Strain Distribution Analysis Using PED Technique at the Interface of L2₁ Precipitates in Al-Cr-Fe-Ni-Ti Complex Concentrated Alloy System

In an alloy system with embedded nano-precipitates in matrix, the lattice match between the precipitates and the matrix plays a very important role in determining the mechanical properties at high temperatures. This microstructure is commonly observed in superalloy systems based on Ni-, Co-, (FeNi)-, etc., and have been developed for high-temperature load-bearing applications with excellent mechanical strength and creep resistance. At this time, the high-temperature mechanical properties of the superalloy are induced from the precipitation hardening and solid solution strengthening of the nano-precipitates. In particular, it is known that the stress field caused by the small lattice mismatch between the precipitate and the matrix interferes with the dislocation motion, thereby increasing the strength and creep resistance. It is important to understand the dislocation distribution and stress distribution at the precipitate interface because the degree of matching of the strain field depends on the size of the precipitate and the lattice mismatch between the precipitate and the matrix.<br/>In this study, the strain distribution at the precipitate interface in the Fe superalloy(Al_xCr_13.3Fe_71.5-xNi_11.2Ti₄, x =8, 10, 12, 14, 16 at.%), which varies according to the Al content, was analyzed. In order to analyze all five alloys varying from Al 8 at.% to 16. at.% with the same normal crystal axis, TEM lamella samples were prepared using FIB in grains having a [001] normal axis using EBSD. For each sample, the lattice mismatch could be calculated by analyzing the dislocation gap at the precipitate interface in the alloy observed in the high-resolution HAADF STEM images, which could be compared with the lattice mismatch calculated by XRD technique. In addition, in order to numerically confirm the relationship between the dislocation and the strain field appearing at the interface, the strain distribution mapping for the precipitates of the Al 8, 12, 16 at.% was carried out. Precession electron diffraction (PED) is a technique that can numerically represent the presence or absence of strain at each point in the image by precessing the electron beam. When the PED method was used to map the precipitates and the matrix, it was possible to obtain the strain distribution results in the xx and yy directions, respectively.<br/>As the content of Al increased, the shape of the precipitates changed from a cube shape to a tetragonal shape. The shape of the precipitate infers its growth mechanism depends on the lattice mismatch. In HRSTEM image analysis, the dislocation density decreased with increasing Al contents. In particular, dislocations did not form at the interface between the matrix and precipitate in the 16 Al sample. The absence of dislocation leads to the higher strain field at the interface than that of other alloys, which is clearly observed in the strain map using PED. The precipitates having high strain field around in the 16 Al sample is expected to be the origin of the high strength of the alloy.