Effect of milling temperature on mechanical alloying in the immiscible Cu-Ta system

Elemental powder blends with atomic composition of Cu_100-x Ta_x (x = 10, 30, 50, 70, and 90) were ball milled in a SPEX mill at several temperatures (room temperature (RT), liquid nitrogen temperature (LN₂T), -80°C, and 95°C) to examine the effect of milling temperature on the extent of alloying and microstrucrural refinement. For the Cu-rich powders (10 < x < 50), high-energy ball milling to steady state at all temperatures produced a mixture of nanocrystalline Cu and Ta with no observable extension of mutual solid solubility. Compared with milling at RT, cryomilling (LN₂T) caused further refinement of Cu crystallites, while the same steady-state grain size was reached for Ta crystallites. On the Ta-rich side (50 < x < 90), ball milling at all temperatures led to refined Cu and Ta grain sizes. Partial amorphization seemed to be present, which apparently increased in extent with increasing contamination from the milling media upon extended milling. Very similar results were obtained for milling at RT and LN₂T. It was concluded that high-energy ball milling at LN₂T did not drastically enhance the amorphization reaction between Cu and Ta nor extend their mutual solubility. The limited power of cryomilling to alloy immiscible elements such as Cu-Ta is explained as a consequence of the inability to fully suppress, during energetic collisions, the atomic mobility responsible for phase separation even when the milling is conducted at the nominal LN₂T. The temperature dependence of milling-induced microstrucrural refinement and alloying is analyzed in terms of the dynamics of the generation and annihilation of the nonequilibrium vacancies in an externally driven system. It is predicted that externally forced mixing as well as diffusion assisted by high-energy ball milling can be merely weakly temperature dependent between RT and LN₂T. As a result, the extension of solubility by using cryomilling is feasible only in limited systems, and this process cannot be expected to alloy all immiscible elements.