To maximize the mechanical properties of Ni-base superalloys, solution heat treatment is essential to sufficiently homogenize the dendritic segregations formed during solidification. To investigate the homogenization behavior during solution heat treatment, a Ni-base single crystal superalloy, TMS-238, was heat treated under various conditions; temperatures ranging from 1573 to 1613 K for times ranging from 2 to 100 h. After solution heat treatment, the average concentrations of Re, an element that exhibits the highest degree of segregation, in dendrite core and inter-dendritic regions were analyzed. From these results, apparent diffusion constants, D app , were determined based on a proposed homogenization model. Obtained D app values were significantly smaller than the diffusion constant of Re in Ni, strongly suggesting that the apparent diffusion coefficients should be obtained experimentally when using the target alloy.

It is required to reduce the lifetime cost of turbine blades. To achieve the cost reduction, a refining and recycling method of scrapped turbine blades is proposed. For the establishment of the method, desulfurization mechanism of Ni-base superalloy by solid CaO was studied. 500 g of superalloy containing sulfur was heated in a vacuum induction furnace and kept at 1600 °C. A CaO rod was inserted into the molten alloy and held for 600 s. After the experiment, sulfur content in the alloy decreased from 200 ppm to 54 ppm. On the surface of the CaO rod after the experiment, only Ca, O, Al, and S were found by EPMA analysis. Especially, Al and S were distributed at the surface and grain boundaries of the rod. By powder XRD analysis, CaO, CaS and 3CaO･Al 2 O 3 were identified as constituent phases on the rod. The desulfurization mechanism of superalloy at 1600 °C is supposed to be three steps: (1) Al and S in the alloy react with CaO to generate CaS and Al 2 O 3 , respectively. (2) Al 2 O 3 melts with CaO as liquid slag. (3) CaS is captured by the slag, therefore, sulfur is removed from the alloy.