Characterization of a Nano-scale γ′ Phase in HPT-Disk P/M Superalloy HGN300 by Small-Angle X-Ray Scattering

Quantitative evaluation of the size and volume fraction of γ′ particles has been studied for the first time by Small-Angle X-ray Scattering (SAXS) in the newly developed HGN300 superalloy employing different aging conditions. In addition to γ′ particles of about 20 nm in diameter formed during cooling, particles with D (diameter) < 10 nm form on aging. The average size of the γ′ particles with D < 10 nm increases on prolonging the time at a fixed temperature as well as by increasing temperature for fixed lengths of time. However, their size remains in the range under 10 nm except for the highest temperature and the longest time. The γ′ particle volume fraction also increase on increasing the aging time at a fixed temperature but is roughly constant after 24 h of heating, independent of temperature. Ultra Small-Angle Scattering (USAXS) reveals that the volume fraction of γ′ particles in the size range from 20 to 100 nm in diameter increase in the early stage of aging, while it decreases in the late stage. These results suggest that the disappearance of the particles in the size range of 20 < D < 100 nm stimulate the formation of γ′ particles with D < 10 nm in the single step aging. Comparison with Scanning Electron Microscope (SEM) images indicates that the regions with a low number density of γ′ particles in the size range from 20 to 100 nm in SEM images are the sites where the particles with D < 10 nm form. An increase in the number density of γ′ particles with 20 < D < 100 nm occurs in the following second step aging and cause an increase of the Vickers hardness. Consequently, statistically representative parameters obtained by SAXS in a larger sample volume than that viewed by direct observations show that the formation pathway of γ′ particles with 20 < D < 100 nm after two steps aging is through the dissolution of γ′ particles with 20 < D < 100 nm formed during cooling and the growth of newly formed particles with D < 10 nm.