Volume 58, Issue 3

The present work will introduce a novel two-step etching procedure for grain analysis of intermetallic iron aluminides. A grain boundary etching technique from the literature will be complemented by subsequent grain-contrast etching to visualize different grains in polarized light. The use of an XY-table for the acquisition of MosaiX images also enables grain analysis over the entire cross-section of the sample.

This work developed a systematic method for a metallographic preparation of single powder particles with diameters of approx. 20 to 40 μm. It was motivated by the objective of understanding additive manufacturing processes such as Laser Powder Bed Fusion. A fundamental aspect of the relationship between manufacturing, structure, and properties is the correlation of rapid solidification and resulting microstructure. During powder-based additive manufacturing processes, cooling rates up to 1 MK/s are attained. A thermal analysis determining the characteristics of solidification at such rapid cooling rates can be performed with the aid of chip sensor-based, dynamic Differential Fast Scanning Calorimetry, DFSC. For this purpose, the heat flow during the solidification of single powder particles is measured and, for instance, the solidification onset temperature is evaluated as a function of cooling rate. It is thus possible to estimate the undercooling which has a significant impact on the resulting structure. Subsequently, cross sections of single powder particles must be prepared for the analysis of the resulting structure.

In this paper, the evolution of the microstructural and performance of IN783 hightemperature bolts in service was studied. The results showed that under high-temperature service environment, with the increase of service time, the superalloy of IN783 bolt and the precipitation quantity of second phase increased. The second phase mainly existed in the form of particles and rods, which were distributed in an obvious oriented way. With the extension of service time, the strength of the alloy increased, but the hardness decreased, the plasticity and toughness decreased significantly, so it is difficult for the IN783 high-temperature bolts to reach the normal level of safe service. Therefore, the IN783 high-temperature bolts that have been in service for a long time should be strictly supervised and replaced appropriately to reduce losses.

A first-stage turbine vane was received in the laboratory directly from fabrication, prior to its use in engine service. The part had not yet been covered with its customary coating system that protects these parts against hot corrosion. A first visual inspection revealed multiple cracks on the airfoil’s hot gas path side, fairly centered in the part. After cutting the part open, it soon became apparent that the cracking was even more severe inside, suggesting crack initiation from that cooled side. Fractography allowed to determine liquation cracking as the metallurgical failure mechanism. Since the part was received immediately after pre-heat before plasma coating, that process step was concluded to have caused the cracking.