Formation of two-phase coupled microstructure in AISI 304 stainless steel during directional solidification
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Y.S. Yanga) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
J.J. Guo School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
J.C. Ma and W.H. Tong Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China (Received 19 August 2008; accepted 4 November 2008)
Formation and evolution details of a two-phase coupled microstructure in AISI 304 stainless steel are studied by quenching method during directional solidification. Results show that the coupled growth microstructure, which is composed of thin lathlike ferrite (d) and austenite (g), crystallizes first in the form of colony from the melt. As solidification develops, the retained liquid transforms into austenite gradually. On cooling, solid-state transformation from ferrite to austenite results in the disappearance of part thinner ferrites and the final two-phase coupled microstructure is formed after the solid-state transformation. The formation mechanism of the two-phase coupled microstructure is analyzed based on the nucleation and constitutional undercooling criterion (NCU) before steady-state growth of each phase is reached.
I. INTRODUCTION
Microstructures of austenitic stainless steels have been the object of considerable renewed interest within the last few years because the mechanical properties and corrosion resistance of this alloy are basically determined by the complicated as-cast microstructures.1–6 However, the solidification behavior of austenitic stainless steels is complicated because of the occurrence of a variety of ferrite morphologies during the solidification and subsequent solid-state transformation. Interpretation of the formation mechanism of the different ferrite morphologies is difficult due to the nonequilibrium solidification conditions and the subsequent solid-state transformation on cooling.6–10 Many efforts have been devoted to reveal the formation mechanism of various ferrite morphologies. Frederiksson11 studied the development of the skeletal ferrite morphology and concluded that the skeletal ferrite was resulted from a diffusion-controlled reaction with partitioning of Ni to the austenite and Cr to the ferrite, leaving only a Cr-rich, Ni-lean ferrite core along the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0282 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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dendrite center. Ma et al.12 studied the microstructure evolution in AISI 304 stainless steel during directional solidification by a quenching method. They concluded that the phase transformation sequence in this system is the precipitation of primary ferrite dendrites, ferrite-austenite eutectic reaction, and the direct formation of austenite occur in sequence during directional solidification of the austenitic stainless steel. As solidification develops, the eutectic ferrites disappear and the final morphology of the ferrite is skeletal, resulting from the primary dendritic
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