A new approach to the coloured visualisation of the grain structure of intermetallic NiAl alloys

Intermetallic phases on the basis of NiAl demonstrate an excellent chemical resistance and a good thermodynamic stability in combination with a high m elting point. As these alloys have in addition comparatively low densities and also high thermal conductivity, they appear to be promising candidates for applications in modern ecological and economical energy conversion syst ems. In the manufacture of polycrystalline components for the high temperat ure area of stationary gas turbines, the size, above all, of the individual grains is therefore of great importance. Various grain refining mechanisms are being investigated in the frame of cu rrent research projects. To test their effectiveness it was necessary to im prove the metallography of NiAl alloys in this respect. In a series of experiments it was shown that using an etchant based on ferr ous chloride, a differentiation in the colour of the grains in the microstr ucture of the NiAl alloy was possible with polarised light. Such a microgra ph can be analysed directly by the interactive image analysis system (IBAS US 400). This technique results in a very detailed grain size analysis.