Tllrce Ni-base \vrought alloys with different hardening mechanisms (Inconel 706, Waspaloy and Inconel 617) are candidates for steam turbine disc applications with tenlperatures up to 700 "C and were examined in respect to their microstructure and ~nicrostructural stability. The materials were investigated after different heat treatments and after short and long term ageing by metallography, scanning and transmission electron microscopy. Special DTA experiments has been perfo~nled to confirm the knonn TTT diagrams and to study the initial precipitation of different phases and their growth, as function of cooling rate after the heat treatment. The amount and size of precipitations uere evaluated by image analysis and stereological methods. The Nb containing alloy Inconel 706 shomrs a complex microstructure containing y', y" and q phases which are stable under long term service up to 620 "C. At higher temperatures a strong particle coarsening and phase transfornlation u a s observed. Waspaloy is hardened by y' particles with a bimodal size distribution. After ageing at 700 "C and higher a coarsening was observed by loss of the bimodal size distribution. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M13C6 carbides. After long te1-111 ageing at temperatures of 650 "C to 750 "C the carbides tended to form carbide films along the grain boundaries and at 700 OC to 750 "C ;/' precipitated to hon~ogeneously distributed particles with low coarsening under long term ser\rice.

Vol. 56 the vagueness with which the concept of critical complex is encumbered. For the relatively small variations in water activity in question, the experimental data do not exclude the possibility that log k + Holog amo rather than log k + Ho is constant, although agreement is definitely less good for the former relation. It is also possible that there is some more or less accidental compensation between the factors a

An intense localized heat source inside a microscope could be useful in the improvement of electron microscopic in-situ investigations. For this reason a JEOL scanning electron microscope was combined with a pulsed Nd-glass laser which allowed power densities up to about lo9 W/cm2 on the specimen under Q-switch conditions. The mechanical and electrical adaptations necessary for the SEM-to-laser coupling, as well as some tests of the method involving the new equipment are discussed in detail in this paper. Various applications to materials are reviewed, including melting processes, thermal cracking and surface hardening of tungsten carbide alloys and other hard materials.

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