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

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.


Strengthening contributions.Increase in yield stress as a function of precipitate radius at 973 K. Experimental points are obtained from the 0.2% yield stress measurements (Fig. 4), and the theoretical lines are calculated from Eqs [1, 2, 3, 4, 5] for the Orowan stress (σOR) and shearing stress due to the ordering (Δσ1), lattice mismatch (Δσ2), and modulus mismatch (Δσ3) contributions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5384073&req=5

f7: Strengthening contributions.Increase in yield stress as a function of precipitate radius at 973 K. Experimental points are obtained from the 0.2% yield stress measurements (Fig. 4), and the theoretical lines are calculated from Eqs [1, 2, 3, 4, 5] for the Orowan stress (σOR) and shearing stress due to the ordering (Δσ1), lattice mismatch (Δσ2), and modulus mismatch (Δσ3) contributions.

Mentions: Assuming the presence of a coherent single-L21-precipitate, the dependence of the strength increase on the precipitate size has been evaluated using Eqs [1, 2, 3], as shown in Fig. 7, based on APB energy (γAPB = 0.058 J/m2) of the L21-Ni2TiAl for the (110) plane42, ΔG = 22.0 GPa, which is determined by the elastic constants of the FEM modeling in Table 2, and δ = 0.0077 for HPSFA at 973 K, determined from the current ND experiments (Table 1). As can be seen, the strength increase from the shearing contributions increases with increasing the size of the precipitate. The strength increase of the shearing contributions from the current calculation is expected to be under-estimated, since the precipitate of HPSFA is a two-phase coupled-structure of the B2 and L21. Furthermore, the APB energy of the B2 for the (110) plane is 0.5J/m2, which is higher than that of the L21 phase42. Therefore, the actual strength increase curve of the shearing mechanism is believed to be higher than the calculated curve, which is much higher than the experimental value of the yield strength of HPSFA and SPSFA. These trends indicate that the precipitate shearing is unlikely to happen in the current alloys.


High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies
Strengthening contributions.Increase in yield stress as a function of precipitate radius at 973 K. Experimental points are obtained from the 0.2% yield stress measurements (Fig. 4), and the theoretical lines are calculated from Eqs [1, 2, 3, 4, 5] for the Orowan stress (σOR) and shearing stress due to the ordering (Δσ1), lattice mismatch (Δσ2), and modulus mismatch (Δσ3) contributions.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f7: Strengthening contributions.Increase in yield stress as a function of precipitate radius at 973 K. Experimental points are obtained from the 0.2% yield stress measurements (Fig. 4), and the theoretical lines are calculated from Eqs [1, 2, 3, 4, 5] for the Orowan stress (σOR) and shearing stress due to the ordering (Δσ1), lattice mismatch (Δσ2), and modulus mismatch (Δσ3) contributions.
Mentions: Assuming the presence of a coherent single-L21-precipitate, the dependence of the strength increase on the precipitate size has been evaluated using Eqs [1, 2, 3], as shown in Fig. 7, based on APB energy (γAPB = 0.058 J/m2) of the L21-Ni2TiAl for the (110) plane42, ΔG = 22.0 GPa, which is determined by the elastic constants of the FEM modeling in Table 2, and δ = 0.0077 for HPSFA at 973 K, determined from the current ND experiments (Table 1). As can be seen, the strength increase from the shearing contributions increases with increasing the size of the precipitate. The strength increase of the shearing contributions from the current calculation is expected to be under-estimated, since the precipitate of HPSFA is a two-phase coupled-structure of the B2 and L21. Furthermore, the APB energy of the B2 for the (110) plane is 0.5J/m2, which is higher than that of the L21 phase42. Therefore, the actual strength increase curve of the shearing mechanism is believed to be higher than the calculated curve, which is much higher than the experimental value of the yield strength of HPSFA and SPSFA. These trends indicate that the precipitate shearing is unlikely to happen in the current alloys.

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.