Microstructure and temperature dependence of intergranular strains on diffractometric macroscopic residual stress analysis
Introduction
Residual stresses on different length scales significantly influence the mechanical behaviour of components [1]. Beside macroscopic residual stresses, which may affect the service life time of high performance components positively or negatively, the role of intergranular strains and stresses has been at the center of attention in recent years. Intergranular residual strains occur on the micro-structural level and originate from non-uniform deformation of differently oriented, adjacent grains under macroscopic (thermo-) mechanical loading. Thus, knowledge of initiation and evolution of intergranular strains gives valuable information on microscopic deformation behaviour [2], [3].
Furthermore, intergranular strains may influence the outcome of diffractometric measurements to determine macroscopic residual stress and may lead to large spurious stesses. Relative strain and stress values are determined during diffractometeric macroscopic residual stress analysis by comparison of 2θ values measured on the component of interest and a macroscopic stress-free reference value 2θ0. Often it is usually assumed that the intergranular stress state is constant in the sample of interest and by use of a reference sample is thus canceled out during analysis. That this assumption is not always fully fulfilled is shown for example in a relaxation study on an Inconel 718 (IN 718) disc [4], where the intergranular strains changed during the cut of the macro stress free reference sample. This led to spurious strains of up to 25% of the actual macroscopic stress value.
The disregard of spurious strains in residual stress evaluation may cause high costs by overestimating the occurring stress states or even worse by unexpected component failures. The current state-of-the art methods to investigate the development of intergranular strains are in-situ mechanical load tests using diffraction, where the evolution of lattice spacings is observed [5], [6], [7], [8].
Most of the in-situ experiments were conducted under ambient conditions, concentrating on the basic mechanisms of how intergranular strains and stresses build-up. However, few studies exist, where the formation of intergranular strains has been investigated at higher temperatures. This is especially important for high performance alloys (e.g. Ni-base superalloys) as they are mainly used in high temperature applications, such as for components of gas turbines. At elevated temperatures, thermally activated processes, like dislocation climb, accelerate the plastic deformation process [9].
Many in-situ experiments studied the micro mechanical behaviour in the creep regime, which starts at about . In contrast, only a few in-situ high temperature experiments under (quasi) static conditions are reported in literature [10], [11], [12]. Beside the intergranular strains for each family, quantities like diffraction elastic constants can be evaluated from quasi static experiments. These values are important input parameters for (i) material models used to predict mechanical behaviour of components and (ii) analyzing and interpreting macroscopic residual stresses determined by diffraction methods.
In this contribution we compare the lattice strain evolution of IN 718 as a function of the applied load at two different temperatures. The lattice strains are observed in quasi static in-situ mechanical tests using neutron diffraction. The results are presented for two different industrial relevant microstructures. The microstructures are discussed in detail and the evolution of lattice strains and evolution of intergranular strains for both sample states are presented and discussed. Particularly their significance on macroscopic residual stress analysis is emphasized. The diffraction elastic constants for the measured reflections are presented for both temperature and sample states.
Section snippets
In-situ mechanical load tests
In-situ mechanical load tests combine the mechanical testing of a sample with diffraction experiments, which monitors varying lattice spacings with changing mechanical load [2].
Typically, the stress vs. total sample strain curve is determined in a tensile test experiment. The Young׳s modulus E is given by the slope of the stress strain curve in the elastic regime . Combined with the strain in the direction perpendicular to the load , the Poisson׳s ratio is given by .
Material
The nickel based superalloy IN 718 is an important high performance alloy for the aerospace industry [15]. Its main applications are in blades, discs and shafts in the high temperature regions of turbines. The remelted and converted casing of IN 718 prematerial used in this study was provided by Böhler Schmiedetechnik GmbH & Co KG. Table 1 shows its chemical composition.
The TTT diagram of IN 718 shows a multitude of possible precipitates [16]. The main precipitates, which are formed in the fcc γ
Experimental details
The evolution of lattice strains with increasing mechanical load at room temperature for the measurement direction parallel to the load direction was observed at the E3 neutron diffractometer in Berlin [23] by in-situ mechanical testing. The wavelength was and a gauge volume given by the irradiated volume of 4×4×6 mm3 (width×height×depth) was chosen.
The lattice strain evolution perpendicular to the direction of applied mechanical load at room temperature as well as the in-situ tests at
Results
Fig. 7 shows the lattice strain evolution of the measured lattice planes as a function of the applied stress for all measured sample states and temperatures. Each figure shows both measurement directions: parallel and perpendicular to the applied load direction.
Discussion
At least one of two criteria should be fulfilled to minimize the influence of intergranular strains on macroscopic residual stress analysis, depending on the {hkl} reflection used. Firstly quantitative numbers of strain values can be evaluated. The smaller the value, the smaller the build-up of intergranular strains, and thus the lower the influence on macroscopic residual stress analysis. The second criteria relies on the fact that for diffractometric macroscopic residual stress analysis the
Conculsion
By combining different experimental techniques it was possible to characterise two different samples of IN 718 with respect to their microstructure and to study the formation of intergranular strains at ambient temperature and at T=550 °C using neutron diffraction. The results show in particular:
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The diffraction elastic constants (XEC) for IN718 follow the trend predicted by the Kröner model for pure nickel. This applies to the data at ambient temperatures as well as at T=550 °C. In addition it is
Acknowledgments
We gratefully acknowledge the DFG for funding this research within projects WE 2351/11-1 and PE 580/7-1. In addition the authors thank the German neutron source FRM II for providing beam time at instruments STRESS-SPEC and SPODI and the instrument scientists M. Hölzel and A. Senyshyn for their support during and after the powder diffraction measurement. The authors also would like to thank M. von Zimmermann for the support during texture measurements at HASYLAB.
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Work done at: Paul Scherrer Institute, Materials Science and Simulation, NUM/ASQ, 5232 Villigen PSI, Switzerland.