The two-dimensional scattering patterns for micro-structural changes during creep observed by SAXS are different from that obtained by SANS technique. The changes in morphology and size characteristics of different regions of the secondary γ' precipitates have been demonstrated by the variation of SAXS scattering intensity. The results show that the secondary γ' precipitates have two types of feature sizes, which have similar trends of change during the creep process characterized by decreasing in the first and second stage and increasing in the final stage. As comparison shows, the larger γ' precipitates have more noticeable changes. The elements of the secondary γ' precipitates diffuse seriously in the second stage with the character of creep 15 h, the surfaces of γ' phase are blurred, and interfaces of two-phase become clear again in the final stage. Due to the increasing size or the reducing number of the secondary γ' phase, the total ares of interfaces between the secondary γ' precipitates and matrix phase then decreases.
Nickel-based superalloys with multimodal γ′ size distribution microstructure have usually been widely used in many extreme engineering applications due to their excellent mechanical properties at high temperatures. Understanding the underlying mechanisms is vital to provide some insight into improving the performance. Up to now, a well understanding of the deformation mechanisms of unimodal microstructures has been obtained. However, it remains mysterious for superalloys with multimodal microstructures due to the complex size and distribution of γ′ particles. In this study, bimodal superalloys of GH4738 and GH4720Li alloys were selected. In GH4738 alloy, the fine and coarse γ′ particles are 32 nm and 221 nm, respectively, in which the coarse γ′ particles are all intragranular. In GH4720Li alloy, the fine and coarse γ′ particles are 37 nm and 1800 nm, respectively, in which most of the coarse γ′ particles are intergranular. In-situ neutron diffraction assisted by a 2-site elastic-viscoplastic self-consistent (EVPSC) model was employed to elucidate the deformation mechanisms. The effects of γ′ particles in different locations on the partitioning of interphase stress and deformation behaviour have been revealed through comparing with other unimodal superalloys. The results indicate that load partitioning between γ and γ′ phases is distinct between the unimodal and bimodal superalloys. On the one hand, premature load partitioning is observed before microscopic yielding, indicating that γ phase starts to deform plastically in elastic region. It could be attributed to the much lower critical stress of Orowan looping for intragranular coarse γ′ particles or the dislocations emitted by the straight interface plane of intergranular coarse γ′ particles in the bimodal superalloys. On the other hand, the extent of load partitioning at 15% strain for GH4738 and GH4720Li alloys is much lower than that of the unimodal superalloys with coarse γ′ particles, demonstrating that the effect of coarse γ′ particles on load partition is weakened by the shearing effect of fine γ′ particles. Those findings could contribute to a better understanding of deformation mechanisms in superalloys with multimodal γ′ size distributions and shine a spotlight on material design to modulate intergranular and interphase stresses.
A new thermal neutron detector with the domestically produced THGEM (THick Gas Electron Multiplier) was developed as an alternative to 3He to meet the needs of the next generation of neutron facilities. One type of Au-coated THGEM was designed specifically for the neutron detection. A detector prototype had been developed and the preliminary experimental tests were presented, including the performance of the Au-coated THGEM working in the Ar/CO2 gas mixtures and the neutron imaging test with 252CF source, which would provide the reference of experimental data for the research in future.
In this paper,A MySAS package,which is verified on Windows XP,can easily convert two-dimensional data in small angle neutron and X-ray scattering analysis,operate individually and execute one particular operation as numerical data reduction or analysis,and graphical visualization.This MySAS package can implement the input and output routines via scanning certain properties,thus recalling completely sets of repetition input and selecting the input files.On starting from the two-dimensional files,the MySAS package can correct the anisotropic or isotropic data for physical interpretation and select the relevant pixels.Over 50 model functions are fitted by the POWELL code using x~2 as the figure of merit function.
To achieve efficient design and accurate simulation of neutron spin flipping, a fast numerical calculation method was introduced to facilitate the processes parameter optimization and flipper design. Magnetic field models and measured magnetic data can be directly imported into the simulation. To test the proposed new simulation software, three experimental examples were performed and compared with the measured data. The software developed showed good accuracy.
According to layout of a triple-axis spectrometer (TAS), neutron beam parameters at the sample position, such as the homogeneity, divergence and flux, were calculated by Monte-Carlo simulation. The results show that using a double-focusing monochromator, the beam flux is 1.54 times larger, but the neutron distribution is worse, than using a flat monochromator, while the vertical maximum divergence increases to 4°. Vertical divergence of the neutrons decreases with increasing focusing lengths, while the beam homogeneity does not change obviously. A pyrolytic graphite (PG) filter is suitable for neutrons in wavelengths of less than 0.4 nm, while a Be filter is better than PG filter for neutrons in wavelengths of longer than 0.4 nm.
With the diversification of service environment, especially the applications facing high-speed deformation and impact load, it has important theoretical and practical value to clarify the deformation mechanisms of superelastic NiTi alloys under dynamic and impact loadings. In this paper, we use neutron diffraction technique to study the bulk structural responses of superelastic NiTi alloy after dynamic impacts, and comparatively carry out in-situ quasi-static loading neutron diffraction experiments, focusing on structural characterizations. The objective is to (1) statistically obtain the parameters such as crystal structure, phase transformation fraction, dislocation density and crystal orientation distribution of NiTi alloy after impact loading, (2) compare and analyze the influence of strain rate on martensitic transformation, plastic deformation and texture evolution in the range of ~10 -5 s -1 to 3×10 3 s -1 , (3) give the differences in deformation mechanism of NiTi alloy at various strain rates. Further, we combine the experimental results of neutron diffraction with constitutive model, select and derive a phenomenological model describing the strain rate dependent deformation process of NiTi alloy through experimental knowledge, and validate the model with the quantified experimental results. Benefiting from the unique nondestructive and deep penetration of neutron beams into materials, neutron diffraction can provide bulk structural parameters with enough macroscopic statistics, which effectively complements the surface or micro-area analysis by conventional experimental methods. The results show that impact loading obviously prevents the stress-induced martensitic transformation of NiTi alloy, and the austenite parent phase undergoes more significant slip deformation during high-speed deformation. In contrast, the diffraction line broadening due to dislocation multiplication is hardly observed in the parent phase under quasi-static conditions. Through the estimation and comparison of temperature effect, it is ruled out that the strain rate effect is caused by the adiabatic temperature rise. The constitutive model constructed according to the experimental knowledge captures the main deformation characteristics, most of the parameters are determined experimentally, and the model results well describe the strain rate dependent mechanical properties of the superelastic NiTi alloy.
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