High strength nanocrystalline Cu–Co alloys with high tensile ductility

Abstract

A supersaturated single-phase Cu-26 at.% Co alloy was produced by high-pressure torsion deformation, leading to a nanocrystalline microstructure with a grain size smaller than 100 nm. The nonequilibrium solid solution decomposed during subsequent isothermal annealing. In situ high-energy X-ray diffraction was used to map changes linked to the separating phases, and the development of a nanoscale Cu-Co composite structure was observed. To gain further information about the relationship of the microstructure and the mechanical properties after phase separation, uniaxial tensile tests were conducted on as-deformed and isothermally annealed samples. Based on the in situ diffraction data, different isothermal annealing temperatures were chosen. Miniaturized tensile specimens with a round cross section were tested, and an image-based data evaluation method enabled the evaluation of true stress-strain curves and strain hardening behavior. The main results are as follows: all microstructural states showed high strength and ductility, which was achieved by a combination of strain-hardening and strain-rate hardening.Andrea Bachmaier received her PhD in Materials Science at the University of Leobenin 2011. For her PhD research, she followed a novel strategy to produce supersaturated solidsolutions in immiscible alloy systems by a new two-step severe plastic deformation (SPD) process to obtain bulk specimens directly. Following graduation she spent two years in industry where she led a large research project funded by the Austrian Research Promotion Agency on advanced high-strength steels. In 2013, she was awarded an Erwin-Schrödinger scholarship that allowed her to work on the mechanisms behind bulk mechanical alloying, the decomposition process of supersaturated solid solutions and its influence on the thermal stability as well as on functional and mechanical properties. She leads a research group at the Erich Schmid Institute of Materials Science in Leoben Austria. Her current research primarily focuses on the generation of metastable materials, novel nanocomposites and nanocrystalline metal-matrix composites by SPD, and ERC Starting Grant funded program on SPD processed nanoscale magnetic materials. 58

Microstructure Evolution and Mechanical Stability of Supersaturated Solid Solution Co-Rich Nanocrystalline Co-Cu Produced by Pulsed Electrodeposition

“…It is assumed that the cobalt content is also an important influencing factor on the improvement of yield strength in nanocrystalline Co-Cu. For example, the PEDprocessed nanocrystalline Co-Cu (28 at.% Cu) exhibits a higher yield strength, compared to the HPTprocessed nanocrystalline Co-Cu (74 at.% Cu) tested by tensile test (σy ≈ 0.8 GPa) [9], whereas this significant difference can mainly be attributed to the higher Co content in the PED-processed material. Different mechanical test methods also contribute to the significant different values of yield strength.…”

“…Investigations into the thermal stability and microstructure evolution of nanocrystalline Co-Cu have been performed in previous works, mostly at temperatures beyond 400 • C [6][7][8], but more research about microstructural changes at lower temperatures is required. The information about the mechanical properties of nanocrystalline Co-Cu is also limited [9][10][11]. On the other hand, the information about the mechanical properties of single-phase nanocrystalline Co and Cu is widely available in the literature [12][13][14][15][16][17].…”

Microstructural Changes Influencing the Magnetoresistive Behavior of Bulk Nanocrystalline Materials

“…The annealing temperatures of 150 °C, 300 °C, and 400 °C were investigated. This was done in combination with an annealing treatment at 600 °C, where well-advanced decomposition of Co and Cu can be expected [26]. The length of all annealing treatments was 1 h. While the annealing treatment at 150 °C was performed in air followed by quick cooling, all other annealing treatments were performed in vacuum with subsequent slow furnace cooling.…”

Generalised stacking fault energies of copper alloys - density functional theory calculations

“…Co: Both the as-deformed and annealed Cu-26at.% Co alloy samples combined high tensile strength withgood ductility andductile fracture behaviour [74], which is achieved by nanoscale composite structure as a result of annealing supersatured solid solution. SFE/USFE ratio is higher than in Cu, which means that PDEis impede.…”