The Precipitation of Nitrogen during Isothermal Aging in a Quenched Fe–N Alloy

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Abstract
The precipitation of nitrogen during isothermal aging between 100 and 300°C in a quenched Fe-0.022 wt%N alloy was studied by means of electrical resistance measurement. The precipitation of α″- and γ′-phases and the transition from α″- to γ′-phase were elucidated by optical and transmission electron microscopy. The alloy was made by nitrogenizing molten iron with nitrogen gas. The nucleation of α″-phase is completed on dislocations and in ferrite matrix during quenching or the very early period of aging at below 225°C. The precipitation of nitrogen proceeds by a gradual growth of α″-disks whose number decreases monotonously with aging time. The rate controlling process of the precipitation is considered to be the diffusion of single nitrogen atoms. The nucleation of γ′-phase is completed before appreciable growth of the γ′-phase during aging at above 200°C. The rate of growth of γ′-phase is controlled by the diffusion of single nitrogen atoms as considered in the case of the α″-phase precipitation. Plates of γ′-phase are formed on the grain boundary of ferrite, at the interface between inclusions and the ferrite matrix, and in the ferrite matrix in close succession with aging time. The size of the average γ′-plate is about 10 μ in length, about 5 μ in width and about 0.1 μ in thickness. The precipitation of nitrogen proceeds by the increase in the number of γ′-plates with aging time.

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