Thermo-mechanical Processing

We introduce a new experimental approach to the compositional and thermo-mechanical design and rapid maturation of bulk structural materials. This method, termed rapid alloy prototyping (RAP), is based on semi-continuous high throughput bulk casting, rolling, heat treatment and sample preparation techniques. 45 Material conditions, i.e. 5 alloys with systematically varied compositions, each modified by 9 different ageing treatments, were produced and investigated within 35 h. This accelerated screening of the tensile, hardness and microstructural properties as a function of chemical and thermo-mechanical parameters allows the highly efficient and knowledge-based design of bulk structural alloys. The efficiency of the approach was demonstrated on a group of Fe–30Mn–1.2C–xAl steels which exhibit a wide spectrum of structural and mechanical characteristics, depending on the respective Al concentration. High amounts of Al addition (>8 wt.%) resulted in pronounced strengthening, while low concentrations (<2 wt.%) led to embrittlement of the material during ageing.

A new class of ternary in situ metal matrix composites (MMCs) with high strength and high electrical conductivity consisting of heavily co-deformed Cu, Cr, and Ag is introduced. Three alloys are investigated in detail, namely, Cu–10wt.%Cr–3wt.%Ag, Cu–10wt.%Cr–1wt.%Ag, and Cu–4.5wt.%Cr–3wt.%Ag. The alloys were produced by inductive melting and chill casting. Because Cu–Cr and Cu–Cr–Ag alloys with hypereutectic Cr content are less ductile than previously investigated Cu–Nb, Cu–Ag, and Cu–Nb–Ag alloys, special attention was placed on optimizing microstructure with respect to both strength and ductility using thermal and thermo-mechanical processing schemes. These included various combinations of swaging, heavy wire deformation (using different lubricants), solution annealing at different temperatures followed by quenching, and aging at different temperatures. Optimized processing allows one to attain maximum wire strains of η=8.48 (η=ln(A0/A), A: wire cross-section). The wires have very high strength (for instance Cu–10wt.%Cr–3wt.%Ag: 1260 MPa at a strain of η=8.48) and good electrical conductivity (62% of the conductivity of pure Cu (IACS) at a strain of η=2.5 after solution treatment). Up to wire strains of η≈8.5 the strength is equal to that of Cu–20wt.%Nb. The wire strength is much higher than predicted by the linear rule of mixtures. The investigation presents the evolution of microstructure during the various thermo-mechanical treatments and relates the results to the observed mechanical and electrical properties. The strength is discussed in terms of Hall–Petch-type hardening.