Elsevier

Materials Letters

Volume 61, Issue 27, November 2007, Pages 4963-4966
Materials Letters

Phase transformation in Cu–Cr–Zr–Mg alloy

https://doi.org/10.1016/j.matlet.2007.03.085Get rights and content

Abstract

After solid solution treatment, the effects of aging processes on the microstructure and electrical conductivity of Cu–Cr–Zr–Mg lead frame alloy were investigated in order to determine the phase transformation and the time–temperature-transformation (TTT) behavior. The results show that aged at 470 °C the fine precipitation of an ordered CrCu2(Zr,Mg) phase is found in copper matrix as well as fine Cr and Cu4Zr, aged at 550 °C the phase CrCu2(Zr,Mg) is decomposed into Cu4Zr and Cr phase, aged at 600 °C the precipitates is bcc Cr only. The phase transformation kinetics equation was deduced from the Avrami empirical formula based on the linear relationship between the electrical conductivity and the volume fraction of the phase transformation.

Introduction

Cu–Cr–Zr–Mg alloys possess good thermal and electrical conductivity and relatively higher strength, so they have attractive considerable interest in their application to integrated circuit lead frame [1], [2]. Aging process is an effective treatment for lead frame copper alloys, with the aim of raising their strength and conductivity [3], [4], [5]. The high hardness of Cu–Cr–Zr–Mg alloys is due to precipitation and dispersion strengthening, and the excellent electrical conductivity is attributed to the very low solubility of Cr and Zr in Cu. There are great studies on the precipitation phase of the Cu–Cr–Zr alloy. In Ref. [6] the precipitates within the grains were indexed to be Hesuler phase CrCu2(Zr,Mg). At the grain boundary Cu4Zr was indexed. Huang [7] found Cu51Zr14 in the matrix of Cu–Cr–Zr alloy. Refs. [8], [9] showed that three phases Cr, Cu5Zr and Cu should exist in the system. There has been no unanimous agreement on the precipitation phase of the alloy. On the other hand, the researches concerning aging transformation kinetics for Cu–Cr–Zr–Mg alloys are seldom reported. Therefore, it is of great value to study the phase transformation and its kinetics for Cu–Cr–Zr–Mg alloys for selection of the heat treatment processes and for the optimization of properties. Owing to the sensitivity of conductivity to the precipitation, the phase transformation kinetics is dealt with by analyzing the variation of the conductivity.

Section snippets

Experimental procedures

The alloy Cu–0.3Cr–0.15Zr–0.05 Mg was produced in a vacuum induction furnace with electrolytic copper with purity of 99.9%, pure chromium, zirconium and magnesium with purity of 99.5% respectively as charge materials. The billet was homogenized at 900 °C for 2 h and hot-rolled to a thickness of 5 mm. The specimens cut from plate were solution-treated at 920 °C for 1 h in an argon atmosphere and water-quenched. The aging treatments were carried out in a tube electric resistance furnace under a

Results

Upon aging after solid solution, the Cu–Cr–Zr–Mg alloy first undergoes the phase transformation and precipitation process due to its supersaturation. This phenomenon is proved by the change of conductivity on aging. Fig. 1 reveals that the electrical conductivity increases with increasing the time and temperature. The higher temperature and the longer time bring about more precipitates. The growth of precipitates reduces the contents of the solute atom in the matrix and results in a continuous

Discussion

Due to the phase transformation and precipitation from the supersaturated solid solution in Cu–Cr–Zr–Mg alloy, the electrical conductivity increases with increasing the time and temperature as shown in Fig. 1. By measuring the changes in electrical conductivity on aging, the kinetics of phase transformation can be dealt with [11]. Thus the work is aimed at defining TTT curves for the alloy to relate the microstructures observed to the properties.

Conclusions

  • (1)

    After the Cu–Cr–Zr–Mg alloy solid was solution-treated and aged at 470 °C the fine precipitation of CrCu2(Zr,Mg), Cr and Cu4Zr phase is found within the grain, aged at 550 °C the phases of Cu4Zr and Cr exist in the alloy, aged at 600 °C the precipitates are bcc Cr only.

  • (2)

    Phase transformation kinetic equation of Cu–Cr–Zr–Mg alloy aged at 450 °C is deduced and the time–temperature-transformation (TTT) curves are established based on the linearity of the electrical conductivity and the volume

Acknowledgements

This work is funded by Special Foundation for Introducing Talents in Henan University of Science and Technology.

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