Svenja Kinzel Längenänderung der Legierungen Waspaloy und Inconel 718 nach Langzeitauslagerung ISBN: 978-3-8440-4877-3 Price: 45,80 € / 57,30 SFR |
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Within the scope of this work the contraction behavior of Ni-based superalloy Waspaloy could in detail be referred to a combination of different microstructural changes and the results could partially be transferred to Ni-Fe-based alloy Inconel 718. Isothermal annealing of sample rods at temperatures between 450°C and 750°C induces an average relative length contraction of about -2.10-4. It is apparent that contraction is more pronounced for lower temperatures (-3.10-4 at 550°C) than for higher ones (-1.10-4 at 750°C). Within the first 300 hours of annealing the contraction reaches about 70-75% of the value measured after 10,000 hours. This means the major part of the effect takes place at the beginning of long term annealing but even after 10,000 hours no saturation occurs. On the basis of lattice parameter measurements it could be found that within the first 300 hours a significant lattice parameter decrease of matrix and gamma´ phase emerged. Longer annealing time does not cause further lattice contraction. This sample behavior can be explained by temperature dependence of phase fractions and phase compositions. Thermodynamic ca1culations as well as stereological analysis of micrographs show a decrease of stable gamma´-phase content with increasing temperature. In parallel, TEM-EDS measurements and calculated phase fractions show concentration fluctuations due to the different precipitate fraction, which cause contraction of the lattice parameter. Furthermore, within the first 100 hours at temperatures up to 650°C the formation or Ni-Cr rich domains could be observed. As these domains exhibit a smaller lattice parameter than the matrix they contribute to the more pronounced contraction at lower temperatures. While XRD measurements point to the formation of Ni₃Cr, TEM-EDS measurements reveal a composition of (Ni,Co)₂Cr. Stress relief heat treatment at higher temperatures (815°C) after annealing shows that the effect of contraction is reversible. It causes an increase in lattice parameter and the Ni-Cr domains formed at lower temperatures are dissolved by the influence of higher temperatures. The sample elongation caused by stress relief heat treatment therefore is more pronounced for samples annealed at lower temperatures. The combination of lattice parameter change and formation of Ni-Cr rich domains explains the sample behavior within the first 300 hours. The further, slowly proceeding contraction for longer annealing times refers to carbide transformation. Particles of TiC and Ti(N,C) slowly dissolve in a peritectic reaction. An extrapolation of the results shows that complete dissolution will be reached after 14,000,000 hours (= 1,600 years). Dissolution of these phases leads to sample contraction reaching values of about -4,0.10-4 after 10,000 hours. A contrary effect is caused by the concurrent formation of Cr23C6, which consumes the free C-atoms from carbide dissolution. Formation of this carbide type with 1.4% volume fraction is finished after 16,000,000 h (= 1,800 years). Within 10,000 h the phase formation leads to sample elongation of about +3,4.10-4: The combination of both effects almost cancels out and therefore only causes contraction of about -0,6.10-4 , which is about 30% of the measured contraction. When the carbide transformation is complete, no further length change is expected. Although conditions are more complicated for alloy Inconel 718 due to the presence of additional phases (gamma" und delta-phase) the results can transferred in principal. The measured sample contraction is lower than for Waspaloy. Within the investigated temperature range the calculated phase diagram shows significant changes only for the phase fraction of gamma´ phase. As volume fraction of gamma´ precipitates is small (<10%) for Inconel 718, the compositional fluctuations caused by its dissolution are low. The measured length contraction at 450°C and 550°C therefore is only about -1,3.10-4. Temperatures of 650°C and above lead to formation of 8 phase, which enhances the contraction to -2,6.10-4 for standard heat treated samples. The reason for the lower impact of this effect for short-term heat treated sample could not fully be resolved in this work. A detailed investigation of the influence of gamma" phase an intense use of TEM techniques is necessary. Additionally, alloy Inconel 718 also shows low phase fractions of carbides and carbonitrides. Dissolution of (Ti,Nb)(C,N) has a bigger impact on contraction but is nearly stable while a slight decrease in (Nb,Ti)C content is measu,red. This causes a contraction of -0,6•10-4, which is way smaller than contraction caused by carbide dissolution in Waspaloy. This contributes to the reason, why overall contraction of Inconel 718 is less pronounced. The combination of several microstructural changes comprehensively explains the effect of contraction of Ni-based superalloys. The major part of the effect is caused by lattice parameter changes due to temperature dependent changes in stable phase fractions and compositions. The further minor contraction is referred to carbide transformation process. | |
Source: INIS International Nuclear Information System DE17F6418 | |
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