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High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies

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ABSTRACT

The ferritic Fe-Cr-Ni-Al-Ti alloys strengthened by hierarchical-Ni2TiAl/NiAl or single-Ni2TiAl precipitates have been developed and received great attentions due to their superior creep resistance, as compared to conventional ferritic steels. Although the significant improvement of the creep resistance is achieved in the hierarchical-precipitate-strengthened ferritic alloy, the in-depth understanding of its high-temperature deformation mechanisms is essential to further optimize the microstructure and mechanical properties, and advance the development of the creep resistant materials. In the present study, in-situ neutron diffraction has been used to investigate the evolution of elastic strain of constitutive phases and their interactions, such as load-transfer/load-relaxation behavior between the precipitate and matrix, during tensile deformation and stress relaxation at 973 K, which provide the key features in understanding the governing deformation mechanisms. Crystal-plasticity finite-element simulations were employed to qualitatively compare the experimental evolution of the elastic strain during tensile deformation at 973 K. It was found that the coherent elastic strain field in the matrix, created by the lattice misfit between the matrix and precipitate phases for the hierarchical-precipitate-strengthened ferritic alloy, is effective in reducing the diffusional relaxation along the interface between the precipitate and matrix phases, which leads to the strong load-transfer capability from the matrix to precipitate.

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Temporal evolution of stress relaxation.(a) Temporal evolution of the macroscopic stress for SPSFA and HPSFA, and evolution of the elastic strain of the Fe and L21 phases, derived from (b) single-peak and (c) whole-peak fitting approaches, during the stress relaxation of SPSFA at 973 K.
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f8: Temporal evolution of stress relaxation.(a) Temporal evolution of the macroscopic stress for SPSFA and HPSFA, and evolution of the elastic strain of the Fe and L21 phases, derived from (b) single-peak and (c) whole-peak fitting approaches, during the stress relaxation of SPSFA at 973 K.

Mentions: During the ND data measurement at 973 K, a certain amount of stress relaxation occurs for both alloys (Fig. 4). The stress relaxation at high temperatures has been studied to understand deformation mechanisms at high-temperatures, such as creep mechanisms3233. The kinetics of this stress relaxation is known to be largely governed by dislocation processes and possibly grain boundary sliding in materials32. In the present alloys, the grain size of HPSFA and SPSFA are estimated to be larger than 200 μm, and, thus, the stress relaxation caused by the grain-boundary sliding is not expected to be significant. Instead, the phase-interface between the matrix and precipitate could play an important role in the stress relaxation, and, hence, the load-transfer between the constitutive phases at high temperatures. However, the holding modes applied during the relaxation are slightly different [actuator displacement control in Fig. 3(a), and strain gauge control in Fig. 3(b)], which could result in the different relaxation behavior. Therefore, to verify this hypothesis, we conducted stress relaxation tests on the HPSFA and SPSFA at 973 K. The samples were loaded to 150 MPa at 973 K and held in displacement holding mode for about 4 hours. The ND data was collected for the SPSFA in every 20 mins, and the ND data was analyzed using the Rietveld whole-peak fitting and single fitting approaches using the GSAS program developed at the Los Alamos National Laboratory30 to obtain the averaged lattice parameters and (hkl) plane spacing of the Fe and L21 phases, respectively. The temporal evolution of the stress relaxation and the elastic strain at 973 K for SPSFA are displayed in Fig. 8.


High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies
Temporal evolution of stress relaxation.(a) Temporal evolution of the macroscopic stress for SPSFA and HPSFA, and evolution of the elastic strain of the Fe and L21 phases, derived from (b) single-peak and (c) whole-peak fitting approaches, during the stress relaxation of SPSFA at 973 K.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5384073&req=5

f8: Temporal evolution of stress relaxation.(a) Temporal evolution of the macroscopic stress for SPSFA and HPSFA, and evolution of the elastic strain of the Fe and L21 phases, derived from (b) single-peak and (c) whole-peak fitting approaches, during the stress relaxation of SPSFA at 973 K.
Mentions: During the ND data measurement at 973 K, a certain amount of stress relaxation occurs for both alloys (Fig. 4). The stress relaxation at high temperatures has been studied to understand deformation mechanisms at high-temperatures, such as creep mechanisms3233. The kinetics of this stress relaxation is known to be largely governed by dislocation processes and possibly grain boundary sliding in materials32. In the present alloys, the grain size of HPSFA and SPSFA are estimated to be larger than 200 μm, and, thus, the stress relaxation caused by the grain-boundary sliding is not expected to be significant. Instead, the phase-interface between the matrix and precipitate could play an important role in the stress relaxation, and, hence, the load-transfer between the constitutive phases at high temperatures. However, the holding modes applied during the relaxation are slightly different [actuator displacement control in Fig. 3(a), and strain gauge control in Fig. 3(b)], which could result in the different relaxation behavior. Therefore, to verify this hypothesis, we conducted stress relaxation tests on the HPSFA and SPSFA at 973 K. The samples were loaded to 150 MPa at 973 K and held in displacement holding mode for about 4 hours. The ND data was collected for the SPSFA in every 20 mins, and the ND data was analyzed using the Rietveld whole-peak fitting and single fitting approaches using the GSAS program developed at the Los Alamos National Laboratory30 to obtain the averaged lattice parameters and (hkl) plane spacing of the Fe and L21 phases, respectively. The temporal evolution of the stress relaxation and the elastic strain at 973 K for SPSFA are displayed in Fig. 8.

View Article: PubMed Central - PubMed

ABSTRACT

The ferritic Fe-Cr-Ni-Al-Ti alloys strengthened by hierarchical-Ni2TiAl/NiAl or single-Ni2TiAl precipitates have been developed and received great attentions due to their superior creep resistance, as compared to conventional ferritic steels. Although the significant improvement of the creep resistance is achieved in the hierarchical-precipitate-strengthened ferritic alloy, the in-depth understanding of its high-temperature deformation mechanisms is essential to further optimize the microstructure and mechanical properties, and advance the development of the creep resistant materials. In the present study, in-situ neutron diffraction has been used to investigate the evolution of elastic strain of constitutive phases and their interactions, such as load-transfer/load-relaxation behavior between the precipitate and matrix, during tensile deformation and stress relaxation at 973 K, which provide the key features in understanding the governing deformation mechanisms. Crystal-plasticity finite-element simulations were employed to qualitatively compare the experimental evolution of the elastic strain during tensile deformation at 973 K. It was found that the coherent elastic strain field in the matrix, created by the lattice misfit between the matrix and precipitate phases for the hierarchical-precipitate-strengthened ferritic alloy, is effective in reducing the diffusional relaxation along the interface between the precipitate and matrix phases, which leads to the strong load-transfer capability from the matrix to precipitate.

No MeSH data available.


Related in: MedlinePlus