RESUMO
A temporal evolution of clusters associated with age hardening behavior in a Cu-Ni-P alloy during ageing at 250 °C for up to 100 ks after solution treatment has been carried out. A three-dimensional atom probe (3DAP) analysis has showed that Ni-P clusters are present in the as-quenched condition, and that the cluster density increases as the ageing time increases. The clusters have a wide range of Ni/P ratios when they are relatively small, whereas larger clusters exhibit a narrow distribution of the Ni/P ratio, approaching a ratio of approximately two. These results would indicate that the clusters with various Ni/P ratios form at the early stage of precipitation and the ratio approaches a value identical to that of the equilibrium phase at 250 °C as the clusters enlarge during ageing.
RESUMO
The nature of the initial carbides formed during the early stages of the tempering of steels is still a matter of debate. Conventionally, the main transition carbide is described as epsilon carbide, with a composition of approximately Fe(2.4)C. However, earlier one-dimensional atom probe (1DAP) results indicated the existence of carbon-rich regions having much lower carbon contents, with maxima of around 10at%. There was some uncertainty about the interpretation of the 1DAP results, because of possible problems with alignment of the aperture and with trajectory aberration effects. We have therefore re-visited this topic, using the three-dimensional (3D) atom probe, and studying both a model Fe-Ni-C alloy and a well-known engineering steel (AISI4340). We demonstrate that, for both materials, low-temperature (20-150 degrees C) aging produces carbon-rich regions with average peak carbon contents of up to 10%. We show for the first time the three-dimensional structure of these carbon-rich regions, and demonstrate that fine-scale faulting exists within them.
RESUMO
Zirconium alloys are used in the nuclear industry as fuel rod cladding. They are chosen for this role because of their good mechanical properties and low thermal neutron absorption. Oxidation of these alloys by coolant is one of the chief limiting factors of the fuel burn-up efficiency. The aim of the present study is to understand these oxidation mechanisms. As a first step, a fundamental study of the oxidation of commercially pure zirconium has been conducted using the 3D atom probe (3DAP). The current generation of 3DAPs allows both voltage and laser pulsing, providing data sets of many millions of ions. According to the literature the only stable oxide of zirconium is ZrO(2). However, the 3DAP shows that an initial layer a few nanometres thick forms with a composition of ZrO(1-)(x) when subjected to light oxidation. This result confirms and extends the work of Wadman et al. [Colloque de Physique 50 (1989) C8 303; Journal de Physique, 11 (1988) C6 49] and Wadman and Andrén [in: C.M. Euchen, A.M. Garde (Eds.), Zirconium in the Nuclear Industry: Ninth Symposium, ASTM STP 1132, ASTM, USA, 1991, p. 461], who used 1DAP techniques, obtaining reduced data sets. Segregation of hydrogen to the metal-oxide interface and a distinct ZrH phase were observed in this study. A novel kinetics study of the room temperature oxidation of zirconium showed the ZrO layer to be non-protective over the time period investigated (up to 1h).
RESUMO
Over the last few years there have been significant developments in the field of three-dimensional atom probe (3DAP) analysis. This article reviews some of the technical compromises that have led to different instrument designs and the recent improvements in performance. An instrument has now been developed, based around a novel reflectron configuration combining both energy compensation and focusing elements, that yields a large field of view and very high mass resolution. The use of laser pulsing in the 3DAP, together with developments in specimen preparation methods using a focused ion-beam instrument, have led to a significant widening in the range of materials science problems that can be addressed with the 3DAP. Recent studies of semiconductor materials and devices are described.
