Volume 52, Issue 12

The present article is considered the continuation of the two review articles “Intermetallische γ-Titanaluminid-Basislegierungen aus metallographischer Sicht” (“Intermetallic γ titanium aluminide based alloys from a metallographic point of view”) by H. Clemens and F. Jeglitsch in Pract. Metallogr. 37 (2000) 194 – 217 and “Intermetallische γ-Titanaluminid-Basislegierungen aus metallographischer Sicht – eine Fortsetzung” (“Intermetallic γ titanium aluminide based alloy from a metallographic point of view – a continuation”) by H. Clemens and S. Mayer in Pract. Metallogr. 48 (2011) 64 – 100. With reference to a γ TiAl based alloy specifically developed for aircraft engines, the so-called TNM alloy, this third and concluding part describes how materialographic methods, in combination with a targeted use of complementary investigation methods, have contributed to their development, manufacture, and processing, and, eventually, to their industrial launch. In this context, the alloy development strategy is discussed and representative microstructures and nanostructures are shown and described after different processing and heat treatment processes. Selected case studies from materialographic examinations are presented and interpreted using, for each and every of these aspects, approaches from the fields of physical metallurgy und metal physics.

Leakages often occur in the copper tube – finned heat exchangers of cooling and air conditioning units, the dimensions of which are on the microscopic scale thus making the location and cause very difficult to determine. In the case presented here the leakages found were located using a Helium leak detector. After removing the fins of the heat exchanger, because the points of failure were not visible to the naked eye, a dye penetrant test was carried out on the affected area (Penetrations test or PT), which led to the discovery of three very small indications on one section of the tube. Subsequent microscopic laboratory examination revealed the cause to be microscopically small holes in the wall of the tube caused by corrosion through from the internal surface of the coolant side of the tube. The corrosion mechanism was identified as Formicary Corrosion otherwise referred to as Ant-Nest Corrosion. For this type of corrosion to occur, which according to the specialist literature is often found in Cooling or Air Conditioning units, an aqueous electrolyte or electrolytic film containing Carbonic acid and Oxygen is required which can lead to damage occurring within weeks to months. In the case described in this article, the unintentional ingress of an aqueous oxygenated acidic solution of Glycol into the heat exchanger was most probably responsible for the damage.