Precipitation of austenite particles at grain boundaries during aging of Fe-Mn-Ni steel

Abstract

Precipitation of austenite particles at grain and lath boundaries after aging treatment of a Fe-8Mn-7Ni alloy was investigated by selected area electron diffraction (SAD), X-ray energy dispersive spectrometry (EDS) in a scanning transmission electron microscope (STEM), and high-resolution (HRTEM) analysis. High spatial-resolution (2 to 5 nm) EDS analysis revealed no significant segregation of alloying elements at grain boundaries but the precipitation of very fine particles of Mn-and Nirich phase. Detailed EDS, SAD, and HRTEM analyses all confirmed that these particles are austenite phase, which have a Kurdjumov-Sachs (K-S) orientation relationship with one of the adjacent grain. The concentration of Mn and Ni in austenite, measured by EDS, varied from ϳ15 pct to a maximum of ϳ30 pct. Low-voltage scanning electron microscopy (SEM) fractographs also revealed the presence of very fine, second-phase precipitates on the fracture surface of the embrittled alloys.

Electron microscopy study on the grain-boundary precipitation during aging of Fe-10Ni-5Mn steel

“…The necessary procedure to differentiate austenite from the -MnNi phase has been documented in our previous work. [21] b. Precipitation of -MnNi intermetallics Figure 6 shows another type of grain-boundary precipitate observed after aging for 30 minutes at 480°C and the diffraction pattern obtained from particle A in Figure 6. These particles are smaller and more numerous than the austenite particles, as can be seen in Figure 4.…”

“…Recent results of high-resolution energy-dispersive spectrometry analysis does not agree with the segregation of the alloying elements at the grain boundaries. [21] An increase in the Mn and Ni peaks in Auger analysis is interpreted as the result of precipitation of fine austenite particles at the grain boundaries. However, embrittlement of the grain boundaries in Fe-Mn-Ni alloys cannot be well explained by the precipitation of fine austenite particles at the grain boundaries.…”

“…The first report in this program investigated the precipitation of austenite particles at prior austenite grain boundaries and is reported elsewhere. [21] This article reports the results of more detailed diffraction and EELS analysis work on the second precipitation phase. The relationship between the second precipitation phase to embrittlement and the de-embrittlement of Fe-10Ni-5Mn alloy is also discussed.…”

Microstructure and Mechanical Properties of Mn-Containing Maraging Steels

“…on the grain boundary segregation and brittleness was investigated extensively though they met with little success generally ( . Interestingly, attempts recurred in the last decade using dedicated transmission electron microscopes (TEM) which recognized pronounced grain boundary precipitation reactions in contrast to the earlier reports (Ref [18][19][20]. Moreover, Mn-containing maraging steels have lately drawn considerable attention due to the accelerated clustering of alloying elements in the presence of Mn addition ( Ref 21,22).…”

Transmission Electron Microscopy Study on the Grain Boundary Precipitation of an Fe-Ni-Mn Maraging Steel

“…[11][12][13] However, it has recently been found that pronounced grain boundary precipitation causes intergranular brittleness in these steels. For instance, Mun et al [14] reported precipitation of fcc austenite particles at grain boundaries in an Fe-8Mn-7Ni (wt pct) maraging steel. Lee et al [15] then reported precipitation of the fct h-NiMn phase at grain boundaries of an Fe-10Ni-5Mn (wt pct) steel.…”