Preparation and chemical analysis of high purity iron-zinc alloys
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We have produced and chemically analyzed a series of high purity iron-zinc alloys with iron concentrations in the range 4 - 2 7 wt. %. The technique involved slow diffusion of high purity zinc with small particle iron powder. We produced alloys within the four main iron-zinc phases, Zeta, Delta, Gamma-1, and Gamma to aid in the identification of the intermetallic phases formed in commercially produced galvanneal steel coatings. The diffusion technique produces iron-zinc alloys which are suitable as instrument calibration standards for galvanneal producers and users. A chemical titration technique which measures iron concentration to within 0.5 wt. % was also refined. In determining the bulk iron concentrations of the samples, we have compared the accuracy of the wet chemical technique with Induction Coupled Plasma spectroscopy. Homogeneity was measured using an electron microprobe and a scanning transmission electron microscope. The data show that samples are homogeneous to greater than 98% of the mean iron concentration.
I. INTRODUCTION The demand to improve the corrosion resistance of steel sheet, particularly for use within the automotive industry, has led to a dramatic increase in the use of coated steels in place of ordinary cold-rolled sheet steel. Consequently, industrial interest in the processing of zinc alloy coatings has risen over the past decade.' Hot-dip galvanized and galvannealed sheet steel are two products in use today. These processes involve the use of zinc and zinc-alloy coatings to protect the steel through the sacrificial or galvanic mechanism. They are an economic way to apply the zinc, and today's continuous coating lines are capable of producing materials having wellcontrolled coating thickness and uniformity. Galvanneal steel differs from galvanized steel in that it results from post-dip annealing of the zinc-coated steel sheet, thereby allowing the interdiffusion of the iron and zinc to form an iron-zinc intermetallic coating. The iron content in the coating depends primarily on the anneal temperature and time. Within this alloy coating, the four different ironzinc phases, Zeta, Delta, Gamma-1, and Gamma, may be present. In order to manufacture the most advantageous coating, it is important to identify which phases form during the galvannealing process, to understand the properties of each phase, and to know how to control the formation of any particular phase or phases in order to obtain optimal material performance. Positive identification of each phase and the fraction present in a galvanneal coating is very difficult. The primary cause of this difficulty is the lack of high quality data on the crystal structure and the related microstructure of the separate iron-zinc phases. This is due to the fact that 2454
J. Mater. Res., Vol. 10, No. 10, Oct 1995
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it is very difficult to produce pure and homogeneous iron-zinc alloys. In order to accurately study the microstructural properties of the iron-zinc intermetallic alloys, it is nec
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