Hot deformation mechanisms in a powder metallurgy nickel-base superalloy IN 625

The hot working behavior of the nickel-base superalloy IN 625 produced by hot extrusion of a powder metallurgy (P/M) compact has been studied by comrpession testing in the temperature range 900-1200 deg C and true strain rate range 0.001-100 s exp -1 . At strain rates < approx0.1 s exp -1 , the stress-strain curves exhibited near steady-state behavior, while at higher strain rates, the flow stress reached a peak before flow softening occurred. The processing maps developed on the basis of the temperature and strain rate and strain dependence of the flow stress exhibited three domains. (1) The first domain occurs at lower strain rates ( < 0.01 s exp -1 ) and temperatures > approx1050 deg C. The peak efficiency and the temperature at which it occurs have increased with strain. The microstructurre of the specimen deformed in this domain exhibited extensive wedge cracking. (2) The second domain occurs in the intermediate range of strain rates (0.01-0.1 s exp -1 ) and temperatures < 1050 deg C, and in this domain, microstructural observations indicated dynamic recrystallization (DRX) of gamma containing delta precipitates and carbide particles resulting in a fine-grained structure. (3) The third domain occurs at higher strain rates ( > 10 s exp -1 ) and temperatures > 1050 deg C, with a peak efficiency of approx42% occurring at 1150 deg C and 100 s exp -1 . Microstructural observations in this domain revealed features such as irregular grain boundaries and grain interiors nearly free from annealing twins, which are typical of DRX of homogeneous gamma phase. The instability map revealed that flow instability occurs at strain rates > 1 s exp -1 and temperatures < 1050 deg C, and this is manifested as intense adiabatic shear bands. These results suggest that bulk metal working of this material may be carried out in the high strain rate domain where DRX of homogeneous gamma occurs. On the other hand, for achieving a fine-grained product, finishing operations may be done in the intermediate strain rate domain. The wedge cracking domain and the regime of instability must be totally avoided for achieving defect-free products.