PRECIPITATION INDUCED GRAIN BOUNDARY MIGRATION IN AUSTENITIC ALLOYSD.N. YOON1, M.F. HENRY2 et M.R. JACKSON2
1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. 150, Chyungryang, Seoul, Korea
2 General Electric Corporate Research and Development Center, PO. Box 8, Schenectady, NY 12301, U.S.A.
In 316 stainless steels some grain boundaries with abnormally high curvatures are found after solution treatment and they are observed to migrate over relatively large distances during cooling and isothermal aging at 650°C. When the carbon content is high (0.059 wt.%) and the grain size large, most of the grain boundaries appear to migrate with the precipitation of carbides at 750°C. In Ni-37Co-13Al and Ni-10Ti alloys (by wt.%), discontinuous precipitation occurs during isothermal aging and produces serrated grain boundaries during slow cooling after solution treatment. In Ni-10Ti alloy, the lamellar precipitates produced by discontinuous precipitation are straight and parallel to Widmanstätten precipitates at high temperatures (800° and 900°C), but at 750°C the lamellar precipitates are not straight and do not show any apparent preferred orientation relationship with the matrix. This result shows that the pucker mechanism of Tu and Turnbull cannot be applied generally to this alloy. The grain boundary serration in Ni-base superalloys is predicted to occur by the same migration processes observed in these alloys.