Bulk Mg-Cu-Y-Al Alloys in the Amorphous, Supercooled Liquid and Crystalline States
- PDF / 288,556 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 42 Downloads / 204 Views
Bulk Mg-Cu-Y-Al alloys in the amorphous, supercooled liquid and crystalline states S. Linderoth, N. Pryds, M. Eldrup, A.S. Pedersen, M. Ohnumaa, T.-J. Zhoub, L. Gerwardb, J.Z. Jiangb, and C. Lathec Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark a National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark c HASYLAB am DESY, D-22603 Hamburg, Germany ABSTRACT Bulk Mg-Cu-Y-Al alloys, prepared by casting into a wedge-shaped copper mold, have been studied in the as-prepared, the supercooled liquid, and the crystalline states. In the as-prepared state x-ray diffraction of sub-millimeter sized regions were performed using a focused x-ray beam. The phase composition of the cross section as well as of the surface of the wedge-shaped specimen was investigated as a function of position. The cooling history of the alloy was experimentally determined and compared to results of a control-volume finite-difference modelling study. The experimentally determined and the calculated cooling rates were correlated with the observed amorphous/crystalline structure. The transition from an amorphous to a crystalline state was followed by x-ray diffraction studies as a function of time at specific temperatures in the region between the glass transition and the crystallization temperature. Based on these results a temperature-time-phase diagram was constructed. The dependence of external pressure on the crystallisation temperature was investigated by in situ high-temperature and highpressure x-ray powder diffraction by using synchrotron radiation. The investigations form the basis for a selection of the optimum temperature in the supercooled liquid region for performing deformation/shaping of the Mg-based alloys.
INTRODUCTION The Mg-Cu systems have been found to be glass formers, in particular in the 10-25 at% Cu range [1]. Sommer et al. [1] proposed that a large negative enthalpy of mixing added to the stabilisation of the melt, and hence the amorphous structure, and that a large difference between atomic radii favoured glass formation. Mostly, the glass transition temperature (Tg) can not observed before the material crystallizes. Kim et al. [2] reported that by the addition of Y the glass transition and the crystallisation could be observed separately in differential scanning calorimetry (DSC) experiments, i.e. the melt spun ribbons possessed a supercooled liquid region. Soon after, Inoue et al. [3] demonstrated that amorphous alloys could be produced in thicknesses up to 4 mm. The thick alloy specimens could be made by simple mold casting methods. The possibility of producing thick amorphous Mg-based samples opened up the arena for a new type of lightweight alloys with high strength. The large supercooled liquid region (∆T=Tx-Tg), where Tx is the crystallisation temperature, allowed for large deformations, because the alloys in this temperature interval possess ideal Newtonian flow [4, 5]. Defor
Data Loading...
