Part VIII – August 1969 – Communications - Some Observations of Matrix Precipitation in a 45-Fe 35-Ni 20-Cr Alloy

ALLOYS with nominal composition of 45-Fe 35-Ni 20-Cr, ASTM-B-163, known under the commercial names of Incoloy 800 and Sanicro 31 (Alloy 800), are often employed at elevated temperatures for extended periods. Therefore, knowledge of the aging behavior of these alloys is of considerable importance. Significant decreases in ductility and increases in yield strength have been observed in the 45-Fe 35-Ni 20-Cr alloys containing about 0.4 pct Ti and 0.4 pct A1 after long-term aging at 540" to 700"~."~ However, no direct evidence has been presented for precipitation phenomena that might result in such changes. Strengthening effects have been attributed to the effects of local or-dering.3 Since y' precipitation (Ni3A1, Ti) is known to occur in similar alloys which contain more titanium and aluminum,4-7 it was decided to explore the possibility that y' might precipitate in these alloys and might be responsible for the changes in mechanical properties on aging. This note reports some observations of two distict types of matrix precipitates made by means of transmission electron microscopy, in a 45-Fe 35-Ni 20-Cr alloy containing 0.31 pct Ti and 0.4 pct Al. The steel used in this investigation was obtained from the Sand-vik Steel Works; it is designated Sanicro-31 (Alloy 800). The chemical composition of the alloy is given in Table I. Aging of solution-treated material at 700" and 800°C for 130 and 100 hr, respectively, produced the matrix precipitates shown in Figs. l(a) and l(b). It is apparent from these micrographs that the precipitate particles occurred in the form of thin square plates parallel to the matrix cube planes. Extraction replication and selected-area electron diffraction showed that the precipitates were M23C6. Coherency strains, consistent with the semicoherent nature of small M23C6 precipitates,' were evident for certain imaging conditions. In addition to matrix precipitation, preferential precipitation of M23C6 on dislocations was common, as has been reported previously for austenitic stainless steels.'" Specimens aged for longer times (-300 hr) at 700°C exhibited a different type of coherent matrix precipitate in addition to the M23C6. An example is shown in Figs. 2(a) and 2(b). In addition to the few large M23C6 precipitates, small precipitates exhibiting strain contrastl0,ll are present. The "line of no contrast" of the strain field of a given precipitate lies along differ-