Size-dependent stress response of nanoscale B2 intermetallic precipitates revealed by in-situ high-energy X-ray diffraction

In-situ high-energy X-ray diffraction experiments under uniaxial loading revealed the stress distribution among austenite, ferrite, and nanoscale B2-(Ni,Fe)Al intermetallic precipitates embedded in the ferrite phase of an Al-added lightweight steel. Stress analysis based on the lattice strains induced by uniaxial tensile loading, while assuming a uniaxial stress state within the grains and neglecting residual stresses, indicated earlier yielding of austenite and the development of higher stresses in ferrite. Remarkably, at an applied true stress of nearly 1.0 GPa, stresses up to about 5.8 GPa were determined within the B2 precipitates. The stress level within the B2 precipitates, which exhibited a bimodal size distribution, was strongly size-dependent, with the finer population experiencing higher stresses. Due to the low Schmid factor for $\left\{hkl\right\}〈100〉$ slip as the preferred slip system in B2, plastic deformation of B2 in this hard orientation was enabled by $〈111〉$ slip, aided by the penetration of $\frac{1}{2}〈111〉$ dislocations gliding on $\left\{110\right\}$ planes in the cube-on-cube-related ferrite. The high stresses in B2 upon loading along the $〈100〉$ direction raised the stress level in the surrounding ferrite, which is a likely cause of $\left\{100\right\}$ cleavage in embrittled body-centered cubic steels. This study enhances our understanding of the micromechanical behavior of precipitation-strengthened alloys and elucidates how matrix-precipitate interactions influence macroscopic mechanical properties.