Typical Ni-based Superalloy Research Articles

Investigation on the fracture mechanism and constitutive relations of a typical Ni-based superalloy

The hot tensile behavior of a typical Ni-based superalloy is elucidated using a microcomputer-controlled electronic universal testing machine at various deformation temperatures (920–1010 °C) and strain rates (0.001–0.01 s−1). Further, the influence of deformation parameters on the fracture characteristics is clarified. The results reveal that the tensile stress speedily rises to a peak level, beyond which it continuously declines until the final rupture. Moreover, numerous dimples exist on the fracture surface, and tenacity nests appear between adjacent dimples, suggesting that micro-void coalescence is the primary failure mechanism. Meanwhile, almost completed dynamic recrystallization (DRX) at elevated deformation temperature and slow strain rate promotes the intergranular fracture of the tested superalloy. Besides, an Arrhenius-type (AT) constitutive model and an artificial neural network (ANN) combined with particle swarm optimization (PSO) algorithms are established and validated. The maximum value of the mean absolute relative error (MARE) for the AT model is 9.74%, while it decreases to 4.20% for the PSO-ANN model. The correlation coefficient (R) increases from 0.9050 for the AT model to 0.9737 for the PSO-ANN model. Overall, it is demonstrated that the established PSO-ANN model has a high prediction performance and is an effective model for reproducing the hot tensile behavior of the tested superalloy.

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

Investigation on the Fracture Mechanism and Constitutive Relations of a Typical Ni-Based Superalloy

Investigation on the Fracture Mechanism and Constitutive Relations of a Typical Ni-Based Superalloy

Study of Grain Growth in a Ni-Based Superalloy by Experiments and Cellular Automaton Model.

The grain growth behavior in a typical Ni-based superalloy was investigated using isothermal heat treatment experiments over a holding temperature range of 1353–1473 K. The experimental results showed that the grain structure continuously coarsened as the holding time and holding temperature increased during heat treatment. A classical parabolic grain growth model was used to explore the mechanism of grain growth under experimental conditions. The grain growth exponent was found to be slightly above 2. This indicates that the current grain growth in the studied superalloy is mainly governed by grain boundary migration with a minor pinning effect from the precipitates. Then, the grain growth in the studied superalloy during isothermal heat treatment was modelled by a cellular automaton (CA) with deterministic state switch rules. The microscale kinetics of grain growth is described by the correlation between the moving velocity and curvature of the grain boundary. The local grain boundary curvature is well evaluated by a template disk method. The grain boundary mobility was found to increase with increasing temperature. The relationship between the grain boundary mobility and temperature has been established. The developed CA model is capable of capturing the dependence of the grain size on the holding time under different holding temperatures.

A new refractory Ni7Nb2 phase identified in Laves eutectic regions by TEM study

The dissolution behavior of γ/Laves eutectic within a typical Ni-based superalloy (Inconel 718) was investigated by in-situ and conventional transmission electron microscopy (TEM) techniques. The results show that the Laves phase dissolved firstly at 700°C, but a new Ni7Nb2 phase possessing similar composition with δ-Ni3Nb phase precipitated near Laves phase did not dissolve at temperatures up to 950°C. The new Ni7Nb2 phase with lattice parameters: a=0.473 nm, b=0.794 nm, c=1.21 nm and β=91.3°, shows a C2/m space group symmetry. The crystal structure of the new Ni7Nb2 phase can be understood by “triple layers” as the Ni7Zr2-type structure does, stacking perpendicular to the (001) planes. Instead of stacking with one single shift vector s corresponding to the Ni7Zr2-type structure, the stacking sequences of the new Ni7Nb2 can be characterized by alternate occurrence of +s and -s. Moreover, the structural change of γ/Laves eutectic constituent before and after heat treatment was investigated. The Ni7Nb2 could remain after heat treatment. Since the experimental Ni7Nb2 structure was shown to be unstable in the Ni-Nb binary system by calculation, it is suggested that Zr, Si and Co promote the formation of Ni7Nb2 phase in superalloys according to the segregation behaviors of elements. Moreover, as Ni7Zr2 is considered to be detrimental to the mechanical properties, Ni7Nb2 phase should also have negative influences and should be eliminated through possible approaches. The finding of the new phase should shed light on the microstructural controlling and heat treatment process formulating of superalloys.

Numerical modelling on the plastic flow and interfacial self-cleaning in linear friction welding of superalloys

The self-cleaning effect of linear friction welding (LFW) is believed to be a crucial means to remove surface contaminants for achieving a satisfactory property of the resultant joints. To systematically investigate the self-cleaning process, a fully-coupled computational model was established based on an integrated approach considering dual plastic bodies for simulating the LFW of a typical Ni-based superalloy, GH4169. Since the removal of surface contaminants is determined by the material flow during LFW, the plastic flow behaviour was studied followed by the introduction of contaminant removal process and parameter optimisation. Results showed the material flow rate was at the highest level when two components were at the maximum relative displacement. The forging load continuously extruding the softened materials from the interface gave the greatest contribution to the material flow and formation of the flash. The removal rate of contaminants was initially increased followed by a subsequent decrease after reaching the peak during the welding process. The removal of 95 % surface contaminants was found to be a criterion to determine the critical welding parameter in simulation. The critical shortening length for removing the contaminants was decreased with increasing friction pressure, but an opposite trend was observed with the oscillation frequency. Validation by experiments suggested that the model provided reasonable and accurate predictions for flash formation, material flow, and final joint property in LFW of GH4169 superalloy. The present developed model could be utilised to select welding parameters of the LFWed Ni-based superalloy joint with a significant simplification to the manufacturing process.

Interdiffusion coefficients of alloying elements in a typical Ni-based superalloy

To study the precipitation reactions, homogenization of the as-cast structure, thermomechanical processing, and high-temperature creep of superalloys, understanding the interdiffusion parameters of alloying elements is a vital requirement. This requires consistent diffusion parameters, which should be obtained by one technique. Accordingly, in the present work, based on the concept of residual segregation index during homogenization treatment of as-cast Inconel 718 superalloy, the activation energy (Q) for interdiffusion of Nb, Mo, Cr, Ti, and Fe were determined as ~ 232, 286, 276, 256, and 280 kJ/mol, respectively. Moreover, the pre-exponential constant (D0) for Nb, Mo, Cr, Ti, and Fe were determined as 0.0037, 0.288, 0.283, 0.0430, and 0.31 cm2/s, respectively. Based on these results, the diffusion coefficient of Mo in austenite was found to be the lowest value, which was consistent with the fact that the diffusion of Mo is the controlling micro-mechanism of homogenization.

The nonlinear unloading behavior of a typical Ni-based superalloy during hot deformation: a new elasto-viscoplastic constitutive model

The nonlinear unloading behavior of a typical Ni-based superalloy is investigated by hot compressive experiments with intermediate unloading–reloading cycles. The experimental results show that there are at least four types of unloading curves. However, it is found that there is no essential difference among four types of unloading curves. The variation curves of instantaneous Young’s modulus with stress for all types of unloading curves include four segments, i.e., three linear elastic segments (segments I, II, and III) and one subsequent nonlinear elastic segment (segment IV). The instantaneous Young’s modulus of segments I and III is approximately equal to that of reloading process, while smaller than that of segment II. In the nonlinear elastic segment, the instantaneous Young’s modulus linearly decreases with the decrease in stress. In addition, the relationship between stress and strain rate can be accurately expressed by the hyperbolic sine function. This study includes two parts. In the present part, the characters of unloading curves are discussed in detail, and a new elasto-viscoplastic constitutive model is proposed to describe the nonlinear unloading behavior based on the experimental findings. While in the latter part (Chen et al. in Appl Phys A. doi: 10.1007/s00339-016-0385-0 , 2016), the effects of deformation temperature, strain rate, and pre-strain on the parameters of this new constitutive model are analyzed, and a unified elasto-viscoplastic constitutive model is proposed to predict the unloading behavior at arbitrary deformation temperature, strain rate, and pre-strain.

The nonlinear unloading behavior of a typical Ni-based superalloy during hot deformation: a unified elasto-viscoplastic constitutive model

The nonlinear unloading behavior of a typical Ni-based superalloy during hot deformation: a unified elasto-viscoplastic constitutive model

New Constitutive Model for Hot Deformation Behaviors of Ni-Based Superalloy Considering the Effects of Initial δ Phase

The hot deformation behaviors of a typical Ni-based superalloy are investigated by uniaxial tensile tests over wide ranges of strain rate and deformation temperature. The experimental results show that the flow stress is sensitive to strain, strain rate, and deformation temperature. Especially, initial δ phase (Ni3Nb) has a special effect on the flow stress. The initial δ phase can enhance the work-hardening behavior and result in the increased peak stress at relatively small strains. With the further straining, the initial δ phase can stimulate the dynamic recrystallization and promote the dynamic-softening behaviors. Considering the synthetical effects of deformation temperature, strain, strain rate, and initial δ phase on the hot deformation behaviors, a new phenomenological constitutive model is proposed. In the proposed model, the peak stress and material constant are expressed as functions of Zener-Hollomon parameter and the initial content of δ phase. A good agreement between the predicted and measured results shows that the proposed model can give an accurate and precise estimate of the hot deformation behaviors for the studied Ni-based superalloy.

Study of dynamic recrystallization in a Ni-based superalloy by experiments and cellular automaton model

The hot compressive behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rates of 0.001–1s−1 and deformation temperatures of 920–1040°C. It is found that the dynamic recrystallization is the main softening mechanism for the studied superalloy during hot deformation. The deformation temperature and strain rate have a significant influence on the dynamically recrystallized grain size. Based on the experimental results, an inverse power law equation is established to describe the relationship between the dynamically recrystallized grain size and the steady-state flow stress. A cellular automaton model with probabilistic state switches is established to simulate the dynamic recrystallization behaviors of the studied superalloy. The flow stress and the dynamically recrystallized grain size can be well predicted by the established model. Then, the dynamic recrystallization kinetic and the evolutions of the average grain size and grain boundary fraction are studied based on the simulated results. The simulated results show that the dynamic recrystallization is initially heterogeneous, and gradually becomes homogeneous with the increase of the volume fraction of dynamic recrystallization. With the increase of strain, the average grain size decreases, while the grain boundary fraction increases. Furthermore, the average grain size and the grain boundary fraction remain relatively constant when the deformation is under a steady state.

Constitutive models for high-temperature flow behaviors of a Ni-based superalloy

The high-temperature deformation behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rate of 0.001–1s−1and temperature of 920–1040°C. The experimental results show that the deformation behaviors of the studied superalloy are significantly affected by the deformation temperature, strain rate and strain. The flow stress increases with the increase of strain rate or the decrease of deformation temperature. The flow stress firstly increases with the strain to a peak value, showing the obvious work hardening behaviors. Then, the stress decreases with the further straining, indicating the dynamic flow softening behaviors. Considering the coupled effects of deformation temperature, strain rate and strain on the hot deformation behaviors of the studied Ni-based superalloy, the phenomenological constitutive models are established to describe the work hardening-dynamic recovery and dynamic softening behaviors. In the established models, the material constants are expressed as functions of the Zener–Hollomon parameter. The established constitutive models can give good correlations with the experimental results, which confirm an accurate and precise estimation of the flow stress for the studied Ni-based superalloy.

Effects of initial δ phase on hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy

Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920–1010°C and strain rate range of 0.01–0.001s−1. The effects of initial δ phase (Ni3Nb) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids.

Hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy

The hot tensile deformation behaviors and fracture characteristics of a typical Ni-based superalloy are studied by uniaxial tensile tests under the deformation temperature range of 920–1040°C and strain rate range of 0.01–0.001s−1. Effects of deformation parameters on the flow behavior, microstructural evolution and fracture characteristics are discussed in detail. The results show that the flow behaviors are significantly affected by the deformation temperature, strain and strain rate. Under relatively low deformation temperatures (920, 950 and 980°C), the flow curves are composed of three distinct stages, i.e., work hardening, steady stress and flow softening stages. The flow curves show the typical DRX characteristics under relatively high deformation temperatures (1010 and 1040°C). With the increase of deformation temperature or the decrease of strain rate, the fraction of recrystallized grains increases. The synthetical effects of localized necking and microvoid coalescence cause the fracture of the studied superalloy under all the deformation conditions. δ phase (Ni3 Nb) and carbides are the nucleus for the formation of microvoids. Also, δ phase plays an important role in the coalescence of microvoids.

Hot deformation behavior and processing map of a typical Ni-based superalloy

The hot compressive deformation behaviors of a typical Ni-based superalloy are investigated over wide ranges of forming temperature and strain rate. Based on the experimental data, the efficiencies of power dissipation and instability parameters are evaluated and processing maps are developed to optimize the hot working processing. The microstructures of the studied Ni-based superalloy are analyzed to correlate with the processing maps. It can be found that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress significantly decreases. The changes of instability domains may be related to the adiabatic shear bands and the evolution of δ phase(Ni3Nb) during the hot formation. Three optimum hot deformation domains for different forming processes (ingot cogging, conventional die forging and isothermal die forging) are identified, which are validated by the microstructural features and adiabatic shear bands. The optimum window for the ingot cogging processing is identified as the temperature range of 1010–1040°C and strain rate range of 0.1–1s−1. The temperature range of 980–1040°C and strain rate range of 0.01–0.1s−1can be selected for the conventional die forging. Additionally, the optimum hot working domain for the isothermal die forging is 1010–1040°C and near/below 0.001s−1.