MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF NI-11.8 AT-PERCENT TI SINGLE-CRYSTALS

The decomposition kinetics of Ni-11.8 at.% Ti single crystals (f.c.c. structure) have been investigated by ageing at 853 and 1073 K. Between 60 and 500 h of ageing at 853 K, concentration waves were developed, which grew in amplitude and wavelength with increasing ageing time. During ageing at 1073 K the stable, hexagonal eta phase was formed after about 4 h. Compressive and tensile tests have been performed at room temperature. The samples aged for 500 h at 853 K were found to have a low ductility in tensile tests and a very high yield strength in compressive and tensile tests. A marked serrated flow was found in compressive tests. Regularly dissociated screw dislocations were observed in samples aged for 250 and 500 h at 853 K and deformed in compression. A model is suggested which describes the formation of ABAC stacking sequences of the stable eta phase out of regularly dissociated dislocations, if an external stress is applied. In compressive tests of samples aged for 4 h and 24 h at 1073 K shearing of eta plates was observed. In tensile tests, however, the eta plates were found to act as dislocation sources. We assume, that the eta plates might be responsible for the compression-tension hardening asymmetry of this ageing stage and the decreasing ductility in tensile tests.