Direct observation of boron segregation at grain boundaries in astrology by 3D atomic tomography

Abstract

Rapid, continuous heat treatment for control of microstructure is a widely used technology for ferrous alloys. For example, induction and, to a lesser extent, laser and electron beam methods are common for the surface hardening of steels [1]. Induction heating has also been applied for tempering of quench-hardened steels, solution annealing of stainless steels, and recrystallization annealing of cold worked carbon, electrical, and stainless sheet steels [1,2]. On the other hand, rapid heat treatment applications have been limited for nonferrous materials. Induction heating has been applied for full (and partial) annealing of cold rolled, non-heat treatable aluminum alloys on a production scale [3]. For titanium and titanium aluminide alloys, various rapid heating techniques have been investigated for beta annealing, recrystallization annealing of cold worked sheet alloys, and other special processes, albeit only on a limited laboratory scale and only from an empirical standpoint [4-7]. The objective of the present work was to analyze the kinetics of beta grain growth during rapid, continuous heating of a conventional alpha-beta titanium alloy. The analysis was based on approximate, closed-form theoretical expressions derived by Bourell and Kaysser [8] and Soper and Semiatin [9] as well as a fully numerical, computer-based approach. The problem and approach discussed here differs from previous investigations of grain growth during continuous heating and cooling [10-13], most of which have been for austenite grain growth in the heat-affected zone during welding of steels. In this regard, the main features of the present work are (1) the very high heating rates involved, (2) the avoidance of the application of complex numerical intergration schemes, and (3) the avoidance of using isothermal grain growth kinetic data to fit continuous heating results