Experiment and simulation on the thermal instability of a heavily deformed Cu–Fe composite

Abstract

The thermal instability of the Fe fibers in the heavily deformed Cu–12.8 wt.%Fe composites is investigated experimentally and numerically. The fiber evolution is characterized by a field emission scanning electron microscopy (FESEM). The results show that the dominant instability of the Fe fibers is the longitudinal boundary splitting which is determined by the greater cross sectional aspect ratio (width/thickness, w/t) and the larger ratio of boundary to interfacial energy (γ_B/γ_S). The longitudinal boundary splitting makes the ribbon-like Fe fibers evolve into a series of cylindrical fibers. Then the cylindrical Fe fibers undergo the instability process in terms of the breakup, growth and coarsening concurrently. The breakup times are accurately predicted by the Rayleigh perturbation model. The growth process primarily contributes to the higher increasing rate of the fiber radius during isothermal annealing at 700 °C than that calculated by the coarsening theory developed for cylindrical fibers, since the Cu-matrix of composites is highly supersaturated after casting/cold-working process.

Introduction

The Cu-based in situ composites play an important role in the modern industry particularly for the attractive combination of high strength and high electrical conductivity [1], [2], [3]. Many researches concentrate on the Cu-bcc (Nb, Cr, Fe, Ta, V, etc.) deformed composites prepared by the “casting/cold working” process. The composites are attractive for such applications as transmission lines, lead frames, connectors and other electrical devices. In practical applications, the Cu-based composites undergo a temperature pulse produced by the ohmic heating such as the Cu–Nb composites used in magnets [4], which definitely causes significant microstructure changes, especially in the morphology of reinforced fibers. Consequently, the combined properties and service life of the composites will be degraded. Therefore, it is worthwhile to investigate the thermal instability of the fibers.

The Cu–Fe system is of particular interest and value because of the relatively lower cost and industrially accessible melting temperature of iron as an alloying element compared to the other possible bcc phases [5]. Many researches [6], [7], [8], [9] have been carried on the thermal instability of Cu–Fe composites. Malzahn Kampe et al. [6] firstly propose three thermal instability models of the Fe fibers including cylinderization, boundary splitting and edge spheroidization after analyzing the microstructure evolution of Cu–14.3 vol.%Fe composites at the moderate deformation strain (η = 5.09). Courtney et al. [7] conduct an analysis on the required times of the three instability processes. In addition, the instability models proposed by Malzahn Kampe also have been applied to other Cu-bcc composites [10], [11], [12], [13]. However, these three instability models are proposed on the basis of a moderate draw strain. If the draw strain is heavier, the morphologies of Fe fibers will be further developed and the instability processes become more complicated. In this paper, the microstructural instabilities of the Fe fibers in the deformed Cu–12.8 wt.%Fe composites with a high draw strain of 8.2 are investigated. Both the Rayleigh perturbation and Ostwald ripening models are introduced to analyze the thermal instabilities of the Fe fibers in the heavily deformed Cu–Fe composites.