Unusual kinetics of strain-induced diffusional phase transformations in Cu-Cr-Zr alloy

Abstract

This paper reports experimental results demonstrating nonmonotonic changes of the solid solution concentration in the process of high-pressure torsion of the preliminary aged Cu-Cr-Zr alloy. The solid solution concentration which is very low in the initial state passes through a maximum before it finally stabilizes at a lower value. Such a behavior is, strictly speaking, impossible for a purely diffusion-controlled process under stationary conditions. Observations on the evolution of the second phases particles in the course of deformation suggest a possible mechanism behind this phenomenon. Severe deformation causes refinement of the particles initially present in the alloy by, most probably, quasi-brittle fracture, what creates fragments with sharp edges and makes possible their partial dissolution by Gibbs-Thomson mechanism. The morphology and sizes of the partially dissolved fragments as well as of newly precipitated particles make them less susceptible to fracture than those formed by the preliminary aging. So, under severe deformation, unlike the usually considered models, a "dissolving" subset of particles evolves not only due to diffusion; in the other words, the deformation creates a difference between "dissolving" and "precipitating" subsets of particles. As combined fracture and dissolution transform the initial ensemble of particles, the dissolution gradually slows down unlike the precipitation, which rate is controlled by the solution concentration and density of precipitation sites. As a result, these processes first reach a transitional balance, corresponding to the maximum concentration, and later a stable dynamic equilibrium on its lower level.

Influence of the Chromium Content in Low-Alloyed Cu–Cr Alloys on the Structural Changes, Phase Transformations and Properties in Equal-Channel Angular Pressing

“…The lattice parameter analysis by X-ray diffraction (XRD) showed that during ECAP processing, the lattice parameter was restored insignificantly (Table 2). The results are in the domain of statistical error, but based on the previous studies of alloys of this class [1,19,21,22], it can be suggested that there is a process of deformation-stimulated decay of the supersaturated SS. The Cu-0.2Cr and Cu-1.1Cr alloys showed almost the same level of lattice restoration during ECAP processing (~0.0006 Å).…”

“…The particles thus identified by X-ray analysis are chromium. The specimens were subjected to aging at a temperature of 450 • C [10,19,21,22], which is more characteristic for this class of alloys. Aging up to one hour was completed with an increment of 30 min, and further aging up to 5 h was performed with an increment of 1 h. Micro-hardness and electrical conductivity were measured at each increment.…”

“…When using SPD technologies, the kinetics of phase transformations changes, which leads to a high level of functional properties in low-alloyed copper alloys. The authors of [21][22][23][24][25][26][27] found that under SPD conditions, the second-phase particles could undergo the process of deformation-induced dissolution. Thereby, under SPD conditions, there can be additional supersaturation of the solid solution (SS), which increases the lifetime of precipitation hardening in the case of subsequent post-deformation heat treatment.…”

“…Therefore, scientists [1,6,13,19,21,22,[28][29][30] paid great attention to investigating the structure and the kinetics of phase transformations in copper alloys with chromium concentration of 0.64-0.74 wt.% at the threshold of maximum solubility and at 960-1070 • C. These papers study the processes of the mutual influence of the dislocation structure and second-phase particles during deformation. For example, it is found that dislocations are the predominant location where particles are generated, but the dispersed nanoparticles released are obstacles to dislocation migration, which decelerates relaxation of the deformed structure [6,7,13,21].…”

Effects of Severe Plastic Deformation and Ultrasonic Treatment on the Structure, Strength, and Corrosion Resistance of Mg-Al-Zn Alloy

“…Particles of the second phases can also promote corrosion development [ 12 ]. It is known that SPD methods can promote dynamic aging when the volume fraction of second-phase precipitates increases [ 13 , 14 , 15 ].…”