Producción CyT
Precipitation and growth of gamma prime phase in Ni-base superalloys
Capitulo de Libro
Fecha:
2010Editorial y Lugar de Edición:
NOVA Science PublishingLibro:
Heat treatments, Theory and applications (pp. 262-292)NOVA Science Publishing
ISBN:
978-1-61324-684-9Resumen *
Abstract Nickel-base superalloys are complex multicomponent alloys, characterized by a high number of alloying elements, widely used in applications where high temperature strength and corrosion resistance are required. Traditionally, two heat treatments are used for these nickel-base superalloys. First, a solution heat treatment has the objective to dissolve the precipitate phases, principally great volume interdendritic gamma-prime precipitates and carbides and homogenize the microstructure. The second stage is one or more precipitate and ageing heat treatments to obtain the precipitation of the hardening phase gamma-prime. The strength of the nickel-base superalloys is achieved through precipitation of the gamma-prime: Ni_{3}left(Al,Ti ight) phase coherently precipitated in a FCC matrix, and strongly depends on volume fraction, particle size and distribution of these precipitates. These parameters can be modified through different heat treatments and also during service at high temperatures. For this reason, it is very important to predict the kinetics growth owing to its technological application at high temperatures. In this chapter, we have performed a detailed study of the gamma-prime precipitate coarsening kinetics behaviour in IN-713C and CMSX-2 nickel-base superalloy for long ageing times. For IN-713C we developed two sets of heat treatments, which provided different initial distribution of such precipitates, and then we carried out ageing heat treatments of samples at constant temperature, in short and then long aging times, until 20.000 hours. Tensile properties were evaluated in the range of 873-1123K for both heat treatments developed. CMSX-2 is known as a first generation alloy with high gamma-prime volume fraction, containing refractory elements (Ta + W + Mo). For this alloy, we studied the resulting microstructure which appears as a consequence of the different heat treatments applied. The solution treatment in a CMSX-2 alloy permits to reach a solutioning structure up to 99% of gamma/gamma´ ratio between the matrix and remnant euthectic gamma-prime. For this reason, a second post solution heat treatment is needed to ensure a total solution. Finally, a precipitation heat treatment is used to obtain a convenient gamma-prime structure, size and distribution. In this way, three different sizes of gamma-prime particles were artificially aged up to 1000 h at different temperatures to obtain, in each case, the growth volumetric rate. The results are consistent with a linear relationship with Ostwald ripening, theory in which large particles grow at the expense of small ones by a diffusion-controlled mechanism. The activation energy of growth process was determined, corresponding to a value of 260 kJ/mol, which is in the magnitude order of diffusion of alloying elements in nickel. Información suministrada por el agente en SIGEVAPalabras Clave
solution and precipitationHeat treatmentsNi-base alloys