Supplementary material for:
Design of a Co-Al-W-Ta alloy series with varying γ′ volume fraction and their thermophysical properties
N. Volza, #, F. Xuea, A. Bezolda, C.H. Zenka, S.G. Friesb,*, J. Schreuerc, S. Neumeiera, M. Gökena
a Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Department of Materials Science & Engineering, Institute I, 91058 Erlangen, Germany
b Ruhr-Universität Bochum (RUB), Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), 44801 Bochum, Germany
c Ruhr-Universität Bochum (RUB), Institut of Geology, Mineralogy and Geophysics, 44801 Bochum, Germany
* now at: Ruhr-Universität Bochum (RUB), Materials Research Department, 44801 Bochum, Germany
#corresponding author: N. Volz, nicklas.volz@fau.de
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Supplementary material
Microstructures after different solution heat treatment temperatures
The homogenized microstructure of the experimental alloys after solution heat treatments above 1350 °C is shown in Fig. S1 (a)-(f). After the heat treatment at 1372 °C, the pure γ solution is observed in VF0-VF60, as shown in the representative microstructure of VF60 (see Fig. S1 (a)), which indicates that the as-cast interdendritic Laves phase with an area fraction of 2.6 % dissolved already in the matrix (Fig. 2 (d) and Fig. S1 (a)). Although Laves and μ phases dissolve to different extents in VF80 and VF100 at 1372 °C (Fig. S1 (b)-(c)), they exhibit a more globular morphology but similar contrast, composition and crystallographic structure compared to that in as-cast condition (Fig. 2 (b) and (c)). It is noteworthy that the μ phase, which exists only in VF100 after solidification, is newly formed in VF80 as an equilibrium phase and increases to about 15.7 % in area fraction in VF100 at 1372 °C (Fig. S1 (c)). When the homogenization temperature rises to 1390 °C, the γ single-phase microstructure remains in VF0-VF40, but eutectic pools and micropores with significant size close to or more than 100 μm are frequently found in VF60, as indicated in Fig. S1 (d). These microstructural features are also observed in VF80 and VF100 with increasing area fraction and size. The eutectic pools
are often next to the Laves phase in all these three alloys, and indicate that the incipient melting preferentially occurs in the Ta enriched interdendritic areas. An obvious shrinkage is also observed in VF80 sample and more significant in VF100 sample after heat treatment at 1390 °C, probably resulting from the solidification contraction during cooling from 1390 °C. With the occurrence of melting in VF60-VF100 at 1390 °C, the equilibrium area fraction of Laves and µ phases is difficult to determine due to the existence of complicated solidified microstructure and of pores.
Fig. S1: Microstructure of VF60, VF80 and VF100. (a)-(c) at 1372 °C/24 h and (d)-(f) at 1390 °C/24 h. The temperatures were chosen according to the onset and end of the additional peak in the DSC data marked in Fig. 3.
Based on the results of the DSC measurements and microstructures at temperatures from 1372 °C to 1390 °C, the small endothermic peak in the DSC heating curves of VF80-100 is believed to be associated with the incipient melting of γ matrix or γ+Laves phase. The TS in VF80-VF100 is thus around 1372 °C, which is not obviously changed between these two alloys but still consistent with decreasing TS in present alloy series. To avoid a potential incipient melting and formation of micro pores when bulk SX rods are heat treated, a conservative choice of 1350 °C is made for the homogenization heat treatment temperature for all experimental alloys.
