produced mainly by precipitation of chromium and refractory metal carbides. Small additions of zirconium and boron improve the morphology and stability of these carbides.
Carbides in superalloys perform three functions. First, grain boundary carbides, when properly formed, strengthen the grain boundaries, prevent or retard grain boundary sliding and permit stress relaxation along the grain boundaries. Second, a fine distribution of carbides precipitated within the grains increases strength; this is especially important in the cobalt based alloys that cannot be strengthened by y' precipitates. Third, carbides can tie-up certain elements that would otherwise promote phase instability during service. Since carbides are harder and more brittle than the alloy matrix, their distribution along the grain boundaries will affect the high temperature strength, ductility and creep performance of nickel based alloys. There is an optimum amount and distribution of carbides along the grain boundaries. If there are no carbides along the grain boundaries, voids will form and contribute to excessive grain boundary sliding. However, if a continuous film of carbides is present along the grain boundaries, continuous fracture paths will result in brittleness. Thus, the optimum distribution is a discontinuous chain of carbides along the grain boundaries, since the carbides will then hinder grain boundary sliding without adversely affecting ductility. Multistage heat treatments are often used to obtain the desired grain boundary distribution, along with a mix of both small and large y' precipitates, for the best combination of strength at intermediate and high temperatures.
Some of the important carbides are MC, M23C6, M6C and M7C3. In nickel based alloys, M stands for titanium, tantalum, niobium or tungsten. MC carbides usually form just below the solidification temperature during ingot casting and are usually large and blocky, have a random distribution and are generally not desirable.4 However, MC carbides tend to decompose during heat treatment into other more stable carbides, such as M23C6 and/or M6C. In the M23C6 carbides, M is usually chromium but can be replaced by iron and to a smaller extent by tungsten, molybdenum or cobalt, depending on the alloy composition. M23C6 carbides can form either during heat treatment or during service at temperatures between 1400 and 1800° F. They can form from either the degeneration of MC carbides or from soluble carbon in the matrix. M23C6 carbides tend to precipitate along the grain boundaries and enhance stress rupture properties. M6C carbides form at temperatures in the range of 1500-1800° F. They are similar to the M23C6 carbides and have a tendency to form when the molybdenum and tungsten contents are high. Although not nearly as prevalent as the other carbides, M7C3 carbides also precipitate on the grain boundaries, and are beneficial if they are discrete particles but detrimental if they form a grain boundary film.
Over the years, a number of undesirable topologically closed-packed (TCP) phases have appeared either during heat treatment or in-service, the most important being a, ^ and Laves. These phases, which usually form as thin plates or needles, can lead to lower stress rupture strengths and a loss in ductility.
They also remove useful strengthening elements from the matrix, such as the refractory elements molybdenum, chromium and tungsten, which reduces both solid solution strengthening and the y/y' mismatch. Modern computer modeling programs (e.g., Phacomp) are capable of predicting their occurrence so they can be avoided in alloy design. As with most high performance alloys, hydrogen, oxygen and nitrogen are considered detrimental and are held to very low levels.
Superalloys are used in the cast, rolled, extruded, forged and powder produced forms as shown in the overall process flow in Fig. 6.4. Wrought alloys are generally more uniform with finer grain sizes and superior tensile and fatigue properties, while cast alloys have more alloy segregation and coarser grain sizes
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