Volume 49, Issue 3

Intermetallic titanium aluminides are employed in aircraft engines and automobile engines because of their low density and excellent high-temperature properties. Today's TiAl-based alloys are multi-phase alloys of a complex structure which mainly consist of γ-TiAl, α 2 -Ti 3 Al and low fractions of a β o -TiAl phase. An example of such an alloy is the so-called TNM alloy which exhibit a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at %). In this alloy, solidification takes place via the β-phase, with the consequence of a fine-grained and nearly segregation-free microstructure. In spite of that, the cast microstructure also contains coarser grains which can act as crack initiators at room temperature and will reduce the deformation capability during tensile tests. Within the framework of this paper, heat treatment studies were conducted on a cast and hot isostatically pressed material with the primary aim of a microstructural homogenization in order to reduce the crack-initiating microstructural components and, hence, increase its fracture elongation at room temperature. In further heat treatments, microstructures with balanced mechanical properties were adjusted.

A new method for preparation of metallographic samples, based on application of phthalocyanine blue diluted with nitrocellulose resin, is presented. A thin layer of the dye was applied onto cast iron or carbon steel samples and alloys prior to observation under optical microscopy. Highly resolved coloured micrographs were obtained for quenching structures or isothermal phase transformation of carbon iron alloys.

Current collector rings (X12Cr13 stainless steel, Mat. No. 1.4006) exhibited different susceptibilities for staining, if provided from different manufacturers. The electrochemical potentiokinetic reactivation test (EPR test) revealed sensitisation as the reason for the insufficient corrosion behaviour. In the initial state, steel SS-1 shows a tempered structure and some ferrite with a sensitisation of max. 2.7%; steel SS-2 with a ferritic structure and carbides at the grain boundaries contains MnS and Ti(C,N) and exhibits sensitisation of ≈80%. After establishing a defined starting condition for both steels by solution annealing at 1050°C for 30 min, the steels were hardened in oil, air or cooled slowly in the furnace. Moreover, such oil-quenched samples were annealed at various conditions (800 or 750°C for 45 or 24 h), and the influence on sensitisation was determined via EPR test. The oil-quenched samples revealed the lowest degrees of sensitisation. Some unusual EPR results for SS-2 could be correlated with the dissolution of MnS during the test.

Signs of segregation found during the goods inwards inspection by the users of hot rolled bars of X20Cr13 turbine blade steel led to difficulties in the customer – supplier relationship, due to their unknown effect on the strength and reliability of the finished component. The aim of this investigation was to characterise the micro structural inhomogeneities metallographically, to discuss the metallurgical causes and possible consequences, and to suggest possible corrective of action.