Small-angle X-ray Scattering Studies on Aging Precipitation of High-strength Soft-magnetic Stainless Steels

We investigated the effect of alloying elements Ni, Al, and Mo on the mechanical properties of precipitation-hardened soft-magnetic stainless steels, whose aging condition and chemical composition were varied. Thermodynamic calculations suggested that the aging treatments led to precipitation of a β-NiAl compound with B2 structure and Fe2Mo-type Laves phases. Although coarse precipitates, which are less effective for precipitation hardening, were observed in bright-field images obtained by transmission electron microscopy (TEM), the B2-type precipitates were detected in TEM electron diffraction patterns. By changing the concentration of the alloying elements, the B2-type β-NiAl precipitates were mainly responsible for the precipitation hardening, and the Vickers hardness was hardly affected by the Mo content. Small-angle X-ray scattering analysis was used to determine the size and number density of the fine β-NiAl precipitates, which contributed to the hardening. The high-density β-NiAl precipitates grew to a few nanometers in radius after an adequate period of aging, suggesting that the β-NiAl precipitates were responsible for the precipitation-hardening characteristics. On the other hand, the size of the precipitates was less affected by the amounts of Ni and Al, and the number density decreased with decreasing Ni and Al content. The improvement in hardness resulting from the β-NiAl precipitates had a linear relationship with the square root of the product of the precipitate size and number density.