| 1. |
- Chai, Guocai, et al.
(författare)
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Mechanisms of Hydrogen Induced Stress Crack Initiation and Propagation in Super Duplex Stainless Steels
- 2009
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Ingår i: STEEL RESEARCH INTERNATIONAL. - 1611-3683. ; 80:7, s. 482-487
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Tidskriftsartikel (refereegranskat)abstract
- Austenitic and ferritic duplex stainless steels, DSS, have recently suffered from hydrogen stress induced cracking, HISC, in subsea components with a cathodic protection. This paper provides discussions on possible HISC mechanisms. HISC initiation can occur at the ferritic grain boundaries and phase boundaries at a stress lower than the yield strength, but dominantly at phase boundaries at a stress higher than the yield strength. EBSD analysis shows that HISC in DSS results from the interaction between the dynamic plasticity by creep and hydrogen diffusion. A model on the formation of microstresses in these two phases under creep conditions is proposed, which explains why HISC occurs mainly in the ferritic phase. Discontinuous two-dimensional HISC paths were observed. The austenitic phase acts as obstacles for crack propagation. The fracture covers "valleys" and "peaks" with the cleavage ferrite and the austenite with microfacets or striations due to the hydrogen-enhanced localized-plasticity.
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| 2. |
- Chai, Guocai, et al.
(författare)
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Fatigue Crack Branching Behavior in Dual Phase Material
- 2009
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue crack branching behaviour in a dual phase steel has been investigated using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack branching results mainly from the extrusions and intrusions of slip bands developed in the grains. The number of crack branches formed depends strongly on the loading condition and the microstructure of the material. The in-situ observation confirms that the formation of crack branches can significantly reduce the crack propagation rate that leads to crack growth retardation in the main mode I crack path. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.
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| 3. |
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| 4. |
- Cong, DY, et al.
(författare)
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Crystal structures and textures of hot forged Ni48Mn30Ga22 alloy investigated by neutron diffraction technique
- 2005
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Ingår i: Materials Science and Technology. - 0267-0836 .- 1743-2847. ; 21:12, s. 1412-1416
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Tidskriftsartikel (refereegranskat)abstract
- A ferromagnetic shape memory alloy of Ni48Mn30Ga22 prepared by induction melting was successfully hot forged. Strong textures and a large anisotropy of in plane plastic flow were developed during the hot forging process. The crystal structures, both in austenitic and martensitic states, were investigated by means of neutron powder diffraction technique. The result suggests that Ni48Mn30Ga22 has a cubic L2(1) Heusler structure at room temperature, the same as that in the stoichiometric Ni2MnGa. When cooled to 243 K, the Ni48Mn30Ga22 alloy changes into a seven layered orthorhombic martensitic structure. No substantial change of the neutron diffraction pattern was observed upon further cooling to 19 K, indicating that there is no intermartensitic transformation in the investigated alloy, which is different from the transformation processes in the Ni-Mn-Ga alloys with higher martensitic transformation temperatures.
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| 5. |
- Cong, D Y, et al.
(författare)
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Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys
- 2006
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Ingår i: Metallurgical and Materials Transactions. A. - 1073-5623 .- 1543-1940. ; 37A:5, s. 1397-1403
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Tidskriftsartikel (refereegranskat)abstract
- Three ferromagnetic shape-memory alloys with the chemical compositions of Ni53Mn25Ga22, Ni48Mn30Ga22, and Ni48Mn25Ga22Co5 were prepared by the induction-melting and hot-forging process. The crystal structures were investigated by the neutron powder diffraction technique, showing that Ni53Mn25Ga22 and Ni48Mn25Ga22Co5 have a tetragonal, I4/mmm martensitic structure at room temperature, while Ni48Mn30Ga22 has a cubic, L2(1) austenitic structure at room temperature. The development of textures in the hot-forged samples shows the in-plane plastic flow anisotropy from the measured pole figures by means of the neutron diffraction technique. Significant texture changes were observed for the Ni48Mn25Ga22Co5 alloy after room temperature deformation, which is due to the deformation-induced rearrangements of martensitic variants. An excellent shape-memory effect (SME) with a recovery ratio of 74 pct was reported in this Ni48Mn25Ga22Co5 polycrystalline alloy after annealing above the martensitic transformation temperature, and the "shape-memory" influence also occurs in the distributions of grain orientations.
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| 6. |
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| 7. |
- Deng, Jiangning, et al.
(författare)
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Texture Evolution in Heavily Cold-Rolled FeCo-2V Alloy during Annealing
- 2009
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Ingår i: JOURNAL OF MATERIALS SCIENCE and TECHNOLOGY. - 1005-0302. ; 25:2, s. 219-224
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Tidskriftsartikel (refereegranskat)abstract
- The recrystallization texture evolution in heavily cold-rolled (93%) FeCo-2V alloy with annealing temperature and time was investigated by X-ray diffraction and electron backscatter diffraction. It was found that the orientation density of a-fiber texture component fluctuates with increasing annealing temperature and time. The transmission electron microscopy images show that abundant precipitates appear inside the recrystallized grains and around the grain boundaries. The amount and size of the precipitates also vary with annealing temperature and time. The enhancement of the a-fiber coincides well with the increase of number density of fine precipitates, indicating that the fine precipitates facilitate the development of a-fiber. The annealing texture evolution observed in the FeCo alloy could be attributed to the facilitating effect of the precipitates on the development of a-fiber and the ordering process.
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| 8. |
- Deng, JN, et al.
(författare)
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Textures and local textures in severely cold-rolled and annealed ultra-fine-grained FeCo alloy
- 2005
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Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 495-497, s. 731-736
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Tidskriftsartikel (refereegranskat)abstract
- We find that a severely rolled FeCo alloy has anomalous enhancement of the rotated-cube {100}< 011 > texture component and a decrease of the {111} components after annealing, which is contrast to the recrystalliization behaviors reported in traditional BCC metals and alloys. The local texture measurements show that two kinds of grains with obviously different orientations, i.e. {100} and {111}, are heterogeneously distributed in the deformed specimen and the migration of high-angle grain boundaries is observed after annealing in the disordering temperature region.
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| 9. |
- Hossain, S., et al.
(författare)
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A study of the generation and creep relaxation of triaxial residual stresses in stainless steel
- 2007
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Ingår i: International Journal of Solids and Structures. - : Elsevier BV. - 0020-7683 .- 1879-2146. ; 44:9, s. 3004-3020
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents results from a numerical and experimental research programme motivated by the need to predict creep damage generated by multi-axial states of stress in austenitic stainless steels. It has been hypothesized that highly triaxial residual stress fields may be sufficient to promote creep damage in thermally aged components, even in the absence of in-service loads. Two prerequisites to test this hypothesis are the provision of test specimens containing a highly triaxial residual stress field and an accurate knowledge of how this residual stress field relaxes due to creep. Creep damage predictions may then be made for these specimens and compared to damage observed in experiments. This paper provides solutions to both of these prerequisites. Cylindrical and spherical test specimens made from type 316H stainless steel are heated to 850 degrees C and then quenched in water. Finite element predictions of the residual stress state, validated by extensive neutron diffraction measurements, are presented which confirm the high level of triaxiality present in the specimens. The specimens are then thermally aged at 550 degrees C and numerical predictions of the residual stress relaxation are given, again validated by extensive neutron diffraction measurements. The results confirm the validity of the creep relaxation models employed. In addition, the results show the influence of specimen size and permit comparisons to be made between three different types of neutron diffractometers.
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| 10. |
- Jia, N, et al.
(författare)
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Direct experimental mapping of microscale deformation heterogeneity in duplex stainless steel
- 2008
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 491:1-2, s. 425-433
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Tidskriftsartikel (refereegranskat)abstract
- In situ tensile test has been performed with the electron back-scattering diffraction (EBSD) technique for characterizing the deformation heterogeneity at microscopic level of a duplex stainless steel consisting of austenite and ferrite. It was observed that, as deformation proceeded, the fraction of low-angle boundaries continuously increased and strain gradient developed at some grain boundaries and twin boundaries, as well as in the interior of some grains. The in situ experiments quantitatively captured the change of grain-orientation-dependent plastic behavior in respective phases and the strain partition between duplex phases as a function of applied strain. Using a visco-plastic self-consistent (VPSC) model incorporating the accommodation of micromechanical properties of grains with different orientations in two phases, the evolution of microstresses/microstrains at various length scales was simulated and discussed in detailed within the material undergoing plastic deformation. The experimental observations are well explained by the VPSC model. The present investigations provide the in-depth understanding of anisotropic micromechanical behaviors of the duplex steel. © 2008 Elsevier B.V. All rights reserved.
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| 11. |
- Jia, N, et al.
(författare)
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Micromechanical behavior and texture evolution of duplex stainless steel studied by neutron diffraction and self-consistent modeling
- 2008
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 56:4, s. 782-793
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Tidskriftsartikel (refereegranskat)abstract
- Microscopic incompatibility-induced stresses in a duplex stainless steel undergoing plastic deformation are elucidated using a visco-plastic self-consistent model. The model considers not only the grain-orientation-dependent stresses and phase-to-phase interactions, but also texture evolution during deformation. The parameters used for describing the micromechanical behavior of the two-phase polycrystalline material are directly derived from the neutron diffraction data. A reliable prediction of the evolution of grain orientation distributions for both phases at small deformations is achieved after considering various micromechanical interactions of the studied material. © 2007 Acta Materialia Inc.
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| 12. |
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| 13. |
- Jia, N, et al.
(författare)
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Tensile Deformation Behavior of Duplex Stainless Steel Studied by In-Situ Time-of-Flight Neutron Diffraction
- 2008
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Ingår i: METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE. - : Springer Science and Business Media LLC. - 1073-5623. ; 39A:13, s. 3134-3140
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Tidskriftsartikel (refereegranskat)abstract
- For a duplex alloy being subjected to deformation, the different mechanical behaviors of its constituent phases may lead to a nonuniform partition of stresses between phases. In addition, the grain-orientation-dependent elastic/plastic anisotropy in each phase may cause grain-to-grain interactions, which further modify the microscopic load partitioning between phases. In the current work, neutron diffraction experiments on the spectrometer for materials research at temperature and stress (SMARTS) were performed on an austenite-ferrite stainless steel for tracing the evolution of various microstresses during tensile loading, with particular emphasis on the load sharing among grains with different crystallographic orientations. The anisotropic elastic/plastic properties of the duplex steel were simulated using a visco-plastic self-consistent (VPSC) model that can predict the phase stress and the grain-orientation-dependent stress. Material parameters used for describing the constitutive laws of each phase were determined from the measured lattice strain distributions for different diffraction {hkl} planes as well as the laboratorial macroscopic stress-strain curve of the duplex steel. The present investigations provide in-depth understanding of the anisotropic micromechanical behavior of the duplex steel during tensile deformation.
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| 14. |
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| 15. |
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| 16. |
- Martinez-Perez, M L, et al.
(författare)
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Measurement and modelling of residual stresses in straightened commercial eutectoid steel rods
- 2005
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 53:16, s. 4415-4425
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Tidskriftsartikel (refereegranskat)abstract
- Neutron strain scanning measurements on a eutectoid steel rod that has been subjected to standard industrial coiling and straightening operations are presented. Strains were determined non-destructively using two different diffractometers, one at a steady-state neutron source and the other at a pulsed spallation neutron source, with measurements made in both the ferrite and cementite components of the pearlitic microstructure. The residual stress state is explained in terms of a simplified analytical model for a two-phase material, which takes into account the successive loading operations contributing to residual stress. The results show that residual stresses generated by bending–straightening operations are significant and are likely to play an important role in the mechanical properties of the final wires.
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| 17. |
- Nie, Z H, et al.
(författare)
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Direct evidence of detwinning in polycrystalline Ni-Mn-Ga ferromagnetic shape memory alloys during deformation
- 2008
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Ingår i: JOURNAL OF APPLIED PHYSICS. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 104:10, s. 103519-
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Tidskriftsartikel (refereegranskat)abstract
- In situ time-of-flight neutron diffraction and high-energy x-ray diffraction techniques were used to reveal the preferred reselection of martensite variants through a detwinning process in polycrystalline Ni-Mn-Ga ferromagnetic shape memory alloys under uniaxial compressive stress. The variant reorientation via detwinning during loading can be explained by considering the influence of external stress on the grain/variant orientation-dependent distortion energy. These direct observations of detwinning provide a good understanding of the deformation mechanisms in shape memory alloys.
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| 18. |
- Peng, Ru, 1960-, et al.
(författare)
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Crystal structure and phase transformation in Ni53Mn 25Ga22 shape memory alloy from 20 K to 473 K
- 2005
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 87:11
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structures, magnetic structures, and phase transformation of the off-stoichiometric Ni53 Mn25 Ga22 were studied by neutron powder diffraction at different temperatures. It is shown that Ni53 Mn25 Ga22 has a tetragonal I4/mmm structure from 20 K to 403 K. An abrupt jump in unit-cell volume around room temperature, corresponding to an endothermic peak in the differential scanning calorimetry curve, was observed. This indicates a pretransformation in the martensitic phase of Ni53 Mn25 Ga22, which is completely different from the phase transformation in the stoichiometric Ni2 MnGa. The sequence of structural transformation in Ni53 Mn25 Ga22 is closely related to its intrinsic temperature-dependent magnetic structure. © 2005 American Institute of Physics.
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| 19. |
- Peng, Ru, 1960-, et al.
(författare)
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Determination of Grain-Orientation-Dependent Stress in Coatings
- 2005
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Ingår i: Diffusion and defect data, solid state data. Part A, Defect and diffusion forum. - 1012-0386 .- 1662-9507. ; 105, s. 107-112
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Tidskriftsartikel (refereegranskat)abstract
- Quantitative interpretations of the so-called non-linear lattice strain distributions observed in coatings and thin films are important not only for determining the macro- and micro-stress fields, but also for inferring the active mechanisms of grain interactions during various deposition processes. In this paper, we present a method, which determines simultaneously both the macro- and micro-stress fields in the coatings and thin films. This method is extended from the previous stress-orientation distribution function (SODF) analysis method, which has already been used for residual stress analysis in bulk materials subjected to rolling and fatigue deformation. The validity of analysis method is demonstrated through measurements of lattice strains by high-energy x-ray and analysis of grain-orientation-dependent stresses in a CrN coating
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| 20. |
- Peng, Ru, 1960-, et al.
(författare)
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In situ neutron diffraction study of micromechanical interactions and phase transformation in Ni-Mn-Ga alloy under uniaxial and hydrostatic stress
- 2008
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:10
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Tidskriftsartikel (refereegranskat)abstract
- This paper deals with the experimental study of stress-induced phase transformation in a polycrystalline Ni-Mn-Ga alloy under uniaxial compression and its powder under hydrostatic compression. In situ neutron diffraction experiments were employed to follow changes in the structure and lattice strains caused by the applied stresses. Large lattice strains that are dependent on the lattice planes or grain orientations were observed in the parent Heusler phase for both the bulk material and the powder sample. The development of such anisotropic strains and the influence of external load conditions are discussed in the paper. © IOP Publishing Ltd.
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| 21. |
- Peng, Ru, et al.
(författare)
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In-situ X-ray diffraction study of load partitioning and microyielding for the super duplex stainless steel SAF2507 (UNS S32750)
- 2006
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Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 524-525, s. 847-852
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Tidskriftsartikel (refereegranskat)abstract
- The deformation behaviour of the super duplex stainless steel SAF2507 (UNS S32750) under successive uniaxial tensile loading-unloading was investigated with respect to load sharing and inter-phase interactions. The steel consists of 58% austenite and 42% ferrite in volume. By insitu X-ray diffraction experiment the evolution of phase-specific stresses with applied load was monitored for three successive loading-unloading cycles with the maximum total strains being 0.34%, 0.75% and 1.63%, respectively. It was found that yielding occurred earlier in the austenitic phase than in the ferritic phase during the first loading cycle. In the followed loading cycles, both phases yielded under larger but similar applied stresses. Due to a similar behavior of the phases in the elasto-plastic regime inter-phase interactions were relatively weak. Low microstresses induced by the plastic straining resulted in somewhat larger stresses in the ferritic phase.
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| 22. |
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| 23. |
- Peng, Ru, 1960-, et al.
(författare)
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Interactions between the phase stress and the grain-orientation-dependent stress in duplex stainless steel during deformation
- 2006
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 54:15, s. 3907-3916
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Tidskriftsartikel (refereegranskat)abstract
- The development of phase stress and grain-orientation-dependent stress under uniaxial compression was investigated in a duplex stainless steel consisting of austenite and ferrite. Using in situ neutron diffraction measurements, the strain response of several h k l planes to the applied compressive stress was mapped as a function of applied stress and sample direction. Analysis based on the experimental results and elastoplastic self-consistent simulations shows that phase stresses of thermal origin further increase during elastic loading but decrease with increased plastic deformation. Grain-orientation-dependent stresses become significant in both austenite and ferrite after loading into the plastic region. After unloading from the plastic regime, a considerable intergranular stress remains in the austenitic phase and dominates over the phase stress. This study provides fundamental experimental inputs for future micromechanical modeling aiming at the evaluation and prediction of the mechanical performance of multiphase materials. © 2006 Acta Materialia Inc.
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| 24. |
- Peng, Ru Lin, et al.
(författare)
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Micromechanical Interactions in a Superduplex Stainless Steel Subjected to Low Cycle Fatigue Loading
- 2008
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Ingår i: Fatigue and Fracture of Engineering Materials and Structures. - : Wiley. - 1460-2695 .- 8756-758X. ; 31:10, s. 892-901
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Tidskriftsartikel (refereegranskat)abstract
- Micromechanical interactions in an austenitic-ferritic SAF 2507 steel under lowcycle fatigue loading was studied by experiment and simulation. Neutron diffraction measurements of residual lattice strains were made on specimens unloaded from different cyclic deformation stages, namely cyclic hardening, softening and saturation. With self-consistent modelling, the micromechanical behaviour of the constituent phases was studied for the first loading cycle. The evolution of the residual lattice strain distributions with cyclic loading and the development of phase stresses have been analysed with respect to the initial residual stress field and the different mechanical properties of the constituent phases.
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| 25. |
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| 26. |
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| 27. |
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| 28. |
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| 29. |
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| 30. |
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| 31. |
- Peng, Ru, et al.
(författare)
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On the development of grain-orientation-dependent and inter-phase stresses in a super duplex stainless steel under uniaxial loading
- 2006
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Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 524-525, s. 917-922
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Tidskriftsartikel (refereegranskat)abstract
- Microstresses due to intergranular and inter-phase interactions in an austenitic-ferritic super duplex steel (SAF 2507) under uniaxial compressive deformation have been studied by in-situ neutron diffraction experiments. Lattice strains of several hkl planes of austenite respective ferrite were mapped as a function of sample direction at a number of load levels during loading into the plastic regime and unloading. The analysis of the experimental results has shown that during loading both grain-orientation-dependent and inter-phase stresses were generated under plastic deformation that was inhomogeneous at the microstructural level. Residual stresses depending on the grain-orientation and phase have been found after unloading. The results also indicate stronger intergranular interactions among the studied hkl planes of austenite than those of ferrite.
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| 32. |
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| 33. |
- Peng, Ru, 1960-, et al.
(författare)
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Residual stress analysis in both As-deposited and annealed CrN coatings
- 2005
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Ingår i: Residual Stresses VII. - Uetikon-Zürich : Trans Tech Publications Inc.. - 0878499695 ; 490-491, s. 643-648
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Konferensbidrag (refereegranskat)abstract
- In this paper, we report on residual stress analysis in physical vapour deposited (PVD) CrN coatings. Two 9 μm thick coatings were grown on tool steel substrates with bias voltages of-50 V and -300 V, respectively. High-energy (E=80 keV) synchrotron radiation measurements have been performed to investigate residual stresses in both as-deposited and annealed CrN coatings. To understand the origins of non-linear distribution of lattice strain versus sin2Ψ for certain (hkl) planes in both coatings, a stress orientation distribution function (SODF) analysis has been carried out, which yields grain-orientation-dependent residual stresses. The results are compared to previous analyses using Reuss and Vook-Witt models on the as-deposited coatings.
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| 34. |
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| 35. |
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| 36. |
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| 37. |
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| 38. |
- Wang, Y.D., et al.
(författare)
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Textures and compressive properties of ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 prepared by isothermal forging process
- 2006
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Ingår i: Journal of Materials Research. - : Springer Science and Business Media LLC. - 0884-2914 .- 2044-5326. ; 21:3, s. 691-697
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Tidskriftsartikel (refereegranskat)abstract
- A ferromagnetic shape-memory alloy Ni48Mn25 Ga22CO5 was prepared by the induction melting and isothermal forging process. Dynamic recrystallization occurs during the isothermal forging. The deformation texture was studied by the neutron diffraction technique. The main texture components consist of (110) [11¯2] and (001) [100], which suggested that in-plane plastic flow anisotropy should be expected in the as-forged condition. The uniaxial compression fracture strain in the forged alloy reaches over 9.5%. The final room-temperature fracture of the polycrystalline Ni48Mn25Ga22CO5 is controlled mainly by intergranular mode.
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| 39. |
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