Analisi microchimica mediante SPEM di una superlega di nichel dopo prove di creep

The continue demand to increase the efficiency and reliability of aeronautical and/or energy gas turbines has led to develop new Ni superalloys having a greater volume fraction of characteristic hardening g' phase respect to the g phase. Nonetheless the considerable mechanical resistance of these materials is due to the specific microstructural mutual arrangement and shape of the two phases, too. In particular the g'+ g structure is not stable during creep: when at higher temperatures the alloy is subjected to a mono-axial tensile stress at the direction <001>, the original cuboidal g' structure changes to an anisotropic lamellar structure (rafts), perpendicular to the load direction. This effect depends on the lattice misfit between the two phases. The lattice parameters of the phases are temperature dependent, since the thermal expansion coefficient of g' is lower than that of g, and the partition of chemical elements between the two phases changes with temperature because of diffusion phenomena. In addition, the relative composition of the two phases can be influenced by the stress state during creep. This work presents the potentiality of high resolution Scanning PhotoEmission Microscopy (SPEM) on the study of the diffusion phenomena taking place between g and g' phases of a Ni superalloy after creep. In particular the results on the single-crystal superalloy CM186LC, before and after creep tests at temperature of 800°C and 900°C, are reported, indicating that SPEM can be useful to study the aforesaid microstructural evolution with creep for the material. The XPS measurements with high spatial resolution and the SPEM analysis, operating both in punctual spectroscopy modality and image configuration, have been carried out at the beam-line ESCA-microscopy at the synchrotron Elettra in Trieste (Italy). By XPS analysis it has been possible to quantify the present elements and their chemical state, whilst by SPEM technique the element distribution on the two phases ?'and ? has been determined. In particular the latter technique is able to analyze the compositions of the two different phases without surface chemical etching. In the considered case it has been observed as the formation of the rafts during creep occurs especially with the diffusion of W e Hf from ? phase to ?' phase. The variations of the relative quantities for the elements into the phases have been observed at the higher creep test temperature, that is 900 °C. W is homogeneously distributed, whilst Hf is totally in ?'.

Lo scopo di questo lavoro è mostrare le potenzialità delle analisi micro-chimiche di superficie, in particolare la spettroscopia Scanning PhotoEmission Microscopy (SPEM) ad alta risoluzione, nello studio dei fenomeni diffusivi che hanno luogo fra le fasi g e g' nelle superleghe di Ni a seguito del creep. Si riportano le analisi condotte su una superlega monocristallina, CM186LC, prima e dopo prove di creep alle temperature di 800 e 900 °C. Le misure di fotoemissione ai raggi X (XPS) ad elevata risoluzione spaziale sono state effettuate presso la beam-line ESCA-microscopy del sincrotrone Elettra di Trieste, in cui è stato utilizzato lo SPEM, che opera in modalità sia di immagine che di spettroscopia puntuale, producendo una microsonda a raggi X di diametro inferiore a 50 nm. L'alta risoluzione permette di esaminare separatamente la composizione chimica della fase rinforzante g' e della matrice g caratterizzanti la superlega. In questo modo è possibile studiare la partizione degli elementi di lega tra le fasi nel materiale vergine e la sua evoluzione dopo le prove di creep.