Microstructure and mechanical properties of Cr–Ta–Si Laves phase-based alloys at elevated temperatures

Abstract

The microstructure and mechanical properties of a series of Cr–Cr₂Ta-based alloys with nominal compositions (90-x)Cr–9Ta–xSi, where x = 4, 6, 10 and 14 at%, have been investigated. The alloys predominantly consisted of a microstructure of primary C14 Cr₂Ta Laves phase dendrites and a eutectic intergrowth of an A2 Cr rich solid solution and the Laves phase. The Laves phase was seen to adopt the C14 polytype in all ternary alloys, with no evidence of transformation to the C15 polytype. Fine, solid-state precipitates of the Laves phase were also observed within the solid solution. At higher silicon contents, a third intermetallic phase, Cr₃Si, with the A15 crystal structure, was also identified. The overall hardness of the alloys at room temperature increased with Si content and hence, the volume fraction of the Laves phase. Compression tests were conducted on samples of the alloys at 1000 and 1100 °C under a strain rate of 10⁻⁴ s⁻¹ in air. Amongst the ternary alloys, the alloy with 14 at% Si showed the highest yield strength of ~440 MPa at 1000 °C and ~290 MPa at 1100 °C. The yield strength of the alloy with 6 at% Si was also found to reduce monotonically with increasing temperature from ambient to 1100 °C. Transmission electron microscopy of the post-compression microstructures revealed that deformation was primarily accommodated by the A2 Cr-rich solid solution, with a high density of dislocations in the phase compared to that in the intermetallic phase. Within the C14 Laves phase, deformation was seen to be mediated by the movement of a/3-type synchro-Shockley partial dislocations, consistent with previous studies, bounding stacking faults on the basal plane.

Keywords:

• intermetallics
• Laves phase
• deformation
• dislocation
• transmission
• electron microscopy

Acknowledgements

Mr Kevin Roberts, University of Cambridge and Tobias Hoffman, University of Erlangen-Nurnberg are thanked for their assistance with the preparation of the alloys and mechanical testing.

Additional information

Funding

Funding. The work was carried out under the financial support provided by Rolls-Royce plc and the Engineering and Physical Sciences Research Council, UK under the Rolls-Royce/ESPRC Strategic Partnership (EP/H500375/1).