Search

Cryogenic high energy ball milling has been used to synthesize nanocrystalline Cu–14Al, Cu–12Al–2Zr and Cu–12Al–2Y alloys by mechanical alloying. The alloys were studied with the aim of comparing the effect of substituting Y and Zr in place of Al, in Cu–Al alloys, on the grain size stability at elevated temperatures. The as-milled alloys were subjected to annealing at various temperatures between 200 and 900 °C and the resulting grain morphology has been studied using X-ray diffraction and transmission electron microscopy. The addition of Y results in significantly reduced susceptibility to grain growth whereas in case of CuAl and CuAlZr alloys, the susceptibility to grain growth was much higher. The hardness is substantially increased due to Zr and Y addition in the as-milled CuAl powders. However, the hardness of Cu–12Al–2Zr gradually decreases and approaches that of Cu–14Al alloy after the annealing treatment whereas in case of Cu–12Al–2Y alloy, the relative drop in the hardness is much lower after annealing. Accordingly, the efficacy of grain size stabilization by Y addition at high homologous temperatures has been explained on the basis of a recent thermodynamic stabilization models.

Alloys of copper (Cu) and zirconium (Zr) were generated by mechanical alloying via cryogenic, high-energy ball milling and then annealed to a maximum temperature of 1000 °C. The addition of only 1 at% Zr to Cu was found effective at stabilizing the grains in the nanocrystalline state to homologous temperatures in excess of 0.85. When Zr was added in concentrations of 2 and 5 at%, the alloys underwent substantial hardening during annealing, but grain size stability was not enhanced. The mechanism of grain size stabilization was investigated using thermodynamic and kinetic modeling. Zr is predicted to significantly reduce the grain boundary energy of Cu via segregation, but simplifications in the thermodynamic model do not capture high temperature behavior. Kinetically, good correlation between calculation and experimental observation was found by applying estimations for the limiting grain size and Orowan strengthening via second-phase pinning. Both thermodynamic and kinetic mechanisms may be active during annealing, but kinetic parameters appear to be sufficient in explaining the excellent stability of nanocrystalline Cu, even at low Zr concentration.

Copper (Cu) was cryogenically milled with tungsten (W) in a high-energy ball mill. The process created W particles dispersed in a nanocrystalline Cu matrix. These “alloys” were then annealed to a maximum temperature of 800 °C. The addition of W stabilized the Cu at∼40 nm during annealing to 400 °C for a 1 at% W composition and to 600 °C for 10 at% W. As evidenced through hardness measurement, the W provided a significant increase in strength over pure Cu, and the 10 at% W material maintained a 2.6 GPa hardness after annealing at 800 °C. The stabilization and strengthening mechanisms are compared against theoretical prediction and found to be in good agreement. Although the strength and stability are significantly improved over pure Cu, the maximum benefit was hindered by an extremely broad W particle size distribution (∼5–5000 nm). For the 10 at% W alloy, only half of the added W was reduced to nanoscale where kinetic pinning and strengthening become most effective.

A series of Cu– x Nb ( x = 1–15 at.% 2 ) alloys have been investigated to study the metastable solid solubility extension of Nb in Cu by mechanical alloying. Analysis of X-ray diffraction and Gibbs free energy change confirmed that 7.5% of Nb was metastably dissolved in Cu after 8 h of milling at room temperature although Cu–Nb is a system with positive heat of mixing. The solid solubility could be extended up to 10% after enhancing milling duration to 16 h. Detailed thermodynamic analysis revealed that the additional energy stored during mechanical alloying could overcome the required energy barrier as per Miedema's model for the formation of disordered solid solution. The extended solid solubility has been explained along with the other possible mechanisms. Extensive annealing experiments and structural investigation revealed that the supersaturated solid solution is completely stable up to 400 °C. The matrix grains were stabilized and retained their size, ∼25 nm, even after annealing at 600 °C. Microhardness measurement and grain size analysis show that the dissolution of Nb in Cu has a larger strengthening effect than that of free Nb in the compositions.