Combined growth of α-Al and Bi in a Al-Bi-Cu monotectic alloy analyzed by in situ X-ray radiography

Abstract

Al-based monotectic alloys show an interesting combination of wear resistance and mechanical strength. While self-lubricating bismuth (Bi) guarantees an adequate wear resistance, the modification with copper (Cu) can increase the ability to support load. Tailoring the micromorphology of the α-Al phase as well as the distribution of the inclusions of Bi embedded in the α-Al matrix is of prime importance in order to improve the properties. In this paper, various solidification experiments are performed with the Al-3.2wt.%Bi-3wt.%Cu monotectic alloy. Real-time radiographs could be acquired during solidification by using the laboratory device SFINX (Solidification Furnace with IN situ X-radiography). The processing conditions could be chosen in such a way that both cellular and dendritic morphologies of the α-Al phase could be clearly seen in a single test. Micromorphological transitions from dendrites to cells are recognized for samples solidified from different cooling rates, i.e. 0.025 K/s and 0.01 K/s. Measurements of the α-Al dendrite velocity and growth parameters of the Bi droplets during solidification have also been extensively performed. A comprehensive model for the cellular-to-dendritic (CDT) transition of Al-based monotectic alloys is outlined. The model considers the interaction between α-Al and Bi droplets during solidification and its impact on the microstructure.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects

“…During solidification, crystals often undergo morphological transitions associated with thermal or solutal perturbation. In situ studies of these transitions have mainly focused on the instability of a planar solid/liquid interface [33,34,37,38], the cellular-to-dendritic transition [35,[37][38][39] and the columnar-to-equiaxed transition (CET) [40,[120][121][122][123][124].…”

“…Unlike the aforementioned works that largely focused on X-ray imaging techniques, the current work instead presents a comprehensive review of knowledge provided by in situ X-ray imaging, for better understanding of solidification theories. Thanks to the high spatial (sub-micron) and temporal (microseconds) resolution offered by third-generation synchrotron sources, significant progress has been made in the understanding, validation and development of theories and models for near-equilibrium solidification, including solute suppressed nucleation (SSN) of both primary solid-solution α-Al [9][10][11][12][13][14] and secondary ordered intermetallics [15,16], dendritic growth of α-Al [17][18][19][20][21][22] and faceted, twin plane re-entrant (TPRE) growth of Fe-rich intermetallics [23][24][25][26], crystal fragmentation [27][28][29][30][31][32], morphological transition [33][34][35][36][37][38][39][40] and defect formation [41][42][43][44][45][46][47]. The focus of solidification research on Al alloys derives from the relatively easy-to-achieve melting temperatures of around 660 °C, and excellent absorption contrast between Al and typical alloying elements such as Cu and Zn.…”

Investigation of Al-20Sn-10Cu alloy directional solidification by laboratory X-radiography

“…Recently, new opportunities have arisen with the improvement of compact microfocus sources and X-ray sensitive detectors that enable in-situ and time-resolved radiography to be used in laboratory devices, with a sufficient spatial and time resolutions to distinguish the microstructure features [11], enabling different orientations of the sample with respect to gravity [12] and compatible with microgravity platforms [13,14]. Such a laboratory device was successfully used by Xavier et al to analyse the morphology of the solidification interface at different cooling rates for an Al-Bi-Cu monotectic alloy [15]. The present work reports on the solidification of an Al-20wt.%Sn-10wt.%Cu sample observed in situ using the SFINX (Solidification Furnace with IN-situ X-radiography) laboratory device.…”