Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging
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3/12/04
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Cu Precipitation in a Prestrained Fe-1.5 Wt Pct Cu Alloy during Isothermal Aging C. ZHANG, M. ENOMOTO, T. YAMASHITA, and N. SANO The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn–Hilliard nonclassical nucleation theory and the Langer–Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.
I. INTRODUCTION
THE age hardenability of Cu precipitates in steel has attracted considerable attention for many years.[1,2] One of the major applications of Cu precipitation is the strengthening of low-carbon and ultra-low-carbon steels for the automotive industry.[3–9] These steels have the potential of achieving good formability and high strength by the control of Cu precipitation. Deformation before aging treatment is expected to reduce the time and temperature of aging and simplify post-heattreatment processes. Thus, several studies were made of the characteristics of age hardening of predeformed Cu bearing steels.[5,8,9] It is noted that the Fe-Cu system was regarded as one of the most suitable systems for studying the precipitation process because of the high solubility of Cu in iron at high temperature and the absence of intermediate phases.[10,11] In spite of these earlier efforts, some issues regarding precipitation behavior are not settled. For example, the concentration of Cu particles was first reported to be less than unity by atom probe field ion microscope (APFIM),[12] while positron annihilation[13] and small angle neutron scattering (SANS)[14,15] tend to support that the particles are pure Cu. Also, the role of dislocations in nucleating Cu particles is not unequivocally understood. Some authors reported that dislocations has no catalytic effect on the nucleation of Cu particles.[5,11] In this article, Cu precipitation was studied both experimentally and by computer simulation. An emphasis is placed on the influence of predeformation on the precipitation during aging at relatively low temperatures. The concentration of Cu nucleus and the activa
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