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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.

No MeSH data available.


DF-TEM images of the precipitates-strengthened ferritic alloys.Dark-field (DF) transmission-electron-microscopy (TEM) images showing the microstructures of (a) SPSFA, and (b) HPSFA (ppt stands for precipitate).
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f1: DF-TEM images of the precipitates-strengthened ferritic alloys.Dark-field (DF) transmission-electron-microscopy (TEM) images showing the microstructures of (a) SPSFA, and (b) HPSFA (ppt stands for precipitate).

Mentions: Recently, two-phase NiAl/Ni2TiAl-precipitate and single-phase Ni2TiAl-precipitate-strengthened ferritic alloys have been developed via the addition of Ti to the NiAl-precipitate strengthened ferritic alloys15161718. The microstructures of the alloys, such as chemistry and morphology of the constitutive phases within the precipitates, have been systematically characterized using transmission-electron microscopy (TEM) and atom-probe tomography (APT)181920. The microstructural features of the 4-wt.% and 2-wt.%-Ti alloys aged at 973 K for 100 hours are displayed in Fig. 1. Based on the APT and TEM results, the single-phase Ni2TiAl-precipitate-strengthened ferritic alloy (SPSFA) is hardened by single L21-Ni2TiAl precipitates in the bcc-Fe matrix1518. The two-phase NiAl/Ni2TiAl-precipitate-strengthened ferritic alloy is reinforced by parent L21-Ni2TiAl precipitates, which is further divided by sub-structures of the B2-NiAl phase18. Such a two-phase precipitate-strengthened ferritic alloy has been described as hierarchical-precipitate-strengthened ferritic alloy (HPSFA)18, characterized by the relative chemical ordering, spatial dimensions of the constitutive phases, and their spatial distribution17. Both alloys often contain a small Fe inclusion within the precipitates, which was previously determined by energy-dispersive X-ray spectroscopy1819. It was suggested that the hierarchical precipitate structure is effective to retain the coherent interface, whereas the single Ni2TiAl precipitate tends to form misfit dislocation at the interface in order to accommodate the large lattice mismatch1820.


High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies
DF-TEM images of the precipitates-strengthened ferritic alloys.Dark-field (DF) transmission-electron-microscopy (TEM) images showing the microstructures of (a) SPSFA, and (b) HPSFA (ppt stands for precipitate).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: DF-TEM images of the precipitates-strengthened ferritic alloys.Dark-field (DF) transmission-electron-microscopy (TEM) images showing the microstructures of (a) SPSFA, and (b) HPSFA (ppt stands for precipitate).
Mentions: Recently, two-phase NiAl/Ni2TiAl-precipitate and single-phase Ni2TiAl-precipitate-strengthened ferritic alloys have been developed via the addition of Ti to the NiAl-precipitate strengthened ferritic alloys15161718. The microstructures of the alloys, such as chemistry and morphology of the constitutive phases within the precipitates, have been systematically characterized using transmission-electron microscopy (TEM) and atom-probe tomography (APT)181920. The microstructural features of the 4-wt.% and 2-wt.%-Ti alloys aged at 973 K for 100 hours are displayed in Fig. 1. Based on the APT and TEM results, the single-phase Ni2TiAl-precipitate-strengthened ferritic alloy (SPSFA) is hardened by single L21-Ni2TiAl precipitates in the bcc-Fe matrix1518. The two-phase NiAl/Ni2TiAl-precipitate-strengthened ferritic alloy is reinforced by parent L21-Ni2TiAl precipitates, which is further divided by sub-structures of the B2-NiAl phase18. Such a two-phase precipitate-strengthened ferritic alloy has been described as hierarchical-precipitate-strengthened ferritic alloy (HPSFA)18, characterized by the relative chemical ordering, spatial dimensions of the constitutive phases, and their spatial distribution17. Both alloys often contain a small Fe inclusion within the precipitates, which was previously determined by energy-dispersive X-ray spectroscopy1819. It was suggested that the hierarchical precipitate structure is effective to retain the coherent interface, whereas the single Ni2TiAl precipitate tends to form misfit dislocation at the interface in order to accommodate the large lattice mismatch1820.

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.