Abstract:
Low thermal stability of grain structure is the main factor preventing the use of nanocrystalline (NC) materials at elevated temperatures. In this paper, a comparative study of the thermal stability of NC grain sizes of pure $\alpha$-Fe and Fe-20Cr, Fe-19.5Cr-0.5N (at.%) ferritic alloys obtained by mechanical alloying was carried out. It has been shown that the Fe-20Cr binary alloy is much more thermally stable than NC $\alpha$-Fe. However, the Fe-19.5Cr-0.5N NC alloy does not show a strong increase in thermal stability compared to Fe-20Cr, despite the fact that Cr$_2$N particles are precipitated during the heating process. It is shown that the actual size of Cr$_2$N particles (30 nm according to the broadening of X-ray diffraction peaks) significantly exceeds the critical particle size $d^*$ = 8 nm above which, according to Gladman, there is no pinning of grain boundaries. Effective strategies for increasing the thermal stability of nitrogen microdoped Fe-Cr alloys are discussed within the critical size model of pinning particles and the thermokinetic grain growth model.
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