Phase Transformations in Diluted Al-Sc-based Alloys and Their Investigation

Al-based alloys have a long tradition of use especially as a lightweight material for traffic systems. The limits of conventional casting have not yet been reached. One of the recent aims in the research of Al-based alloys is to realize the potential of non-traditional alloying elements such as transitions metals (TM) – e.g. Sc, Zr, Mn, Cr, etc. and their combinations. In this contribution we describe the excellent improvement of Al-Sc-based alloys properties (weldability, corrosion resistance, hardening and increase of recrystallization temperature) due to the presence of coherent, nanometer size Al3Sc precipitates. We present also one of possible methods in an investigation of solid solution decomposition in Al-Sc-based alloys – electrical resistivity measurements as a response to isochronal and isothermal annealing. Introduction In the early years aluminum alloys were developed by a “try and error” method. It was detected before the World War I, that an age hardening is possible in an Al alloy (called Duralumin alloy which contained additions of copper, magnesium and manganese) at room temperature. These useful properties of the material were utilized for instance in the famous “Zeppelin” airship [1, 2]. Commercial Al-based alloys were the primary material of choice for structural components of aircraft since about 1930 [1]. Now, more than seventy years after, polymer matrix composites (PMC) are being used in modern commercial aircraft, e.g. for the horizontal stabilizer of the Airbus A340 and the Boeing 777, but conventional aluminum alloys still continue to be the primary material of choice for airframes (e.g. Boeings 7X7 comprise from more than 70 wt. % Al) [1]. Al-Sc-based alloys exhibit a unique combination of high strength and plasticity, corrosion resistance and weldability [3 7]. Mechanical properties of these alloys depend on their microstructure and their phase composition, so its knowledge provides an opportunity to improve material properties by thermal treatment. Electrical resistometry is successfully used in the investigation of phase development as a cheap and fast method, which decreases a number of required specimens for a direct observation by transmission electron microscopy (TEM). Phase transformation in diluted Al-Sc-based alloys The content of solutes is limited in aluminum alloys containing transition metals (transition metals – TM – metals with unpent electron shell, scandium has only one electron on the 3d shell and atomic radius of 0.165 nm). The TM content does not exceed one percent due to the very low solubility of transition metals in solid aluminum – see binary phase diagrams – Fig. 1 (or e.g. figures in [8]). This figure also shows that for Al-Mn, Al-Cr and Al-Sc systems a temperature of invariant transformations, both eutectic and peritectic ones, is close to that of aluminum melting (660.32 C). In the case of solidification of aluminum solid solution, lines of solidus and liquidus come up closer to each other, i.e. solid solution has a very narrow solidification range. Four intermetallic compounds exist in the Al-Sc system: Al3Sc, Al2Sc, AlSc and AlSc2. The Al3Sc phase forms an eutectics with Al at ~ 655°C and ~ 0.38 at. % Sc (0.47 wt. %) [3 5, 8]. The crystal structures and parameters of intermediate compounds are given in Tab. 1. The Al3Sc phase is also reported to have an orthorhombic structure with parameters: 0.50299nm = a , , [9], nevertheless, this result is solitary in the literature. nm 98945 . 0 = b nm 31263 . 0 = c WDS'05 Proceedings of Contributed Papers, Part III, 637–642, 2005. ISBN 80-86732-59-2 © MATFYZPRESS