MECHANICAL-PROPERTIES AND STRENGTHENING OF A NI-25MO-8CR ALLOY CONTAINING NI-2(MO,CR) PRECIPITATES

The mechanical properties and mechanisms of deformation and strengthen ing 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 reporte d. 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 compressio n and to failure in tension. Thr deformation structures were observed by transmission electron microscopy. A two-fold increase in strength a nd tremendous strain hardening are observed as the short-range (SRO) t o long-range ordering (LRO) transformation proceeds, although ductilit y remains high even in well-aged samples. Major contribution to streng thening and strain hardening comes from the precipitation of a high vo lume Fraction of Ni-2(Mo,Cr) precipitates, with hardening in the solut ion treated samples and those aged for short periods bring associated with the presence of SRO in the matrix. A transition in deformation mo de, from glide of unit dislocations in planar arrays to profuse twinni ng, is observed as a function of aging time and imposed strain, twinni ng being observed in samples containing Ni-2(Mo,Cr) precipitates. A se mi-quantitative model developed on thr basis of precipitate size and m ode of deformation (shearing, twinning, bypassing) is able to satisfac torily account for this transition. A presentation and discussion of t hese results, as well as those of the mechanisms of strengthening and strain hardening, are provided. Copyright (C) 1996 Acta Metallurgica I nc.