GTD-111 is an Ni-based superalloy used in manufacturing of gas turbine blades. The superalloy attains appropriate high temperature strength through precipitation hardening. In this research the effect of isothermal aging on the morphology evolution and characteristics of precipitates was investigated. Two different heat treatment cycles were applied to the specimens. In cycle I all samples were solution-treated at 1 200 °C for 4 h and water quenched to obtain supersaturated solid solution. A microstructure containing fine γ′ precipitates was produced after heat treatment cycle I. The second heat treatment, cycle II, consisted of reheating of the specimens in the precipitate partial dissolution zone at 1 150 °C for 2 h to get coarse primary precipitates, and then water quenching to obtain fine secondary precipitates. The precipitate coarsening in the fine and duplex size precipitate distribution was studied by means of aging of the specimens for various times at 1 000 °C. The results showed that during long-term annealing of the single size distribution (Cycle Ι), precipitates line up to reduce interactive free energy and grow to fairly coarse sizes along a direction. In cycle ΙΙ, the precipitates were split into a group of eight small cuboids or pairs of parallel plates and a large number of γ′ particles were closely aligned along a direction.
Morphology of 0 precipitates affects mechanical properties of Ni-base superalloys. In this research study, the coarsening behavior of 0 precipitates in Ni-base superalloy GTD-111 was investigated. The alloy was solution-treated at 1200 C for 4 h and water quenched to obtain single size distribution of the precipitates. Aging at 1000 C up to 200 h was carried out to examine the effect of aging time on the growth of 0 precipitates. The shape of 0 precipitates changed from sphere to oval and then to cube as the aging time was increased. Moreover, the coarsening of 0 precipitates started fast initially, whereas it slowed down as the growth continued, contrary to the Lifshitz-Slyozov-Wagner theory. The decelerated coarsening of 0 precipitates was attributed to the effect of elastic interaction energy and decrease in driving force. Finally, the activation energy for diffusion of solute atoms or the growth of 0 precipitates in GTD-111 alloy was calculated.Keywords: coarsening behavior; 0 precipitate; elastic interaction energy; GTD-111 Ni-base alloy
IntroductionNickel-base superalloys are widely used in applications requiring strength at high temperature. The excellent mechanical properties of these alloys at elevated temperature are mainly due to the high volume fraction of 0 coherent precipitates. The phase with L1 2 ordered structure is an intermetallic compound, Ni 3 (Al,Ti), which precipitates in fcc matrix.Since the deformation behavior of Ni-base superalloys is sensitively affected by the size of 0 precipitates, the coarsening behavior of 0 precipitates in Ni-base superalloys during high-temperature exposure has been extensively investigated.The coarsening behavior of 0 precipitates in Ni-base superalloys is known as Ostwald ripening, in which the larger precipitates grow at the expense of smaller precipitates in order to reduce the total interfacial energy while maintaining a constant volume fraction of the precipitates [1]. The classical theory that describes the Ostwald ripening of precipitates is known as the Lifshitz-Slyozov-Wagner (LSW) theory, which has been based on the
We propose a novel processing technique called Stair Rolling Treatment (SRT) to generate a graded sheet with a functionally graded microstructure along its longitude. The SRT process generates a longitudinal plastic strain gradient by the cold rolling of a longitudinally variable-thickness plate. Microstructural and mechanical characterisation of the 304L stainless steel SRT sheets, are performed by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron channelling contrast imaging (ECCI), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) techniques, microhardness measurements, and uniaxial tensile tests. Mechanical characterisations show that the gradients of phase and grain size lead to mechanical incompatibility and hence non-uniform stress and strain during tensile tests. The annealed SRT sheet samples demonstrate a good combination of strength and ductility.
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