S. Li

Rotating bending fatigue behaviour of a Ni3Al-based single crystal alloy IC6SX at 760°C

Abstract The high cycle fatigue behaviour at 760°C of a Ni3Al-based single crystal alloy IC6SX has been studied in the present investigation. The specimens for the fatigue tests were prepared by screw selection crystal method in a directional solidification furnace. The rotating bending fatigue tests of the specimens was carried out at the temperature of 760°C in air, the stress ratio of R (σmax/σmin) was −u20091, and the rotating speed of the fatigue tests was 6500u200aru2009min−u20091 (108u200aHz). The stress–fatigue life cycle (Su2009−u2009N f) curve was obtained based on the fatigue tests, and the fracture surfaces were examined using scanning electron microscopy (SEM). The results showed that the fatigue cracks initiated from the surface defects of the specimens and the cracks propagated along the (111) crystal plane. The fatigue fracture was composed of three different characteristic regions corresponding to the three fatigue stages, including fatigue crack initiation stage, steady-state propagation stage and the fast propagation stage. The research results also showed that the rotating bending fatigue performance of alloy IC6SX is superior to that of the second-generation single crystal nickel-based superalloys containing 3u200awt-% Re.

Two-phase microstructural evolution at high temperatures for γ′-richen single crystal superalloys

Abstract The two-phase microstructural evolution and the remained fractions of γ′ were investigated by scanning electron microscope (SEM) for a series of γ′-richen single crystal superalloys from the IC alloys system. At alloys IC31 and IC32, high levels of the dissolution of γ′ cause the remarkable damages of two-phase microstructure, especially when the fraction drops to less than 45%, the γ′ phase becomes sphere significantly and the two-phase interface distorts fairly. And the relationship between different degrading stages and the remained fractions are discussed systemically. The dissolution temperatures, Tsol, of IC alloys determined by differential scanning calorimetry (DSC) are further considered. It is found that there is a good linear relationship between the Tsol and the remained fraction of the γ′ at 1200°C and the slide keeps fixed as about 0·5%/°C. Meanwhile, the remained fraction of the γ′ seems to play an important role on the mechanical properties, and a method with Tsol used to estimate the properties at 1200°C is proposed.

Effect of heat treatment and long-term age on microstructure of a Ni3Al-based single crystal superalloy

Abstract A new Re-containing Ni3Al-based single crystal superalloy was recently developed by our group. In order to obtain uniform microstructure and further improve mechanical properties of superalloy, the heat treatment regime (i.e. solution treatment and age treatment) was investigated by experimental analysis and dynamics calculation (by JMatPro), and the optimal heat treatment regime was 1335–1355°C/16u200ah, air cooling (AC)+1140°C/2u200ah, AC+870°C/32u200ah, AC. Meanwhile, completely heat-treated experimental alloy was long-term aged at 1150°C for 25–1000u200ah, which showed very good microstructural stability and the coarsening behaviour of γ′ phases agreed well with the typical Lifshitz–Slyozov–Wagner (LSW) theory.

Effects of Mo and Ta in Ni3Al based single crystal alloys on their stress–rupture properties at intermediate temperatures

Abstract The effects of the addition of Mo and Ta elements to Ni3Al based single crystal (SC) alloys on their microstructures and stress–rupture properties were investigated. The results of this research indicate that in a Ni3Al based SC alloy with an added 3%Ta, the volume fraction, mean size and cubical degree of the γ′ phase increased and the segregation ratios of the refractory elements were not totally altered, compared to the effects seen after the addition of 3%Mo, as analysed using scanning electron microscopy (SEM) and energy dispersive spectroscopy. Moreover, the lattice mismatch of the γ/γ′ phase in the Ni3Al based SC alloy became smaller, as observed using X-ray diffraction. The stress–rupture lives of the Ni3Al based SC alloys were ∼330 and 260 h respectively. The results suggest that Mo promotes an increase in the stress–rupture property and that the effect of Ta is less than that of Mo. Analysis through transmission electron microscopy illustrated that, in the alloy with Mo, a multiple slip of <011> [111] dislocations was developed to form dense dislocation networks and a few of the dislocations sheared into γ′ by short <112> [111] stacking faults ribbons. In the alloy with Ta, the dislocations sheared into the γ′ precipitates by coupled dislocation pairs without dense dislocation networks. These differing results are mostly due to larger lattice mismatch and denser dislocation networks formed through the addition of Mo.

Effect of process parameters on microstructures and properties of DZ125 superalloy solidified by LMC

Abstract High thermal gradient in the interface region of melting and solidified metal during the unidirectional solidification has significant impact on the microstructures and mechanical properties of the castings. In this paper, DZ125 superalloy was solidified by directional equipment associated with liquid metal cooling medium of Ga–In alloy. Several process parameters have been chosen in order to understand the effect of thermal gradient on the castings. Comparing with traditional high rate solidification prepared DZ125 superalloy, the results show that element segregation such as Al, Ti, W and Mo was decreased with high thermal gradients or high withdraw rates, the precipitation of low melting phase in the interdendritic region was suppressed and the sizes of eutectic and γ′ phase were both refined. Stress–rupture test was carried out at 980°C/235 MPa after heat treatment. Obvious stress–rupture improvement has been observed, and the stress–rupture life decrease at high withdrawal rate due to the column grain discontinuity and deflection.

The effect of the γ′ morphology on stress-rupture properties of IC21 alloy at 850°C

Abstract In this study, the effect of the γ′ morphology on stress-rupture properties of single crystal alloy IC21 at 850°Cu2009500u200aMPa−u20091 was investigated. Special heat treatment (HT) with different cooling procedure and holding temperature were conducted to samples before the stress-rupture test. The longest stress-rupture life and the smallest elongation were obtained after HT3 treatment. Microstructure examination shows that micro-raft was built during intermediate temperature stress-rupture test after HT3 treatment. Moreover, transmission electron microscopy (TEM) analysis of the stress-ruptured samples reviews that with the change of the γ′ morphology, the dominant deformation mechanism has been changed from the shearing of the <u2009112> super-partial dislocations to the Orowan by-passing mechanism.

The influence of thermal exposure on the microstructure of a Ni3Al-based single crystal superalloy

Abstract The effect of thermal exposure on the microstructure of an Ni3Al-based single crystal superalloy was investigated. The Ni3Al-based single crystal superalloy used in this experiment contained a high content of molybdenum (Mo) and a low content of chromium (Cr), which had good high-temperature strength and oxidation resistance. The alloy was exposed at 1020–1200°C for different time periods. The microstructure after thermal exposure was examined by scanning electron microscope (SEM) and field emission microscope (FEM). The results indicated that precipitation of needle-like topologically close-packed (TCP) phases was only observed at certain intermediate temperature intervals (1020–1080°C). Moreover, once the TCP phases precipitated, they would not dissolve back even after thermal exposed for 1000 h. No precipitation phases were observed after high-temperature thermal exposure at 1100–1200°C for 25 h.

Precipitation of γ′ phases during slow cooling of nickel based superalloy

Abstract In this paper, the precipitation behaviour of γ′ phases during cooling after solution treatment has been investigated. An experimental superalloy with a large γ′ volume fraction was employed, and the average cooling rate was ca. 5°C min−1. In order to investigate the forming of precipitates at different temperature stages, interrupt quenching was used during the cooling process. As the results show, the γ′ precipitates change from monomodal to multimodal size distributions with quenching temperature. On the one hand, the large γ′ phases were mainly formed by coalescing and merging. On the other hand, small γ′ in the matrix were continuously precipitated during the cooling process, and the nucleation was not shut off until quenching at 1100°C. The precipitation mechanism is also discussed in this paper.

δ-Mo49Ni51 phase in a Ni3Al based single crystal superalloy during thermal exposure

Abstract In the paper, the hexagonal δ-Mo49Ni51 phase had been investigated in a Mo-rich Ni3Al based single crystal superalloy with Re additions in 3–5 wt%. The δ-Mo49Ni51 phase was identified by transmission electron microscopy (TEM), scanning electron micrograph (SEM) and electron probe micro-analysis (EPMA). Before thermal exposure, the microstructure of the alloy was the cuboidal ordered γ′ phase in coherent with the disordered γ matrix without any precipitation. During thermal exposure, the strip-like, faceted, plate-shaped δ-Mo49Ni51 phases significantly were precipitated. By the means of the composition analysis of TEM/EDS and EPMA/EDS, the elements Ni, Mo and Re were rich obviously in δ-Mo49Ni51 phase. Furthermore, the crystallography relationships between the δ-Mo49Ni51 phase and γ phase have been reported.