[en] Above T_max∼1050 K, the yield strength σ_t of the commercial γ'-strengthened nickel-base superalloys NIMONIC PE16, NIMONIC 105, and of many similar superalloys decreases drastically. In order to elucidate the physical reasons for this softening effect, σ_t has been measured as a function of the deformation temperature and of the plastic strain rate. Moreover, the configurations of dislocations in deformed specimens have been studied by transmission electron microscopy (TEM). The results are as follows: in NIMONIC 105, the relevant dislocation processes change at T_max: below T_max pairs of dislocations with Burgers vectors of the type (1/2)<1 1 0> shear the strengthening γ'-precipitates athermally whereas above T_max single (1/2)<1 1 0>-dislocations overcome them thermally activated. The effects of changes of the γ'-precipitate dispersion with temperature are negligible. In NIMONIC PE16, however, both mechanisms--changes of the γ'-dispersion and thermally activated dislocation processes--strongly affect σ_t above T_max

[en] In the present study, Ni50.7at.%-Ti shape memory alloys were prepared by vacuum induction melting using graphite crucibles. During melting the carbon concentration of the alloy increased and TiC carbides formed during solidification. The microstructure of the as-cast NiTi alloys was examined using scanning and transmission electron microscopy (SEM, TEM). Only eutectic TiC fibers were found in a low carbon NiTi alloy. For higher carbon concentrations, primary TiC carbides were observed in addition to the fine eutectic TiC phase. TEM results suggest that there is no specific orientation relationship between primary TiC carbides and B2. For the eutectic TiC phase, however, the following orientation relationship could be identified for low and high carbon contents in the NiTi alloy: B2 [001]||TiC [001], B2 (200)||TiC (220), B2 (11-bar 0)||TiC (200), and B2 (110)||TiC (020). It was also found that the interface between TiC and B2 is semi-coherent. During a subsequent post-cast heat treatment new carbides formed with different morphology (sizes and shapes) but with the same orientation relationship. Experimental evidence including SEM and TEM micrographs and diffraction patterns is presented to document these findings

[en] The nickel-base alloy NIMONIC PE16 is strengthened by coherent spherical nano-scale precipitates of the L1₂-long range ordered γ'-phase. Along grain boundaries precipitate free zones (PFZs) may form, which lower the yield strength drastically. In this study it has been attempted to eliminate this softening effect by double aging treatments: the main γ'-precipitate dispersion is grown at 1079 K and subsequently finer γ'-particles are precipitated at 949 K - in between the larger 1079 K-γ'-precipitates and in the PFZs formed at 1079 K. Unfortunately softening by PFZs could not be suppressed by these procedures. In addition, multiple strengthening of finely grained, PFZ-free NIMONIC PE16-polycrystals by γ'-precipitates, by atoms dissolved in the matrix, and by grain boundaries has been modeled and computer simulated