Interaction of the solute niobium or molybdenum with grain boundaries in α-iron
Introduction
It has been well known that a small addition of alloying elements such as Nb and Mo in low alloy steels retards phase transformation [1], [2] and recrystallisation [3]. However, it has not been clear whether the retardation is due to a solute drag effect or the pinning of boundaries by fine carbonitride precipitates or even a combination of the two, because most of the researches have been carried out in the presence of small amounts of C and N. In order to estimate the quantitative contribution of solute and precipitates to the inhibition of interface migration, it is necessary to determine the interaction between the solutes and interfaces. The interaction energy E and Gibbs free energy of segregation ΔGb are the important parameters that determine the solute drag behaviour [4], [5], [6]. However, in spite of a considerable number of experimental and theoretical investigations regarding solute drag, we are still in doubt about the distribution of solute atoms on stationary interfaces and on migrating interfaces. This is because experimental techniques for studying solute distributions with sufficient spatial resolution have been lacking. A suitable experimental technique with atomic resolution is now available, as a result of the recent development of the three-dimensional atom probe (3DAP). This has an advantage over Auger electron spectroscopy (AES), energy dispersive X-ray analysis (EDS) in transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) in terms of spatial resolution and analysable elements. The difficulty in bringing the interfaces to the apex region of an atom probe specimen has long been known as a serious practical problem in applying atom probe technique to the study of grain boundaries. However, a recent application of focused ion beam milling to the preparation of atom probe specimens [7] allows interfaces of interest to be positioned within the apex region more easily.
The purpose of this paper is to estimate the interaction of solute Nb or Mo with grain boundaries of α-Fe by using 3DAP. The factors that may affect the width of solute segregation profile at grain boundaries are discussed first, and then the interaction across the grain boundaries is estimated. The difference in the interaction between Nb and Mo, solute distribution within grain boundaries, and the artefacts that may degrade the spatial resolution of segregation profile is also discussed.
Section snippets
Experimental
The materials investigated are Fe–Nb and Fe–Mo binary alloys with ultra-low C and N concentration (Table 1). Composition analysis was performed by spark-source atomic emission spectro-chemical analysis technique. The concentrations of Nb (0.09 at.%) and Mo (0.18 at.%) are both within the solubility limit in α-Fe at 800 °C where the isothermal annealing was carried out. Specimens were first heated to 800 °C and subsequently hot-compressed to a strain of 1.5 and held at temperature for 10 000 s,
Nb and Mo distribution across grain boundaries of Fe
Fig. 1 shows a ladder profile across a large-angle grain boundary (φ=40°, =[−0.80, 0.42, 0.44]) and 3D atom map for Nb in the Fe–0.09Nb binary alloy. Nb is segregated at the grain boundary with the width of the segregated region measured as 2.4 nm. Obvious grain boundary segregation of C and other trace elements was not observed. The mean concentration of Nb was 0.58±0.19 at.% at the grain boundary and 0.03±0.02 at.% in matrix. The angle between the grain boundary plane and direction of
Summary
The segregation of solute Nb or Mo at large-angle grain boundaries in α-Fe was quantified by 3DAP by making use of the FIB technique for specimen preparation.
The measured width of solute segregation was broader when the direction of atom probe analysis was not perpendicular to the boundary plane, mainly due to a local magnification effect at grain boundaries. The relationship between the width of grain boundary segregation and the direction of analysis revealed that the degradation due to the
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