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

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СКАЧАТЬ 2.15 Iron‐carbon phase diagram demonstrating the main phases under atmospheric pressure.

      Source: Caesar (2019), https://commons.wikimedia.org/wiki/File:Iron_carbon_phase_diagram.svg. Licensed under CC BY‐SA 4.0.

An iron‑carbon phase that is metastable and so does not appear on the phase diagram but is highly important to engineering materials is called martensite. Martensite is a body‐centered tetragonal phase, like BCC, iron, but with an elongated c‐axis due to the placement of carbon atoms at 0, 0, ½ sites. It is extremely hard and strong, but is also brittle. Tempering of martensite forms a two‐phase mixture of ferrite and carbide with the carbide present as small particles rather than platelets. Tempered martensite is stronger and tougher than either pearlite or martensite. Specialized heat treatment of the iron‐carbide system can produce other mixtures, such as bainite (a plate‐like mixture where the carbide within the plates is particulate rather than platelet like) and a spheroidite (nearly spherical iron carbide). Each mixture can give rise to different properties.

      The majority of steels are not plain carbon steels. Rather, they are alloyed steels that are modified to obtain different properties with the addition of alloying elements, such as manganese, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, and niobium. For example, stainless steels contain a minimum of 11% chromium in order to resist corrosion through the formation of a self‐repairing (in the presence of oxygen) passive layer of chromium oxide on the surface. Chromium addition with little to no nickel produces ferritic (BCC) stainless steel similar to carbon steel. Nickel is added to stainless steels to form austenitic stainless steels. Nickel (as well as manganese) helps stabilize the austenitic (FCC) structure and results in high toughness and high strength throughout a broad temperature range. For example, grade 316 stainless steel nominally contains (in wt.%) 17 Cr, 12 Ni, 2 Mn, 1 Si, 0.1 C, and 2.5 Mo with balance Fe (Carter and Paul 1991).

      Stainless steel can also be precipitation hardened to increase strength and corrosion resistance. The composition of such material can become quite complex. For example, Discaloy is a common precipitation‐hardened stainless steel with a nominal composition (in wt.%) of 26 Ni, 13.5 Cr, 2.75 Mo, 1.75 Ti, 0.90 Mn, 0.80 Si, 0.08 C, and 0.005 B with balance Fe.

      The complexity of iron‐base alloys has enabled them to be remarkably versatile. However, as the boundaries to high‐temperature use are pushed even higher, the search for alternative materials has led to remarkable superalloy materials outside of the iron system.

      

2.8.2 Nickel‐base Alloys

      Nickel is the fifth most abundant element on Earth, but most of it is located almost 3000 km below the surface (United States Geological Survey 2012). In Earth's crust, magmatic sulfide deposits (principal ore mineral pentlandite, (Ni,Fe)₉S₈) and laterite deposits (ore mixtures of nickeliferous limonite, (Fe,Ni)O(OH), and garnierite – a mixture of hydrous nickel and nickel‐rich silicates) (United States Geological Survey 2012). Roughly 3000 nickel‐base alloys are in use, forming products for numerous industries, including energy, chemical, petrochemical, and aerospace industries. Ni‐base systems have risen as systems of choice for tailored superalloys largely due to the nature of the precipitates and resulting properties that can be achieved in the system. As such, a deeper look into nickel‐base alloys is critical to frame later discussion in the text.

      Nickel‐base alloys tend to have a fully austenitic (FCC) structure and the ability to maintain good tensile, rupture, and creep properties to much higher temperatures than (BCC) systems. They are often used in load‐bearing structures and nickel‐base superalloys have been reported being used to highest homologous temperature (up to T _m = 0.9) of any common alloy system (Bowman 2000). Like Fe‐base alloys, the properties of Ni‐base alloys can be tailored through the addition of many other elements, including chromium, iron, cobalt, molybdenum, tungsten, tantalum, aluminum, titanium, zirconium, niobium, rhenium, yttrium, vanadium, carbon, boron, and hafnium. Nickel‐base alloys can be solid‐solution or precipitation strengthened, but the latter is generally used for more demanding applications.

The properties of Ni‐base superalloys have largely arisen through their unique phases and the ability to tailor the properties through the addition of various elements. Ni‐base superalloys are generally composed of a gamma (γ) phase, which forms an austenitic solid‐solution matrix composed of the alloying elements. As the alloys are cooled from the melt, gamma prime (γ′) phases begin to precipitate within the γ‐matrix (Figure 2.16) (Betteridge and Heslop 1974; Ross and Sims 1987). γ′ phase is a GCP phase generally composed of Ni₃(Al,Ti) in Ni‐base superalloys. It is the main strengthening phase in the alloy and has strong coherency with the matrix, which allows for ductility. The precipitates can take on several different geometries, including cuboidal, spheres, platelets, or combinations thereof. However, to decrease their energy states, they often align along the <100> directions and form cuboidal structures (Sabol 1969). Many Ni‐base superalloys can thus be described as having ordered γ′ particles within a disordered γ‐matrix.

      The γ–γ′ system forms the basic structure of Ni‐base superalloys, but several other phases can also be present:

       A gamma double prime (γ″), often with the composition of Ni3Nb or Ni3V, can form small disk‐like precipitates with a GCP body‐centered tetragonal structure. These precipitates form with the (001) planes of the precipitate (γ″) parallel to the {001} family in the matrix (γ) as a result of lattice mismatch. They can strengthen the superalloy at lower temperatures compared to the γ′ phase, but often dissolve at higher temperatures.Figure 2.16 Schematic of cuboidal γ′ (Ni3(Al,Ti)) precipitates within a γ [FCC‐structured Ni, Co, and Fe solid solution]. In this schematic, the characteristic precipitate size is shown as dγ′.Sources: Betteridge and Heslop (1974); Ross and Sims (1987).

       In addition to GCP phases, several phases can form, and many of these can be undesirable due to poor mechanical properties, incoherency with the matrix, and morphologies, such as needle‐like structures that can nucleate cracks as well as depleting the materials ability to form the desired γ′ particles (Belan 2016).

       Carbide phases are also formed from carbon and reactive and refractory elements, such as titanium, tantalum, and hafnium (e.g. TiC, TaC, or HfC). With heat treatment (and/or service), the carbides can decompose to lower metal carbides, such as M23C6 and M6C, and often aggregate to the grain boundaries. Many carbides have an FCC structure and are known to support strengthening at grain boundaries in polycrystalline materials (Sabol 1969). However, their role in single crystal material properties is still under debate (Bowman 2000).

       Cr and Al in the systems can also protect against oxidation and corrosion by forming protective oxide phases.

The addition of other elements can further tailor the properties of the alloy. For example, not only does higher Al and Ti result in higher volume fractions of the γ′‐phase, but also the addition of cobalt reduces the solubility of Al and Ti in the NiCr matrix and further promotes precipitation. This improves strength at high temperatures, as well as the workability of these alloys. Other elements are added to improve microstructural stability, strength, and deformation characteristics. The creep‐rupture life of these alloys can be increased significantly by the careful control of composition and microstructure. For different uses, variations in composition and process are critical leading to a variety of different nickel‐base superalloys in use. An example of the chemical compositions of some well‐known nickel‐base alloys is given in Table СКАЧАТЬ