Volume 53, Issue 11

Metallographic preparation is an art, especially when relevant information hidden at the micro- or nano-scale has to be unraveled. In particular, electron backscattered diffraction (EBSD) is a technique that provides valuable information of the microstructure of bulk and coatings. Indeed, information regarding evolution of the morphology, size and orientation of grains is very relevant for the design and fabrication of new commercial coatings. Metallographic preparation for EBSD analysis of hard coatings consisting of Al 2 O 3 and TiCN layers is presented in this work. In particular, preparation timing during grinding and polishing is discussed. EBSD data is presented and correlated with the metallographic preparation parameters. The main conclusions of this work highlight the importance of systematic preparation of all the successive metallographic procedures and the understanding of the quality parameters associated with the EBSD scans and data processing.

The oxidation of 99 wt.-% purity (low purity – main impurities were Fe and Si) and 99.999 wt.-% purity (high purity) Al foil was studied using a thermo-gravimetric method. The Al oxide was characterized with several techniques such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The Al foils were oxidized in a 50 % O 2 – 50 % Ar mixture between 773 and 843 K. Microscopic examination (SEM) of the oxide revealed that “pitting” occurred on the low purity Al. High Si and Fe concentration were found on the surface or near-surface of the oxide. The presence of Fe, as precipitates, on the surface of the oxide accounts for the pitting of the oxide. XPS inspection revealed the formation of γ-Al 2 O 3 with the presence of some Al hydroxide.

WC-Co bi-layer functionally graded composites were produced by powder metallurgy techniques. The influence of WC particle size and the grain growth inhibitor on the formation of the functionally graded composite was investigated. SEM images show that all sintered samples present a graded microstructure after sintering, with two side regions of extra-coarse and coarse WC-Co and a clear, dense interface without defects or other phases. EBSD results showed a change of WC particle size depending on the processing and the addition of Mo 2 C. Cobalt binder distribution corresponds to WC particle size. Hardness values correlate to WC particle size and binder content. It is shown that tailored hardness/toughness can be produced by adjusting the WC particle size and binder content on both sides of the bi-layer composite.

This work presents the analysis of a fractured impeller used in slurry pumps at the Collahuasi copper mine in Chile. Although the impeller must run more than 1 800 h, it was fractured under working conditions before having reached 1 500 h, generating a catastrophic failure of the pumping system. The microstructure of the impeller material was analyzed by optical microscopy, scanning electron microscopy (SEM) and microhardness. The composition of the impeller material was studied by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman spectroscopy. It was found that the impeller material corresponds to cast iron with high content of chromium, typically called white cast iron, which usually exhibits high wear and corrosion resistance. After analyzing the overall failure, we conclude that the fracture was mainly produced by the existence of high-impact materials in the slurry pulp, which produced an imbalance of the impeller rotation.