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In the present work, AlN precipitation and growth were investigated in a FeCrAl stainless steel with high aluminum content (Al wt pct = 4.07). The effect of the cooling rate on the number and size distribution of AlN particles was analyzed with the directional solidification method and the field emission scanning electron microscope. A coupled model of solutes micro-segregation and diffusion-controlled inclusion growth was also established in combination with the interaction between the inclusions and advancing solidifying interface, to characterize the precipitation and growth of AlN during solidification of FeCrAl stainless steel. The results indicate that the AlN particles with the hexagonal wurtzite crystal structure belong to the P63mc space group. AlN precipitates in the mushy zone during solidification. The growth of AlN is controlled by the diffusion of nitrogen. The increased cooling rate could increase the number density of AlN and reduce the size of AlN particles. The coupled model can be used to predict AlN size distribution and solutes distribution in solid and liquid phases during solidification. [ABSTRACT FROM AUTHOR]

The effect of cooling rate on the evolution of AlN inclusions precipitated during solidification in FeCrAl stainless steel was investigated using an experimental study and thermodynamic and kinetic calculations. The number and size of AlN inclusions precipitated under different cooling rates were examined with the feature function of the field-emission scanning electron microscope. A model combining micro-segregation and the diffusion-controlled growth model was set up to determine the mechanism of AlN particle growth. The results showed that AlN precipitates in the mushy zone. The size of AlN particles decreases and the number of AlN particles increases with increasing cooling rate, whereas the volume fraction is essentially unchanged. The AlN particles grow during solidification after the content of solutes in molten steel has exceeded the concentration in equilibrium with AlN. The nitrogen content varies significantly with the cooling rate during solidification. Increasing the cooling rate and reducing the nitrogen content in the molten steel can reduce the AlN particle size in FeCrAl alloys as the growth time decreases. [ABSTRACT FROM AUTHOR]

In order to further investigate the solidification process of FeCrAl alloy for automobile exhaust gas purifying systems, the phase transformation and precipitation in FeCrAl alloy with different Al contents were studied using differential scanning calorimetry (DSC), Thermo-Calc calculation and high temperature in-situ X-ray diffraction (XRD). The Thermo-Calc calculation results manifests that the complete solidification process of FeCrAl alloy includes the transformations from L to α-(Fe, Cr) and from α-(Fe, Cr) to α (Fe-rich ferrite)+α′ (Cr-rich ferrite) as well as the precipitation of (Fe, Cr) 7 C 3 , (Fe, Cr) 23 C 6 , and σ. The calculation results with 6.49 wt% Al content are reasonable agreement with the DSC results. The solidification process investigation of FeCrAl alloy with different Al content indicates that the decrease of Al content in FeCrAl alloy increases the forming temperature of α′ phase and the precipitation temperature of (Fe, Cr) 23 C 6 and narrows the precipitation temperature range of (Fe, Cr) 7 C 3 . In addition, the investigations using high temperature in-situ XRD further validate the separation of α (Fe-rich) and α′ (Cr-rich) as well as the absence of ferrite to austenite transformation in FeCrAl alloy during solidification. [ABSTRACT FROM AUTHOR]