When exposed for long time at elevated temperatures of 430°C and 650°C the nickelbase superalloy EI 698 VD can experience a significant decrease in the creep. The cause of the creep degradation of nickel-base superalloy has never been unequivocally explained, but it has generally attributed to the microstructural instability at prolonged high temperature exposure. The creep lifetime decrease was more expressive as time of thermal exposure was extended up to 52 000 hours. In this article, the creep life data, generated on prior long thermally exposed wrought nickelbase superalloy were related to the microstructural changes observed using SEM and TEM analysing techniques. Qualitative and quantitative structure analyses provided supporting evidence concerning changes associated with grain boundary carbide precipitation and with volume fraction changes of gamma prime the exposure time prolonged. In order to clarify the role of gamma prime distribution modification in thermally exposed superalloy on creep degradation the SANS (Small Angle Neutron Scattering) diffraction experiment was employed in the characterization of the gamma prime morphology and the size distribution with respect to the period of thermal exposure.
is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. a b s t r a c tIn situ neutron diffraction loading experiments were carried out on a cold-rolled dual-phase (a-phase, $ 10% b-phase) Zr-2.5%Nb alloy at room temperature. The specimens were cut at different angles from the rolling direction (RD) towards the transverse direction (TD), thus the loading axis changes gradually from the rolling to transverse direction. Due to the strong texture of the studied alloy, and unidirectional nature of deformation twinning, the changing loading direction with respect to initial texture has a significant impact on the collaborative slip-twinning deformation mode in the hexagonal close-packed (hcp) a-phase. The present neutron diffraction results provide direct evidence of {1 À 1.2}/1 À 1. À 1S ''tensile'' twins in the a-phase of dual-phase Zr-2.5%Nb alloy at room temperature.Additionally, TEM analysis was employed to confirm the presence of ''tensile'' twins, and determine if other type of twins were present. It is further clear from the neutron diffraction results that applied load is gradually transferred from the plastically softer a-phase to the plastically harder b-phase which acts as a reinforcing phase having a yield strength in the range 750-900 MPa depending on the loading direction.
High strength and ductility of the TRIP steels is often attributed to the transformation induced plasticity effect resulting from the strain induced martensitic transformation of the retained austenite in the bainite microstructure. The present work reports results of in-situ neutron diffraction experiments focused on monitoring the phase evolution in two TRIP steel samples (two different thermomechanical treatments) subjected to tensile loading at room temperature. Comparison of the single lineprofile analysis of reactor data (TKSN-400 at NPI Rez) and multi lineprofile analysis of data obtained at spallation neutron source (diffractometer ENGIN-X at ISIS RAL Chilton) suggests that the former can be used in the first approximation for in-situ monitoring of the phase evolution in TRIP steels subjected to mechanical loads.
An excellent combination of high strength and fomiability can be oblained in a Si-Mn TRIP steels when processed by thermomechanical treatmetit consisting of high temperature deformation followed by isothennal holding in the bainite region and cooling to room temperature. Microsiructure of these steels consists of ferrite. hainite and retained austenite. The combination of high strength and ductility is attributed to the transformation-induced plasticity (TRIP) effect resulting from the strain induced trans form at ion of the retained austenite to martensite. These steels exhibit tensile strengths ranging 800-10(X)MPa and elongation up to 710%. In the present paper, the influence of transformation conditions on volume fraction of ferrite and retained austenite in selected 0.2CI.9Sil.45Mn TRIP steel was investigated. Based on results of in situ high-temperature neutron diffraction experiments, the set of specimens was prepared and their transformation characteristics at room temperature were studied again by neutron diffraction experiments realized in situ upon tensile tests. The reported neutron diffraction method has been found as an efficient tool for characterization of austenite transformation proceeding in TRIP steels during combined thermomechanical treatments.
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