Strengthening in MULTIPHASE (MP35N) alloy: Part I. ambient temperature deformation and recrystallization

The microstructure and mechanical properties of the cobalt-based MP35N alloy (35 pct Co, 35 pct Ni, 20 pct Cr, and 10 pct Mo) were investigated following room-temperature deformation. It was found that the material showed marked work hardening at strains greater than about 0.15 to 0.2, which correlated with the formation of platelike structures on {111} planes. The initial yield strength of 390 MPa was increased to 1385 MPa by cold drawing 48 pct. Electron microscopy showed evidence that the platelike structures were both faulted face-centered cubic (fcc) twins and hexagonal martensite. Secondary strengthening from 1385 to 1935 MPa was achieved by annealing the cold-drawn MP35N at 650 °C for 4 hours. However, no structural change could be detected that could account for this large secondary strengthening. This failure lends indirect support to the proposal that this secondary strengthening could arise from solute partitioning between the fcc matrix and hexagonal martensite. Cold-worked MP35N resisted recrystallization up to a critical softening temperature of 1083 K (810 °C), at which the alloy lost 50 pct of its hardness during a 1-hour anneal. Nucleation occurred readily at and above this temperature. Growth of the new grains at temperatures close to the critical softening temperature appeared to be strongly inhibited by the martensite plates but not by twins. Residual dislocation densities were seen in the recrystallized grains in the vicinity of the disappearing martensite plates. It is proposed that these dislocations arise from local changes in lattice parameters arising from prior solute segregation to the martensite plates. A revision of the earlier solute partitioning model of secondary strengthening is proposed here — that the partitioning occurs locally, adjacent to the preexisting plates. During postanneal deformation, higher stresses are then required for fresh martensite plates (and twins) to be nucleated from the existing plates. Ultrafine grain sizes of only about 1 μm could be produced in MP35N by successive cycles of deformations and anneals at temperatures close to the critical softening temperature.