Maicang Zhang

HIGH-TEMPERATURE OXIDATION OF FGH96 P/M SUPERALLOY

High temperature oxidation behaviors of FGH96 P/M superalloy have been studied in air at temperatures ranging from 600 to 1000°C. By means of isothermal oxidation testing. X-ray diffraction, SEM (scanning electron microscopy), and EDS (energy dispersive X-ray spectroscopy) analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800°C. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr 2 O 3 , TiO 2 , and a little amount of NiCr 2 O 4 . The oxidation is controlled by the transmission of chromium, titanium, and oxygen through the oxide scale.

Solidification characteristics and hot tearing susceptibility of Ni-based superalloys for turbocharger turbine wheel

Abstract The solidification characteristics and the hot tearing susceptibility were investigated on two Ni-based superalloys for turbocharger turbine wheel, K418 and K419. The segregation behaviors of the alloying elements and the precipitation phases were also studied. The results show that the solidification behavior of K419 alloy is complicated when compared with K418 due to the interdendritic segregation of many kinds of strong interdendritic partitioning elements in the remaining liquid at the final stage of solidification. The segregation of multiple elements in interdendritic liquid results in an extremely low solidus in K419. A long residual liquid stage is found during the solidification of K419, giving rise to reduced cohesion strength of dendrites and increased sensitivity to hot tearing. A hot tearing susceptibility coefficient (HTS) criterion is proposed based on a hot tearing sensitive model. The HTS value of K419 alloy is larger than that of K418 alloy.

A Study on the Recrystallization Behavior of Ni-Based Alloy G3 During Hot Deformation

An integrated microstructure evolution model of thermomechanical processing was developed in terms of dynamic recrystallization (DRX), post-dynamic recrystallization (PDRX) and grain growth. Hot compression tests were carried out on a Gleeble-1500 thermal simulator under different conditions to model DRX, PDRX and short-time grain growth during the post-deformation and cooling process. Furthermore, in combination with the established microstructure evolution models, an elastic–plastic finite element model was built using DEFORM-2D software to simulate the microstructure evolution during the hot extrusion process. The simulation result was compared with the microstructure of a hot-extruded pipe of alloy G3 manufactured in a factory. The simulation results agree well with the experimental ones, validating the accuracy of the established microstructure evolution model. Furthermore, the finite element simulation is an effective method for hot deformation analysis, which can provide theoretical guidance for the optimization manufacturing parameters.

Constitutive behavior and processing maps of low expansion GH909 superalloy

The hot deformation behavior of GH909 superalloy was studied systematically using isothermal hot compression tests in a temperature range of 960 to 1040°C and at strain rates from 0.02 to 10 s−1 with a height reduction as large as 70%. The relations considering flow stress, temperature, and strain rate were evaluated via power-law, hyperbolic sine, and exponential constitutive equations under different strain conditions. An exponential equation was found to be the most appropriate for process modeling. The processing maps for the superalloy were constructed for strains of 0.2, 0.4, 0.6, and 0.8 on the basis of the dynamic material model, and a total processing map that includes all the investigated strains was proposed. Metallurgical instabilities in the instability domain mainly located at higher strain rates manifested as adiabatic shear bands and cracking. The stability domain occurred at 960–1040°C and at strain rates less than 0.2 s−1; these conditions are recommended for optimum hot working of GH909 superalloy.

Stress Relaxation Behavior Comparison of Typical Nickel-Base Superalloys for Fasteners

In the present work, stress relaxation behavior of three typical nickel-base Superalloys: Alloy 718, Waspaloy and AEREX 350, were investigated in a temperature range of 600–800 °C for up to 10 h. The effects of stress relaxation parameters on behavior were analyzed and stress relaxation characteristics of the three superalloys are compared and mechanisms revealed by FESEM and TEM observation. The study results show that the stress relaxation property of Alloy 718 is very sensitive to temperature. It is very stable at 650 °C, but decreases extremely with temperature increasing to 750 °C as a result of serious microstructure degeneration. Moreover, stress relaxation stability is related with initial stress, and appropriate increase of initial stress under normal service temperature can increase the stress relaxation limit of Alloy 718. The stress relaxation resistance of Waspaloy decreases with increasing temperature. The increase of initial stress and initial strain is beneficial for stress relaxation resistance of Waspaloy, but the influencing degree is related with temperature. Furthermore, Waspaloy with heat treatment A (1020 °C × 4 h/AC + 845 °C × 4 h/AC + 760 °C × 16 h/AC) shows better stress relaxation resistance than that with heat treatment B (1080 °C × 4 h/AC + 845 °C × 24 h/AC + 760 °C × 16 h/AC). In addition, stress relaxation stability of AEREX 350 is the best among the three superalloys in the temperature range of 600–800 °C on the whole. The combined effect of γ′ phase and η phase guarantee the stress relaxation property of AEREX 350.

The Correlation Between Grain Orientation Evolution and Stress Rupture Properties of Waspaloy

AbstractThe grain orientation evolution of Waspaloy with different initial grain sizes under the same stress rupture tests were systematically investigated by electron back-scattered diffraction (EBSD). Correlation between the grain orientation evolution and the changes of stress rupture properties were also studied. The results show that the specimens with different initial grain sizes present different grain orientation evolution behaviors during the constant stress loading process, and the stress rupture lives of alloy are closely related to the orientation changes. Specimen with large initial grain size generally exhibits a small lattice rotation, in the [101] orientation and consequently, a long stress rupture life. On the converse, the fine grain specimen exhibits significantly shorter life with much more [111] orientation, since the lattice is obviously rotated and the deformation has little coordination. Further analysis shows that initial grain size affects the misorientation distribution in the grain, which has a significant influence on the stress rupture life. The lower the misorientation angle and the higher the proportion of the low-angle misorientation is, the longer the rupture time it exhibits.