PhD Thesis Defense: Margaret Wu

Monday, September 16, 2019, 5:00–7:00pm

Rm 201 (Rett's Rm), MacLean ESC

“Microstructure and Mechanical Performance of Fe40.2Ni11.3Mn30Al7.5Cr11 and Annealed Fe40.4Ni11.3Mn34.8Al7.5Cr6 with and without 1.1% carbon”

Abstract

There is an increasing demand for modern aerospace, automotive, and power generation systems to become more energy-efficient and environmentally benign. One way of achieving this goal is to develop structural materials that can maintain their corrosion resistance and mechanical properties at elevated service temperatures. Fe40.4Ni11.3Mn34.8Al7.5Cr6 (at. %) containing 1.1 % carbon (CHEA) is a single-phase, f.c.c. high entropy alloy that exhibits good strength and ductility. However, its high-temperature phase stability has not been examined until now. It is the objective of this thesis to investigate the effect of carbon on the microstructure and mechanical performance of annealed Fe40.4Ni11.3Mn34.8Al7.5Cr6. For the CHEA, the presence of (Mn,Cr)23C6 carbides as grain boundary (GB) lamellar colonies at 773K, and as matrix and GB precipitates at higher temperatures led to low (<2%) ductility compared to the un-doped HEA which contained matrix and GB Heusler-phase lamellae at 773K and b.c.c. matrix precipitates at 1073K.

To establish the processing conditions necessary for the large-scale manufacture and utilization of the CHEA’s properties, the material was hot-rolled and annealed. Dense dislocation walls indicated that the alloy had not fully recrystallized, and the deformation substructure and large fraction of Σ3 twin boundaries contributed to the high ambient yield strength (~523 MPa). Despite its substantial dislocation density, the material demonstrated an improvement in elongation to fracture (~33%) compared to the negligible ductility observed for the solely annealed CHEAs.

A novel multi-phase Fe40.2Ni11.3Mn30Al7.5Cr11 alloy with a b.c.c.+B2/f.c.c. structure and displaying an excellent combination of high strength and ductility has been developed. Post- deformation transmission electron micrographs revealed that the f.c.c phase accommodated plastic strain via wavy slip and deformed before the b.c.c. regions, which were reinforced by B2 particles and acted as obstacles to moving dislocations. When annealed at 1173K, b.c.c. and σ particles contributed to rapid age-hardening, while increased ageing temperature led to σ dissolution and a reduction in the b.c.c./B2 volume fraction. A consequent drop in ambient yield strength from the as-cast value of ~593 MPa to ~486 MPa and ~228 MPa for specimens annealed at 1223K and 1273K, respectively, was observed alongside an increase in ductility. An understanding of the strengthening mechanisms in a multi-phase alloy with 11% Cr is vital for its future development as a high-performance, corrosion-resistant structural material.

Thesis Committee

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.