Effect of grain orientation and precipitates on the superelasticity of Fe-Ni-Co-Al polycrystalline alloys

In this paper, the evolution of microstructure in Fe-Ni-Co-Al polycrystalline alloys and its effects on mechanical properties were systematically investigated. The results show that the proportion of high-angle grain boundaries (HAGBs) is 98.1 % with 51.2 % of Sigma 3 GBs during the solid solution (SS) process. The distribution of Sigma 3 GBs and other HAGBs (non-Sigma 3 HAGBs) are relatively dispersed, which prevents the formation of a specific orientation. In addition, there is no discernible anisotropy in GB migration during the SS process, ultimately resulting in the absence of texture. In contrast, during the directional recrystallization (DR) process, the directional migration of GBs is driven by the temperature gradient. Random high-angle grain boundaries (RHAGBs) with Sigma values exceeding 29 possess higher GB energy and migration rate compared to low-angle grain boundaries (LAGBs) and HAGBs, making them more susceptible to GB migration. In the grain incubation area, the proportion of RHAGBs between grains oriented {111}<214> (grain 2 and grain 5) and their surrounding grains are 90 % and 75 %. These grains have the growth advantage and eventually form the main orientation {111}<214>. While the proportion of RHAGBs between grains oriented {114}<214> (grain 11) and its surrounding grains is only 65 %, characterized by lower GB energy and migration rate. Therefore, these grains eventually form the minor orientation {114}<214>. The coherent L1(2)-Ni3Al (gamma ') precipitates were observed in both the SS-AG (SS after aging) and DR-AG (DR after aging) treatments. The DR-AG alloy, characterized by {111}<214> and {114}<214> orientations and gamma ' precipitates, exhibited a 3.6 % recoverable strain and a 1.2 % superelastic strain, whereas no evident superelastic behavior was found in the SS-AG alloy, which featured a random texture and gamma ' precipitates.

Automated summary

This paper systematically investigated the evolution of microstructure in Fe-Ni-Co-Al polycrystalline alloys and its effects on mechanical properties. The results revealed that the migration of grain boundaries in different processes is driven by different factors, which impacts the grain orientation and precipitate formation. In the process of directional recrystallization, grains with specific orientations grow in the grain boundary region and form the dominant orientation, while grains with lower migration rate form the minor orientation. The alloy produced through directional recrystallization exhibited good recoverable strain and superelastic strain, while the alloy produced through solid solution treatment showed no evident superelastic behavior.