AMP_04_May_June_2021_Digital_Edition

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 6 3 4120 alloy modified with the addition of 0.12 wt% niobi- um and 0.30 wt% molybdenum. The specimen was heat treated following hot rolling to 900°C and water quenched. Figure 1a highlights the distribution of niobium and car- bon within the region captured by the atom probe needle. The image indicates carbon segregation at what is inter- preted to be a prior austenite grain boundary. Notably, sev- eral Nb-rich precipitates are present along the boundary, directly demonstrating the Zener pinning effect of these precipitates. The proximity histogram, presented in Fig. 1b, suggests molybdenum incorporation into the microalloy precipitates and an absence of Mo at the precipitate-ma- trix interface. This observation is contrary to previous speculation that molybdenum segregates to precipitate grain boundaries. This type of information enables further understanding of the role of various alloying approaches on precipitation behavior. STEM-HAADF IMAGING TO UNDERSTAND SILICON EFFECTS IN NITRIDED STEELS Nitriding is a thermochemical process where nitrogen is introduced into the surface of a component via a heated gas, molten salt, or plasma. Increased amounts of nitro- gen lead to solid solution strengthening and precipitation hardening if strong nitride forming elements such as vana- dium, niobium, molybdenum, aluminum, and chromium are present in sufficient quantities. Both nitrogen solid solution and nitride precipitates lead to the development of compressive residual stresses. Other elements such as manganese and silicon have been shown to form nitrides, but they are generally assumed to make minimal contri- butions to strengthening in the nitride case in commer- cial alloys. The influences of vanadium and silicon on the case hardness and residual stress in experimental medium car- bon steels after nitriding were recently reported by ASP- PRC researchers [4] . It was shown through conventional transmission electron microscopy (CTEM) bright field (BF) and dark field (DF) imaging that increases in vanadium content led to increases in vanadium containing nitride precipitation. The increased vanadium nitride precipita- tion significantly increased both case hardness and com- Fig. 2 — (a) Conventional transmission electron microscopy bright field (CTEM-BF) image of the nitrided case region of a 1.57 wt% Si alloy. (b) Scanning transmission electron microscopy high angle annular dark field (STEM-HAADF) image showing amorphous Si containing nitrides not visible in the CTEM-BF image. (c) Influence of Si content on the volume fraction of the amorphous Si containing nitrides. (d) Influence of Si content on case hardness and compressive residual stress in the nitride case. Plots adapted from J. Klemm-Toole et al. [4] . (a) (b) (c) (d) 10 11