Deformation-induced supersaturation of immiscible binary systems
Research output: Thesis › Doctoral Thesis
Metals immiscible in thermodynamic equilibrium can be forced into a metastable supersaturated state through plastic deformation as evidenced in numerous studies. Despite detailed investigations in the last decades, the underlying processes and the key parameters controlling deformation-induced supersaturation are still a controversial issue. To understand how deformation-induced supersaturation is realized the fundamental deformation behaviors in immiscible composites need to be known. Many system-related characteristics such as hardness level, volume fraction and geometrical arrangement of the constituent phases can induce complex interacting deformation processes. In order to study the deformation behavior and related supersaturation processes a systematic variation of the potential limiting parameters during high-pressure torsion is conducted on two different immiscible systems. Cu-Ag and Ag-Ni were chosen because they coincide concerning lattice structures and positive heat of mixing, but differ in their saturation hardness levels of the constituent phases. In the Cu-Ag composites with initial phase dimension in the micrometer range the hardening behavior and microstructural evolution can be divided in three distinct stages. In the first stage in the beginning of the deformation process co-deformation of the individual phases takes place leading to a lamellar microstructure independent of composition and deformation temperature. In the second stage instead pronounced differences are observed. In composites with low and high Ag content in the second stage further co-deformation takes place, resulting in a single-phase supersaturated solid solution in the third stage, also referred to as saturation regime. In the medium composition range where the constituent phases are available in nearly equal volume fraction, the high number of phase boundaries restricts dislocation activity. As a consequence, pronounced shear band formation governs the refinement and hardening process in the second stage. Large strains are accommodated within shear bands accompanied by a strong refinement and supersaturation, while the lamellar matrix remains dual-phase and only partial supersaturation is achieved. The final composite is composed of Cu-rich, Ag-rich and single-phase supersaturated solid solution regions. To obtain a homogeneous single-phase supersaturated solid solution in the medium composition range lowering of the processing temperature to liquid nitrogen is necessary. At elevated temperatures decomposition is promoted due to thermodynamic driving forces and phase-separated composites evolve independent of composition. In general, a transition from complete single-phase supersaturated alloys at low deformation temperatures to fully phase-separated composites at elevated temperatures occurs. The temperature window for homogenous supersaturation depends on the volume fraction of phases. In the Ag-Ni system the constituent phases have markedly different saturation hardness levels. The deformation behavior is therefore governed by strong strain localization in the softer Ag phase and elongation and repeated fracturing of the harder Ni particles. The effect of strain localization becomes more pronounced when the volume fraction of Ni is decreased. In a Ag-2.3wt%Ni composite the fragmentation of Ni is extremely delayed and retained Ni powder particles are still observed even at the highest applied strain level. Consequently, it takes high strains to reach the critical phase dimension required for dissolution and the supersaturation process is retarded as well. In conclusion, a fundamental relation between the deformation mechanisms in the composites and the resulting degree of supersaturation exists, because inhomogeneous deformation modes can limit or delay the supersaturation process.
|Translated title of the contribution||Verformungsinduzierte Übersättigung in unmischbaren binären Systemen|
|Publication status||Published - 2016|