Aluminium – Iron – Zinc

Abstract

Constitutional equilibria in the Al-Fe-Zn system is very important for the production of high quality Zncoatings in steels by a process commonly known as hot-dip galvanizing. As a result, a large number of experimental studies have been carried out to determine the phase equilibria. The earlier results [1922Fue, 1934Fue, 1945May, 1947May, 1953Geb, 1953Ray, 1961Ren] on the phase equilibria were reviewed several times [1943Mon, 1952Han, 1961Phi, 1969Wat, 1976Mon]. [1953Ray] studied the solidification using about 150 ternary alloys, and also reported isothermal sections at 350 and 370°C. [1961Ren] investigated the phase equilibria in alloys containing up to 20 mass% Al and 20 mass% Fe. They reported isothermal sections at 600, 400°C and at room temperature. The most comprehensive study was carried out by [1970Koe] and [1971Koe]. They investigated a large number of alloys containing up to 60 mass% (Fe+Zn). The alloys were prepared using Armco-grade Fe and 99.99 mass% Al and Zn. The ternary alloys were prepared by adding either Fe or Zn to a master alloy of Fe:Al 50:50 or to pure Al. The solidification path and the isothermal sections were determined by means of thermal analysis, X-ray diffraction and microstructural investigations. They presented a reaction scheme, liquidus surface, nine isothermal sections in the temperature range of 250 to 700°C, and four temperature-composition sections. [1973Ure1] investigated the partial isothermal section at 450°C by means of metallography and electron microprobe analysis. They carried out equilibration experiments using solid Al-Fe intermetallic (FeAl, FeAl2, Fe2Al5, or Fe4Al13) and either liquid Zn or Zn-1.71Al (mass%) alloy. Prepared samples in evacuated capsules were held at 450°C for 800 h followed by quenching in iced water. These results were critically assessed by [1992Gho] and [1992Rag]. Recently, there has been a renewed interest in the phase equilibria, particularly the Zn corner around 450°C, due to very stringent quality control requirements of galvanized steel sheets for the automotive industry. As a result, recent studies are focused primarily in experimental determination [1990Che, 1992Per, 1994Tan, 1995Tan2, 1996Tan, 1997Gyu, 1997Uwa1, 1999Tan] and CALPHAD modeling [1991Bel, 1992Per, 1999Cos, 2001Gio, 2002Bai] of phase equilibria of the Zn corner in the temperature range of 450 to 470°C. Due to rapid interfacial reaction between steel and liquid Al-Zn alloys, the importance of metastable equilibria [1991Bel, 1992Per, 2002Bai], diffusion path [1992Per, 1998Ada, 1998Uch1, 1998Uch2, 2002Bai], and the mechanism of phase transformations [1994Lin, 1995Lin1, 1995Lin2, 1995Tan1, 1995Yam2, 1997Mcd, 1997Mor, 1997Ser, 1998Ada, 1998Uch1, 1998Uch2, 1998Yam, 2002Bai] during interfacial reaction have also been elucidated. [1990Che] prepared three ternary alloys using Al, Fe and Zn powders of unspecified purity. The final heat treatment of the alloys was annealing at 450°C for about 10 h. The phase equilibria were determined by XRD and SEM/EDX techniques. [1991Bel] determined the stable and metastable solubility limits of Fe in liquid (Zn) 447 to 480°C. [1992Per] determined the metastable and stable isothermal sections at 450°C based on the interfacial reaction studies between solid Al-Fe and liquid Al-Zn alloys. They used Al-Fe alloys containing 5, 29 and 36 at.% Al, and liquid Al-Zn alloys containing 0.12, 0.22, 0.39 and 11.2 at.% Al. Both short time (less than 30 min) and long time (1000 h) experiments were carried out. The phase compositions were determined by SEM/EDX technique. [1994Tan] reported an isothermal section of Zn corner at 470°C. Tang [1995Tan2, 1996Tan] reported the phase equilibria at 450°C by combining the results of [1990Che] and his experimental data of the Zn-corner. [1997Uwa1] prepared four ternary alloys by dry ball milling. They used elemental powders of following purity: 99.5% Al, 99.9+% Fe and 99.9% Zn. The ball milled powders were annealed at 300, 400 and 570°C for 3 h. They used DSC to study phase transformations, and XRD to identify the phases. Some of the controversial results of [1997Uwa1] have been the subject of extensive discussions [1997Tan, 1997Uwa2, 1998Tan, 1998Uwa]. [2000Tan] determined the Fe solubilities in dilute liquid Al-Zn alloys in the temperature range of 450 to 480°C. He prepared 16 ternary alloys containing up to 0.1 mass% Fe and up to 0.23 mass% Al using 99.5% pure Fe