Phase separation of a supersaturated nanocrystalline Cu–Co alloy and its influence on thermal stability

Abstract

The thermal decomposition behavior, the microstructural evolution and its influence on the mechanical properties of a supersaturated Cu-Co solid solution with ~100 nm average grain size prepared by severe plastic deformation is investigated under non-isothermal and isothermal annealing conditions. Pure fine-grained Cu and Co exhibit substantial grain growth upon annealing, whereas the Cu-Co alloy is thermally stable at the same annealing temperatures. The annealed microstructures are studied by independent characterization methods, including scanning electron microscopy, electron energy loss spectroscopy and atom probe tomography. The phase separation process in the Cu-Co alloy proceeds by the same mechanism, but on different length scales: a fine scaled spinodal-type decomposition is observed in the grain interior, simultaneously Co and Cu regions with a larger scale are formed near the grain boundary regions. Subsequent grain growth at higher annealing temperatures results in a microstructure consisting of the pure equilibrium phases. Such mechanisms can be used to tailor nano structures to optimize certain properties. Published in

Deformation‐Induced Supersaturation in Immiscible Material Systems during High‐Pressure Torsion

“…This is indicated by chemical modulations inside the grains in form of an array of alternating Cu and Co, which was analyzed by EELS and APT, for details see ref. [56] This type of decomposition is observed inside the grains, because diffusion distances are very limited at 400 C and beneath and the grain size is relatively large (compared to the other investigated systems, which all exhibit grain sizes well below 50 nm. The formed structures of phase-separated Cu and Co regions in the nanometer regime, together with Cu and Co particles, which formed at grain boundaries, show an extraordinary high longterm stability.…”

“…The hardness after annealing for 30 min is stable up to 600 C, at annealing temperatures between 200 and 400 C even a hardness increase is observed (Figure 14a). In immiscible systems, this phenomenon is a common feature (also documented in Cu-Fe, [49] Cu-Ag, [53] and Cu-Co [56] ), however, the details of this hardening during annealing are still unclear.…”

“…[56] The grain size of this composite is about %100 nm. During annealing at medium temperatures (400 C), a process similar to spinodal decomposition presumably takes place.…”

“…With ongoing deformation, these dislocations arrange to new grain A. Bachmaier studied materials science and is currently doing her Post-doc at the Erich Schmid Institute, Austria. Grain size in saturation [nm] Cu-Ag (fcc-fcc) [53,54] Cu-6/9Ag %355 %30-50 nm Cu-16/22Ag %380 %20-40 nm Cu-37/46Ag %340 %10 nm Cu-26/25Co %280 %100 nm Cu-Co (fcc-hcp) [55][56][57][58] Cu-54/50Co %400 <50 nm Cu-76/75Co %450 <50 nm Cu-17/17Fe %330 <50 nm Cu-Fe (fcc-bcc) [49] Cu-53/53Fe %440 <50 nm Cu-73/73Fe %445 <50 nm Cu-87/87Fe %640 <50 nm Cu-Cr (fcc-bcc) [32,59] Cu-47/47Cr %450 %10-20 nm Ni-6/9Ag %700 %30-50 nm Ni-Ag (fcc-fcc) [60] Ni-19/26Ag %600 %10-20 nm Ni-35/46Ag [61][62][63] Cu-51/58W %550 %5-15 nm boundaries, which subdivide the initial grains and finally transform the material into an UFG structure.…”

Three orthogonal ultrasounds fabricate uniform ternary Al-Sn-Cu immiscible alloy

“…Therefore, great efforts have been made to explore novel methods to produce the desired uniform composite structure. In recent years, liquid state methods, such as rapid solidification7 and melt spinning8, semi-solid state technologies such as rheo-diecasting2, and solid state methods like mechanical alloying9 and severe plastic deformation1011 are well developed to process immiscible alloys. Nevertheless, the formation of homogenous composite structure for bulk immiscible alloys remains to be a great challenge.…”