize phase compositions, sizes, and distribution of phases.

Figure 2.19 Schematic of primary and secondary α‐phases within a β‐phase matrix in Ti alloys.

Titanium alloy Ti‐6Al‐2Sn‐4Zr‐6Mo (or Ti‐6246) is a solution‐strengthened heat‐treatable α‐β alloy with higher β‐stabilizer content. It was introduced in 1966 (Eylon et al. 1984), but has more recently been optimized and designed for use as forgings in the intermediate temperature sections of gas turbine engines, particularly in compressor disks, impellers, and fan blades (Wood and Favor 1972; Boyer et al. 1994). The morphology of the alloy can be changed by thermomechanical processing (Uginet 1994). Ti‐6246 processing routes have been shown to generate three different types of microstructures: a β‐annealed (“lamellar”) microstructure, an α‐β‐recrystallized microstructure (“bimodal”), and a through β‐transus‐processed (“necklace”) microstructure (Sauer and Lütjering 2001). The as‐received microstructure commonly used is a lamellar structure containing primary‐α (grains) at prior β‐grain triplets and secondary‐α (lamellae of α) in a transformed β‐matrix (Figure 2.19). It can consist of acicular (needle‐like) or platelet microstructures. The grain boundaries and platelet boundaries in materials with acicular microstructure contain thin layers of softer β‐phase (BCC). The mechanical properties of this alloy depend primarily on the colony size of secondary‐α lamellae. Recently, novel friction stir processing has been used to optimize the microstructure to reduce primary grains and produce a very fine structure with acicular orthorhombic or α″‐laths with nanoprecipitates