Strain-Induced Martensitic Transformation Kinetic in Austempered Ductile Iron (ADI)
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INTRODUCTION
AUSTEMPERED ductile iron (ADI) is a cast iron with a distinct microstructure-labeled ausferrite consisting of plate-like ferrite, carbon-enriched retained austenite, and nodular graphite.[1] This unique microstructure is responsible for the excellent mechanical properties of ADI compared to cast iron as it has increased yield strength, wear resistance, and good ductility. In combination with the relatively low manufacturing costs and its weight-saving potential as a cast material, ADI has provided engineers an alternative to steel or aluminum alloys and is being used in a wide range of applications in automotive and heavy engineering industries.[1] This ausferrite matrix is formed by carrying out a heat treatment sequence of austenitization, quenching to holding temperature, austempering, and subsequent cooling to room temperature. During austenitization at temperatures between Tc = 1123 K (850 C) and Tc = 1223 K (950 C), the initial ferrite or pearlite structure transforms to austenite in which carbon diffuses from the graphite nodules. After reaching equilibrium in carbon uptake, the material is quenched to the austempering temperature typically between X.H. LI, M. HOELZEL, W. PETRY, and M. HOFMANN are with the Heinz Maier-Leibnitz Zentrum (MLZ) FRM II, Technische Universita¨t Mu¨nchen, Lichtenbergstr. 1, 85747 Garching, Germany. Contact e-mail: [email protected] P. SAAL and W. VOLK are with the Institute of Metal Forming and Casting (utg), Technische Universita¨t Mu¨nchen, Walther-Meibner-Str. 4, 85747 Garching, Germany. W.M. GAN is with the German Engineering Materials Centre at MLZ, Helmholtz-Zentrum Geesthacht, 85747 Garching, Germany. Manuscript submitted 1 February, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
TAus = 523 K (250 C) and TAus = 723 K (450 C). Once this austempering temperature is reached, a two-phase transformation proceeds. In stage I, ferrite platelets grow into the austenite matrix grains, and since the carbon solubility in ferrite is very low, the excess carbon is redistributed into the austenite. Thus, the austenite becomes stabilized against transformation to martensite when finally cooled to room temperature. After prolonged austempering the undesired stage II, transformation sets in where the high-carbon austenite eventually decomposes into ferrite and iron carbides. The kinetics of the associated phase transformations during the heat treatment and the relationship between the resulting microstructure and mechanical properties are still topics of extensive investigation.[2–5] Following this heat treatment, the ensuing microstructure of ADI consists of acicular ferrite and highly carbon-enriched retained austenite. The carbon-enriched austenite is metastable and several reports exist indicating a partial transformation to martensite when ADI is subjected to deformation.[6–8] Formation of martensite in ADI was first observed on the fractured surface of tensile specimens and the surface of cold rolled specimens using X-ray diffraction and microscopy,[2] while
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