The Precipitation and Strengthening Behavior of Ni₂(Mo,Cr) in HASTELLOY C-22HS Alloy, a Newly Developed High Molybdenum Ni-Base Superalloy

Abstract

The precipitation and strengthening behavior has been studied in a newly developed high molybdenum Ni-base superalloy, HASTELLOY ® C-22HS ® alloy, for long time exposures of 1000hrs at 600, 625 and 650ºC. The homogeneous precipitation of the long range ordered (LRO) phase Ni 2 (Mo,Cr) with an average size on the order of 10nm after a specially designed heat treatment contributes an excellent strengthening effect for this new superalloy. The grain boundary precipitation of high molybdenum containing phases such as M 6 C and μ phase has been studied via isothermal exposures in a wide temperature range of 600-1050ºC. The T-T-T curve of grain boundary phase precipitation has been also determined. This newly developed Ni-base superalloy with excellent corrosion resistance and high strength can be used in the intermediate temperature range below 650ºC.

Formation of microstructures responsible for remarkable properties of the Ni- and Co-based superalloys

“…This type of interaction excludes the possibility of the Ni 2 (Mo, Cr) phase formation in the alloy. Actually, the attempt of the authors [21] to electrolytically extract particles of Ni 2 (Mo, Cr) from the solid solution of the Hastelloy Ce22HS alloy, and then to determine their chemical composition did not lead to the desired results: the amount of chromium in the anodic deposit was much lower than expected.…”

“…11) shows that the precipitate are chemical compounds of Ni 2 Mo with a structure of the Рt 2 Mo type (arrow points out the reflection from which the dark-field image was obtainedinset). Since such a compound in the Ni-20 at.% Mo binary alloy is not a stable phase [11], the authors [21] supposed that in the NieMoeCr alloys, the Ni 2 Mo compound has the chemical composition of Ni 2 (Mo, Cr), in which atoms of chromium may replace some of the atoms of molybdenum, to strengthen the stability of this compound. However, this supposition is in conflict with the results [19,20], from which it becomes clear that in alloys of the NieCr system (where the Cr concentration prevails), the tendency to phase separation takes place at all temperatures.…”

Sub-Kelvin magnetic and electrical measurements in a diamond anvil cell with in situ tunability

“…Hastelloy C22HS is also predominantly in the FCC phase, although precipitation hardening forms Ni 2 (Mo, Cr) particles as large as 10 nm in extensively precipitated (∼1000 h) specimens. 46 We found only a trace presence of Ni 2 (Mo, Cr) with a volume ratio less than 0.1% (Fig. 2) for a heat treatment time of 24 h. Udimet L605 and Elgiloy both showed a significant volume (∼10%) of minor phases in addition to the FCC majority phase.…”

Enhanced stability of the strengthening phase Ni2(Cr,Mo) in Ni–Cr–Mo alloys by adjacent instability

“…It means that the addition of Mo would decrease the stability of the Ni 2 (Cr,Mo) phase. In fact, the stability of the Ni 2 (Cr,Mo) phase is so subtle that only well controlled heat treatments can give the precipitation of the ordered Ni 2 (Cr,Mo) phase [5][6][7]9].…”