Nickel-based superalloys are known for their use at high temperatures and loads. Due to the interaction of external loads and internal stresses caused by the misfit between precipitate and matrix, the so-called "rafting" occurs, with the precipitates coalescing into elongated rafts. Since both the γ’ rafts and the γ matrix are fully interconnected, i.e. a bicontinuous network has formed, this phenomenon can be used to produce open-porous metallic membranes. For this purpose, the single-crystalline alloy CMSX-4 is thermo-mechanically treated under creep stress and then one of the two phases is electrochemically extracted.

In order to reduce costs and realize larger specimen dimensions, in addition to membranes made of single-crystalline, those made of polycrystalline starting material are studied. Recent publications investigate the development of superalloy membranes by incoherent growth of the particles. This results in membranes with relatively large pores, a porosity of 44 % and an ultimate tensile strength of 120 MPa.

In this work, we show polycrystalline membranes with a directionally coarsened structure. By means of separate mechanical and thermal treatment, a rafted structure similar to the directionally coarsened one in CMSX-4 is also formed in the polycrystalline alloy. In contrast to the uniaxial creep load during the production of directionally coarsened single-crystalline membranes, a directional rafted structure is formed here from originally cubic γ' precipitates by repeated rolling and ageing.

The development of the microstructure during the thermo-mechanical treatment is shown by micrographs of the specimen and cross-sections of the extracted membranes, as well as quantitatively evaluated on the basis of the length-related flow resistivity. In addition, the mechanical properties are determined in tensile tests using an optical strain measurement system.