Superplastic deformation and microstructure evolution in PM IN-100 superalloy

The superplastic deformation behaviour of PM IN-100 alloys consolidated by hot isostatic pressing (HIP) was investigated in compression tests at temperatures between 1323 and 1373K. The microstructural changes were observed using scanning electron microscopy. In the high strain rate region, grain refinement occurs due to dynamic recrystallization, resulting in the work softening type stress-strain curves. At low strain rates, grain growth occurs during deformation corresponding to work hardening. The strain rate sensitivity index,m, reaches a maximum value (m = 0.6) at the optimum strain rate which depends on the test temperature. The grain size dependence coefficient,p, was determined to be 2.0. The activation energy for deformation was 348kJ mol^–1. The rate-controlling mechanism of superplasticity in as HI Ped IN-100 seems to be the grain-boundary sliding controlled by volume diffusion rather than grain-boundary diffusion.     

Microstructural changes during the fractional melting of IN-100 cast nickel-base superalloy

The application of fractional melting to IN-l00 cast nickel-basesuperalloy can reduce its microporosity and lower the amount of γ+ γ’ eutectic. It can also homogenzze the dendritic eutectic and grain boundary segregations, which are greatly znfluenced by the applied pressure and maximum pressing temperature. After fractional melting the . . size and morphology of γ’ and the eutectic are slightly different, and some M₂₃C₆ preczpztates out. Chemical composition is little changed below 1323°C.     

Effect of boron addition on the microstructure and mechanical properties of K4750 nickel-based superalloy

The effect of boron addition at 0,0.007 wt.% and 0.010 wt.% on the microstructure and mechanical properties of K4750 nickel-based superalloy was studied.The microstructure of the as-cast and heat-treated alloys was analyzed by SEM,EPMA,SIMS and TEM.Lamellar M₅ B₃-type borides were observed in boroncontaining as-cast alloys.After the full heat treatment,boron atoms released from the decomposition of M₅ B₃ borides were segregated at grain boundaries,which inhibited the growth and agglomeration of M₂₃C₆ carbides.Therefore,the M₂₃C₆ carbides along grain boundaries were granular in boron-containing alloys,while those were continuous in boron-free alloys.The mechanical prope rty analysis indicated that the addition of bo ron significantly improved the tensile ductility at room tempe rature and stress rupture properties at 750℃/430 MPa of K4750 alloy.The low tensile ductility at room temperature of 0 B alloy was attributed to continuous M₂₃C₆ carbides leaded to stress concentration,which provided a favorable location for crack nucleation and propagation.The improvement of the stress rupture properties of boron-containing alloys was the result of the combination of boron segregation increased the cohesion of grain boundaries and granular M₂₃C₆ carbides suppressed the link-up and extension of micro-cracks.     

Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000 °C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900 °C. At temperatures up to 1000 °C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C → M₂₃C₆ + Me (W, Ni, Cr, Mo). At temperatures below 900 °C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000 °C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000 °C.     

Effect of element Ru on microstructure and creep behaviour of single crystal nickel-based superalloy

By means of creep property measurement and microstructure observation, the influence of element Ru on the microstructure and creep behaviour of single crystal nickel-based superalloy is investigated. The results show that the plate-like μ phase is precipitated along {111} plane of the Ru-free alloy during thermal exposure at 1080°C and creep at 980°C. And the precipitation of μ phase in the alloy with 6(wt)%Mo and 6(wt)%W may be restrained by adding 2% Ru element. Compared to Ru-free alloy, the creep life of the 2% Ru single crystal alloy at 980°C200 MPa increases form 123 h to 333h. Compared to the 2% Ru alloy, the precipitated plate-like μ phase in the Ru-free alloy may promote the initiation and propagation of cracks along the γ matrix up to fracture, which is thought to be the main reason of the alloy having a lower creep resistances and shorter lifetime.     

Effect of strain rate on microstructure evolution of a nickel-based superalloy during hot deformation

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s⁻¹ at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s⁻¹, the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ3 boundaries at the intermediate strain rate of 1 s⁻¹, while the fractions of Σ3 boundaries were much higher at both the lower strain rates ($\dot{\epsilon }⩽0.1{\text{s}}^{-1}$) and higher strain rates ($\dot{\epsilon }⩾5{\text{s}}^{-1}$).     

Effects of aging treatments on the Microstructure of the Ni-base superalloy IN-738

The effects of aging treatments on the microstructure of the Ni-base superalloys IN-738 HC and IN-738 LC were investigated. Changes in (i) size, morphology, and distribution of the γ′-phase, (ii) the amount of M₂₃C₆-carbide, and (iii) the occurrence of additional phases and phase instabilities were quantitatively determined. The results indicate the possibility to deduce an unknown thermal history from the observed microstructure.     

Plastic deformation behavior of a nickel-based superalloy on the mesoscopic scale

Nickel-based superalloys have become the key materials of micro-parts depending on excellent mechanical properties at high temperatures. The plastic deformation behavior is difficult to predict due to the occurrence of size effect on the mesoscopic scale. In this paper, the effect of specimen diameter to grain size ratio(D/d) on the flow stress and inhomogeneous plastic deformation behavior in compression of nickel-based superalloy cylindrical specimens was investigated on the mesoscopic scale. The results showed that when D/d is less than 9.7, the flow stress increases with the grain size. Aiming at this phenomenon, a flow stress size effect model considering compressive strain partitioning was established.The calculated flow stress values agree well with the experimental values, thus revealing the effect of D/d on the flow stress in compression of nickel-base superalloy on the mesoscopic scale. The inhomogeneous plastic deformation during compression deformation increases with the grain size. The end surface profiles evolve from a regular circular shape to an irregular shape with the grain size. The surface folding phenomenon occurs only in partially compressed specimen with a few grains across the diameter.Crystal plasticity finite-element(CPFE) simulation of compression deformation on the mesoscopic scale real-time displayed the evolution of microstructure. The study of this paper has important guiding significance for understanding the influence of D/d on the compression deformation behavior of nickel-based superalloy on the mesoscopic scale.     

Phase stability of B1-type carbides in nickel-based alloys

In nickel-based superalloys, the B1-type monocarbides contain several kinds of transition elements. At elevated temperatures, the monocarbides transform to other carbides, such as M₂₃C₆ and M₆C as a result of the reaction with the alloy. The phase stability of monocarbides was investigated by X-ray diffraction techniques. The degree of stability is strongly affected by the composition of the monocarbides and is related generally to the magnitude of the lattice parameter of the monocarbides. That is, the larger the lattice parameters of the monocarbides, the higher their stability in nickel-based alloys.     

Effect of transition metal elements on the structural and optical properties of ZnO nanoparticles

Undoped and transition metal (TM)-doped ZnO nanoparticles (Zn_0.98X_0.02O-NPs, X = Mn, Cr, Co and Fe) were synthesized from a metal nitrate precursor and gelatin by a sol–gel method. The compounds were synthesized at calcination temperatures of 550^°C for 6 h. The synthesized undoped/doped ZnO-NPs were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). XRD results indicated that the sample products were crystalline with a hexagonal wurtzite phase. SEM images showed the ZnO-NPs nearly spherical shapes and a non-uniform shape for doped ZnO-NPs. The crystalline development in the ZnO-NPs was investigated by X-ray peak broadening. The size–strain plot (SSP) method was used to study the individual contributions of crystallite sizes and lattice strain of the undoped and doped ZnO-NPs. The obtained results showed that strain of the NPs plays an important role in peak broadening; moreover, the mean crystalline size of the undoped and doped ZnO-NPs estimated from the SEM and the SSP method was highly inter-correlated. Finally, optical properties of the samples were studied by a UV–Vis spectrometer.     

Microstructure and tensile properties of nickel-based superalloy K417G bonded using transient liquid-phase infiltration

Nickel-based superalloy K417G samples with 2.2 mm gap are bonded by transient liquid-phase infiltration (TLI). The microstructure and tensile properties of TLI joint have been investigated. The results show that the nonuniform microstructure is obtained in the TLI joints. The voids, Cr₅B₃ borides and (Cr, W)B borides are presented in the TLI region. The no isothermal solidification of the transient TLI joint leads to the formation of Cr₅B₃ borides. The tensile properties of TLI joint samples are lower than that of base sample, especially the ductility. The observation of fracture surfaces show that the Cr₅B₃ borides are the main cause, which leads to the decrease of ductility. The Cr₅B₃ borides can act as the crack initiation and also accelerate the propagation of cracks during the deformation.     

Effect of postweld heat treatment on development of interfacial structures in nickel-based transition joints

Abstract

The interfacial structures between 2·25Cr–1Mo steel and Inconel 182 weld metal have been studied after post-weld heat treatments (PWHTs) at 700°C and subsequent aging at 630°C. This aging temperature accelerates the changes in interfacial structure that occur during power-station operation, and thus provides a method of studying the effects of the initial PWHT. The paper shows that during PWHT for ≤8 h at 700°C, arrays of carbide particles develop in the ferritic steel, parallel and very close to the weld metal interface, and that these arrays continue to grow during aging at 630°C for 6000 h. However, the precipitate sizes after PWHT are small compared with those developed during the subsequent aging. For longer heat treatments, up to 100 h at 700°C, the I interfacial precipitates develop to significant sizes, but growth then ceases because of the limited carbon migration from the 2·25Cr–1Mo steel. Aging at 630°C then causes carbon redistribution in the heat-affected zone, resulting in an incubation period before further interfacial precipitate growth can occur. The net result is that after aging for ≥500 h at 630°C, PWHTs of 2–100 h at 700°C have negligible effects on the interfacial-precipitate sizes and distributions, compared with those found in similarly aged as-welded specimens.

MST/119     

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

Prediction of oxidation-assisted crack growth has been carried out for a nickel-based superalloy at elevated temperature based on finite element analyses of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. The material constitutive behaviour, implemented in the finite element code ABAQUS via a user-defined material subroutine (UMAT), was described by a unified viscoplastic model with non-linear kinematic and isotropic hardening rules. Diffusion of oxygen was assumed to be controlled by two parameters, the oxygen diffusivity and deformation-assisted oxygen mobility. Low frequencies and superimposed hold periods at peak loads significantly enhanced oxygen concentration near the crack tip. Evaluations of near-tip deformation and oxygen concentration were performed, which led to the construction of a failure envelop for crack growth based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure envelop was then utilised to predict crack growth rates in a compact tension (CT) specimen under fatigue-oxidation conditions for selected loading ranges, frequencies and dwell periods. The predictions from the fatigue-oxidation failure envelop compared well with the experimental results for triangular and dwell loading waveforms, with marked improvements achieved over those predicted from the viscoplastic model alone. The fatigue-oxidation predictions also agree well with the experimental results for slow-fast loading waveforms, but not for fast-slow waveforms where the effect of oxidation is much reduced.