Analysis of Nimonic 263 creep behaviour : [Analisi del comportamento a creep della superlega Nimonic 263]

An adequate understanding of the complex mechanisms operating during creep is still lacking. Discrimination among rival theories is often impossible because the experimental data cover only a limited stress/temperature field. In this work the creep behaviour of the nickel base superalloy C263 has been studied at constant load and temperature at 750-30MPa/600-950°C. The experimental results have shown a very strong dependence of the creep curve shape with the applied stress/temperature, particularly: o At 600 °C, during the initial loading, an instantaneous plastic strain, increasing with the applied stress, occurs. The creep curve is characterised by a long and large decelerating primary creep stage. The creep strain due to the primary stage, ?pr, is always larger than the calculated elastic strain, ?/E, obtained during the initial loading. The long primary creep is interrupted by the fracture, before significant secondary and tertiary stages appear. o At 700 and 800°C, the size of the primary creep is definitely smaller compared to the tests at 600°C. It generally results ?pr< ?/E. After the decelerating primary stage, a clear accelerating creep phase appears. Its contribution to the creep curve increases with the decrement of the applied stress up to dominate the whole creep curve. The accelerated stage exhibits a linear relationship between strain rate and strain. o In the 900-950°C temperature range, the negligible decelerating primary creep ends in a minimum creep rate followed by an accelerated strain stage. At the lowest applied stresses, the rapidly accelerating stage is followed by a long steady state. Then, a further accelerating stage leads to fracture through necking and loss of section. The creep results have been modelled using coupled differential equations of the Kachanov form, that are consistent with physical deformation mechanisms of the studied alloy. The parameters of the proposed constitutive equation are functions of microscopic parameters like the density of mobile dislocations, but they are also related to easily measurable characteristics of the creep curves, so that they can be determined using macroscopic creep data only. The use of differential formalism of the equations allows to describe the creep behaviour also at variable stress and temperature, and to cope also with more complex load/temperature histories as, for example, variable loading creep, constant strain rate and stress relaxation tests without appealing to arbitrary strain or time hardening rules.

Il comportamento a creep della superlega Nimonic 263 è stato studiato a carico e temperatura costante nell'intervallo 750-30MPa/600-950°C. I risultati sperimentali hanno mostrato che la forma della curva di creep dipende fortemente dalle sollecitazioni applicate. Nelle prove eseguite a sollecitazioni superiori al carico di snervamento, le curve di creep consistono essenzialmente nel solo stadio primario/decelerante, mentre per sollecitazioni inferiori, ed in particolare ai più bassi carichi ed elevate temperature qui studiati, lo stadio primario diventa molto piccolo e breve, ed altri stadi di deformazione dominano le curve di creep. In questo lavoro si dimostra che un'unica e semplice equazione costitutiva, basata sulla moltiplicazione e annichilazione delle dislocazioni mobili, è in grado di descrivere e interpolare correttamente le curve di creep in tutto l'intervallo di sollecitazioni/temperature esplorato.