[en] The precipitation behavior of μ phase in Ni-base single crystal superalloys was investigated by in situ transmission electron microscopy (TEM). A layer-by-layer growth process with a ledge propagation mechanism was first observed during in situ precipitation. Three types of μ phase with different morphologies were found, which grow along [001]_μ with (001)_μ planar defects, [-111]_μ with (1–12)_μ planar defects, as well as both directions with mixed planar defects. High-resolution TEM image and established atomic models reveal a basic growth mechanism of μ phase by stacking on (001)_μ plane and randomly forming coherent planar defects, while the nucleation of incoherent (1–12)_μ planar defects at the early stage of precipitation plays an important role in affecting the basic growth mechanism. The frequent faults during the stacking process of the sub-unit layers within μ lattice should be responsible for the defect formation. -- Graphical abstract: In situ transmission electron microscopy (TEM) investigations reveal the layer-by-layer growth mechanism of μ phase precipitated in Ni-base single crystal superalloys. Three types of μ phase with different morphologies were formed at 1050 °C, which grows along [001]_μ with (001)_μ planar defects, [-111]_μ with (1–12)_μ planar defects, as well as both directions with mixed planar defects respectively. Formation of (001)_μ micro-twin and stacking fault is the essential feature for precipitated μ phase, while nucleation of incoherent (1–12)_μ planar defects plays an important role in changing growth method. Display Omitted

[en] Precipitation of topologically close-packed (TCP) phase in Ni-base single crystal superalloys with additions of platinum group metals (PGMs) was investigated by in situ transmission electron microscopy (TEM). Pt, Ir and Ru additions are all helpful to suppress the precipitation of TCP phase, among which Ru has the largest efficiency while Pt has the least one. Comparing with the TCP precipitation in non-PGMs alloys, addition of Ru mainly retards the nucleation rate of TCP phase, but Pt and Ir mainly reduces the growth rate of TCP precipitates. Elemental partitioning investigation reveals that Pt and Ir prefer partitioning to the surrounding γ matrix rather than in the precipitated μ phase, and Ru distributes homogenously in both γ matrix and μ phase. Cr distribution in precipitated μ phase is higher in Pt/Ir-containing alloys than in Ru-containing alloy, which is supposed to be related to the different metallic bonding energy of Cr–Pt, Cr–Ir and Cr–Ru. - Highlights: • In situ TEM was applied to study the TCP suppression effects by PGMs addition. • Pt, Ir and Ru additions all suppress TCP precipitation in a sequence of Ru >Ir> Pt. • Ru suppresses TCP precipitation mainly by retarding its nucleation rate. • Pt and Ir suppress TCP precipitation mainly by reducing its growth rate. • PGMs segregate differently in TCP precipitates and affect the partition of Cr.

[en] Graphical abstract: In situ transmission electron microscopy (TEM) investigations reveal the homogeneous nucleation of topologically close-packed (TCP) phase in Ni-base single crystal superalloy at elevated temperatures, which tends to grow toward primary γ/γ′ interface for element supply. Ru addition can decrease both the nucleation and growth rate, hence to suppress TCP precipitation effectively. Higher exposure temperature reduces the nucleation rate but increases the growth rate, finally accelerates the TCP precipitation. - Highlights: • In situ TEM was first applied to study the precipitation behavior of TCP phase. • The TCP phase nucleates homogeneously in γ matrix without preferential sites. • The TCP phase tends to grow toward primary γ/γ′ interface for element supply. • Ru addition can decrease both the nucleation and growth rate of TCP precipitation. • Higher temperature reduces nucleation rate but increases growth rate of TCP phase. - Abstract: In situ transmission electron microscopy was first applied to study the precipitation behavior of topologically close-packed (TCP) phase at elevated temperature in Ru-free and Ru-containing Ni-base single crystal superalloys. Ru addition can decrease both the nucleation and growth rate, hence to suppress TCP precipitation effectively. Higher exposure temperature reduces the nucleation rate but increases the growth rate, finally accelerates the precipitation. The TCP phases nucleate homogeneously in γ matrix without preferential sites, but tend to grow toward primary γ/γ′ interface for element supply. An orientation relationship is kept during nucleation and growth of μ phase in γ matrix, which is [1 1 0]_μ // [0 0 1]_γ and (0 0 1)_μ // (−1 1 0)_γ

[en] Additive manufacturing (AM) of single crystal superalloys has got some progress in recent researches, but there are few reports on eliminating recrystallization or mechanical properties of single crystal superalloys by AM. In this work, single-crystal samples of SRR99 were fabricated at high temperature by laser melting deposition (LMD). Owing to the high temperature of substrate in the deposition process, recrystallization in the following heat treatment process was eliminated. The contrast sample of SRR99 was prepared by directional solidification, and the microstructure evolution during heat treatment and tensile property of deposited samples were analyzed. The results showed that the shapes of the γ′ phase became irregular after solution treatment in deposited samples. After a solid solution and aging treatment, the γ′ phase size is larger and the γ′ volume fraction is slightly lower in deposited samples than in the contrast sample. As a result, the yield and tensile strength of deposited samples are slightly lower than that of contrast samples, but the plasticity of deposited samples is better. (paper)

[en] Highlights: • γ' coarsening and TCP phase precipitation are characterized after long-term exposure. • The TCP phase does not embrittle the alloy. • The severe γ' coarsening is the main factor in the degradation of the life time. Microstructural degradation after thermal exposure of a 5% Re-containing single crystal (SX) superalloys has been studied. The alloy was subjected to thermal exposure at temperature of 1100 °C for periods of 100, 500, and 1000 h. The microstructure of the alloy was degraded after long-term thermal exposure, mainly in γ' coarsening, the precipitation topologically close packed (TCP) phases. Moreover, the stress-rupture properties of the samples before and after long-term thermal exposures were characterized. The results showed that the γ' coarsening and TCP formation after exposed for 500 h, which led to a damaged of the stress-rupture life time. Besides, the growth of pores during exposure also influences the stress-rupture life.

[en] The tensile properties and microstructures of 1050 aluminum alloy prepared by equal channel angular pressing at cryogenic temperature (cryoECAP) after electric current annealing at 90–210 °C for 3 h were investigated by tensile test, electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM). An unexpected annealing-induced strengthening phenomenon occurs at 90–210 °C, due to a significant decrease in the density of mobile dislocations after annealing, and thus a higher yield stress is required to nucleate alternative dislocation sources during tensile test. The electric current can enhance the motion of dislocations, lead to a lower dislocation density at 90–150 °C, and thus shift the peak annealing temperature from 150 °C to 120 °C. Moreover, the electric current can promote the migration of grain boundaries at 150–210 °C, result in a larger grain size at 150 °C and 210 °C, and thus causes a lower yield stress. The sample annealed with electric current has a lower uniform elongation at 90–120 °C, and the deviation in the uniform elongation between samples annealed without and with electric current becomes smaller at 150–210 °C. - Highlights: • An unexpected annealing-induced strengthening phenomenon occurs at 90–210 °C. • The d. c. current can enhance the motion of dislocations at 90–150 °C, and thus shift the peak annealing temperature from 150 °C to 120 °C. • The d. c. current can promote the grain growth at 150–210 °C, and thus cause a lower yield stress. • The DC annealed sample has a lower uniform elongation at 90–120 °C.

[en] The topological close packing phases in nickel-base single crystal superalloy aged at 1100 °C for 1000 h was investigated by the spherical aberration-corrected transmission electron microscope. There are R phases precipitated in the matrix, with the hexagonal lattice parameter as a = b = 11.154 Å, c = 17.782 Å. There are lamellar contrasts at the tip of the R phase, which were recognized to be {1-10-1} 〈01-1-1〉 type growth twins. The HRTEM results confirm that the formation of growth twins in R phase can release misfit stress to coordinate the distorted relationship between the R phase and the matrix. (paper)

[en] Highlights: • Constitutive equations were built segmentally considering the γ′ phases dissolution. • The undissolved γ particles promote DRX at γ sub-solvus temperature. • The MT formation mechanism at high temperatures was related to low SFE and LAGBs. In this work, hot compression behaviors and deformation mechanisms of a Ni–Co-based superalloy prepared using directional solidification (DS) were investigated by carrying out the isothermal compression tests within the temperature range of 1080–1190 °C and the strain rate range of 0.001–1 s^-1 under the true strain of 0.693. The effects of temperature and strain rate on dynamic recrystallization (DRX) were analyzed, and constitutive equations were established. The activation energies for γ+γ dual-phase and the γ single-phase regions of Ni–Co-based superalloy with columnar grains were 1190 kJ mol^-1 and 415 kJ mol^-1, respectively. The results showed that the dominant deformation mechanisms were closely related to the strain rate and temperature. DRX was accelerated at low strain rate conditions, while the DRX process was apparently sluggish at strain rates higher than 1 s^-1. The undissolved γ phase interacted with dislocations and accelerated the DRX of the matrix during γ sub-solvus temperature (1080–1120 °C) deformation. Continuous dynamic recrystallization (CDRX) nucleation proved to play a dominant role. During γ super-solvus temperature (1150–1190 °C) deformation, the dominant dynamic softening mechanism of the alloy is discontinuous dynamic recrystallization (DDRX). Local migration of grain boundaries (GBs) causes an increase of misorientation within the deformed grains and induced the formation of subgrain boundaries near the GBs, which subsequently evolved into high angle grain boundaries (HAGBs) and participated in DDRX. Furthermore, during high-temperature deformation in Ni–Co-based superalloys, the relatively rare microtwins (MTs) were detected under compression at 1100 °C/1 s^-1.

[en] Highlights: • Brooke formula is only valid for estimating the lattice misfit relaxed by the dislocation network. • The two historical Brooke formulas overestimate the magnitude of the misfit. • The modified Brooke formula can correctly estimate the misfit. The validity of the Brooke formula in estimating the lattice misfit based on dislocation network structure in Ni-based superalloys is investigated. The misfit value of a sample subjected to prolonged thermal exposure as derived from the popular Brooke formula is below that determined by X-ray diffraction (XRD) method, whereas the misfit estimated using the Brooke formula for a sample subjected to creep is greater than that determined by XRD method. These discrepancies imply the invalidity of the popular Brooke formula in two aspects: that a dislocation network is not always mature to fully relax the lattice misfit leading underestimate the misfit and that using the full magnitude of the Burgers vector overestimates the misfit. To overcome this problem, a modified Brook formula using the {100} projection of the edge component of a Burgers vector as the effective magnitude to relieve the misfit is proposed. The misfit derived from the modified Brook formula is in good agreement with the values determined by XRD method for a number of Ni-based superalloys studied.

[en] Highlights: • The dominant factor of competitive grain growth during directional solidification was systematically investigated. • A new model of competitive grain growth based on the solute field was proposed. • A new mechanism of primary spacing evolution in the directional columnar solidification structure is highlighted. -- Abstract: To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.