Investigation On Phase Diagram Thermodynamics In The Cu-Cr-Zr(-Ag/Ti) System As Well As Its Aging Microstructure

Aging strengthen Cu-Cr-Zr based copper alloy is one of the most promising high-performance copper alloys,and it is widely used in electronics industry,railways,aerospace and other fields due to their excellent strength and electrical conductivity.So far,most high-performance copper alloys in China still rely on imports.In order to develop new copper alloys with independent intellectual property rights,realize the optimal control of thermodynamic parameter,it is essential to establish an accurate thermodynamic database of the multicomponent Cu alloys.Furthermore,the addition of Ag or Ti in Cu-Cr-Zr alloy can modulate the interaction between elements during the aging precipitation process,and has been demonstrated to improve the mechanical property of Cu-Cr-Zr alloys remarkably.Based on the CALPHAD（CALculation of PHAse Diagram）method,a multivariate thermodynamic database was established.Furthermore,combined with the microstructure characterization,property measurement and strengthening mechanism calculation,the effect of Ag or Ti addition on thermodynamics and kinetics in the aging process of the Cu-Cr-Zr alloys can be explored.In this work,the Cu-Cr-Zr（-Ag/Ti）systems have been used as the research objects,and the major innovative research achievements of the present work are:（1）The thermodynamic parameters of the Cr-Ag binary system was recalculated based on literature evaluation.The isothermal sections at1000°C and 750°C and the vertical sections at 50 at.%Zr,x（Zr）/x（Ag）=9/11 and x（Ag）/x（Cr）=4/1 were constructed for the Cr-Zr-Ag ternary system,and the thermodynamic parameters were determined using the CALPHAD method.Then the thermodynamic parameters of Cu-Cr,Cu-Zr,Cu-Ag,Cr-Zr and Zr-Ag binary systems as well as Cu-Cr-Zr,Cu-Cr-Ag and Cu-Zr-Ag ternary systems were collected from literature.A set of Cu-Cr-Zr-Ag thermodynamic database was constructed based on the thermodynamic parameters of the binary and ternary subsystems.（2）Based on the temperature-composition gradient experiment,First-principles calculations and literature evaluation,the thermodynamic parameters of the Cu-Zr-Ti ternary system were re-optimized by the CALPHAD method.Moreover,based on literature evaluation,the thermodynamic parameters of the Cu-Cr-Ti ternary system were evaluated by the CALPHAD method.The temperature-composition gradient experimental method can greatly reduce time spent conducting experiments and provides a novel strategy to determine phase diagram.The solidification path and phases in the alloy can be accurately determined through a single test cycle,and the change of phase composition was determined as a function the temperature gradient.Then the thermodynamic parameters of Cu-Ti,Cr-Ti and Zr-Ti binary systems as well as Cr-Zr-Ti ternary system were collected from literature.A set of Cu-Cr-Zr-Ti thermodynamic database was constructed based on the thermodynamic parameters of the binary and ternary subsystems.（3）A set of Cu-Cr-Zr-Ag-Ti thermodynamic databases was constructed from the thermodynamic parameters of Cu-Cr-Zr-Ag,Cu-Cr-Zr-Ti,Ag-Ti and Cu-Ag-Ti.Based on thermodynamic and kinetic analysis,the effect of Ti addition on the structural evolution of Cu-Cr-Zr alloy during aging process was studied.Firstly,the influence of Ti contents（0.25,0.6and 1.02 wt.%）on the microstructure,mechanical and electrical properties of Cu-Cr-Zr alloys has been investigated experimentally.The hardness and yield strength have a distinct and positive correlation with Ti content,indicating that the increase of Ti content can improve the mechanical properties.Meanwhile,Ti content exhibited a strong yet negative correlation with the value of electrical conductivity.In peak aging treatment,the precipitate of 0.25Ti and 0.6Ti alloys was Fcc-Cr,while the precipitates of 1.02Ti alloy were Fcc-Cr andβ′-Cu₄Ti.Secondly,the effect of Ti on the strengthening mechanism was evaluated based on four terms:dislocation,solid solution,grain boundary and precipitation strengthenings.A strengthening model considering the size distributions of particles was adopted to estimate precipitate strengthening.Finally,combined with the thermodynamic database and experimental results,the thermodynamic and kinetic analysis of Cu-Cr-Zr-Ti alloy was carried out.Based on thermodynamic analysis,the addition of Ti could reduce the nucleation barrier of Fcc-Cr,thereby promoting the nucleation of Fcc-Cr.Furthermore,it was found that the nucleation energy barrier of Fcc-Cr was much lower than that ofβ′-Cu₄Ti and Bcc-Cr,indicating the Fcc-Cr precipitates were preferentially formed.Based on kinetic analysis,the addition of Ti element can increase the activation energy of precipitation growth process of alloys,hence hindering the growth of precipitates.（4）Based on the relevant literature revolution and the thermodynamic database in this work,combined with thermodynamic and kinetic analysis,the effect of Ag addition on the structural evolution of Cu-Cr-Zr alloy during aging process was discussed.Based on thermodynamic analysis,the addition of Ag could increase the nucleation barrier of Fcc-Cr,thus making the precipitation process of Fcc-Cr difficult.Based on kinetic analysis,the addition of Ag element can decrease the activation energy of precipitation growth process of alloys,thereby promoting the growth of precipitates.In this work,a thermodynamic database of multicomponent Cu alloys was established,and the relationship of"composition/aging process-microstructure-property"in age-hardening copper alloys was systematically investigated.This provides a novel strategy and a reference method for the design of high-performance copper alloys.