Mechanical properties and strengthening of a Ni-25Mo-8Cr alloy containing Ni-2(Mo,Cr) precipitates

The mechanical properties and mechanisms of deformation and strengthening of Haynes Alloy 242(tm), a nominal Ni-25Mo-8Cr (in wt%) containing Ni-2(Mo,Cr) precipitates in a face-centered cubic matrix, are reported. Both solution treated, as well as aged samples (550-750 degrees C, 1-1200 h) were deformed to permanent strains of 1 and 6% in compression and to failure in tension. Thr deformation structures were observed by transmission electron microscopy. A two-fold increase in strength and tremendous strain hardening are observed as the short-range (SRO) to long-range ordering (LRO) transformation proceeds, although ductility remains high even in well-aged samples. Major contribution to strengthening and strain hardening comes from the precipitation of a high volume Fraction of Ni-2(Mo,Cr) precipitates, with hardening in the solution treated samples and those aged for short periods bring associated with the presence of SRO in the matrix. A transition in deformation mode, from glide of unit dislocations in planar arrays to profuse twinning, is observed as a function of aging time and imposed strain, twinning being observed in samples containing Ni-2(Mo,Cr) precipitates. A semi-quantitative model developed on thr basis of precipitate size and mode of deformation (shearing, twinning, bypassing) is able to satisfactorily account for this transition. A presentation and discussion of these results, as well as those of the mechanisms of strengthening and strain hardening, are provided. Copyright (C) 1996 Acta Metallurgica Inc.