The role of phase transitions in the evolution of dispersion particles in chromium bronzes upon the equal channel angular pressing

Abstract

Samples of a diluted dispersion-strengthening alloy of the Cu-Cr-Zr system were processed with a combination of some severe plastic deformation methods. Preliminary, the samples were quenched into water from a pre-melting temperature to create a supersaturated solid solution concentration. In the course of the experiment the average sizes and distribution density of particles with different chemical compositions undergo an evolution that indicates that phase transformations occur even during the cold deformation. A decomposition of the solid solution due to the deformation-induced acceleration of diffusion certainly contributes to this evolution, but the whole complex of the observed results cannot be explained only with this relatively well-studied phenomenon and implies the presence of the deformation-induced dissolution of the second-phase particles. A possible mechanism of the dissolution linking this process to a mechanical fragmentation of the particles in the course of severe plastic deformation is discussed. A dependence of the dissolution efficiency on the composition and related morphology of the particles was also noted. A conclusion on the simultaneously occurring of the kinetically opposite phase transitions is also confirmed by an observation of changes in the lattice constant of copper matrix that are non-monotonous versus the accumulated strain. The crystalline structure refinement upon the severe plastic deformation proceeds through selforganization of dense dislocation pileups into grain boundaries what implies a reverse influence of the second-phase particles, which are effective obstacles to dislocation motion, on the structural transformations. Thus, there exists an interdependence between strain mechanisms and phase transitions such as the second phase particles precipitation and their dissolution in the copper matrix that influences the structure refinement, the dispersion particles distribution in the bulk and, consequently, the strength and other properties of the material. This influence may be especially complicated in alloys that like the one under study have a second phase consisted of particles of many different compositions.

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.…”

“…A combination of the precipitation and deformation methods of hardening allows for a more than two-fold increase in the strength of precipitation-hardening copper alloys [1,6,[18][19][20][21]. Initially, hardening occurs due to an increased number of defects and micro-structure refinement during the deformation treatment and then by means of precipitation hardening during metal aging.…”

“…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]. The dimensions and nature of the distribution of second-phase particles have a substantial effect on the mechanical properties of the material.…”