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

Gian Song,Yanfei Gao,Shu Yan Zhang,Peter K Liaw,Lin Li,Zhiqian Sun,Bjørn Clausen

doi:10.1038/srep45965

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.

Highlights

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 alloys[15,16,17,18]
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 distribution[17]
It was suggested that the elastic strain field in the matrix created by misfitting precipitates of HPSFA plays a critical role in enhancing the elastic interaction between the precipitate and mobile dislocations, and, the creep resistance

Summary

Introduction

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 alloys[15,16,17,18]. 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 phase[18]. 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 distribution[17]. We have characterized in-situ dynamic evolution of elastic/plastic response of constitutive phases in hierarchical- and single-precipitate-strengthened ferritic alloys during tensile/relaxation deformation at 973 K, by state-of-the-art neutron diffraction at the Spallation Neutron Source, Los Alamos National Laboratory and Rutherford Appleton Laboratory, and crystal-plasticity finite-element model. The response to stress relaxation reveals that the hierarchical-structure-enhanced elastic field is effective in reducing the diffusional relaxation along the interface between the precipitate and matrix phases

Methods

Results

Conclusion