Comparison of intergranular strain formation of conventional and newly developed nickel based superalloys

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Abstract

In this paper we report about the formation of intergranular strains in superalloy Haynes 282 studied by neutron diffraction during uniaxial tensile testing. The results gained from the initial bar material are compared to results of a peak-aged sample state at ambient temperature. A comparison of the results with data from a fully aged IN 718 alloy sample shows that intergranular strains are much lower in Haynes 282 than in the conventional nickel based superalloy. In contrast to IN 718 the formation of intergranular residual strains between bar material and heat treated sample state shows no significant differences. The relevance of the results to the macroscopic residual stress analysis by neutron diffraction on both continuous wavelength and spallation neutron sources is discussed in detail.

Introduction

Conventional nickel based superalloys are mainly used for high temperature applications. For example Inconel 718 (IN 718, chemical composition given in Table 1) shows excellent mechanical properties (yield strength, ductility, creep strength) at temperatures up to 650 °C and is thus often used as structural materials in the high temperature regions of gas turbines. In polycrystalline materials the mechanical behavior is based on the presence of a multitude of precipitates within the fcc matrix phase, namely γ (fcc, Ni3(Al,Ti)), γ (bct, Ni3Nb) and δ-phase (orthorhombic, Ni3Nb) [1].

Increasing effort has been put into developing new materials in the last couple of years, in order to raise the operation temperature in power engines for increased fuel efficiency. New materials have to combine high temperature creep strength, oxidation resistance and thermally stable microstructures with good fabrication properties, such as ductility and weldability. Besides new material classes, such as Nb-Si-X or Re based alloys, the recently developed nickel based superalloy Haynes 282 is considered as a substitute for conventional nickel based superalloys like Rene-41 and Waspaloy [2], [3]. The machinability of Haynes 282 for example by radius end milling is comparable to the machinability of aged IN 718 [4] while having very good creep resistance [2]. The excellent mechanical properties of Haynes 282 are based on a high solid solution hardening favored by the alloying elements Co, Cr and Mo. The chemical composition of Haynes 282 is given in Table 1. Aging of this alloy leads to the precipitation of the nano crystalline γ phase (fcc, Ni3(Ti,Al)) [5]. Cast Haynes 282 also exhibits the tetragonal σ phase (Cr, Ni, Mo and Co) [6].

It is well known that additional phases in materials may influence the build up of residual strains on different length scales. The proper determination of macroscopic stresses is crucial for predicting the service life time of components. A powerful method for the determination of stresses and strains is neutron diffraction. Due to the high penetration depth of neutrons information may be gained non-destructively up to a depth of several centimeters. It turned out, that in the diffractometric determination of macroscopic stresses, the residual stresses and strains occurring on the microstructural level between grains (intergranular stresses/strains) may lead to an erroneous estimation of macroscopic residual stresses and thus of the service life time predictions of high performance components [7], [8]. Consequently, a lattice parameter, which is not or only little affected by intergranular strains should be used for macroscopic residual stress analysis by diffraction. For Haynes 282 the formation of intergranular and interphase residual strains and stresses is largely unknown. The evolution of intergranular strains with increasing stress is directly observable in in-situ diffraction experiments, where a defined macroscopic stress state, applied through a mechanical test rig, is being subtracted from the measurement results [9]. The remaining strains are caused by intergranular stresses [10].

In this study we report on the formation of intergranular strains in the nickel based superalloy Haynes 282 studied by neutron diffraction during tensile testing. The results of the bar material and the fully aged material are compared to results obtained on fully aged IN 718 samples. The {hkl} dependent Young's modulus is discussed in detail for all three sample states. In contrast to observations made for IN 718 the formation of intergranular strains seems to be not affected by the aging process. Additionally, accumulation of intergranular strains is less pronounced in Haynes 282 than in IN 718. Conclusions are drawn regarding the impact of the results on the macroscopic residual stress analysis by neutron diffraction.

Section snippets

Material

To study the evolution of intergranular micro stresses under mechanical load and temperature, round tensile test specimens of Haynes 282 were produced by EDM (electrical discharge machining). One sample of the initial bar material was used in its original state to serve as reference material. Further test samples were solution annealed at 1080 °C for 2 h and air cooled (AC) down to room temperature. The annealing was followed by an aging heat treatment: 8 h at 780 °C followed by AC to room

Experimental details

All in-situ neutron diffraction experiments on the Haynes 282 samples were carried out at the time-of-flight beam line POLDI of the swiss neutron source SINQ@PSI [14]. A gauge volume of 3.8×10×3.8mm3 (width×height×depth) was chosen for all measurements. The depth dimension of the gauge volume was realized by a collimator for good spatial resolution. The corresponding neutron diffraction experiments of the IN 718 sample were performed at the E3 neutron diffractometer in Berlin [15]. The

Results

Table 3 shows the diffraction elastic constants Ehkl derived from the slope of the stress to lattice strain curves in the linear elastic regime of the material. The values for sample state HeatTreat show similar values as the sample state BarMaterial, which means – as expected – that the heat treatment did not change the elastic stiffness of the material. The values are about 5% higher as the corresponding values calculated with the Kröner model for pure nickel due to the alloying elements.

Discussion

The intergranular strains in both Haynes 282 sample states are identical within the error bars of the measurement (±150μϵ30MPa) (Fig. 3). This result is in contrast to observations for IN 718, where changes in the microstructure of the sample due to heat treatments appear to have a significant influence on the accumulation of intergranular strains [8], [13].

In IN 718 the γ phase was found to be responsible for large values of intergranular micro strains. For the {220} and {200} lattice planes

Conclusion

The following conclusions may be drawn from the results of the in situ tension tests on IN 718 and Haynes 282 samples:

  • 1.

    For beam lines operating with a continues neutron beam the use of the {111} and {311} Bragg reflections are the most favorable ones for all three sample states. This finding is consistent with the recommendation in the technical note of the ISO committee. If there is no reflection of the {111} and {311} family accessible, the use of the {220} Bragg reflection might deliver

Acknowledgments

We gratefully acknowledge the Swiss neutron source SINQ for providing beam time at POLDI. Furthermore, the authors thank the German neutron source FRM II for providing beam time at the powder diffractometer SPODI. We also thank the German neutron source in Berlin BER of the Helmholtz Association for providing beam time at the neutron diffractometer E3 and R.C. Wimpory for his technical and scientific support during the measurements.

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Present address: KNMF and IAM-WK, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

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