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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 macroscopic stress.Temporal evolution of macroscopic stress for (a) SPSFA and (b) HPSFA during the whole in-situ tension experiments at 973 K. Temporal evolution of relative stress relaxation for (a) SPSFA and (b) HPSFA during ND measurements with respect to plastic strain.
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f4: Temporal evolution of macroscopic stress.Temporal evolution of macroscopic stress for (a) SPSFA and (b) HPSFA during the whole in-situ tension experiments at 973 K. Temporal evolution of relative stress relaxation for (a) SPSFA and (b) HPSFA during ND measurements with respect to plastic strain.

Mentions: Figure 4 shows the temporal evolution of the applied stress in the plastic deformation region (Fig. 3) where the ND measurements were carried out with the displacement or strain gauge holding mode. The averaged stress was estimated during each holding step, and marked as red squares in Fig. 4a and b. Note that the holding time (about 20 mins) of SPSFA for the ND measurements is longer than that of HPSFA (about 12 mins). This is because the holding time was required to obtain the statistically-reliable ND data in each ND instrument. Figure 4c and d show the temporal evolution of the relative stress decay with different plastic strains. It can be seen that the total amount of the stress decay in each alloy is independent upon the plastic strains considered. The stress relaxation curve can be divided into three regimes, initial rapid decline, transition, and asymptotic regions, as observed in stress relaxation experiments at high temperatures3233. Figure 4c and d provide the insight into the efficiency of the stress relaxation varying with the tested alloys. For example, the total stress decay of SPSFA is greater than that of HPSFA. The magnitude of the stress decay of SPSFA for 700 seconds is about 70 MPa, which is about 20 MPa higher than that of HPSFA, as indicated by blue arrows in Fig. 4c and d. This suggests that the stress relaxation occurs more easily for SPSFA at 973 K, as compared to HPSFA.


High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies
Temporal evolution of macroscopic stress.Temporal evolution of macroscopic stress for (a) SPSFA and (b) HPSFA during the whole in-situ tension experiments at 973 K. Temporal evolution of relative stress relaxation for (a) SPSFA and (b) HPSFA during ND measurements with respect to plastic strain.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Temporal evolution of macroscopic stress.Temporal evolution of macroscopic stress for (a) SPSFA and (b) HPSFA during the whole in-situ tension experiments at 973 K. Temporal evolution of relative stress relaxation for (a) SPSFA and (b) HPSFA during ND measurements with respect to plastic strain.
Mentions: Figure 4 shows the temporal evolution of the applied stress in the plastic deformation region (Fig. 3) where the ND measurements were carried out with the displacement or strain gauge holding mode. The averaged stress was estimated during each holding step, and marked as red squares in Fig. 4a and b. Note that the holding time (about 20 mins) of SPSFA for the ND measurements is longer than that of HPSFA (about 12 mins). This is because the holding time was required to obtain the statistically-reliable ND data in each ND instrument. Figure 4c and d show the temporal evolution of the relative stress decay with different plastic strains. It can be seen that the total amount of the stress decay in each alloy is independent upon the plastic strains considered. The stress relaxation curve can be divided into three regimes, initial rapid decline, transition, and asymptotic regions, as observed in stress relaxation experiments at high temperatures3233. Figure 4c and d provide the insight into the efficiency of the stress relaxation varying with the tested alloys. For example, the total stress decay of SPSFA is greater than that of HPSFA. The magnitude of the stress decay of SPSFA for 700 seconds is about 70 MPa, which is about 20 MPa higher than that of HPSFA, as indicated by blue arrows in Fig. 4c and d. This suggests that the stress relaxation occurs more easily for SPSFA at 973 K, as compared to HPSFA.

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